
                  UNIVERSITY OF CAMBRIDGE COMPUTING SERVICE

                SPECIFICATION OF THE EXIM MAIL TRANSFER AGENT

                                      by

                                 Philip Hazel

University Computing Service
New Museums Site
Pembroke Street
Cambridge CB2 3QH
United Kingdom

phone:  +44 1223 334600
fax:    +44 1223 334679
email:  ph10 at cus.cam.ac.uk

Edition for Exim 4.30, December 2003

                  Copyright (c) University of Cambridge 2003



                                   CONTENTS


1. Introduction

  1.1 Exim documentation
  1.2 FTP and web sites, and mailing list
  1.3 Exim training
  1.4 Bug reports
  1.5 Where to find the Exim distribution
  1.6 Wish list
  1.7 Contributed material
  1.8 Limitations
  1.9 Run time configuration
  1.10 Calling interface
  1.11 Terminology

2. Incorporated code

3. How Exim receives and delivers mail

  3.1 Overall philosophy
  3.2 Policy control
  3.3 User filters
  3.4 Message identification
  3.5 Receiving mail
  3.6 Handling an incoming message
  3.7 Life of a message
  3.8 Processing an address for delivery
  3.9 Processing an address for verification
  3.10 Running an individual router
  3.11 Router preconditions
  3.12 Delivery in detail
  3.13 Retry mechanism
  3.14 Temporary delivery failure
  3.15 Permanent delivery failure
  3.16 Failures to deliver bounce messages

4. Building and installing Exim

  4.1 Unpacking
  4.2 Multiple machine architectures and operating systems
  4.3 DBM libraries
  4.4 Pre-building configuration
  4.5 Support for iconv()
  4.6 Including TLS/SSL encryption support
  4.7 Use of tcpwrappers
  4.8 Including support for IPv6
  4.9 The building process
  4.10 Overriding build-time options for Exim
  4.11 OS-specific header files
  4.12 Overriding build-time options for the monitor
  4.13 Installing Exim binaries and scripts
  4.14 Installing info documentation
  4.15 Setting up the spool directory
  4.16 Testing
  4.17 Replacing another MTA with Exim
  4.18 Upgrading Exim
  4.19 Stopping the Exim daemon on Solaris

5. The Exim command line

  5.1 Setting options by program name
  5.2 Trusted and admin users
  5.3 Command line options

6. The Exim run time configuration file

  6.1 Using a different configuration file
  6.2 Configuration file format
  6.3 File inclusions in the configuration file
  6.4 Macros in the configuration file
  6.5 Conditional skips in the configuration file
  6.6 Common option syntax
  6.7 Boolean options
  6.8 Integer values
  6.9 Octal integer values
  6.10 Fixed point number values
  6.11 Time interval values
  6.12 String values
  6.13 Expanded strings
  6.14 User and group names
  6.15 List construction
  6.16 Format of driver configurations

7. The default configuration file

  7.1 Main configuration settings
  7.2 ACL configuration
  7.3 Router configuration
  7.4 Transport configuration
  7.5 Default retry rule
  7.6 Rewriting configuration
  7.7 Authenticators configuration

8. Regular expressions

  8.1 Testing regular expressions

9. File and database lookups

  9.1 Lookup types
  9.2 Single-key lookup types
  9.3 Query-style lookup types
  9.4 Temporary errors in lookups
  9.5 Default values in single-key lookups
  9.6 Partial matching in single-key lookups
  9.7 Lookup caching
  9.8 Quoting lookup data
  9.9 More about dnsdb
  9.10 More about LDAP
  9.11 Format of LDAP queries
  9.12 LDAP quoting
  9.13 LDAP connections
  9.14 LDAP authentication and control information
  9.15 Format of data returned by LDAP
  9.16 More about NIS+
  9.17 More about MySQL, PostgreSQL, Oracle, and Interbase
  9.18 Special MySQL features
  9.19 Special PostgreSQL features

10. Domain, host, address, and local part lists

  10.1 Expansion of lists
  10.2 Negated items in lists
  10.3 File names in lists
  10.4 An lsearch file is not an out-of-line list
  10.5 Named lists
  10.6 Named lists compared with macros
  10.7 Domain lists
  10.8 Host lists
  10.9 Special host list patterns
  10.10 Host list patterns that match by IP address
  10.11 Host list patterns for single-key lookups by host address
  10.12 Host list patterns that match by host name
  10.13 Host list patterns for single-key lookups by host name
  10.14 Host list patterns for query-style lookups
  10.15 Mixing wildcarded host names and addresses in host lists
  10.16 Address lists
  10.17 Case of letters in address lists
  10.18 Local part lists

11. String expansions

  11.1 Literal text in expanded strings
  11.2 Character escape sequences in expanded strings
  11.3 Testing string expansions
  11.4 Expansion items
  11.5 Expansion operators
  11.6 Expansion conditions
  11.7 Combining expansion conditions
  11.8 Expansion variables

12. Embedded Perl

13. Starting the daemon and the use of network interfaces

  13.1 Starting a listening daemon
  13.2 Special IP listening addresses
  13.3 Overriding local_interfaces and daemon_smtp_ports
  13.4 IPv6 address scopes
  13.5 Examples of starting a listening daemon
  13.6 Recognising the local host
  13.7 Delivering to a remote host

14. Main configuration

  14.1 Miscellaneous
  14.2 Exim parameters
  14.3 Privilege controls
  14.4 Logging
  14.5 Frozen messages
  14.6 Data lookups
  14.7 Message ids
  14.8 Embedded Perl Startup
  14.9 Daemon
  14.10 Resource control
  14.11 Policy controls
  14.12 Callout cache
  14.13 TLS
  14.14 Local user handling
  14.15 All incoming messages (SMTP and non-SMTP)
  14.16 Non-SMTP incoming messages
  14.17 Incoming SMTP messages
  14.18 SMTP extensions
  14.19 Processing messages
  14.20 System filter
  14.21 Routing and delivery
  14.22 Bounce and warning messages
  14.23 Alphabetical list of main options

15. Generic options for routers

16. The accept router

17. The dnslookup router

18. The ipliteral router

19. The iplookup router

20. The manualroute router

  20.1 Private options for manualroute
  20.2 Routing rules in route_list
  20.3 Routing rules in route_data
  20.4 Format of the list of hosts
  20.5 How the list of hosts is used
  20.6 How the options are used
  20.7 Manualroute examples

21. The queryprogram router

22. The redirect router

  22.1 Redirection data
  22.2 Forward files and address verification
  22.3 Interpreting redirection data
  22.4 Items in a non-filter redirection list
  22.5 Redirecting to a local mailbox
  22.6 Special items in redirection lists
  22.7 Duplicate addresses
  22.8 Repeated redirection expansion
  22.9 Errors in redirection lists
  22.10 Private options for the redirect router

23. Environment for running local transports

  23.1 Uids and gids
  23.2 Current and home directories
  23.3 Expansion variables derived from the address

24. Generic options for transports

25. Address batching in local transports

26. The appendfile transport

  26.1 The file and directory options
  26.2 Private options for appendfile
  26.3 Operational details for appending
  26.4 Operational details for delivery to a new file
  26.5 Maildir delivery
  26.6 Mailstore delivery
  26.7 Non-special new file delivery

27. The autoreply transport

  27.1 Private options for autoreply

28. The lmtp transport

29. The pipe transport

  29.1 Returned status and data
  29.2 How the command is run
  29.3 Environment variables
  29.4 Private options for pipe
  29.5 Using an external local delivery agent

30. The smtp transport

  30.1 Multiple messages on a single connection
  30.2 Use of the $host variable
  30.3 Private options for smtp
  30.4 How the value of hosts_max_try is used

31. Address rewriting

  31.1 Testing the rewriting rules that apply on input
  31.2 Rewriting rules
  31.3 Rewriting patterns
  31.4 Rewriting replacements
  31.5 Rewriting flags
  31.6 Flags specifying which headers and envelope addresses to rewrite
  31.7 The SMTP-time rewriting flag
  31.8 Flags controlling the rewriting process
  31.9 Rewriting examples

32. Retry configuration

  32.1 Retry rules
  32.2 Choosing which retry rule to use
  32.3 Retry rules for specific errors
  32.4 Retry rule parameters
  32.5 Retry rule examples
  32.6 Timeout of retry data
  32.7 Long-term failures
  32.8 Ultimate address timeout

33. SMTP authentication

  33.1 Generic options for authenticators
  33.2 The AUTH parameter on MAIL commands
  33.3 Authentication on an Exim server
  33.4 Testing server authentication
  33.5 Authentication by an Exim client

34. The plaintext authenticator

  34.1 Using plaintext in a server
  34.2 The PLAIN authentication mechanism
  34.3 The LOGIN authentication mechanism
  34.4 Support for different kinds of authentication
  34.5 Using plaintext in a client

35. The cram_md5 authenticator

  35.1 Using cram_md5 as a server
  35.2 Using cram_md5 as a client

36. The spa authenticator

  36.1 Using spa as a server
  36.2 Using spa as a client

37. Encrypted SMTP connections using TLS/SSL

  37.1 OpenSSL vs GnuTLS
  37.2 Configuring an Exim server to use TLS
  37.3 Requesting and verifying client certificates
  37.4 Configuring an Exim client to use TLS
  37.5 Multiple messages on the same encrypted TCP/IP connection
  37.6 Certificates and all that
  37.7 Certificate chains
  37.8 Self-signed certificates

38. Access control lists

  38.1 Testing ACLs
  38.2 Specifying when ACLs are used
  38.3 ACL return codes
  38.4 Unset ACL options
  38.5 Data for message ACLs
  38.6 Data for non-message ACLs
  38.7 Use of the ACL selection options
  38.8 Format of an ACL
  38.9 ACL variables
  38.10 Condition and modifier processing
  38.11 ACL modifiers
  38.12 ACL conditions
  38.13 Using DNS lists
  38.14 DNS lists keyed on domain names
  38.15 Data returned by DNS lists
  38.16 Variables set from DNS lists
  38.17 Additional matching conditions for DNS lists
  38.18 Negated DNS matching conditions
  38.19 DNS lists and IPv6
  38.20 Address verification
  38.21 Callout verification
  38.22 Additional parameters for callouts
  38.23 Callout caching
  38.24 Sender address verification reporting
  38.25 Redirection while verifying
  38.26 Using an ACL to control relaying
  38.27 Checking a relay configuration

39. Adding a local scan function to Exim

  39.1 Building Exim to use a local scan function
  39.2 API for local_scan()
  39.3 Configuration options for local_scan()
  39.4 Available Exim variables
  39.5 Structure of header lines
  39.6 Structure of recipient items
  39.7 Available Exim functions
  39.8 More about Exim's memory handling

40. System-wide message filtering

  40.1 Specifying a system filter
  40.2 Testing a system filter
  40.3 Contents of a system filter
  40.4 Additional variable for system filters
  40.5 Defer, freeze, and fail commands for system filters
  40.6 Adding and removing headers in a system filter
  40.7 Setting an errors address in a system filter
  40.8 Per-address filtering

41. Customizing bounce and warning messages

  41.1 Customizing bounce messages
  41.2 Customizing warning messages

42. Some common configuration requirements

  42.1 Sending mail to a smart host
  42.2 Using Exim to handle mailing lists
  42.3 Syntax errors in mailing lists
  42.4 Re-expansion of mailing lists
  42.5 Closed mailing lists
  42.6 Virtual domains
  42.7 Multiple user mailboxes
  42.8 Simplified vacation processing
  42.9 Taking copies of mail
  42.10 Intermittently connected hosts
  42.11 Exim on the upstream server host
  42.12 Exim on the intermittently connected client host

43. SMTP processing

  43.1 Outgoing SMTP and LMTP over TCP/IP
  43.2 Errors in outgoing SMTP
  43.3 Variable Envelope Return Paths (VERP)
  43.4 Incoming SMTP messages over TCP/IP
  43.5 Unrecognized SMTP commands
  43.6 Syntax and protocol errors in SMTP commands
  43.7 Use of non-mail SMTP commands
  43.8 The VRFY and EXPN commands
  43.9 The ETRN command
  43.10 Incoming local SMTP
  43.11 Outgoing batched SMTP
  43.12 Incoming batched SMTP

44. Message processing

  44.1 Line endings
  44.2 Unqualified addresses
  44.3 The UUCP From line
  44.4 Resent- header lines
  44.5 The Auto-Submitted: header line
  44.6 The Bcc: header line
  44.7 The Date: header line
  44.8 The Delivery-date: header line
  44.9 The Envelope-to: header line
  44.10 The From: header line
  44.11 The Message-ID: header line
  44.12 The Received: header line
  44.13 The Return-path: header line
  44.14 The Sender: header line
  44.15 Adding and removing header lines
  44.16 Constructed addresses
  44.17 Case of local parts
  44.18 Dots in local parts
  44.19 Rewriting addresses

45. Log files

  45.1 Where the logs are written
  45.2 Logging to local files that are periodically 'cycled'
  45.3 Datestamped log files
  45.4 Logging to syslog
  45.5 Log line flags
  45.6 Logging message reception
  45.7 Logging deliveries
  45.8 Discarded deliveries
  45.9 Deferred deliveries
  45.10 Delivery failures
  45.11 Fake deliveries
  45.12 Completion
  45.13 Summary of Fields in Log Lines
  45.14 Other log entries
  45.15 Reducing or increasing what is logged
  45.16 Message log

46. Exim utilities

  46.1 Finding out what Exim processes are doing (exiwhat)
  46.2 Selective queue listing (exiqgrep)
  46.3 Summarising the queue (exiqsumm)
  46.4 Extracting specific information from the log (exigrep)
  46.5 Cycling log files (exicyclog)
  46.6 Mail statistics (eximstats)
  46.7 Checking access policy (exim_checkaccess)
  46.8 Making DBM files (exim_dbmbuild)
  46.9 Finding individual retry times (exinext)
  46.10 Hints database maintenance (exim_dumpdb, exim_fixdb, exim_tidyd
  46.11 Mailbox maintenance (exim_lock)

47. The Exim monitor

  47.1 Running the monitor
  47.2 The stripcharts
  47.3 Main action buttons
  47.4 The log display
  47.5 The queue display
  47.6 The queue menu

48. Security considerations

  48.1 Building a more 'hardened' Exim
  48.2 Root privilege
  48.3 Running Exim without privilege
  48.4 Delivering to local files
  48.5 IPv4 source routing
  48.6 The VRFY, EXPN, and ETRN commands in SMTP
  48.7 Privileged users
  48.8 Spool files
  48.9 Use of argv[0]
  48.10 Use of %f formatting
  48.11 Embedded Exim path
  48.12 Use of sprintf()
  48.13 Use of debug_printf() and log_write()
  48.14 Use of strcat() and strcpy()

49. Format of spool files

50. Adding new drivers or lookup types



                               1. INTRODUCTION


    "If I have seen further it is by standing on the shoulders of giants."
                                                                (Isaac Newton)


Exim is a mail transfer agent (MTA) for hosts that are running Unix or Unix-
like operating systems. It was designed on the assumption that it would be run
on hosts that are permanently connected to the Internet. However, it can
be used on intermittently connected hosts with suitable configuration
adjustments.

Configuration files currently exist for the following operating systems: AIX,
BSD/OS (aka BSDI), Darwin (Mac OS X), DGUX, FreeBSD, GNU/Hurd, GNU/Linux, HI-
OSF (Hitachi), HP-UX, IRIX, MIPS RISCOS, NetBSD, OpenBSD, QNX, SCO, SCO SVR4.2
(aka UNIX-SV), Solaris (aka SunOS5), SunOS4, Tru64-Unix (formerly Digital
UNIX, formerly DEC-OSF1), Ultrix, and Unixware. However, code is not available
for determining system load averages under Ultrix.

There are also configuration files for compiling Exim in the Cygwin environ-
ment that can be installed on systems running Windows. However, this document
does not contain any information about running Exim in the Cygwin environment.

The terms and conditions for the use and distribution of Exim are contained in
the file NOTICE. Exim is distributed under the terms of the GNU General Public
Licence, a copy of which may be found in the file LICENCE.

The use, supply or promotion of Exim for the purpose of sending bulk,
unsolicited electronic mail is incompatible with the basic aims of the
program, which revolve around the free provision of a service that enhances
the quality of personal communications. The author of Exim regards indiscrimi-
nate mass-mailing as an antisocial, irresponsible abuse of the Internet.

Exim owes a great deal to Smail 3 and its author, Ron Karr. Without the
experience of running and working on the Smail 3 code, I could never have
contemplated starting to write a new MTA. Many of the ideas and user
interfaces were originally taken from Smail 3, though the actual code of Exim
is entirely new, and has developed far beyond the initial concept.

Many people, both in Cambridge and around the world, have contributed to the
development and the testing of Exim, and to porting it to various operating
systems. I am grateful to them all. The distribution now contains a file
called ACKNOWLEDGMENTS, in which I have started recording the names of
contributors.


1.1 Exim documentation

This edition of the Exim specification applies to version 4.30 of Exim.        |
Substantive changes from the 4.20 edition are marked by bars in the right-hand |
margin in the PostScript, PDF, and plain text versions of the document, and by |
green text in the HTML version, as shown by this paragraph. Changes are not    |
marked in the Texinfo version, because Texinfo doesn't support change bars.    |
Minor corrections and rewordings are not marked.                               |

This document is very much a reference manual; it is not a tutorial. The
reader is expected to have some familiarity with the SMTP mail transfer
protocol and with general Unix system administration. Although there are some
discussions and examples in places, the information is mostly organized in a
way that makes it easy to look up, rather than in a natural order for
sequential reading. Furthermore, the manual aims to cover every aspect of Exim
in detail, including a number of rarely-used, special-purpose features that
are unlikely to be of very wide interest.

An 'easier' discussion of Exim which provides more in-depth explanatory,
introductory, and tutorial material can be found in a book entitled The Exim
SMTP Mail Server, published by UIT Cambridge (http://www.uit.co.uk/exim-
book/).

This book also contains a chapter that gives a general introduction to SMTP
and Internet mail. Inevitably, however, the book is unlikely to be fully up-
to-date with the latest release of Exim. (Note that the earlier book about
Exim, published by O'Reilly, covers Exim 3, and many things have changed in
Exim 4.)

As the program develops, there may be features in newer versions that have not
yet made it into this document, which is updated only when the most
significant digit of the fractional part of the version number changes.
However, specifications of new features that are not yet in this manual are
placed in the file doc/NewStuff in the Exim distribution. All changes to the
program (whether new features, bug fixes, or other kinds of change) are noted
briefly in the file called doc/ChangeLog.

This specification itself is available as an ASCII file in doc/spec.txt so
that it can easily be searched with a text editor. Other files in the doc
directory are:

  OptionLists.txt      list of all options in alphabetical order
  dbm.discuss.txt      discussion about DBM libraries
  exim.8               a man page of Exim's command line options
  filter.txt           specification of the filter language
  pcrepattern.txt      specification of PCRE regular expressions
  pcretest.txt         specification of the PCRE testing program
  Exim3.upgrade        upgrade notes from release 2 to release 3
  Exim4.upgrade        upgrade notes from release 3 to release 4

The main specification and the specification of the filtering language are
also available in other formats (HTML, PostScript, PDF, and Texinfo). Section
1.5 below tells you how to get hold of these.


1.2 FTP and web sites, and mailing list

The primary distribution site for Exim is an FTP site, whose contents are
described in "Where to find the Exim distribution" below. In addition, there
is a web site at http://www.exim.org by courtesy of Energis Squared, formerly
Planet Online Ltd, who are situated in the UK. The site is mirrored in a
number of other countries; links to the mirrors are listed on the home page.
The web site contains the Exim distribution, and you can also find the
documentation and the FAQ online there, as well as other relevant material.

Energis Squared also provide resources for the following mailing lists:

  exim-users@exim.org              general discussion list
  exim-announce@exim.org           moderated, low volume announcements list

You can subscribe to these lists, change your existing subscriptions, and view
or search the archives via the mailing lists link on the Exim home page. The
exim-users mailing list is also forwarded to http://www.egroups.com/list/exim-
users, an archiving system with searching capabilities.


1.3 Exim training

From time to time (approximately annually at the time of writing), lecture-
based training courses are run by the author of Exim in Cambridge, UK. The
web site http://www-tus.csx.cam.ac.uk/courses/exim/ has details of any such
courses that are planned.


1.4 Bug reports

Reports of obvious bugs should be emailed to "bugs@exim.org". However, if you
are unsure whether some behaviour is a bug or not, the best thing to do is to
post a message to the exim-users mailing list and have it discussed.


1.5 Where to find the Exim distribution

The master ftp site for the Exim distribution is

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim

Within that directory there are subdirectories called exim3 (for previous Exim
3 distributions), exim4 (for the latest Exim 4 distributions), and Testing for
occasional testing versions. Those mirror sites that I know about are listed
in the file

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/Mirrors

In the exim4 subdirectory, the current release can always be found in files
called

  exim-n.nn.tar.gz
  exim-n.nn.tar.bz2

where n.nn is the highest such version number in the directory. The two files
contain identical data; the only difference is the type of compression. The
.bz2 file is usually a lot smaller than the .gz file. The distributions are
signed with Philip Hazel's GPG key. The corresponding public key is available  |
from a number of keyservers, and there is also a copy in the file:             |
                                                                               |
ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/Public-Key                     |
                                                                               |
The signatures for the tar bundles are in:

  exim-n.nn.tar.gz.sig
  exim-n.nn.tar.bz2.sig

When there is only a small amount of change from one release to the next, a
patch file may be provided, with a final component name of the form

  exim-patch-n.nn-m.mm.gz

For each released version, the log of changes is made separately available in
the directory

  ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/ChangeLogs

so that it is possible to find out what has changed without having to download
the entire distribution.

The main distribution contains ASCII versions of this specification and other
documentation; other formats of the documents are available in separate files
inside the exim4 directory of the FTP site:

  exim-html-n.nn.tar.gz
  exim-pdf-n.nn.tar.gz
  exim-postscript-n.nn.tar.gz
  exim-texinfo-n.nn.tar.gz

These tar files contain only the doc directory, not the complete distribution,
and are also available in .bz2 as well as .gz forms.

The FAQ is available for downloading in two different formats from

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/FAQ.txt.gz
ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/FAQ.html.tar.gz

The first of these is a single ASCII file that can be searched with a text
editor. The second is a directory of HTML files, normally accessed by starting
at index.html. The HTML version of the FAQ (which is also included in the HTML
documentation tarbundle) includes a keyword-in-context index, which is often
the most convenient way of finding your way around.


1.6 Wish list

A wish list is maintained, containing ideas for new features that have been
submitted. From time to time the file is exported to the ftp site:

  ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/WishList

Items are removed from the list if they get implemented.


1.7 Contributed material

At the ftp site, there is a directory called

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/Contrib/

which contains miscellaneous files contributed to the Exim community by Exim
users. There is also a collection of contributed configuration examples in

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim4/config.samples.tar.gz

These samples are referenced from the FAQ.


1.8 Limitations

 .   Exim is designed for use as an Internet MTA, and therefore handles
     addresses in RFC 2822 domain format only. It cannot handle UUCP 'bang
     paths', though simple two-component bang paths can be converted by a
     straightforward rewriting configuration. This restriction does not pre-
     vent Exim from being interfaced to UUCP as a transport mechanism,
     provided that domain addresses are used.

 .   Exim insists that every address it handles has a domain attached. For
     incoming local messages, domainless addresses are automatically qualified
     with a configured domain value. Configuration options specify from which
     remote systems unqualified addresses are acceptable. These are then
     qualified on arrival.

 .   The only external transport currently implemented is an SMTP transport
     over a TCP/IP network (using sockets, including support for IPv6).
     However, a pipe transport is available, and there are facilities for
     writing messages to files and pipes, optionally in "batched SMTP" format;
     these facilities can be used to send messages to some other transport
     mechanism such as UUCP, provided it can handle domain-style addresses.
     Batched SMTP input is also catered for.

 .   Exim is not designed for storing mail for dial-in hosts. When the volumes
     of such mail are large, it is better to get the messages 'delivered' into
     files (that is, off Exim's queue) and subsequently passed on to the dial-
     in hosts by other means.

 .   Although Exim does have some facilities for scanning incoming messages,   |
     these are not comprehensive enough to do full virus or spam scanning.     |
     Such operations are best carried out using additional specialized soft-   |
     ware packages.                                                            |


1.9 Run time configuration

Exim's run time configuration is held in a single text file that is divided
into a number of sections. The entries in this file consist of keywords and
values, in the style of Smail 3 configuration files. A default configuration
file which is suitable for simple online installations is provided in the
distribution, and is described in chapter 7 below.


1.10 Calling interface

Like many MTAs, Exim has adopted the Sendmail command line interface so that
it can be a straight replacement for /usr/lib/sendmail or /usr/sbin/sendmail
when sending mail, but you do not need to know anything about Sendmail in
order to run Exim. For actions other than sending messages, Sendmail-
compatible options also exist, but those that produce output (for example,
-bp, which lists the messages on the queue) do so in Exim's own format. There
are also some additional options that are compatible with Smail 3, and some
further options that are new to Exim. Chapter 5 documents all Exim's command
line options. This information is automatically made into the man page that
forms part of the Exim distribution.

Control of messages on the queue can be done via certain privileged command
line options. There is also an optional monitor program called "eximon", which
displays current information in an X window, and which contains a menu
interface to Exim's command line administration options.


1.11 Terminology

The "body" of a message is the actual data that the sender wants to transmit.
It is the last part of a message, and is separated from the "header" (see
below) by a blank line.

When a message cannot be delivered, it is normally returned to the sender in a
delivery failure message. The term "bounce" is commonly used for this action,
and the error reports are often called "bounce messages". This is a convenient
shorthand for 'delivery failure error report'. Such messages have an empty
sender address in the message's "envelope" (see below) to ensure that they
cannot themselves give rise to further bounce messages.

The term "default" appears frequently in this manual. It is used to qualify a
value which is used in the absence of any setting in the configuration. It may
also qualify an action which is taken unless a configuration setting specifies
otherwise.

The term "defer" is used when the delivery of a message to a specific
destination cannot immediately take place for some reason (a remote host may
be down, or a user's local mailbox may be full). Such deliveries are
"deferred" until a later time.

The word "domain" is sometimes used to mean all but the first component of a
host's name. It is not used in that sense here, where it normally refers to
the part of an email address following the @ sign.

A message in transit has an associated "envelope", as well as a header and a
body. The envelope contains a sender address (to which bounce messages should
be delivered), and any number of recipient addresses. References to the sender
or the recipients of a message usually mean the addresses in the envelope. An
MTA uses these addresses for delivery, and for returning bounce messages, not
the addresses that appear in the header lines.

The "header" of a message is the first part of a message's text, consisting of
a number of lines, each of which has a name such as From:, To:, Subject:, etc.
Long header lines can be split over several text lines by indenting the
continuations. The header is separated from the body by a blank line.

The term "local part", which is taken from RFC 2822, is used to refer to that
part of an email address that precedes the @ sign. The part that follows the @
sign is called the "domain" or "mail domain".

The terms "local delivery" and "remote delivery" are used to distinguish
delivery to a file or a pipe on the local host from delivery by SMTP over
TCP/IP to a remote host.

"Return path" is another name that is used for the sender address in a
message's envelope.

The term "queue" is used to refer to the set of messages awaiting delivery,
because this term is in widespread use in the context of MTAs. However, in
Exim's case the reality is more like a pool than a queue, because there is
normally no ordering of waiting messages.

The term "queue runner" is used to describe a process that scans the queue and
attempts to deliver those messages whose retry times have come. This term is
used by other MTAs, and also relates to the command "runq", but in Exim the
waiting messages are normally processed in an unpredictable order.

The term "spool directory" is used for a directory in which Exim keeps the
messages on its queue - that is, those that it is in the process of
delivering. This should not be confused with the directory in which local
mailboxes are stored, which is called a 'spool directory' by some people. In
the Exim documentation, 'spool' is always used in the first sense.



                             2. INCORPORATED CODE


A number of pieces of external code are included in the Exim distribution.

 .   Regular expressions are supported in the main Exim program and in the
     Exim monitor using the freely-distributable PCRE library, copyright (c)
     2003 University of Cambridge. The source is distributed in the directory
     src/pcre. However, this is a cut-down version of PCRE. If you want to use
     the PCRE library in other programs, you should obtain and install the
     full version from ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre.


 .   Support for the cdb (Constant DataBase) lookup method is provided by code
     contributed by Nigel Metheringham of Planet Online Ltd. which contains
     the following statements:
     _________________________________________________________________________

     Copyright (c) 1998 Nigel Metheringham, Planet Online Ltd

     This program is free software; you can redistribute it and/or modify it
     under the terms of the GNU General Public License as published by the
     Free Software Foundation; either version 2 of the License, or (at your
     option) any later version.

     This code implements Dan Bernstein's Constant DataBase (cdb) spec.
     Information, the spec and sample code for cdb can be obtained from
     http://www.pobox.com/~djb/cdb.html. This implementation borrows some code
     from Dan Bernstein's implementation (which has no license restrictions
     applied to it).
     _________________________________________________________________________

     The implementation is completely contained within the code of Exim. It
     does not link against an external cdb library.

 .   Client support for Microsoft's "Secure Password Authentication" is pro-   |
     vided by code contributed by Marc Prud'hommeaux. Server support was       |
     contributed by Tom Kistner. This includes code taken from the Samba       |
     project, which is released under the Gnu GPL.                             |


 .   Support for calling the Cyrus "pwcheck" and "saslauthd" daemons is        |
     provided by code taken from the Cyrus-SASL library and adapted by         |
     Alexander S. Sabourenkov. The permission notice appears below, in
     accordance with the conditions expressed therein.

     _________________________________________________________________________

     Copyright (c) 2001 Carnegie Mellon University. All rights reserved.

     Redistribution and use in source and binary forms, with or without
     modification, are permitted provided that the following conditions are
     met:

     1.   Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

     2.   Redistributions in binary form must reproduce the above copyright
          notice, this list of conditions and the following disclaimer in the
          documentation and/or other materials provided with the distribution.

     3.   The name 'Carnegie Mellon University' must not be used to endorse or
          promote products derived from this software without prior written
          permission. For permission or any other legal details, please
          contact

            Office of Technology Transfer
            Carnegie Mellon University
            5000 Forbes Avenue
            Pittsburgh, PA  15213-3890
            (412) 268-4387, fax: (412) 268-7395
            tech-transfer@andrew.cmu.edu

     4.   Redistributions of any form whatsoever must retain the following
          acknowledgment:
             This product includes software developed by Computing Services at
             Carnegie Mellon University (http://www.cmu.edu/computing/).

     CARNEGIE MELLON UNIVERSITY DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS
     SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
     FITNESS, IN NO EVENT SHALL CARNEGIE MELLON UNIVERSITY BE LIABLE FOR ANY
     SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER
     RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF
     CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
     CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
     _________________________________________________________________________


 .   The Exim Monitor program, which is an X-Window application, includes
     modified versions of the Athena StripChart and TextPop widgets. This code
     is copyright by DEC and MIT, and their permission notice appears below,
     in accordance with the conditions expressed therein.

     _________________________________________________________________________

     Copyright 1987, 1988 by Digital Equipment Corporation, Maynard,
     Massachusetts, and the Massachusetts Institute of Technology, Cambridge,
     Massachusetts.

                                All Rights Reserved

     Permission to use, copy, modify, and distribute this software and its
     documentation for any purpose and without fee is hereby granted, provided
     that the above copyright notice appear in all copies and that both that
     copyright notice and this permission notice appear in supporting documen-
     tation, and that the names of Digital or MIT not be used in advertising
     or publicity pertaining to distribution of the software without specific,
     written prior permission.

     DIGITAL DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING
     ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL
     DIGITAL BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR
     ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
     WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
     ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
     SOFTWARE.
     _________________________________________________________________________


 .   Some of the code to support the use of maildirsize files for maildir
     deliveries is taken from the Courier Imapd source code. This code is
     released under the GPL.
     _________________________________________________________________________



                    3. HOW EXIM RECEIVES AND DELIVERS MAIL


3.1 Overall philosophy

Exim is designed to work efficiently on systems that are permanently connected
to the Internet and are handling a general mix of mail. In such circumstances,
most messages can be delivered immediately. Consequently, Exim does not
maintain independent queues of messages for specific domains or hosts, though
it does try to send several messages in a single SMTP connection after a host
has been down, and it also maintains per-host retry information.


3.2 Policy control

Policy controls are now an important feature of MTAs that are connected to the
Internet. Perhaps their most important job is to stop MTAs being abused as
'open relays' by misguided individuals who send out vast amounts of unsolici-
ted junk, and want to disguise its source. Exim provides flexible facilities
for specifying policy controls on incoming mail:

 .   Exim 4 (unlike previous versions of Exim) implements policy controls on
     incoming SMTP mail by means of "Access Control Lists" (ACLs). Each list
     is a series of statements that may either grant or deny access. ACLs can
     be used at several places in the SMTP dialogue while receiving a message.
     However, the most common places are after each RCPT command, and at the
     very end of the message. The sysadmin can specify conditions for
     accepting or rejecting individual recipients or the entire message,
     respectively, at these two points (see chapter 38). Denial of access
     results in an SMTP error code.

 .   An ACL is also available for locally generated, non-SMTP messages. In
     this case, the only available actions are to accept or deny the entire
     message.

 .   When a message has been received, either from a remote host or from the
     local host, but before the final acknowledgement has been sent, a locally
     supplied C function called "local_scan()" can be run to inspect the
     message and decide whether to accept it or not (see chapter 39). If the
     message is accepted, the list of recipients can be modified by the
     function.

 .   After a message has been accepted, a further checking mechanism is
     available in the form of the system filter (see chapter 40). This runs at
     the start of every delivery process.

                                                                               |
3.3 User filters                                                               |
                                                                               |
In a conventional Exim configuration, users are able to run private filters by |
setting up appropriate .forward files in their home directories. See chapter   |
22 (about the redirect router) for the configuration needed to support this,   |
and the separate document entitled "Exim's interfaces to mail filtering" for   |
user details. Two different kinds of filtering are available:                  |
                                                                               |
 .   Sieve filters are written in the standard filtering language that is      |
     defined by RFC 2038.                                                      |
                                                                               |
 .   Exim filters are written in a syntax that is unique to Exim, but which is |
     more powerful than Sieve, which it pre-dates.                             |
                                                                               |
User filters are run as part of the routing process, described below.          |


3.4 Message identification

Every message handled by Exim is given a "message id" which is sixteen
characters long. It is divided into three parts, separated by hyphens, for
example "16VDhn-0001bo-D3". Each part is a sequence of letters and digits,
normally encoding numbers in base 62. However, in the Darwin operating system
(Mac OS X) and when Exim is compiled to run under Cygwin, base 36 (avoiding
the use of lower case letters) is used instead, because the message id is used
to construct file names, and the names of files in those systems are not case-
sensitive.

The detail of the contents of the message id have changed as Exim has evolved.
Earlier versions relied on the operating system not re-using a process id
(pid) within one second. On modern operating systems, this assumption can no
longer be made, so the algorithm had to be changed. To retain backward
compatibility, the format of the message id was retained, which is why the
following rules are somewhat eccentric:

 .   The first six characters of the message id are the time at which the
     message started to be received, to a granularity of one second. That is,
     this field contains the number of seconds since the start of the epoch
     (the normal Unix way of representing the date and time of day).

 .   After the first hyphen, the next six characters are the id of the process
     that received the message.

 .   There are two different possibilities for the final two characters:

     (a)  If "localhost_number" is not set, this value is the fractional part
          of the time of reception, normally in units of 1/2000 of a second,
          but for systems that must use base 36 instead of base 62 (because of
          case-insensitive file systems), the units are 1/1000 of a second.

     (b)  If "localhost_number" is set, it is multiplied by 200 (100) and
          added to the fractional part of the time, which in this case is in
          units of 1/200 (1/100) of a second.

After a message has been received, Exim waits for the clock to tick at the
appropriate resolution before proceeding, so that if another message is
received by the same process, or by another process with the same (re-used)
pid, it is guaranteed that the time will be different. In most cases, the
clock will already have ticked while the message was being received.


3.5 Receiving mail

The only way Exim can receive mail from a remote host is using SMTP over
TCP/IP, in which case the sender and recipient addresses are tranferred using
SMTP commands. However, from a locally running process (such as a user's MUA),
there are several possibilities:

 .   If the process runs Exim with the -bm option, the message is read non-
     interactively (usually via a pipe), with the recipients taken from the
     command line, or from the body of the message if -t is also used.

 .   If the process runs Exim with the -bS option, the message is also read
     non-interactively, but in this case the recipients are listed at the
     start of the message in a series of SMTP RCPT commands, terminated by a
     DATA command. This is so-called 'batch SMTP' format, but it isn't really
     SMTP. The SMTP commands are just another way of passing envelope
     addresses in a non-interactive submission.

 .   If the process runs Exim with the -bs option, the message is read
     interactively, using the SMTP protocol. A two-way pipe is normally used
     for passing data between the local process and the Exim process. This is
     'real' SMTP and is handled in the same way as SMTP over TCP/IP. For
     example, the ACLs for SMTP commands are used for this form of submission.

 .   A local process may also make a TCP/IP call to the host's loopback
     address (127.0.0.1) or any other of its IP addresses. When receiving
     messages, Exim does not treat the loopback address specially. It treats
     all such connections in the same way as connections from other hosts.

In the three cases that do not involve TCP/IP, the sender address is
constructed from the login name of the user that called Exim and a default
qualification domain (which can be set by the "qualify_domain" configuration
option). For local or batch SMTP, a sender address that is passed using the
SMTP MAIL command is ignored. However, the system administrator may allow
certain users ('trusted users') to specify a different sender address uncon-
ditionally, or all users to specify certain forms of different sender address.
The -f option or the SMTP MAIL command is used to specify these different
addresses. See section 5.2 for details of trusted users, and the
"untrusted_set_sender" option for a way of allowing untrusted users to change
sender addresses.

Messages received by either of the non-interactive mechanisms are subject to
checking by the non-SMTP ACL, if one is defined. Messages received using SMTP
(either over TCP/IP, or interacting with a local process) can be checked by a
number of ACLs that operate at different times during the SMTP session. Either
individual recipients, or the entire message, can be rejected if local policy
requirements are not met. The "local_scan()" function (see chapter 39) is run
for all incoming messages.

Exim can be configured not to start a delivery process when a message is
received; this can be unconditional, or depend on the number of incoming SMTP
connections or the system load. In these situations, new messages wait on the
queue until a queue runner process picks them up. However, in standard
configurations under normal conditions, delivery is started as soon as a
message is received.


3.6 Handling an incoming message

When Exim accepts a message, it writes two files in its spool directory. The
first contains the envelope information, the current status of the message,
and the header lines, and the second contains the body of the message. The
names of the two spool files consist of the message id, followed by -H for the
file containing the envelope and header, and -D for the data file.

By default all these message files are held in a single directory called input
inside the general Exim spool directory. Some operating systems do not perform
very well if the number of files in a directory gets very large; to improve
performance in such cases, the "split_spool_directory" option can be used.
This causes Exim to split up the input files into 62 sub-directories whose
names are single letters or digits.

The envelope information consists of the address of the message's sender and
the addresses of the recipients. This information is entirely separate from
any addresses contained in the header lines. The status of the message
includes a list of recipients who have already received the message. The
format of the first spool file is described in chapter 49.

Address rewriting that is specified in the rewrite section of the configur-
ation (see chapter 31) is done once and for all on incoming addresses, both in
the header lines and the envelope, at the time the message is accepted. If
during the course of delivery additional addresses are generated (for example,
via aliasing), these new addresses are rewritten as soon as they are
generated. At the time a message is actually delivered (transported) further
rewriting can take place; because this is a transport option, it can be
different for different forms of delivery. It is also possible to specify the
addition or removal of certain header lines at the time the message is
delivered (see chapters 15 and 24).


3.7 Life of a message

A message remains in the spool directory until it is completely delivered to
its recipients or to an error address, or until it is deleted by an
administrator or by the user who originally created it. In cases when delivery
cannot proceed - for example, when a message can neither be delivered to its
recipients nor returned to its sender, the message is marked 'frozen' on the
spool, and no more deliveries are attempted.

An administrator can 'thaw' such messages when the problem has been corrected,
and can also freeze individual messages by hand if necessary. In addition, an
administrator can force a delivery error, causing a bounce message to be sent.

There is an option called "auto_thaw", which can be used to cause Exim to
retry frozen messages after a certain time. When this is set, no message will
remain on the queue for ever, because the delivery timeout will eventually be
reached. Delivery failure reports (bounce messages) that reach this timeout
are discarded. There is also an option called "timeout_frozen_after", which
discards frozen messages after a certain time.

While Exim is working on a message, it writes information about each delivery
attempt to the main log file. This includes successful, unsuccessful, and
delayed deliveries for each recipient (see chapter 45). The log lines are also
written to a separate message log file for each message. These logs are solely
for the benefit of the administrator, and are normally deleted along with the
spool files when processing of a message is complete. The use of individual
message logs can be disabled by setting "no_message_logs"; this might give an
improvement in performance on very busy systems.

All the information Exim itself needs to set up a delivery is kept in the
first spool file, along with the header lines. When a successful delivery
occurs, the address is immediately written at the end of a journal file, whose
name is the message id followed by -J. At the end of a delivery run, if there
are some addresses left to be tried again later, the first spool file (the -H
file) is updated to indicate which these are, and the journal file is then
deleted. Updating the spool file is done by writing a new file and renaming
it, to minimize the possibility of data loss.

Should the system or the program crash after a successful delivery but before
the spool file has been updated, the journal is left lying around. The next
time Exim attempts to deliver the message, it reads the journal file and
updates the spool file before proceeding. This minimizes the chances of double
deliveries caused by crashes.


3.8 Processing an address for delivery

The main delivery processing elements of Exim are called routers and trans-
ports, and collectively these are known as drivers. Code for a number of them
is provided in the source distribution, and compile-time options specify which
ones are included in the binary. Run time options specify which ones are
actually used for delivering messages.

Each driver that is specified in the run time configuration is an "instance"
of that particular driver type. Multiple instances are allowed; for example,
you can set up several different smtp transports, each with different option
values that might specify different ports or different timeouts. Each instance
has its own identifying name. In what follows we will normally use the
instance name when discussing one particular instance (that is, one specific
configuration of the driver), and the generic driver name when discussing the
driver's features in general.

A router is a driver that operates on an address, either determining how its
delivery should happen, by routing it to a specific transport, or converting
the address into one or more new addresses (for example, via an alias file). A
router may also explicitly choose to fail an address, causing it to be
bounced.

A transport is a driver that transmits a copy of the message from Exim's spool
to some destination. There are two kinds of transport: for a local transport,
the destination is a file or a pipe on the local host, whereas for a remote
transport the destination is some other host. A message is passed to a
specific transport as a result of successful routing. If a message has several
recipients, it may be passed to a number of different transports.

An address is processed by passing it to each configured router instance in
turn, subject to certain preconditions, until a router accepts the address or
specifies that it should be bounced. We will describe this process in more
detail shortly. As a simple example, the diagram below illustrates how each
recipient address in a message is processed in a small configuration of three
routers that are configured in various ways.


         address
            |
            |<------------- new addresses -----------------------------
            V                                                         |
    -----------------                -----------------                |
    | first router  |----- yes ----->|     run       |--- accept      |
    | conditions ok?|                | first router  |      |         |
    -----------------                -----------------      |         |
            |                                |              V         |
         no |                           fail |          queue for     |
            |                                V          transport     |
            |                         address bounces                 |
            |                                                         |
            V                                                         |
    -----------------                -----------------                |
    | second router |----- yes ----->|     run       |----redirect ----
    | conditions ok?|                | second router |
    -----------------                -----------------
            |                            |       |
         no |                            |       |
            |<-------- decline -----------       --- fail ---> address
            |                                                  bounces
            V
    -----------------                -----------------
    | third router  |----- yes ----->|     run       |--- accept
    | conditions ok?|                | third router  |      |
    -----------------                -----------------      |
            |                                |              V
         no |                                |          queue for
            |<-------- decline ---------------          transport
            |
            V
     no more routers
     address bounces

To make this a more concrete example, we'll describe it in terms of some
actual routers, but remember, this is only an example. You can configure
Exim's routers in many different ways, and there may be any number of routers
in a configuration.

The first router that is specified in a configuration is often one that
handles addresses in domains that are not recognized specially by the local
host. These are typically addresses for arbitrary domains on the Internet. A
precondition is set up which looks for the special domains known to the host
(for example, its own domain name), and the router is run for addresses that
do not match. Typically, this is a router that looks up domains in the DNS in
order to find the hosts to which this address routes. If it succeeds, the
address is queued for a suitable SMTP transport; it it does not succeed, the
router is configured to fail the address.

The example pictured could be a configuration of this type. The second and
third routers can only be run for addresses for which the preconditions for
the first router are not met. If one of these preconditions checks the domain,
the second and third routers are run only for domains that are somehow special
to the local host.

The second router does redirection - also known as aliasing and forwarding.
When it generates one or more new addresses from the original, each of them is
routed independently from the start. Otherwise, the router may cause an
address to fail, or it may simply decline to handle the address, in which case
the address is passed to the next router.

The final router in many configurations is one that checks to see if the
address belongs to a local mailbox. The precondition may involve a check to
see if the local part is the name of a login account, or it may look up the
local part in a file or a database. If its preconditions are not met, or if
the router declines, we have reached the end of the routers. When this
happens, the address is bounced.

                                                                               |
3.9 Processing an address for verification                                     |
                                                                               |
As well as being used to decide how to deliver to an address, Exim's routers   |
are also used for "address verification". Verification can be requested as one |
of the checks to be performed in an ACL for incoming messages, on both sender  |
and recipient addresses, and it can be tested using the -bv and -bvs command   |
line options.                                                                  |
                                                                               |
When an address is being verified, the routers are run in 'verify mode'. This  |
does not affect the way the routers work, but it is a state that can be        |
detected. By this means, a router can be skipped or made to behave differently |
when verifying. A common example is a configuration in which the first router  |
sends all messages to a message-scanning program, unless they have been        |
previously scanned. Thus, the first router accepts all addresses without any   |
checking, making it useless for verifying. Normally, the "no_verify" option    |
would be set for such a router, causing it to be skipped in verify mode.       |


3.10 Running an individual router

As explained in the example above, a number of preconditions are checked
before running a router. If any are not met, the router is skipped, and the
address is passed to the next router. When all the preconditions on a router
are met, the router is run. What happens next depends on the outcome, which is
one of the following:

 .   "accept": The router accepts the address, and either queues it for a
     transport, or generates one or more 'child' addresses. Processing the
     original address ceases, unless the "unseen" option is set on the router.
     This option can be used to set up multiple deliveries with different
     routing (for example, for keeping archive copies of messages). When
     "unseen" is set, the address is passed to the next router. Normally,
     however, an "accept" return marks the end of routing.

     If child addresses are generated, Exim checks to see whether they are
     duplicates of any existing recipient addresses. During this check, local
     parts are treated as case-sensitive. Duplicate addresses are discarded.
     Each of the remaining child addresses is then processed independently,
     starting with the first router by default. It is possible to change this
     by setting the "redirect_router" option to specify which router to start
     at for child addresses. Unlike "pass_router" (see below) the router
     specified by "redirect_router" may be anywhere in the router
     configuration.

 .   "pass": The router recognizes the address, but cannot handle it itself.
     It requests that the address be passed to another router. By default the
     address is passed to the next router, but this can be changed by setting
     the "pass_router" option. However, (unlike "redirect_router") the named
     router must be below the current router (to avoid loops).

 .   "decline": The router declines to accept the address because it does not
     recognize it at all. By default, the address is passed to the next
     router, but this can be prevented by setting the "no_more" option. When
     "no_more" is set, all the remaining routers are skipped.

 .   "fail": The router determines that the address should fail, and queues it
     for the generation of a bounce message. There is no further processing of
     the original address unless "unseen" is set on the router.

 .   "defer": The router cannot handle the address at the present time. (A
     database may be offline, or a DNS lookup may have timed out.) No further
     processing of the address happens in this delivery attempt. It is tried
     again next time the message is considered for delivery.

 .   "error": There is some error in the router (for example, a syntax error
     in its configuration). The action is as for defer.

If an address reaches the end of the routers without having been accepted by
any of them, it is bounced as unrouteable. The default error message in this
situation is 'unrouteable address', but you can set your own message by making
use of the "cannot_route_message" option. This can be set for any router; the
value from the last router that 'saw' the address is used.

Sometimes while routing you want to fail a delivery when some conditions are
met but others are not, instead of passing the address on for further routing.
You can do this by having a second router that explicitly fails the delivery
when the relevant conditions are met. The redirect router has a 'fail'
facility for this purpose.


3.11 Router preconditions

The preconditions that are tested for each router are listed below, in the
order in which they are tested. The individual configuration options are
described in more detail in chapter 15.

 .   The "local_part_prefix" and "local_part_suffix" options can specify that
     the local parts handled by the router may or must have certain prefixes
     and/or suffixes. If a mandatory affix (prefix or suffix) is not present,
     the router is skipped. These conditions are tested first. When an affix
     is present, it is removed from the local part before further processing,
     including the evaluation of any other conditions.

 .   Routers can be designated for use only when not verifying an address,
     that is, only when routing it for delivery (or testing its delivery
     routing). If the "verify" option is set false, the router is skipped when
     Exim is verifying an address. Setting the "verify" option actually sets   |
     two options, "verify_sender" and "verify_recipient", which independently  |
     control the use of the router for sender and recipient verification. You  |
     can set these options directly if you want a router to be used for only   |
     one type of verification.                                                 |

 .   If the "address_test" option is set false, the router is skipped when
     Exim is run with the -bt option to test an address routing. This can be
     helpful when the first router sends all new messages to a scanner of some
     sort; it makes it possible to use -bt to test subsequent delivery routing
     without having to simulate the effect of the scanner.

 .   Routers can be designated for use only when verifying an address, as
     opposed to routing it for delivery. The "verify_only" option controls
     this.

 .   Certain routers can be explicitly skipped when running the routers to
     check an address given in the SMTP EXPN command (see the "expn" option).

 .   If the "domains" option is set, the domain of the address must be in the
     set of domains that it defines.

 .   If the "local_parts" option is set, the local part of the address must be
     in the set of local parts that it defines. If "local_part_prefix" or
     "local_part_suffix" is in use, the prefix or suffix is removed from the
     local part before this check. If you want to do precondition tests on
     local parts that include affixes, you can do so by using a "condition"
     option (see below) that uses the variables $local_part,
     $local_part_prefix, and $local_part_suffix as necessary.

 .   If the "check_local_user" option is set, the local part must be the name
     of an account on the local host. If this check succeeds, the uid and gid  |
     of the local user are placed in $local_user_uid and $local_user_gid;      |
     these values can be used in the remaining preconditions.                  |

 .   If the "router_home_directory" option is set, it is expanded at this
     point, because it overrides the value of $home. If this expansion were
     left till later, the value of $home as set by "check_local_user" would be
     used in subsequent tests. Having two different values of $home in the
     same router could lead to confusion.

 .   If the "senders" option is set, the envelope sender address must be in
     the set of addresses that it defines.

 .   If the "require_files" option is set, the existence or non-existence of
     specified files is tested.

 .   If the "condition" option is set, it is evaluated and tested. This option
     uses an expanded string to allow you to set up your own custom
     preconditions. Expanded strings are described in chapter 11.

Note that "require_files" comes near the end of the list, so you cannot use it
to check for the existence of a file in which to lookup up a domain, local
part, or sender. However, as these options are all expanded, you can use the
"exists" expansion condition to make such tests within each condition. The
"require_files" option is intended for checking files that the router may be
going to use internally, or which are needed by a specific transport (for
example, .procmailrc).


3.12 Delivery in detail

When a message is to be delivered, the sequence of events is as follows:

 .   If a system-wide filter file is specified, the message is passed to it.
     The filter may add recipients to the message, replace the recipients,
     discard the message, cause a new message to be generated, or cause the
     message delivery to fail. The format of the system filter file is the
     same as for Exim user filter files, described in the separate document
     entitled "Exim's interfaces to mail filtering". (Note: Sieve cannot be    |
     used for system filter files.) Some additional features are available in  |
     system filters - see chapter 40 for details. Note that a message is
     passed to the system filter only once per delivery attempt, however many
     recipients it has. However, if there are several delivery attempts
     because one or more addresses could not be immediately delivered, the
     system filter is run each time. The filter condition "first_delivery" can
     be used to detect the first run of the system filter.

 .   Each recipient address is offered to each configured router in turn,
     subject to its preconditions, until one is able to handle it. If no
     router can handle the address, that is, if they all decline, the address
     is failed. Because routers can be targeted at particular domains, several
     locally handled domains can be processed entirely independently of each
     other.

 .   A router that accepts an address may set up a local or a remote transport
     for it. However, the transport is not run at this time. Instead, the
     address is placed on a queue for the particular transport, to be run
     later. Alternatively, the router may generate one or more new addresses
     (typically from alias, forward, or filter files). New addresses are fed
     back into this process from the top, but in order to avoid loops, a
     router ignores any address which has an identically-named ancestor that
     was processed by itself.

 .   When all the routing has been done, addresses that have been successfully
     handled are passed to their assigned transports. When local transports
     are doing real local deliveries, they handle only one address at a time,
     but if a local transport is being used as a pseudo-remote transport (for
     example, to collect batched SMTP messages for transmission by some other
     means) multiple addresses can be handled. Remote transports can always
     handle more than one address at a time, but can be configured not to do
     so, or to restrict multiple addresses to the same domain.

 .   Each local delivery runs in a separate process under a non-privileged
     uid, and they are run in sequence. Remote deliveries also run in separate
     processes, normally under a uid that is private to Exim ('the Exim
     user'), but in this case, several remote deliveries can be run in
     parallel. The maximum number of simultaneous remote deliveries for any
     one message is set by the "remote_max_parallel" option.

 .   When it encounters a local delivery during a queue run, Exim checks its
     retry database to see if there has been a previous temporary delivery
     failure for the address before running the local transport. If there was
     a previous failure, Exim does not attempt a new delivery until the retry
     time for the address is reached. However, this happens only for delivery
     attempts that are part of a queue run. Local deliveries are always
     attempted when delivery immediately follows message reception, even if
     retry times are set for them. This makes for better behaviour if one
     particular message is causing problems (for example, causing quota
     overflow, or provoking an error in a filter file).

 .   Remote transports do their own retry handling, since an address may be
     deliverable to one of a number of hosts, each of which may have a
     different retry time. If there have been previous temporary failures and
     no host has reached its retry time, no delivery is attempted, whether in
     a queue run or not. See chapter 32 for details of retry strategies.

 .   If there were any permanent errors, a bounce message is returned to an
     appropriate address (the sender in the common case), with details of the
     error for each failing address. Exim can be configured to send copies of
     bounce messages to other addresses.

 .   If one or more addresses suffered a temporary failure, the message is
     left on the queue, to be tried again later. Delivery of these addresses
     is said to be "deferred".

 .   When all the recipient addresses have either been delivered or bounced,
     handling of the message is complete. The spool files and message log are
     deleted, though the message log can optionally be preserved if required.


3.13 Retry mechanism

Exim's mechanism for retrying messages that fail to get delivered at the first
attempt is the queue runner process. You must either run an Exim daemon that
uses the -q option with a time interval to start queue runners at regular
intervals, or use some other means (such as "cron") to start them. If you do
not arrange for queue runners to be run, messages that fail temporarily at the
first attempt will remain on your queue for ever. A queue runner process works
it way through the queue, one message at a time, trying each delivery that has
passed its retry time. You can run several queue runners at once.

Exim uses a set of configured rules to determine when next to retry the
failing address (see chapter 32). These rules also specify when Exim should
give up trying to deliver to the address, at which point it generates a bounce
message. If no retry rules are set for a particular host, address, and error
combination, no retries are attempted, and temporary errors are treated as
permanent.


3.14 Temporary delivery failure

There are many reasons why a message may not be immediately deliverable to a
particular address. Failure to connect to a remote machine (because it, or the
connection to it, is down) is one of the most common. Temporary failures may
be detected during routing as well as during the transport stage of delivery.
Local deliveries may be delayed if NFS files are unavailable, or if a mailbox
is on a file system where the user is over quota. Exim can be configured to
impose its own quotas on local mailboxes; where system quotas are set they
will also apply.

If a host is unreachable for a period of time, a number of messages may be
waiting for it by the time it recovers, and sending them in a single SMTP
connection is clearly beneficial. Whenever a delivery to a remote host is
deferred, Exim makes a note in its hints database, and whenever a successful
SMTP delivery has happened, it looks to see if any other messages are waiting
for the same host. If any are found, they are sent over the same SMTP
connection, subject to a configuration limit as to the maximum number in any
one connection.


3.15 Permanent delivery failure

When a message cannot be delivered to some or all of its intended recipients,
a bounce message is generated. Temporary delivery failures turn into permanent
errors when their timeout expires. All the addresses that fail in a given
delivery attempt are listed in a single message. If the original message has
many recipients, it is possible for some addresses to fail in one delivery
attempt and others to fail subsequently, giving rise to more than one bounce
message. The wording of bounce messages can be customized by the adminis-
trator. See chapter 41 for details.

Bounce messages contain an X-Failed-Recipients: header line that lists the
failed addresses, for the benefit of programs that try to analyse such
messages automatically.

A bounce message is normally sent to the sender of the original message, as
obtained from the message's envelope. For incoming SMTP messages, this is the
address given in the MAIL command. However, when an address is expanded via a
forward or alias file, an alternative address can be specified for delivery
failures of the generated addresses. For a mailing list expansion (see section
42.2) it is common to direct bounce messages to the manager of the list.


3.16 Failures to deliver bounce messages

If a bounce message (either locally generated or received from a remote host)
itself suffers a permanent delivery failure, the message is left on the queue,
but it is frozen, awaiting the attention of an administrator. There are
options which can be used to make Exim discard such failed messages, or to
keep them for only a short time (see "timeout_frozen_after" and
"ignore_bounce_errors_after").



                       4. BUILDING AND INSTALLING EXIM


4.1 Unpacking

Exim is distributed as a gzipped or bzipped tar file which, when upacked,
creates a directory with the name of the current release (for example,
exim-4.30) into which the following files are placed:

  ACKNOWLEDGMENTS   contains some acknowledgments
  CHANGES           contains a reference to where changes are documented
  LICENCE           the GNU General Public Licence
  Makefile          top-level make file
  NOTICE            conditions for the use of Exim
  README            list of files, directories and simple build instructions

Other files whose names begin with README may also be present. The following
subdirectories are created:

  Local             an empty directory for local configuration files
  OS                OS-specific files
  doc               documentation files
  exim_monitor      source files for the Exim monitor
  scripts           scripts used in the build process
  src               remaining source files
  util              independent utilities

The main utility programs are contained in the src directory, and are built
with the Exim binary. The util directory contains a few optional scripts that
may be useful to some sites.


4.2 Multiple machine architectures and operating systems

The building process for Exim is arranged to make it easy to build binaries
for a number of different architectures and operating systems from the same
set of source files. Compilation does not take place in the src directory.
Instead, a "build directory" is created for each architecture and operating
system. Symbolic links to the sources are installed in this directory, which
is where the actual building takes place.

In most cases, Exim can discover the machine architecture and operating system
for itself, but the defaults can be overridden if necessary.


4.3 DBM libraries

Even if you do not use any DBM files in your configuration, Exim still needs a
DBM library in order to operate, because it uses indexed files for its hints
databases. Unfortunately, there are a number of DBM libraries in existence,
and different operating systems often have different ones installed.

If you are using Solaris, IRIX, one of the modern BSD systems, or a modern
Linux distribution, the DBM configuration should happen automatically, and you
may be able to ignore this section. Otherwise, you may have to learn more than
you would like about DBM libraries from what follows.

Licensed versions of Unix normally contain a library of DBM functions
operating via the "ndbm" interface, and this is what Exim expects by default.
Free versions of Unix seem to vary in what they contain as standard. In
particular, some early versions of Linux have no default DBM library, and
different distributors have chosen to bundle different libraries with their
packaged versions. However, the more recent releases seem to have standardised
on the Berkeley DB library.

Different DBM libraries have different conventions for naming the files they
use. When a program opens a file called dbmfile, there are four possibilities:

(1)  A traditional "ndbm" implementation, such as that supplied as part of
     Solaris, operates on two files called dbmfile.dir and dbmfile.pag.

(2)  The GNU library, "gdbm", operates on a single file. If used via its
     "ndbm" compatibility interface it makes two different hard links to it
     with names dbmfile.dir and dbmfile.pag, but if used via its native
     interface, the file name is used unmodified.

(3)  The Berkeley DB package, if called via its "ndbm" compatibility inter-
     face, operates on a single file called dbmfile.db, but otherwise looks to
     the programmer exactly the same as the traditional "ndbm" implementation.

(4)  If the Berkeley package is used in its native mode, it operates on a
     single file called dbmfile; the programmer's interface is somewhat
     different to the traditional "ndbm" interface.

(5)  To complicate things further, there are several very different versions
     of the Berkeley DB package. Version 1.85 was stable for a very long time,
     releases 2.x and 3.x were current for a while, but the latest versions
     are now numbered 4.x. Maintenance of some of the earlier releases has
     ceased. All versions of Berkeley DB can be obtained from

       http://www.sleepycat.com/

(6)  Yet another DBM library, called "tdb", has become available from

       http://download.sourceforge.net/tdb

     It has its own interface, and also operates on a single file.

Exim and its utilities can be compiled to use any of these interfaces. In
order to use any version of the Berkeley DB package in native mode, you must
set USE_DB in an appropriate configuration file (typically Local/Makefile).
For example:

  USE_DB=yes

Similarly, for gdbm you set USE_GDBM, and for tdb you set USE_TDB. An error is
diagnosed if you set more than one of these.

At the lowest level, the build-time configuration sets none of these options,
thereby assuming an interface of type (1). However, some operating system
configuration files (for example, those for the BSD operating systems and
Linux) assume type (4) by setting USE_DB as their default, and the configur-
ation files for Cygwin set USE_GDBM. Anything you set in Local/Makefile,
however, overrides these system defaults.

As well as setting USE_DB, USE_GDBM, or USE_TDB, it may also be necessary to
set DBMLIB, to cause inclusion of the appropriate library, as in one of these
lines:

  DBMLIB = -ldb
  DBMLIB = -ltdb

Settings like that will work if the DBM library is installed in the standard
place. Sometimes it is not, and the library's header file may also not be in
the default path. You may need to set INCLUDE to specify where the header file
is, and to specify the path to the library more fully in DBMLIB, as in this
example:

  INCLUDE=-I/usr/local/include/db-4.1
  DBMLIB=/usr/local/lib/db-4.1/libdb.a

There is further detailed discussion about the various DBM libraries in the
file doc/dbm.discuss.txt in the Exim distribution.


4.4 Pre-building configuration

Before building Exim, a local configuration file that specifies options
independent of any operating system has to be created with the name
Local/Makefile. A template for this file is supplied as the file src/EDITME,
and it contains full descriptions of all the option settings therein. These
descriptions are therefore not repeated here. If you are building Exim for the
first time, the simplest thing to do is to copy src/EDITME to Local/Makefile,
then read it and edit it appropriately.

There are three settings that you must supply, because Exim will not build
without them. They are the location of the run time configuration file
(CONFIGURE_FILE), the directory in which Exim binaries will be installed
(BIN_DIRECTORY), and the identity of the Exim user (EXIM_USER and maybe
EXIM_GROUP as well). The value of CONFIGURE_FILE can in fact be a colon-
separated list of file names; Exim uses the first of them that exists.

There are a few other parameters that can be specified either at build time or
at run time, to enable the same binary to be used on a number of different
machines. However, if the locations of Exim's spool directory and log file
directory (if not within the spool directory) are fixed, it is recommended
that you specify them in Local/Makefile instead of at run time, so that errors
detected early in Exim's execution (such as a malformed configuration file)
can be logged.

If you are going to build the Exim monitor, a similar configuration process is
required. The file exim_monitor/EDITME must be edited appropriately for your
installation and saved under the name Local/eximon.conf. If you are happy with
the default settings described in exim_monitor/EDITME, Local/eximon.conf can
be empty, but it must exist.

This is all the configuration that is needed in straightforward cases for
known operating systems. However, the building process is set up so that it is
easy to override options that are set by default or by operating-system-
specific configuration files, for example to change the name of the C
compiler, which defaults to "gcc". See section 4.10 below for details of how
to do this.

                                                                               |
4.5 Support for iconv()                                                        |
                                                                               |
The contents of header lines in messages may be encoded according to the rules |
described RFC 2047. This makes it possible to transmit characters that are not |
in the ASCII character set, and to label them as being in a particular         |
character set. When Exim is inspecting header lines by means of the "$h_"      |
mechanism, it decodes them, and translates them into a specified character set |
(default ISO-8859-1). The translation is possible only if the operating system |
supports the "iconv()" function.                                               |
                                                                               |
However, some of the operating systems that supply "iconv()" do not support    |
very many conversions. The GNU "libiconv" library (available from              |
http://www.gnu.org/software/libiconv/) can be installed on such systems to     |
remedy this deficiency, as well as on systems that do not supply "iconv()" at  |
all. After installing "libiconv", you should add                               |
                                                                               |
  HAVE_ICONV=yes                                                               |
                                                                               |
to your Local/Makefile and rebuild Exim.                                       |


4.6 Including TLS/SSL encryption support

Exim can be built to support encrypted SMTP connections, using the STARTTLS
command as per RFC 2487. It can also support legacy clients that expect to
start a TLS session immediately on connection to a non-standard port (see the
-tls-on-connect command line option).

If you want to build Exim with TLS support, you must first install either the
OpenSSL or GnuTLS library. There is no cryptographic code in Exim itself for
implementing SSL.

If OpenSSL is installed, you should set

  SUPPORT_TLS=yes
  TLS_LIBS=-lssl -lcrypto

in Local/Makefile. You may also need to specify the locations of the OpenSSL
library and include files. For example:

  SUPPORT_TLS=yes
  TLS_LIBS=-L/usr/local/openssl/lib -lssl -lcrypto
  TLS_INCLUDE=-I/usr/local/openssl/include/

If GnuTLS is installed, you should set

  SUPPORT_TLS=yes
  USE_GNUTLS=yes
  TLS_LIBS=-lgnutls -ltasn1 -lgcrypt

in Local/Makefile, and again you may need to specify the locations of the
library and include files. For example:

  SUPPORT_TLS=yes
  USE_GNUTLS=yes
  TLS_LIBS=-L/usr/gnu/lib -lgnutls -ltasn1 -lgcrypt
  TLS_INCLUDE=-I/usr/gnu/include

You do not need to set TLS_INCLUDE if the relevant directory is already
specified in INCLUDE. Details of how to configure Exim to make use of TLS are
given in chapter 37.


4.7 Use of tcpwrappers

Exim can be linked with the "tcpwrappers" library in order to check incoming
SMTP calls using the "tcpwrappers" control files. This may be a convenient
alternative to Exim's own checking facilities for installations that are
already making use of "tcpwrappers" for other purposes. To do this, you should
set USE_TCP_WRAPPERS in Local/Makefile, arrange for the file tcpd.h to be
available at compile time, and also ensure that the library libwrap.a is
available at link time, typically by including -lwrap in EXTRALIBS_EXIM. For
example, if "tcpwrappers" is installed in /usr/local, you might have

  USE_TCP_WRAPPERS=yes
  CFLAGS=-O -I/usr/local/include
  EXTRALIBS_EXIM=-L/usr/local/lib -lwrap

in Local/Makefile. The name to use in the "tcpwrappers" control files is
'exim'. For example, the line

  exim : LOCAL  192.168.1.  .friendly.domain.example

in your /etc/hosts.allow file allows connections from the local host, from the
subnet 192.168.1.0/24, and from all hosts in "friendly.domain.example". All
other connections are denied. Consult the "tcpwrappers" documentation for
further details.


4.8 Including support for IPv6

Exim contains code for use on systems that have IPv6 support. Setting
HAVE_IPV6=YES in Local/Makefile causes the IPv6 code to be included; it may
also be necessary to set IPV6_INCLUDE and IPV6_LIBS on systems where the IPv6
support is not fully integrated into the normal include and library files.

IPv6 is still changing rapidly. Two different types of DNS record for handling
IPv6 addresses have been defined. AAAA records are already in use, and are
currently seen as the 'mainstream', but another record type called A6 is being
argued about. Its status is currently 'experimental'. Exim has support for
A6 records, but this is included only if you set SUPPORT_A6=YES in
Local/Makefile.


4.9 The building process

Once Local/Makefile (and Local/eximon.conf, if required) have been created,
run "make" at the top level. It determines the architecture and operating
system types, and creates a build directory if one does not exist. For
example, on a Sun system running Solaris 8, the directory build-
SunOS5-5.8-sparc is created. Symbolic links to relevant source files are
installed in the build directory.

If this is the first time "make" has been run, it calls a script that builds a
make file inside the build directory, using the configuration files from the
Local directory. The new make file is then passed to another instance of
"make". This does the real work, building a number of utility scripts, and
then compiling and linking the binaries for the Exim monitor (if configured),
a number of utility programs, and finally Exim itself. The command "make
makefile" can be used to force a rebuild of the make file in the build
directory, should this ever be necessary.

If you have problems building Exim, check for any comments there may be in the
README file concerning your operating system, and also take a look at the FAQ,
where some common problems are covered.


4.10 Overriding build-time options for Exim

The main make file that is created at the beginning of the building process
consists of the concatenation of a number of files which set configuration
values, followed by a fixed set of "make" instructions. If a value is set more
than once, the last setting overrides any previous ones. This provides a
convenient way of overriding defaults. The files that are concatenated are, in
order:

  OS/Makefile-Default
  OS/Makefile-<ostype>
  Local/Makefile
  Local/Makefile-<ostype>
  Local/Makefile-<archtype>
  Local/Makefile-<ostype>-<archtype>
  OS/Makefile-Base

where <ostype> is the operating system type and <archtype> is the architecture
type. Local/Makefile is required to exist, and the building process fails if
it is absent. The other three Local files are optional, and are often not
needed.

The values used for <ostype> and <archtype> are obtained from scripts called
scripts/os-type and scripts/arch-type respectively. If either of the environ-
ment variables EXIM_OSTYPE or EXIM_ARCHTYPE is set, their values are used,
thereby providing a means of forcing particular settings. Otherwise, the
scripts try to get values from the "uname" command. If this fails, the shell
variables OSTYPE and ARCHTYPE are inspected. A number of ad hoc transform-
ations are then applied, to produce the standard names that Exim expects. You
can run these scripts directly from the shell in order to find out what values
are being used on your system.

OS/Makefile-Default contains comments about the variables that are set
therein. Some (but not all) are mentioned below. If there is something that
needs changing, review the contents of this file and the contents of the make
file for your operating system (OS/Makefile-<ostype>) to see what the default
values are.

If you need to change any of the values that are set in OS/Makefile-Default or
in OS/Makefile-<ostype>, or to add any new definitions, you do not need to
change the original files. Instead, you should make the changes by putting the
new values in an appropriate Local file. For example, when building Exim in
many releases of the Tru64-Unix (formerly Digital UNIX, formerly DEC-OSF1)
operating system, it is necessary to specify that the C compiler is called
"cc" rather than "gcc". Also, the compiler must be called with the option
-std1, to make it recognize some of the features of Standard C that Exim uses.
(Most other compilers recognize Standard C by default.) To to this, you should
create a file called Local/Makefile-OSF1 containing the lines

  CC=cc
  CFLAGS=-std1

If you are compiling for just one operating system, it may be easier to put
these lines directly into Local/Makefile.

Keeping all your local configuration settings separate from the distributed
files makes it easy to transfer them to new versions of Exim simply by copying
the contents of the Local directory.

Exim contains support for doing LDAP, NIS, NIS+, and other kinds of file
lookup, but not all systems have these components installed, so the default is
not to include the relevant code in the binary. All the different kinds of
file and database lookup that Exim supports are implemented as separate code
modules which are included only if the relevant compile-time options are set.
In the case of LDAP, NIS, and NIS+, the settings for Local/Makefile are:

  LOOKUP_LDAP=yes
  LOOKUP_NIS=yes
  LOOKUP_NISPLUS=yes

and similar settings apply to the other lookup types. They are all listed in
src/EDITME. In most cases the relevant include files and interface libraries
need to be installed before compiling Exim. However, in the case of cdb, which
is included in the binary only if

  LOOKUP_CDB=yes

is set, the code is entirely contained within Exim, and no external include
files or libraries are required. When a lookup type is not included in the
binary, attempts to configure Exim to use it cause run time configuration
errors.

Exim can be linked with an embedded Perl interpreter, allowing Perl subrou-
tines to be called during string expansion. To enable this facility,

  EXIM_PERL=perl.o

must be defined in Local/Makefile. Details of this facility are given in
chapter 12.

The location of the X11 libraries is something that varies a lot between
operating systems, and of course there are different versions of X11 to cope
with. Exim itself makes no use of X11, but if you are compiling the Exim
monitor, the X11 libraries must be available. The following three variables
are set in OS/Makefile-Default:

  X11=/usr/X11R6
  XINCLUDE=-I$(X11)/include
  XLFLAGS=-L$(X11)/lib

These are overridden in some of the operating-system configuration files. For
example, in OS/Makefile-SunOS5 there is

  X11=/usr/openwin
  XINCLUDE=-I$(X11)/include
  XLFLAGS=-L$(X11)/lib -R$(X11)/lib

If you need to override the default setting for your operating system, place a
definition of all three of these variables into your Local/Makefile-<ostype>
file.

If you need to add any extra libraries to the link steps, these can be put in
a variable called EXTRALIBS, which appears in all the link commands, but by
default is not defined. In contrast, EXTRALIBS_EXIM is used only on the
command for linking the main Exim binary, and not for any associated
utilities. There is also DBMLIB, which appears in the link commands for
binaries that use DBM functions (see also section 4.3). Finally, there is
EXTRALIBS_EXIMON, which appears only in the link step for the Exim monitor
binary, and which can be used, for example, to include additional X11
libraries.

The make file copes with rebuilding Exim correctly if any of the configuration
files are edited. However, if an optional configuration file is deleted, it is
necessary to touch the associated non-optional file (that is, Local/Makefile
or Local/eximon.conf) before rebuilding.


4.11 OS-specific header files

The OS directory contains a number of files with names of the form
os.h-<ostype>. These are system-specific C header files that should not
normally need to be changed. There is a list of macro settings that are
recognized in the file OS/os.configuring, which should be consulted if you are
porting Exim to a new operating system.


4.12 Overriding build-time options for the monitor

A similar process is used for overriding things when building the Exim
monitor, where the files that are involved are

  OS/eximon.conf-Default
  OS/eximon.conf-<ostype>
  Local/eximon.conf
  Local/eximon.conf-<ostype>
  Local/eximon.conf-<archtype>
  Local/eximon.conf-<ostype>-<archtype>

As with Exim itself, the final three files need not exist, and in this case
the OS/eximon.conf-<ostype> file is also optional. The default values in
OS/eximon.conf-Default can be overridden dynamically by setting environment
variables of the same name, preceded by EXIMON_. For example, setting
EXIMON_LOG_DEPTH in the environment overrides the value of LOG_DEPTH at run
time.


4.13 Installing Exim binaries and scripts

The command "make install" runs the "exim_install" script with no arguments.
The script copies binaries and utility scripts into the directory whose name
is specified by the BIN_DIRECTORY setting in Local/Makefile.

Exim's run time configuration file is named by the CONFIGURE_FILE setting in
Local/Makefile. If this names a single file, and the file does not exist, the
default configuration file src/configure.default is copied there by the
installation script. If a run time configuration file already exists, it is
left alone. If CONFIGURE_FILE is a colon-separated list, naming several
alternative files, no default is installed.

One change is made to the default configuration file when it is installed: the
default configuration contains a router that references a system aliases file.
The path to this file is set to the value specified by SYSTEM_ALIASES_FILE in
Local/Makefile (/etc/aliases by default). If the system aliases file does not
exist, the installation script creates it, and outputs a comment to the user.

The created file contains no aliases, but it does contain comments about the
aliases a site should normally have. Mail aliases have traditionally been
kept in /etc/aliases. However, some operating systems are now using
/etc/mail/aliases. You should check if yours is one of these, and change
Exim's configuration if necessary.

The default configuration uses the local host's name as the only local domain,
and is set up to do local deliveries into the shared directory /var/mail,
running as the local user. System aliases and .forward files in users' home
directories are supported, but no NIS or NIS+ support is configured. Domains
other than the name of the local host are routed using the DNS, with delivery
over SMTP.

The install script copies files only if they are newer than the files they are
going to replace. The Exim binary is required to be owned by root and have the
"setuid" bit set, for normal configurations. Therefore, you must run "make
install" as root so that it can set up the Exim binary in this way. However,
in some special situations (for example, if a host is doing no local
deliveries) it may be possible to run Exim without making the binary setuid
root (see chapter 48 for details).

It is possible to install Exim for special purposes (such as building a binary
distribution) in a private part of the file system. You can do this by a
command such as

  make DESTDIR=/some/directory/ install

This has the effect of pre-pending the specified directory to all the file
paths, except the name of the system aliases file that appears in the default
configuration. (If a default alias file is created, its name "is" modified.)
For backwards compatibility, ROOT is used if DESTDIR is not set, but this
usage is deprecated.

Running "make install" does not copy the Exim 4 conversion script
"convert4r4", or the "pcretest" test program. You will probably run the first
of these only once (if you are upgrading from Exim 3), and the second isn't
really part of Exim. None of the documentation files in the doc directory are
copied, except for the info files when you have set INFO_DIRECTORY, as
described in section 4.14 below.

For the utility programs, old versions are renamed by adding the suffix .O to
their names. The Exim binary itself, however, is handled differently. It is
installed under a name that includes the version number and the compile
number, for example exim-4.30-1. The script then arranges for a symbolic link
called exim to point to the binary. If you are updating a previous version of
Exim, the script takes care to ensure that the name exim is never absent from
the directory (as seen by other processes).

If you want to see what the "make install" will do before running it for real,
you can pass the -n option to the installation script by this command:

  make INSTALL_ARG=-n install

The contents of the variable INSTALL_ARG are passed to the installation
script. You do not need to be root to run this test. Alternatively, you can
run the installation script directly, but this must be from within the build
directory. For example, from the top-level Exim directory you could use this
command:

  (cd build-SunOS5-5.5.1-sparc; ../scripts/exim_install -n)

There are two other options that can be supplied to the installation script.

 .   -no_chown bypasses the call to change the owner of the installed binary
     to root, and the call to make it a setuid binary.

 .   -no_symlink bypasses the setting up of the symbolic link exim to the
     installed binary.

INSTALL_ARG can be used to pass these options to the script. For example:

  make INSTALL_ARG=-no_symlink install

The installation script can also be given arguments specifying which files are
to be copied. For example, to install just the Exim binary, and nothing else,
without creating the symbolic link, you could use:

  make INSTALL_ARG='-no_symlink exim' install


4.14 Installing info documentation

Not all systems use the GNU "info" system for documentation, and for this
reason, the Texinfo source of Exim's documentation is not included in the main
distribution. Instead it is available separately from the ftp site (see
section 1.5).

If you have defined INFO_DIRECTORY in Local/Makefile and the Texinfo source of
the documentation is found in the source tree, running "make install"
automatically builds the info files and installs them.


4.15 Setting up the spool directory

When it starts up, Exim tries to create its spool directory if it does not
exist. The Exim uid and gid are used for the owner and group of the spool
directory. Sub-directories are automatically created in the spool directory as
necessary.


4.16 Testing

Having installed Exim, you can check that the run time configuration file is
syntactically valid by running the following command, which assumes that the
Exim binary directory is within your PATH environment variable:

  exim -bV

If there are any errors in the configuration file, Exim outputs error
messages. Otherwise it outputs the version number and build date, the DBM
library that is being used, and information about which drivers and other
optional code modules are included in the binary. Some simple routing tests
can be done by using the address testing option. For example,

  exim -bt <local username>

should verify that it recognizes a local mailbox, and

  exim -bt <remote address>

a remote one. Then try getting it to deliver mail, both locally and remotely.
This can be done by passing messages directly to Exim, without going through a
user agent. For example:

  exim -v postmaster@your.domain.example
  From: user@your.domain.example
  To: postmaster@your.domain.example
  Subject: Testing Exim

  This is a test message.
  ^D

The -v option causes Exim to output some verification of what it is doing. In
this case you should see copies of three log lines, one for the message's
arrival, one for its delivery, and one containing 'Completed'.

If you encounter problems, look at Exim's log files ("mainlog" and "paniclog")
to see if there is any relevant information there. Another source of
information is running Exim with debugging turned on, by specifying the -d
option. If a message is stuck on Exim's spool, you can force a delivery with
debugging turned on by a command of the form

  exim -d -M <message-id>

You must be root or an 'admin user' in order to do this. The -d option
produces rather a lot of output, but you can cut this down to specific areas.
For example, if you use -d-all+route only the debugging information relevant
to routing is included. (See the -d option in chapter 5 for more details.)

One specific problem that has shown up on some sites is the inability to do
local deliveries into a shared mailbox directory, because it does not have the
'sticky bit' set on it. By default, Exim tries to create a lock file before
writing to a mailbox file, and if it cannot create the lock file, the delivery
is deferred. You can get round this either by setting the 'sticky bit' on the
directory, or by setting a specific group for local deliveries and allowing
that group to create files in the directory (see the comments above the
local_delivery transport in the default configuration file). Another approach
is to configure Exim not to use lock files, but just to rely on "fcntl()"
locking instead. However, you should do this only if all user agents also use
"fcntl()" locking. For further discussion of locking issues, see chapter 26.

One thing that cannot be tested on a system that is already running an MTA is
the receipt of incoming SMTP mail on the standard SMTP port. However, the -oX
option can be used to run an Exim daemon that listens on some other port, or
"inetd" can be used to do this. The -bh option and the "exim_checkaccess"
utility can be used to check out policy controls on incoming SMTP mail.

Testing a new version on a system that is already running Exim can most easily
be done by building a binary with a different CONFIGURE_FILE setting. From
within the run time configuration, all other file and directory names that
Exim uses can be altered, in order to keep it entirely clear of the production
version.


4.17 Replacing another MTA with Exim

Building and installing Exim for the first time does not of itself put it in
general use. The name by which the system's MTA is called by mail user agents
is either /usr/sbin/sendmail, or /usr/lib/sendmail (depending on the operating
system), and it is necessary to make this name point to the "exim" binary in
order to get the user agents to pass messages to Exim. This is normally
done by renaming any existing file and making /usr/sbin/sendmail or
/usr/lib/sendmail a symbolic link to the "exim" binary. It is a good idea to
remove any setuid privilege and executable status from the old MTA. It is then
necessary to stop and restart the mailer daemon, if one is running.

Some operating systems have introduced alternative ways of switching MTAs.
For example, if you are running FreeBSD, you need to edit the file
/etc/mail/mailer.conf instead of setting up a symbolic link as just described.
A typical example of the contents of this file for running Exim is as follows:

  sendmail            /usr/exim/bin/exim
  send-mail           /usr/exim/bin/exim
  mailq               /usr/exim/bin/exim -bp
  newaliases          /usr/bin/true

Once you have set up the symbolic link, or edited /etc/mail/mailer.conf, your
Exim installation is 'live'. Check it by sending a message from your favourite
user agent.

You should consider what to tell your users about the change of MTA. Exim may
have different capabilities to what was previously running, and there are
various operational differences such as the text of messages produced by
command line options and in bounce messages. If you allow your users to make
use of Exim's filtering capabilities, you should make the document entitled
"Exim's interface to mail filtering" available to them.


4.18 Upgrading Exim

If you are already running Exim on your host, building and installing a new
version automatically makes it available to MUAs, or any other programs that
call the MTA directly. However, if you are running an Exim daemon, you do need
to send it a HUP signal, to make it re-exec itself, and thereby pick up the
new binary. You do not need to stop processing mail in order to install a new
version of Exim.


4.19 Stopping the Exim daemon on Solaris

The standard command for stopping the mailer daemon on Solaris is

  /etc/init.d/sendmail stop

If /usr/lib/sendmail has been turned into a symbolic link, this script fails
to stop Exim because it uses the command "ps -e" and greps the output for the
text 'sendmail'; this is not present because the actual program name (that is,
'exim') is given by the "ps" command with these options. A solution is to
replace the line that finds the process id with something like

  pid=`cat /var/spool/exim/exim-daemon.pid`

to obtain the daemon's pid directly from the file that Exim saves it in.

Note, however, that stopping the daemon does not 'stop Exim'. Messages can
still be received from local processes, and if automatic delivery is con-
figured (the normal case), deliveries will still occur.



                           5. THE EXIM COMMAND LINE


Exim's command line takes the standard Unix form of a sequence of options,
each starting with a hyphen character, followed by a number of arguments. The
options are compatible with the main options of Sendmail, and there are also
some additional options, some of which are compatible with Smail 3. Certain
combinations of options do not make sense, and provoke an error if used. The
form of the arguments depends on which options are set.


5.1 Setting options by program name

If Exim is called under the name "mailq", it behaves as if the option -bp were
present before any other options. The -bp option requests a listing of the
contents of the mail queue on the standard output. This feature is for
compatibility with some systems that contain a command of that name in one
of the standard libraries, symbolically linked to /usr/sbin/sendmail or
/usr/lib/sendmail.

If Exim is called under the name "rsmtp" it behaves as if the option -bS were
present before any other options, for compatibility with Smail. The -bS option
is used for reading in a number of messages in batched SMTP format.

If Exim is called under the name "rmail" it behaves as if the -i and -oee
options were present before any other options, for compatibility with Smail.
The name "rmail" is used as an interface by some UUCP systems.

If Exim is called under the name "runq" it behaves as if the option -q were
present before any other options, for compatibility with Smail. The -q option
causes a single queue runner process to be started.

If Exim is called under the name "newaliases" it behaves as if the option -bi
were present before any other options, for compatibility with Sendmail. This
option is used for rebuilding Sendmail's alias file. Exim does not have the
concept of a single alias file, but can be configured to run a given command
if called with the -bi option.


5.2 Trusted and admin users

Some Exim options are available only to "trusted users" and others are
available only to "admin users". In the description below, the phrases 'Exim
user' and 'Exim group' mean the user and group defined by EXIM_USER and
EXIM_GROUP in Local/Makefile or set by the "exim_user" and "exim_group"
options. These do not necessarily have to use the name 'exim'.

 .   The trusted users are root, the Exim user, any user listed in the
     "trusted_users" configuration option, and any user whose current group or
     any supplementary group is one of those listed in the "trusted_groups"
     configuration option. Note that the Exim group is not automatically
     trusted.

     Trusted users are always permitted to use the -f option or a leading
     'From ' line to specify the envelope sender of a message that is passed
     to Exim through the local interface (see the -bm and -f options below).
     See the "untrusted_set_sender" option for a way of permitting non-trusted
     users to set envelope senders. For a trusted user, there is never any
     check on the contents of the From: header line, and a Sender: line is
     never added. Furthermore, any existing Sender: line in incoming local
     (non-TCP/IP) messages is not removed.

     Trusted users may also specify a host name, host address, interface
     address, protocol name, ident value, and authentication data when submit-
     ting a message locally. Thus, they are able to insert messages into
     Exim's queue locally that have the characteristics of messages received
     from a remote host. Untrusted users may in some circumstances use -f, but
     can never set the other values that are available to trusted users.

 .   The admin users are root, the Exim user, and any user that is a member of
     the Exim group or of any group listed in the "admin_groups" configuration
     option. The current group does not have to be one of these groups.

     Admin users are permitted to list the queue, and to carry out certain
     operations on messages, for example, to force delivery failures. It is
     also necessary to be an admin user in order to see the full information
     provided by the Exim monitor, and full debugging output.

     By default, the use of the -M, -q, -R, and -S options to cause Exim to
     attempt delivery of messages on its queue is restricted to admin users.
     However, this restriction can be relaxed by setting the
     "prod_requires_admin" option false (that is, specifying
     "no_prod_requires_admin").

     Similarly, the use of the -bp option to list all the messages in the
     queue is restricted to admin users unless "queue_list_requires_admin" is
     set false.

Warning: If you configure your system so that admin users are able to edit
Exim's configuration file, you are giving those users an easy way of getting
root. There is further discussion of this issue at the start of chapter 6.


5.3 Command line options

The command options are described in alphabetical order below.

--     This is a pseudo-option whose only purpose is to terminate the options
       and therefore to cause subsequent command line items to be treated as
       arguments rather than options, even if they begin with hyphens.

--help This option causes Exim to output a few sentences stating what it is.
       The same output is generated if the Exim binary is called with no
       options and no arguments.

-B<type>
       This is a Sendmail option for selecting 7 or 8 bit processing. Exim is
       8-bit clean; it ignores this option.

-bd    This option runs Exim as a daemon, awaiting incoming SMTP connections.
       Usually the -bd option is combined with the -q<time> option, to specify
       that the daemon should also initiate periodic queue runs.

       The -bd option can be used only by an admin user. If either of the -d
       (debugging) or -v (verifying) options are set, the daemon does not
       disconnect from the controlling terminal. When running this way, it can
       be stopped by pressing ctrl-C.

       By default, Exim listens for incoming connections to the standard SMTP  |
       port on all the host's running interfaces. However, it is possible to   |
       listen on other ports, on multiple ports, and only on specific          |
       interfaces. Chapter 13 contains a description of the options that       |
       control this.                                                           |

       When a listening daemon is started without the use of -oX (that is,
       without overriding the normal configuration), it writes its process id
       to a file called exim-daemon.pid in Exim's spool directory. This
       location can be overridden by setting PID_FILE_PATH in Local/Makefile.
       The file is written while Exim is still running as root.

       When -oX is used on the command line to start a listening daemon, the
       process id is not written to the normal pid file path. However, -oP can
       be used to specify a path on the command line if a pid file is
       required.

       The SIGHUP signal can be used to cause the daemon to re-exec itself.
       This should be done whenever Exim's configuration file, or any file
       that is incorporated into it by means of the ".include" facility, is
       changed, and also whenever a new version of Exim is installed. It is
       not necessary to do this when other files that are referenced from the
       configuration (for example, alias files) are changed, because these are
       reread each time they are used.

-bdf   This option has the same effect as -bd except that it never disconnects
       from the controlling terminal, even when no debugging is specified.

-be    Run Exim in expansion testing mode. Exim discards its root privilege,
       to prevent ordinary users from using this mode to read otherwise
       inaccessible files. If no arguments are given, Exim runs interactively,
       prompting for lines of data. Long expressions can be split over several
       lines by using backslash continuations. As in Exim's run time configur-
       ation, whitespace at the start of continuation lines is ignored.

       Each argument or data line is passed through the string expansion
       mechanism, and the result is output. Variable values from the configur-
       ation file (for example, $qualify_domain) are available, but no mess-
       age-specific values (such as $domain) are set, because no message is
       being processed.

-bF <filename>
       This option is the same as -bf except that it assumes that the filter
       being tested is a system filter. The additional commands that are
       available only in system filters are recognized.

-bf <filename>
       This option runs Exim in filter testing mode; the file is the filter
       file to be tested, and a test message must be supplied on the standard
       input. If there are no message-dependent tests in the filter, an empty
       file can be supplied. If a system filter file is being tested, -bF
       should be used instead of -bf. If the test file does not begin with one |
       of the special lines                                                    |
                                                                               |
         # Exim filter                                                         |
         # Sieve filter                                                        |
                                                                               |
       it is taken to be a normal .forward file, and is tested for validity    |
       under that interpretation. See sections 22.4 to 22.6 for a description  |
       of the possible contents of non-filter redirection lists.               |

       The result of an Exim command that uses -bf, provided no errors are
       detected, is a list of the actions that Exim would try to take if
       presented with the message for real. More details of filter testing are
       given in the separate document entitled "Exim's interfaces to mail
       filtering".

       When testing a filter file, the envelope sender can be set by the -f
       option, or by a 'From ' line at the start of the test message. Various
       parameters that would normally be taken from the envelope recipient
       address of the message can be set by means of additional command line
       options. These are:

         -bfd  <domain>            default is the qualify domain
         -bfl  <local_part>        default is the logged in user
         -bfp  <local_part_prefix> default is null
         -bfs  <local_part_suffix> default is null

       The local part should always be set to the incoming address with any
       prefix or suffix stripped, because that is how it appears to the filter
       when a message is actually being delivered.

-bh <IP address>
       This option runs a fake SMTP session as if from the given IP address,
       using the standard input and output. The IP address may include a port
       number at the end, after a full stop. For example:

         exim -bh 10.9.8.7.1234
         exim -bh fe80::a00:20ff:fe86:a061.5678

       Comments as to what is going on are written to the standard error file.
       These include lines beginning with 'LOG' for anything that would have
       been logged. This facility is provided for testing configuration        |
       options for incoming messages, to make sure they implement the required |
       policy. For example, you can test your relay controls using -bh.        |
                                                                               |
       Warning 1: You cannot test features of the configuration that rely on   |
       ident (RFC 1413) callouts. These cannot be done when testing using -bh  |
       because there is no incoming SMTP connection.                           |
                                                                               |
       Warning 2: Address verification callouts (see section 38.21) are also   |
       skipped when testing using -bh. If you want these callouts to occur,    |
       use -bhc instead.                                                       |

       Messages supplied during the testing session are discarded, and nothing
       is written to any of the real log files. There may be pauses when DNS
       (and other) lookups are taking place, and of course these may time out.
       The -oMi option can be used to specify a specific IP interface and port
       if this is important.

       The "exim_checkaccess" utility is a 'packaged' version of -bh whose
       output just states whether a given recipient address from a given host
       is acceptable or not. See section 46.7.

-bhc <IP address>                                                              |
       This option operates in the same way as -bh, except that address        |
       verification callouts are performed if required. This includes consult- |
       ing and updating the callout cache database.                            |

-bi    Sendmail interprets the -bi option as a request to rebuild its alias
       file. Exim does not have the concept of a single alias file, and so it
       cannot mimic this behaviour. However, calls to /usr/lib/sendmail with
       the -bi option tend to appear in various scripts such as NIS make
       files, so the option must be recognized.

       If -bi is encountered, the command specified by the "bi_command"
       configuration option is run, under the uid and gid of the caller of
       Exim. If the -oA option is used, its value is passed to the command as
       an argument. The command set by "bi_command" may not contain arguments.
       The command can use the "exim_dbmbuild" utility, or some other means,
       to rebuild alias files if this is required. If the "bi_command" option
       is not set, calling Exim with -bi is a no-op.

-bm    This option runs an Exim receiving process that accepts an incoming,
       locally-generated message on the current input. The recipients are
       given as the command arguments (except when -t is also present - see
       below). Each argument can be a comma-separated list of RFC 2822
       addresses. This is the default option for selecting the overall action
       of an Exim call; it is assumed if no other conflicting option is
       present.

       If any addresses in the message are unqualified (have no domain),
       they are qualified by the values of the "qualify_domain" or
       "qualify_recipient" options, as appropriate. The -bnq option (see
       below) provides a way of suppressing this for special cases.

       Policy checks on the contents of local messages can be enforced by
       means of the non-SMTP ACL. See chapter 38 for details. The return code
       is zero if the message is successfully accepted. Otherwise, the action
       is controlled by the -oex option setting - see below.

       The format of the message must be as defined in RFC 2822, except that,
       for compatibility with Sendmail and Smail, a line in one of the forms

         From sender Fri Jan  5 12:55 GMT 1997
         From sender Fri, 5 Jan 97 12:55:01

       (with the weekday optional, and possibly with additional text after the
       date) is permitted to appear at the start of the message. There appears
       to be no authoritative specification of the format of this line. Exim
       recognizes it by matching against the regular expression defined by the
       "uucp_from_pattern" option, which can be changed if necessary. The
       specified sender is treated as if it were given as the argument to the
       -f option, but if a -f option is also present, its argument is used in
       preference to the address taken from the message. The caller of Exim
       must be a trusted user for the sender of a message to be set in this
       way.

-bnq   By default, Exim automatically qualifies unqualified addresses (those
       without domains) that appear in messages that are submitted locally
       (that is, not over TCP/IP). This qualification applies both to
       addresses in envelopes, and addresses in header lines. Sender addresses
       are qualified using "qualify_domain", and recipient addresses using
       "qualify_recipient" (which defaults to the value of "qualify_domain").

       Sometimes, qualification is not wanted. For example, if -bS (batch
       SMTP) is being used to re-submit messages that originally came from
       remote hosts after content scanning, you probably do not want to
       qualify unqualified addresses in header lines. (Such lines will be
       present only if you have not enabled a header syntax check in the
       appropriate ACL.)

       The -bnq option suppresses all qualification of unqualified addresses
       in messages that originate on the local host. When this is used,
       unqualified addresses in the envelope provoke errors (causing message
       rejection) and unqualified addresses in header lines are left alone.

-bP    If this option is given with no arguments, it causes the values of all
       Exim's main configuration options to be written to the standard output.
       The values of one or more specific options can be requested by giving
       their names as arguments, for example:

         exim -bP qualify_domain hold_domains

       However, any option setting that is preceded by the word 'hide' in the
       configuration file is not shown in full, except to an admin user. For
       other users, the output is as in this example:

         mysql_servers = <value not displayable>

       If "configure_file" is given as an argument, the name of the run time
       configuration file is output. If a list of configuration files was
       supplied, the value that is output here is the name of the file that
       was actually used.

       If "log_file_path" or "pid_file_path" are given, the names of the
       directories where log files and daemon pid files are written are
       output, respectively. If these values are unset, log files are written
       in a sub-directory of the spool directory called "log", and the pid
       file is written directly into the spool directory.

       If -bP is followed by a name preceded by "+", for example,

         exim -bP +local_domains

       it searches for a matching named list of any type (domain, host,
       address, or local part) and outputs what it finds.

       If one of the words "router", "transport", or "authenticator" is given,
       followed by the name of an appropriate driver instance, the option
       settings for that driver are output. For example:

         exim -bP transport local_delivery

       The generic driver options are output first, followed by the driver's
       private options. A list of the names of drivers of a particular type
       can be obtained by using one of the words "router_list",
       "transport_list", or "authenticator_list", and a complete list of all
       drivers with their option settings can be obtained by using "routers",
       "transports", or "authenticators".

-bp    This option requests a listing of the contents of the mail queue on the
       standard output. If the -bp option is followed by a list of message
       ids, just those messages are listed. By default, this option can be
       used only by an admin user. However, the "queue_list_requires_admin"
       option can be set false to allow any user to see the queue.

       Each message on the queue is displayed as in the following example:

         25m  2.9K 0t5C6f-0000c8-00 <alice@wonderland.fict.example>
                   red.king@looking-glass.fict.example
                   <other addresses>

       The first line contains the length of time the message has been on the
       queue (in this case 25 minutes), the size of the message (2.9K), the
       unique local identifier for the message, and the message sender, as
       contained in the envelope. For bounce messages, the sender address is
       empty, and appears as '<>'. If the message was submitted locally by an
       untrusted user who overrode the default sender address, the user's
       login name is shown in parentheses before the sender address. If the
       message is frozen (attempts to deliver it are suspended) then the text
       '*** frozen ***' is displayed at the end of this line.

       The recipients of the message (taken from the envelope, not the
       headers) are displayed on subsequent lines. Those addresses to which
       the message has already been delivered are marked with the letter D. If
       an original address gets expanded into several addresses via an alias
       or forward file, the original is displayed with a D only when
       deliveries for all of its child addresses are complete.

-bpa   This option operates like -bp, but in addition it shows delivered
       addresses that were generated from the original top level address(es)
       in each message by alias or forwarding operations. These addresses are
       flagged with '+D' instead of just 'D'.

-bpc   This option counts the number of messages on the queue, and writes the
       total to the standard output. It is restricted to admin users, unless
       "queue_list_requires_admin" is set false.

-bpr   This option operates like -bp, but the output is not sorted into
       chronological order of message arrival. This can speed it up when there
       are lots of messages on the queue, and is particularly useful if the
       output is going to be post-processed in a way that doesn't need the
       sorting.

-bpra  This option is a combination of -bpr and -bpa.

-bpru  This option is a combination of -bpr and -bpu.

-bpu   This option operates like -bp but shows only undelivered top-level
       addresses for each message displayed. Addresses generated by aliasing
       or forwarding are not shown, unless the message was deferred after
       processing by a router with the "one_time" option set.

-brt   This option is for testing retry rules, and it must be followed by up
       to three arguments. It causes Exim to look for a retry rule that
       matches the values and to write it to the standard output. For example:

         exim -brt bach.comp.mus.example
         Retry rule: *.comp.mus.example  F,2h,15m; F,4d,30m;

       See chapter 32 for a description of Exim's retry rules. The first
       argument, which is required, can be a complete address in the form
       "local_part@domain", or it can be just a domain name. The second
       argument is an optional second domain name; if no retry rule is found
       for the first argument, the second is tried. This ties in with Exim's
       behaviour when looking for retry rules for remote hosts - if no rule is
       found that matches the host, one that matches the mail domain is
       sought. The final argument is the name of a specific delivery error, as
       used in setting up retry rules, for example 'quota_3d'.

-brw   This option is for testing address rewriting rules, and it must be
       followed by a single argument, consisting of either a local part
       without a domain, or a complete address with a fully qualified domain.
       Exim outputs how this address would be rewritten for each possible
       place it might appear. See chapter 31 for further details.

-bS    This option is used for batched SMTP input, which is an alternative
       interface for non-interactive local message submission. A number of
       messages can be submitted in a single run. However, despite its name,
       this is not really SMTP input. Exim reads each message's envelope from
       SMTP commands on the standard input, but generates no responses. If the
       caller is trusted, or "untrusted_set_sender" is set, the senders in the
       SMTP MAIL commands are believed; otherwise the sender is always the
       caller of Exim.

       The message itself is read from the standard input, in SMTP format
       (leading dots doubled), terminated by a line containing just a single
       dot. An error is provoked if the terminating dot is missing. A further
       message may then follow.

       As for other local message submissions, the contents of incoming batch
       SMTP messages can be checked using the non-SMTP ACL (see chapter 38).
       Unqualified addresses are automatically qualified using
       "qualify_domain" and "qualify_recipient", as appropriate, unless the
       -bnq option is used.

       Some other SMTP commands are recognized in the input. HELO and EHLO act
       as RSET; VRFY, EXPN, ETRN, and HELP act as NOOP; QUIT quits, ignoring
       the rest of the standard input.

       If any error is encountered, reports are written to the standard output
       and error streams, and Exim gives up immediately. The return code is 0
       if no error was detected; it is 1 if one or more messages were accepted
       before the error was detected; otherwise it is 2.

       More details of input using batched SMTP are given in section 43.12.

-bs    This option causes Exim to accept one or more messages by reading SMTP
       commands on the standard input, and producing SMTP replies on the
       standard output. SMTP policy controls, as defined in ACLs (see chapter
       38) are applied.

       Some user agents use this interface as a way of passing locally-
       generated messages to the MTA. In this usage, if the caller of Exim is
       trusted, or "untrusted_set_sender" is set, the senders of messages are
       taken from the SMTP MAIL commands. Otherwise the content of these
       commands is ignored and the sender is set up as the calling user.
       Unqualified addresses are automatically qualified using
       "qualify_domain" and "qualify_recipient", as appropriate, unless the
       -bnq option is used.

       The -bs option is also used to run Exim from "inetd", as an alternative
       to using a listening daemon. Exim can distinguish the two cases by
       checking whether the standard input is a TCP/IP socket. When Exim is
       called from "inetd", the source of the mail is assumed to be remote,
       and the comments above concerning senders and qualification do not
       apply. In this situation, Exim behaves in exactly the same way as it
       does when receiving a message via the listening daemon.

-bt    This option runs Exim in address testing mode, in which each argument
       is taken as an address to be tested for deliverability. The results are
       written to the standard output. If a test fails, and the caller is not
       an admin user, no details of the failure are output, because these
       might contain sensitive information such as usernames and passwords for
       database lookups.

       If no arguments are given, Exim runs in an interactive manner,
       prompting with a right angle bracket for addresses to be tested. Each
       address is handled as if it were the recipient address of a message
       (compare the -bv option). It is passed to the routers and the result
       is written to the standard output. However, any router that has
       "no_address_test" set is bypassed. This can make -bt easier to use for
       genuine routing tests if your first router passes everything to a
       scanner program.

       The return code is 2 if any address failed outright; it is 1 if no
       address failed outright but at least one could not be resolved for some
       reason. Return code 0 is given only when all addresses succeed.

       Warning: -bt can only do relatively simple testing. If any of the
       routers in the configuration makes any tests on the sender address of a
       message, you can use the -f option to set an appropriate sender when
       running -bt tests. Without it, the sender is assumed to be the calling
       user at the default qualifying domain. However, if you have set up (for
       example) routers whose behaviour depends on the contents of an incoming
       message, you cannot test those conditions using -bt. The -N option
       provides a possible way of doing such tests.

-bV    This option causes Exim to write the current version number, compi-
       lation number, and compilation date of the "exim" binary to the
       standard output. It also lists the DBM library this is being used, the
       optional modules (such as specific lookup types), the drivers that are
       included in the binary, and the name of the run time configuration file
       that is in use.

-bv    This option runs Exim in address verification mode, in which each
       argument is taken as an address to be verified. During normal oper-
       ation, verification happens mostly as a consequence processing a
       "verify" condition in an ACL (see chapter 38). If you want to test an
       entire ACL, see the -bh option.

       If verification fails, and the caller is not an admin user, no details
       of the failure are output, because these might contain sensitive
       information such as usernames and passwords for database lookups.

       If no arguments are given, Exim runs in an interactive manner,
       prompting with a right angle bracket for addresses to be verified.
       Verification differs from address testing (the -bt option) in that
       routers that have "no_verify" set are skipped, and if the address is
       accepted by a router that has "fail_verify" set, verification fails.
       The address is verified as a recipient if -bv is used; to test
       verification for a sender address, -bvs should be used.

       If the -v option is not set, the output consists of a single line for
       each address, stating whether it was verified or not, and giving a
       reason in the latter case. Otherwise, more details are given of how the
       address has been handled, and in the case of address redirection, all
       the generated addresses are also considered. Without -v, generating
       more than one address by redirection causes verification to end
       sucessfully.

       The return code is 2 if any address failed outright; it is 1 if no
       address failed outright but at least one could not be resolved for some
       reason. Return code 0 is given only when all addresses succeed.

       If any of the routers in the configuration makes any tests on the
       sender address of a message, you should use the -f option to set an
       appropriate sender when running -bv tests. Without it, the sender is
       assumed to be the calling user at the default qualifying domain.

-bvs   This option acts like -bv, but verifies the address as a sender rather
       than a recipient address. This affects any rewriting and qualification
       that might happen.

-C <filelist>
       This option causes Exim to find the run time configuration file from
       the given list instead of from the list specified by the CONFIGURE_FILE
       compile-time setting. Usually, the list will consist of just a single
       file name, but it can be a colon-separated list of names. In this case,
       the first file that exists is used. Failure to open an existing file
       stops Exim from proceeding any further along the list, and an error is
       generated.

       When this option is used by a caller other than root or the Exim user,
       and the list is different from the compiled-in list, Exim gives up its
       root privilege immediately, and runs with the real and effective uid
       and gid set to those of the caller. However, if ALT_CONFIG_ROOT_ONLY is |
       defined in Local/Makefile, root privilege is retained for -C only if    |
       the caller of Exim is root. This option is not set by default.          |
                                                                               |
       Setting ALT_CONFIG_ROOT_ONLY locks out the possibility of testing a     |
       configuration using -C right through message reception and delivery,    |
       even if the caller is root. The reception works, but by that time, Exim |
       is running as the Exim user, so when it re-execs to regain privilege    |
       for the delivery, the use of -C causes privilege to be lost. However,   |
       root can test reception and delivery using two separate commands (one   |
       to put a message on the queue, using -odq, and another to do the        |
       delivery, using -M).                                                    |
                                                                               |
       If ALT_CONFIG_PREFIX is defined in Local/Makefile, it specifies a       |
       prefix string with which any file named in a -C command line option     |
       must start. In addition, the file name must not contain the sequence    |
       "/../". However, if the value of the -C option is identical to the      |
       value of CONFIGURE_FILE in Local/Makefile, Exim ignores -C and proceeds |
       as usual. There is no default setting for ALT_CONFIG_PREFIX; when it is |
       unset, any file name can be used with -C.                               |
                                                                               |
       ALT_CONFIG_PREFIX can be used to confine alternative configuration      |
       files to a directory to which only root has access. This prevents       |
       someone who has broken into the Exim account from running a privileged  |
       Exim with an arbitrary configuration file.                              |

       The -C facility is useful for ensuring that configuration files are
       syntactically correct, but cannot be used for test deliveries, unless
       the caller is privileged, or unless it is an exotic configuration that
       does not require privilege. No check is made on the owner or group of
       the files specified by this option.

-D<macro>=<value>
       This option can be used to override macro definitions in the configur-
       ation file (see section 6.4). However, like -C, if it is used by an
       unprivileged caller, it causes Exim to give up its root privilege. If   |
       DISABLE_D_OPTION is defined in Local/Makefile, the use of -D is         |
       completely disabled, and its use causes an immediate error exit.        |

       The entire option (including equals sign if present) must all be within
       one command line item. -D can be used to set the value of a macro to
       the empty string, in which case the equals sign is optional. These two
       commands are synonymous:

         exim -DABC  ...
         exim -DABC= ...

       To include spaces in a macro definition item, quotes must be used. If
       you use quotes, spaces are permitted around the macro name and the
       equals sign. For example:

         exim '-D ABC = something' ...

       -D may be repeated up to 10 times on a command line.

-d<debug options>
       This option causes debugging information to be written to the standard
       error stream. It is restricted to admin users because debugging output
       may show database queries that contain password information. Also, the
       details of users' filter files should be protected. When -d is used, -v
       is assumed. If -d is given on its own, a lot of standard debugging data
       is output. This can be reduced, or increased to include some more
       rarely needed information, by following -d with a string made up of
       names preceded by plus or minus characters. These add or remove sets of
       debugging data, respectively. For example, -d+filter adds filter debug-
       ging, whereas -d-all+filter selects only filter debugging. The avail-
       able debugging categories are:

         acl              ACL interpretation
         auth             authenticators
         deliver          general delivery logic
         dns              DNS lookups (see also resolver)
         dnsbl            DNS black list (aka RBL) code
         exec             arguments for "execv()" calls
         expand           detailed debugging for string expansions
         filter           filter handling
         hints_lookup     hints data lookups
         host_lookup      all types of name-to-IP address handling
         ident            ident lookup
         interface        lists of local interfaces
         lists            matching things in lists
         load             system load checks
         local_scan       can be used by "local_scan()" (see chapter 39)
         lookup           general lookup code and all lookups
         memory           memory handling
         pid              add pid to debug output lines
         process_info     setting info for the process log
         queue_run        queue runs
         receive          general message reception logic
         resolver         turn on the DNS resolver's debugging output
         retry            retry handling
         rewrite          address rewriting
         route            address routing
         timestamp        add timestamp to debug output lines
         tls              TLS logic
         transport        transports
         uid              changes of uid/gid and looking up uid/gid
         verify           address verification logic

         all              all of the above, and also -v

       Unfortunately, debugging output from the DNS resolver is written to
       stdout rather than stderr.

       The default (-d with no argument) omits "expand", "filter", "inter-
       face", "load", "memory", "pid", "resolver", and "timestamp". However,
       the "pid" selector is forced when debugging is turned on for a daemon,
       which then passes it on to any re-executed Exims. Exim also automati-
       cally adds the pid to debug lines when several remote deliveries are
       run in parallel.

       The "timestamp" selector causes the current time to be inserted at the
       start of all debug output lines. This can be useful when trying to
       track down delays in processing.

       If the "debug_print" option is set in any driver, it produces output
       whenever any debugging is selected, or if -v is used.

-dropcr
       This is an obsolete option that is now a no-op. It used to affect the   |
       way Exim handled CR and LF characters in incoming messages. What        |
       happens now is described in section 44.1.                               |

-E     This option specifies that an incoming message is a locally-generated
       delivery failure report. It is used internally by Exim when handling
       delivery failures and is not intended for external use. Its only effect
       is to stop Exim generating certain messages to the postmaster, as
       otherwise message cascades could occur in some situations. As part of
       the same option, a message id may follow the characters -E. If it does,
       the log entry for the receipt of the new message contains the id,
       following 'R=', as a cross-reference.

-ex    There are a number of Sendmail options starting with -oe which seem to
       be called by various programs without the leading "o" in the option.
       For example, the "vacation" program uses -eq. Exim treats all options
       of the form -ex as synonymous with the corresponding -oex options.

-F <string>
       This option sets the sender's full name for use when a locally-
       generated message is being accepted. In the absence of this option, the
       user's "gecos" entry from the password data is used. As users are
       generally permitted to alter their "gecos" entries, no security con-
       siderations are involved. White space between -F and the <string> is
       optional.

-f <address>
       This option sets the address of the envelope sender of a locally-
       generated message (also known as the return path). The option can
       normally be used only by a trusted user, but "untrusted_set_sender" can
       be set to allow untrusted users to use it. In the absence of -f, or if
       the caller is not allowed to use it, the sender of a local message is
       set to the caller's login name at the default qualify domain.

       There is one exception to the restriction on the use of -f: an empty
       sender can be specified by any user, to create a message that can never
       provoke a bounce. An empty sender can be specified either as an empty
       string, or as a pair of angle brackets with nothing between them, as in
       these examples of shell commands:

         exim -f '<>' user@domain
         exim -f "" user@domain

       In addition, the use of -f is not restricted when testing a filter file
       with -bf or when testing or verifying addresses using the -bt or -bv
       options.

       Allowing untrusted users to change the sender address does not of
       itself make it possible to send anonymous mail. Exim still checks that
       the From: header refers to the local user, and if it does not, it
       adds a Sender: header, though this can be overridden by setting
       "no_local_from_check".

       White space between -f and the <address> is optional (that is, they can
       be given as two arguments or one combined argument). The sender of a
       locally-generated message can also be set (when permitted) by an
       initial 'From ' line in the message - see the description of -bm above
       - but if -f is also present, it overrides 'From'.

-G     This is a Sendmail option which is ignored by Exim.

-h <number>
       This option is accepted for compatibility with Sendmail, but has no
       effect. (In Sendmail it overrides the 'hop count' obtained by counting
       Received: headers.)

-i     This option, which has the same effect as -oi, specifies that a dot on
       a line by itself should not terminate an incoming, non-SMTP message. I
       can find no documentation for this option in Solaris 2.4 Sendmail, but
       the "mailx" command in Solaris 2.4 uses it. See also -ti.

-M <message id> <message id> ...
       This option requests Exim to run a delivery attempt on each message in
       turn. If any of the messages are frozen, they are automatically thawed
       before the delivery attempt. The settings of "queue_domains",
       "queue_smtp_domains", and "hold_domains" are ignored. Retry hints for
       any of the addresses are overridden - Exim tries to deliver even if the
       normal retry time has not yet been reached. This option requires the
       caller to be an admin user. However, there is an option called
       "prod_requires_admin" which can be set false to relax this restriction
       (and also the same requirement for the -q, -R, and -S options).

-Mar <message id> <address> <address> ...
       This option requests Exim to add the addresses to the list of
       recipients of the message ('ar' for 'add recipients'). The first
       argument must be a message id, and the remaining ones must be email
       addresses. However, if the message is active (in the middle of a
       delivery attempt), it is not altered. This option can be used only by
       an admin user.


-MC <transport> <hostname> <sequence number> <message id>
       This option is not intended for use by external callers. It is used
       internally by Exim to invoke another instance of itself to deliver a
       waiting message using an existing SMTP connection, which is passed as
       the standard input. Details are given in chapter 43. This must be the
       final option, and the caller must be root or the Exim user in order to
       use it.

-MCA   This option is not intended for use by external callers. It is used
       internally by Exim in conjunction with the -MC option. It signifies
       that the connection to the remote host has been authenticated.

-MCP   This option is not intended for use by external callers. It is used
       internally by Exim in conjunction with the -MC option. It signifies
       that the server to which Exim is connected supports pipelining.

-MCQ <process id> <pipe fd>
       This option is not intended for use by external callers. It is used
       internally by Exim in conjunction with the -MC option when the original
       delivery was started by a queue runner. It passes on the process id of
       the queue runner, together with the file descriptor number of an open
       pipe. Closure of the pipe signals the final completion of the sequence
       of processes that are passing messages through the same SMTP
       connection.

-MCS   This option is not intended for use by external callers. It is used
       internally by Exim in conjunction with the -MC option, and passes on
       the fact that the SMTP SIZE option should be used on messages delivered
       down the existing connection.

-MCT   This option is not intended for use by external callers. It is used
       internally by Exim in conjunction with the -MC option, and passes on
       the fact that the host to which Exim is connected supports TLS
       encryption.

-Mc <message id> <message id> ...
       This option requests Exim to run a delivery attempt on each message in
       turn, but unlike the -M option, it does check for retry hints, and
       respects any that are found. This option is not very useful to external
       callers. It is provided mainly for internal use by Exim when it needs
       to re-invoke itself in order to regain root privilege for a delivery
       (see chapter 48). However, -Mc can be useful when testing, in order to
       run a delivery that respects retry times and other options such as
       "hold_domains" that are overridden when -M is used. Such a delivery
       does not count as a queue run. If you want to run a specific delivery
       as if in a queue run, you should use -q with a message id argument. A
       distinction between queue run deliveries and other deliveries is made
       in one or two places.

-Mes <message id> <address>
       This option requests Exim to change the sender address in the message
       to the given address, which must be a fully qualified address or '<>'
       ('es' for 'edit sender'). There must be exactly two arguments. The
       first argument must be a message id, and the second one an email
       address. However, if the message is active (in the middle of a delivery
       attempt), its status is not altered. This option can be used only by an
       admin user.

-Mf <message id> <message id> ...
       This option requests Exim to mark each listed message as 'frozen'. This
       prevents any delivery attempts taking place until the message is
       'thawed', either manually or as a result of the "auto_thaw" configur-
       ation option. However, if any of the messages are active (in the middle
       of a delivery attempt), their status is not altered. This option can be
       used only by an admin user.

-Mg <message id> <message id> ...
       This option requests Exim to give up trying to deliver the listed
       messages, including any that are frozen. However, if any of the
       messages are active, their status is not altered. For non-bounce
       messages, a delivery error message is sent to the sender, containing
       the text 'cancelled by administrator'. Bounce messages are just dis-
       carded. This option can be used only by an admin user.

-Mmad <message id> <message id> ...
       This option requests Exim to mark all the recipient addresses in the
       messages as already delivered ('mad' for 'mark all delivered').
       However, if any message is active (in the middle of a delivery
       attempt), its status is not altered. This option can be used only by an
       admin user.

-Mmd <message id> <address> <address> ...
       This option requests Exim to mark the given addresses as already
       delivered ('md' for 'mark delivered'). The first argument must be a
       message id, and the remaining ones must be email addresses. These are
       matched to recipient addresses in the message in a case-sensitive
       manner. If the message is active (in the middle of a delivery attempt),
       its status is not altered. This option can be used only by an admin
       user.

-Mrm <message id> <message id> ...
       This option requests Exim to remove the given messages from the queue.
       No bounce messages are sent; each message is simply forgotten. However,
       if any of the messages are active, their status is not altered. This
       option can be used only by an admin user or by the user who originally
       caused the message to be placed on the queue.

-Mt <message id> <message id> ...
       This option requests Exim to 'thaw' any of the listed messages that are
       'frozen', so that delivery attempts can resume. However, if any of the
       messages are active, their status is not altered. This option can be
       used only by an admin user.

-Mvb <message id>
       This option causes the contents of the message body (-D) spool file to
       be written to the standard output. This option can be used only by an
       admin user.

-Mvh <message id>
       This option causes the contents of the message headers (-H) spool file
       to be written to the standard output. This option can be used only by
       an admin user.

-Mvl <message id>
       This option causes the contents of the message log spool file to be
       written to the standard output. This option can be used only by an
       admin user.

-m     This is apparently a synonym for -om that is accepted by Sendmail, so
       Exim treats it that way too.

-N     This is a debugging option that inhibits delivery of a message at the
       transport level. It implies -v. Exim goes through many of the motions
       of delivery - it just doesn't actually transport the message, but
       instead behaves as if it had successfully done so. However, it does not
       make any updates to the retry database, and the log entries for
       deliveries are flagged with '*>' rather than '=>'.

       Because -N discards any message to which it applies, only root or the
       Exim user are allowed to use it with -bd, -q, -R or -M. In other words,
       an ordinary user can use it only when supplying an incoming message to
       which it will apply. Although transportation never fails when -N is
       set, an address may be deferred because of a configuration problem on a
       transport, or a routing problem. Once -N has been used for a delivery
       attempt, it sticks to the message, and applies to any subsequent
       delivery attempts that may happen for that message.

-n     This option is interpreted by Sendmail to mean 'no aliasing'. It is
       ignored by Exim.

-O <data>                                                                      |
       This option is interpreted by Sendmail to mean 'set option'. It is      |
       ignored by Exim.                                                        |

-oA <file name>
       This option is used by Sendmail in conjunction with -bi to specify an
       alternative alias file name. Exim handles -bi differently; see the
       description above.


-oB <n>
       This is a debugging option which limits the maximum number of messages
       that can be delivered down one SMTP connection, overriding the value
       set in any smtp transport. If <n> is omitted, the limit is set to 1.

-odb   This option applies to all modes in which Exim accepts incoming
       messages, including the listening daemon. It requests 'background'
       delivery of such messages, which means that the accepting process
       automatically starts delivery process for each message received, but
       does not wait for the delivery process to complete. This is the default
       action if none of the -od options are present.

       If one of the queueing options in the configuration file ("queue_only"  |
       or "queue_only_file", for example) is in effect, -odb overrides it if   |
       "queue_only_override" is set true, which is the default setting. If     |
       "queue_only_override" is set false, -odb has no effect.                 |

-odf   This option requests 'foreground' (synchronous) delivery when Exim has
       accepted a locally-generated message. (For the daemon it is exactly the
       same as -odb.) A delivery process is automatically started to deliver
       the message, and Exim waits for it to complete before proceeding.       |
       However, like -odb, this option has no effect if "queue_only_override"  |
       is false and one of the queueing options in the configuration file is   |
       in effect.                                                              |

-odi   This option is synonymous with -odf. It is provided for compatibility
       with Sendmail.

-odq   This option applies to all modes in which Exim accepts incoming
       messages, including the listening daemon. It specifies that the
       accepting process should not automatically start a delivery process for
       each message received. Messages are placed on the queue, and remain
       there until a subsequent queue runner process encounters them. There    |
       are several configuration options (such as "queue_only") that can be    |
       used to queue incoming messages under certain conditions. This option   |
       overrides all of them and also -odqs. It always forces queueing.        |

-odqs  This option is a hybrid between -odb/-odi and -odq. However, like -odb  |
       and -odi, this option has no effect if "queue_only_override" is false   |
       and one of the queueing options in the configuration file is in effect. |
                                                                               |
       When -odqs does operate, a delivery process is started for each         |
       incoming message, in the background by default, but in the foreground   |
       if -odi is also present. The recipient addresses are routed, and local  |
       deliveries are done in the normal way. However, if any SMTP deliveries
       are required, they are not done at this time, so the message remains on
       the queue until a subsequent queue runner process encounters it.
       Because routing was done, Exim knows which messages are waiting for
       which hosts, and so a number of messages for the same host can be sent
       in a single SMTP connection. The "queue_smtp_domains" configuration
       option has the same effect for specific domains. See also the -qq
       option.

-oee   If an error is detected while a non-SMTP message is being received (for
       example, a malformed address), the error is reported to the sender in a
       mail message. Provided this error message is successfully sent, the
       Exim receiving process exits with a return code of zero. If not, the
       return code is 2 if the problem is that the original message has no
       recipients, or 1 any other error. This is the default -oex option if
       Exim is called as "rmail".

-oem   This is the same as -oee, except that Exim always exits with a non-zero
       return code, whether or not the error message was successfully sent.
       This is the default -oex option, unless Exim is called as "rmail".

-oep   If an error is detected while a non-SMTP message is being received, the
       error is reported by writing a message to the standard error file
       (stderr). The return code is 1 for all errors.

-oeq   This option is supported for compatibility with Sendmail, but has the
       same effect as -oep.

-oew   This option is supported for compatibility with Sendmail, but has the
       same effect as -oem.

-oi    This option, which has the same effect as -i, specifies that a dot on a
       line by itself should not terminate an incoming, non-SMTP message. This
       is the default if Exim is called as "rmail". See also -ti.

-oitrue
       This option is treated as synonymous with -oi.

-oMa <host address>
       A number of options starting with -oM can be used to set values
       associated with remote hosts on locally-submitted messages (that is,
       messages not received over TCP/IP). These options can be used by any
       caller in conjunction with the -bh, -be, -bf, -bF, -bt, or -bv testing  |
       options. In other circumstances, they are ignored unless the caller is
       trusted.

       The -oMa option sets the sender host address. This may include a port
       number at the end, after a full stop (period). For example:

         exim -bs -oMa 10.9.8.7.1234

       An alternative syntax is to enclose the IP address in square brackets,
       followed by a colon and the port number:

         exim -bs -oMa [10.9.8.7]:1234

       The IP address is placed in the $sender_host_address variable, and the  |
       port, if present, in $sender_host_port.                                 |

-oMaa <name>
       See -oMa above for general remarks about the -oM options. The -oMaa
       option sets the value of $sender_host_authenticated (the authenticator
       name). See chapter 33 for a discussion of SMTP authentication.

-oMai <string>
       See -oMa above for general remarks about the -oM options. The -oMai
       option sets the value of $authenticated_id (the id that was             |
       authenticated). This overrides the default value (the caller's login    |
       id) for messages from local sources. See chapter 33 for a discussion of
       authenticated ids.

-oMas <address>
       See -oMa above for general remarks about the -oM options. The -oMas
       option sets the authenticated sender value in $authenticated_sender. It |
       overrides the sender address that is created from the caller's login id
       for messages from local sources. See chapter 33 for a discussion of
       authenticated senders.

-oMi <interface address>
       See -oMa above for general remarks about the -oM options. The -oMi
       option sets the IP interface address value. A port number may be
       included, using the same syntax as for -oMa. The interface address is   |
       placed in $interface_address and the port number, if present, in        |
       $interface_port.                                                        |

-oMr <protocol name>
       See -oMa above for general remarks about the -oM options. The -oMr
       option sets the received protocol value in $received_protocol. However, |
       this applies only when -bs is not used. For interactive SMTP input, the
       protocol is determined by whether EHLO or HELO is used, and is always
       either 'local-esmtp' or 'local-smtp'. For -bS (batch SMTP) however, the
       protocol can be set by -oMr.

-oMs <host name>
       See -oMa above for general remarks about the -oM options. The -oMs
       option sets the sender host name in $sender_host_name. When this option |
       is present, Exim does not attempt to look up a host name from an IP     |
       address; it uses the name it is given.                                  |

-oMt <ident string>
       See -oMa above for general remarks about the -oM options. The -oMt
       option sets the sender ident value in $sender_ident. The default        |
       setting for local callers is the login id of the calling process.

-om    In Sendmail, this option means 'me too', indicating that the sender of
       a message should receive a copy of the message if the sender appears in
       an alias expansion. Exim always does this, so the option does nothing.

-oo    This option is ignored. In Sendmail it specifies 'old style headers',
       whatever that means.

-oP <path>
       This option is useful only in conjunction with -bd or -q with a time
       value. The option specifies the file to which the process id of the
       daemon is written. When -oX is used with -bd, or when -q with a time is
       used without -bd, this is the only way of causing Exim to write a pid
       file, because in those cases, the normal pid file is not used.

-or <time>
       This option sets a timeout value for incoming non-SMTP messages. If it
       is not set, Exim will wait forever for the standard input. The value
       can also be set by the "receive_timeout" option. The format used for
       specifying times is described in section 6.11.

-os <time>
       This option sets a timeout value for incoming SMTP messages. The
       timeout applies to each SMTP command and block of data. The value can
       also be set by the "smtp_receive_timeout" option; it defaults to 5
       minutes. The format used for specifying times is described in section
       6.11.

-ov    This option has exactly the same effect as -v.

-oX <number or string>
       This option is relevant only when the -bd (start listening daemon)      |
       option is also given. It controls which ports and interfaces the daemon |
       uses. Details of the syntax, and how it interacts with configuration    |
       file options, are given in chapter 13. When -oX is used to start a      |
       daemon, no pid file is written unless -oP is also present to specify a  |
       pid file name.                                                          |

-pd    This option applies when an embedded Perl interpreter is linked with
       Exim (see chapter 12). It overrides the setting of the "perl_at_start"
       option, forcing the starting of the interpreter to be delayed until it
       is needed.

-ps    This option applies when an embedded Perl interpreter is linked with
       Exim (see chapter 12). It overrides the setting of the "perl_at_start"
       option, forcing the starting of the interpreter to occur as soon as
       Exim is started.

-q     This option is normally restricted to admin users. However, there is a
       configuration option called "prod_requires_admin" which can be set
       false to relax this restriction (and also the same requirement for the
       -M, -R, and -S options).

       The -q option starts one queue runner process. This scans the queue of
       waiting messages, and runs a delivery process for each one in turn. It
       waits for each delivery process to finish before starting the next one.
       A delivery process may not actually do any deliveries if the retry
       times for the addresses have not been reached. Use -qf (see below) if
       you want to override this. If the delivery process spawns other
       processes to deliver other messages down passed SMTP connections, the
       queue runner waits for these to finish before proceeding.

       When all the queued messages have been considered, the original queue
       runner process terminates. In other words, a single pass is made over
       the waiting mail, one message at a time. Use -q with a time (see below)
       if you want this to be repeated periodically.

       Exim processes the waiting messages in an unpredictable order. It isn't
       very random, but it is likely to be different each time, which is all
       that matters. If one particular message screws up a remote MTA, other
       messages to the same MTA have a chance of getting through if they get
       tried first.

       It is possible to cause the messages to be processed in lexical message
       id order, which is essentially the order in which they arrived, by
       setting the "queue_run_in_order" option, but this is not recommended
       for normal use.

-q<qflags>
       The -q option may be followed by one or more flag letters that change
       its behaviour. They are all optional, but if more than one is present,
       they must appear in the correct order. Each flag is described in a
       separate item below.

-qq... An option starting with -qq requests a two-stage queue run. In the
       first stage, the queue is scanned as if the "queue_smtp_domains" option
       matched every domain. Addresses are routed, local deliveries happen,
       but no remote transports are run. The hints database that remembers
       which messages are waiting for specific hosts is updated, as if
       delivery to those hosts had been deferred. After this is complete, a
       second, normal queue scan happens, with routing and delivery taking
       place as normal. Messages that are routed to the same host should
       mostly be delivered down a single SMTP connection because of the hints
       that were set up during the first queue scan. This option may be useful
       for hosts that are connected to the Internet intermittently.

-q[q]i...
       If the "i" flag is present, the queue runner runs delivery processes
       only for those messages that haven't previously been tried. ("i" stands
       for 'initial delivery'.) This can be helpful if you are putting
       messages on the queue using -odq and want a queue runner just to
       process the new messages.

-q[q][i]f...
       If one "f" flag is present, a delivery attempt is forced for each non-
       frozen message, whereas without "f" only those non-frozen addresses
       that have passed their retry times are tried.

-q[q][i]ff...
       If "ff" is present, a delivery attempt is forced for every message,
       whether frozen or not.

-q[q][i][f[f]]l
       The "l" (the letter 'ell') flag specifies that only local deliveries
       are to be done. If a message requires any remote deliveries, it remains
       on the queue for later delivery.

-q<qflags> <start id> <end id>
       When scanning the queue, Exim can be made to skip over messages whose
       ids are lexically less than a given value by following the -q option
       with a starting message id. For example:

         exim -q 0t5C6f-0000c8-00

       Messages that arrived earlier than "0t5C6f-0000c8-00" are not
       inspected. If a second message id is given, messages whose ids are
       lexically greater than it are also skipped. If the same id is given
       twice, for example,

         exim -q 0t5C6f-0000c8-00 0t5C6f-0000c8-00

       just one delivery process is started, for that message. This differs
       from -M in that retry data is respected, and it also differs from -Mc
       in that it counts as a delivery from a queue run. Note that the
       selection mechanism does not affect the order in which the messages are
       scanned. There are also other ways of selecting specific sets of
       messages for delivery in a queue run - see -R and -S.

-q<qflags><time>
       When a time value is present, the -q option causes Exim to run as a
       daemon, starting a queue runner process at intervals specified by the
       given time value (whose format is described in section 6.11). This form
       of the -q option is commonly combined with the -bd option, in which
       case a single daemon process handles both functions. A common way of
       starting up a combined daemon at system boot time is to use a command
       such as

         /usr/exim/bin/exim -bd -q30m

       Such a daemon listens for incoming SMTP calls, and also starts a queue
       runner process every 30 minutes.

       When a daemon is started by -q with a time value, but without -bd, no
       pid file is written unless one is explicitly requested by the -oP
       option.

-qR<rsflags> <string>
       This option is synonymous with -R. It is provided for Sendmail
       compatibility.

-qS<rsflags> <string>
       This option is synonymous with -S.

-R<rsflags> <string>
       The <rsflags> may be empty, in which case the white space before the
       string is optional, unless the string is "f", "ff", "r", "rf", or
       "rff", which are the possible values for <rsflags>. White space is
       required if <rsflags> is not empty.

       This option is similar to -q with no time value, that is, it causes
       Exim to perform a single queue run, except that, when scanning the
       messages on the queue, Exim processes only those that have at least one
       undelivered recipient address containing the given string, which is
       checked in a case-independent way. If the <rsflags> start with "r",
       <string> is interpreted as a regular expression; otherwise it is a
       literal string.

       Once a message is selected, all its addresses are processed. For the
       first selected message, Exim overrides any retry information and forces
       a delivery attempt for each undelivered address. This means that if
       delivery of any address in the first message is successful, any
       existing retry information is deleted, and so delivery attempts for
       that address in subsequently selected messages (which are processed
       without forcing) will run. However, if delivery of any address does not
       succeed, the retry information is updated, and in subsequently selected
       messages, the failing address will be skipped.

       If the <rsflags> contain "f" or "ff", the delivery forcing applies to
       all selected messages, not just the first; frozen messages are included
       when "ff" is present.

       The -R option makes it straightforward to initiate delivery of all
       messages to a given domain after a host has been down for some time.
       When the SMTP command ETRN is accepted by its ACL (see chapter 38), its
       default effect is to run Exim with the -R option, but it can be
       configured to run an arbitrary command instead.

-r     This is a documented (for Sendmail) obsolete alternative name for -f.


-S<rsflags> <string>
       This option acts like -R except that it checks the string against each
       message's sender instead of against the recipients. If -R is also set,
       both conditions must be met for a message to be selected. If either of
       the options has "f" or "ff" in its flags, the associated action is
       taken.

-t     When Exim is receiving a locally-generated, non-SMTP message on its
       standard input, the -t option causes the recipients of the message to
       be obtained from the To:, Cc:, and Bcc: header lines in the message
       instead of from the command arguments. The addresses are extracted
       before any rewriting takes place.

       If the command has any arguments, they specify addresses to which the
       message is not to be delivered. That is, the argument addresses are
       removed from the recipients list obtained from the headers. This is
       compatible with Smail 3 and in accordance with the documented behaviour
       of several versions of Sendmail, as described in man pages on a number
       of operating systems (e.g. Solaris 8, IRIX 6.5, HP-UX 11). However,
       some versions of Sendmail add argument addresses to those obtained from
       the headers, and the O'Reilly Sendmail book documents it that way. Exim
       can be made to add argument addresses instead of subtracting them by
       setting the option "extract_addresses_remove_arguments" false.

       If a Bcc: header line is present, it is removed from the message unless
       there is no To: or Cc:, in which case a Bcc: line with no data is
       created. This is necessary for conformity with the original RFC 822
       standard; the requirement has been removed in RFC 2822, but that is
       still very new.

       If there are any "Resent-" header lines in the message, Exim extracts
       recipients from all Resent-To:, Resent-Cc:, and Resent-Bcc: header
       lines instead of from To:, Cc:, and Bcc:. This is for compatibility
       with Sendmail and other MTAs. (Prior to release 4.20, Exim gave an
       error if -t was used in conjunction with "Resent-" header lines.)

       RFC 2822 talks about different sets of "Resent-" header lines (for when
       a message is resent several times). The RFC also specifies that they
       should be added at the front of the message, and separated by Received:
       lines. It is not at all clear how -t should operate in the present of
       multiple sets, nor indeed exactly what constitutes a 'set'. In prac-    |
       tice, it seems that MUAs do not follow the RFC. The "Resent-" lines are
       often added at the end of the header, and if a message is resent more
       than once, it is common for the original set of "Resent-" headers to be
       renamed as "X-Resent-" when a new set is added. This removes any
       possible ambiguity.

-ti    This option is exactly equivalent to -t -i. It is provided for          |
       compatibility with Sendmail.                                            |

-tls-on-connect
       This option is available when Exim is compiled with TLS support. It
       makes it possible to support legacy clients that do not support the
       STARTTLS command, but instead expect to start up a TLS session as soon
       as a connection to the server is established. These clients use a
       special port (usually called the 'ssmtp' port) instead of the normal
       SMTP port 25. The -tls-on-connect option can be used to run Exim in
       this way from "inetd", and it can also be used to run a special daemon
       that operates in this manner (use -oX to specify the port). However,
       although it is possible to run one daemon that listens on several
       ports, it is not possible to have some of them operate one way and some
       the other. With only a few clients that need the legacy support, a
       convenient approach is to use a daemon for normal SMTP (with or without
       STARTTLS) and "inetd" with -tls-on-connect for the legacy clients.

-U     Sendmail uses this option for 'initial message submission', and its
       documentation states that in future releases, it may complain about
       syntactically invalid messages rather than fixing them when this flag
       is not set. Exim ignores this option.

-v     This option causes Exim to write information to the standard error
       stream, describing what it is doing. In particular, it shows the log
       lines for receiving and delivering a message, and if an SMTP connection
       is made, the SMTP dialogue is shown. Some of the log lines shown may
       not actually be written to the log if the setting of "log_selector"
       discards them. Any relevant selectors are shown with each log line. If
       none are shown, the logging is unconditional.

-x     AIX uses -x for a private purpose ('mail from a local mail program has
       National Language Support extended characters in the body of the mail
       item'). It sets -x when calling the MTA from its "mail" command. Exim
       ignores this option.



                   6. THE EXIM RUN TIME CONFIGURATION FILE


Exim uses a single run time configuration file that is read whenever an Exim
binary is executed. Note that in normal operation, this happens frequently,
because Exim is designed to operate in a distributed manner, without central
control.

The name of the configuration file is compiled into the binary for security
reasons, and is specified by the CONFIGURE_FILE compilation option. In most
configurations, this specifies a single file. However, it is permitted to give
a colon-separated list of file names, in which case Exim uses the first
existing file in the list.

The run time configuration file must be owned by root or by the user that is
specified at compile time by the EXIM_USER option, and it must not be world-
writeable or group-writeable, unless its group is the one specified at compile
time by the EXIM_GROUP option.

Warning: In a conventional configuration, where the Exim binary is setuid to
root, anybody who is able to edit the run time configuration file has an easy
way to run commands as root. If you make your mail administrators members of
the Exim group, but do not trust them with root, make sure that the run time
configuration is not group writeable.

A default configuration file, which will work correctly in simple situations,
is provided in the file src/configure.default. If CONFIGURE_FILE defines just
one file name, the installation process copies the default configuration to a
new file of that name if it did not previously exist. If CONFIGURE_FILE is a
list, no default is automatically installed. Chapter 7 is a 'walk-through'
discussion of the default configuration.

If a syntax error is detected while reading the configuration file, Exim
writes a message on the standard error, and exits with a non-zero return code.
The message is also written to the panic log.


6.1 Using a different configuration file

A one-off alternate configuration can be specified by the -C command line
option, which may specify a single file or a list of files. However, when -C
is used, Exim gives up its root privilege, unless called by root or the Exim
user (or unless the argument for -C is identical to the built-in value from
CONFIGURE_FILE). -C is useful mainly for checking the syntax of configuration
files before installing them. No owner or group checks are done on a
configuration file specified by -C.

The privileged use of -C by the Exim user can be locked out by setting         |
ALT_CONFIG_ROOT_ONLY in Local/Makefile when building Exim. However, if you do  |
this, you also lock out the possibility of testing a configuration using -C    |
right through message reception and delivery, even if the caller is root. The  |
reception works, but by that time, Exim is running as the Exim user, so when   |
it re-execs to regain privilege for the delivery, the use of -C causes         |
privilege to be lost. However, root can test reception and delivery using two  |
separate commands (one to put a message on the queue, using -odq, and another  |
to do the delivery, using -M).                                                 |
                                                                               |
If ALT_CONFIG_PREFIX is defined in Local/Makefile, it specifies a prefix       |
string with which any file named in a -C command line option must start. In    |
addition, the file name must not contain the sequence "/../". There is no      |
default setting for ALT_CONFIG_PREFIX; when it is unset, any file name can be  |
used with -C.                                                                  |

One-off changes to a configuration can be specified by the -D command line
option, which defines and overrides values for macros used inside the
configuration file. However, like -C, the use of this option by a non-
privileged user causes Exim to discard its root privilege. If DISABLE_D_OPTION |
is defined in Local/Makefile, the use of -D is completely disabled, and its    |
use causes an immediate error exit.                                            |

Some sites may wish to use the same Exim binary on different machines that
share a file system, but to use different configuration files on each machine.
If CONFIGURE_FILE_USE_NODE is defined in Local/Makefile, Exim first looks for
a file whose name is the configuration file name followed by a dot and the
machine's node name, as obtained from the "uname()" function. If this file
does not exist, the standard name is tried. This processing occurs for each
file name in the list given by CONFIGURE_FILE or -C.

In some esoteric situations different versions of Exim may be run under
different effective uids and the CONFIGURE_FILE_USE_EUID is defined to help
with this. See the comments in src/EDITME for details.


6.2 Configuration file format

Exim's configuration file is divided into a number of different parts. General
option settings must always appear at the start of the file. The other parts
are all optional, and may appear in any order. Each part other than the first
is introduced by the word 'begin' followed by the name of the part. The
optional parts are:

 .   "ACL": Access control lists for controlling incoming SMTP mail.

 .   "authenticators": Configuration settings for the authenticator drivers.
     These are concerned with the SMTP AUTH command (see chapter 33).

 .   "routers": Configuration settings for the router drivers. Routers process
     addresses and determine how the message is to be delivered.

 .   "transports": Configuration settings for the transport drivers.
     Transports define mechanisms for copying messages to destinations.

 .   "retry": Retry rules, for use when a message cannot be immediately
     delivered.

 .   "rewrite": Global address rewriting rules, for use when a message arrives
     and when new addresses are generated during delivery.

 .   "local_scan": Private options for the "local_scan()" function. If you
     want to use this feature, you must set

       LOCAL_SCAN_HAS_OPTIONS=yes

     in Local/Makefile before building Exim. Full details of the
     "local_scan()" facility are given in chapter 39.

Blank lines in the file, and lines starting with a # character (ignoring
leading white space) are treated as comments and are ignored. Note: a #
character other than at the beginning of a line is not treated specially, and
does not introduce a comment.

Any non-comment line can be continued by ending it with a backslash. Trailing
white space after the backslash is ignored, and leading white space at the
start of continuation lines is also ignored. Comment lines beginning with #    |
(but not empty lines) may appear in the middle of a sequence of continuation   |
lines.                                                                         |

A convenient way to create a configuration file is to start from the default,
which is supplied in src/configure.default, and add, delete, or change
settings as required.

The ACLs, retry rules, and rewriting rules have their own syntax which is
described in chapters 38, 32, and 31, respectively. The other parts of the
configuration file have some syntactic items in common, and these are
described below, from section 6.6 onwards. Before that, the inclusion, macro,
and conditional facilities are described.


6.3 File inclusions in the configuration file

You can include other files inside Exim's run time configuration file by using
this syntax:

  .include <file name>

or                                                                             |
                                                                               |
  .include_if_exists <file name>                                               |
                                                                               |
on a line by itself. Double quotes round the file name are optional. If you    |
use the first form, a configuration error occurs if the file does not exist;   |
the second form does nothing for non-existent files.                           |

Includes may be nested to any depth, but remember that Exim reads its
configuration file often, so it is a good idea to keep them to a minimum. If
you change the contents of an included file, you must HUP the daemon, because
an included file is read only when the configuration itself is read.

The processing of inclusions happens early, at a physical line level, so, like
comment lines, an inclusion can be used in the middle of an option setting,
for example:

  hosts_lookup = a.b.c \
                 .include /some/file

Include processing happens after macro processing (see below). Its effect is
to process the lines of the file as if they occurred inline where the
inclusion appears.


6.4 Macros in the configuration file

If a line in the main part of the configuration (that is, before the first
'begin' line) begins with an upper case letter, it is taken as a macro
definition, and must be of the form

  <name> = <rest of line>

The name must consist of letters, digits, and underscores, and need not all be
in upper case, though that is recommended. The rest of the line, including any
continuations, is the replacement text, and has leading and trailing white
space removed. Quotes are not removed. The replacement text can never end with
a backslash character, but this doesn't seem to be a serious limitation.

Once a macro is defined, all subsequent lines in the file (and any included
files) are scanned for the macro name; if there are several macros, the line
is scanned for each in turn, in the order in which they are defined. The
replacement text is not re-scanned for the current macro, though it is scanned
for subsequently defined macros. For this reason, a macro name may not contain
the name of a previously defined macro as a substring. You could, for example,
define

  ABCD_XYZ = <<something>>
  ABCD = <<something else>>

but putting the definitions in the opposite order would provoke a configur-
ation error.

Macro expansion is applied to individual lines from the file, before checking
for line continuation or file inclusion (see below). If a line consists solely
of a macro name, and the expansion of the macro is empty, the line is ignored.
A macro at the start of a line may turn the line into a comment line or a
".include" line.

As an example of macro usage, consider a configuration where aliases are
looked up in a MySQL database. It helps to keep the file less cluttered if
long strings such as SQL statements are defined separately as macros, for
example:

  ALIAS_QUERY = select mailbox from user where \
                login=${quote_mysql:$local_part};

This can then be used in a redirect router setting like this:

  data = ${lookup mysql{ALIAS_QUERY}}

In earlier versions of Exim macros were sometimes used for domain, host, or
address lists. In Exim 4 these are handled better by named lists - see section
10.5.

Macros in the configuration file can be overridden by the -D command line
option, but Exim gives up its root privilege when -D is used, unless called by
root or the Exim user.


6.5 Conditional skips in the configuration file

You can use the directives ".ifdef", ".ifndef", ".elifdef", ".elifndef",
".else", and ".endif" to dynamically include or exclude portions of the
configuration file. The processing happens whenever the file is read (that is,
when an Exim binary starts to run).

The implementation is very simple. Instances of the first four directives must
be followed by text that includes the names of one or macros. The condition
that is tested is whether or not any macro substitution has taken place in the
line. Thus:

  .ifdef AAA
  message_size_limit = 50M
  .else
  message_size_limit = 100M
  .endif

sets a message size limit of 50M if the macro "AAA" is defined, and 100M
otherwise. If there is more than one macro named on the line, the condition is
true if any of them are defined. That is, it is an 'or' condition. To obtain
an 'and' condition, you need to use nested ".ifdef"s.

Although you can use a macro expansion to generate one of these directives, it
is not very useful, because the condition 'there was a macro substitution in
this line' will always be true.

Text following ".else" and ".endif" is ignored, and can be used as comment to
clarify complicated nestings.


6.6 Common option syntax

For the main set of options, driver options, and "local_scan()" options, each
setting is on a line by itself, and starts with a name consisting of lower-
case letters and underscores. Many options require a data value, and in these
cases the name must be followed by an equals sign (with optional white space)
and then the value. For example:

  qualify_domain = mydomain.example.com

Some option settings may contain sensitive data, for example, passwords for
accessing databases. To stop non-admin users from using the -bP command line
option to read these values, you can precede the option settings with the word
'hide'. For example:

  hide mysql_servers = localhost/users/admin/secret-password

For non-admin users, such options are displayed like this:

  mysql_servers = <value not displayable>

If 'hide' is used on a driver option, it hides the value of that option on all
instances of the same driver.

The following sections describe the syntax used for the different data types
that are found in option settings.


6.7 Boolean options

Options whose type is given as boolean are on/off switches. There are two
different ways of specifying such options: with and without a data value. If
the option name is specified on its own without data, the switch is turned on;
if it is preceded by 'no_' or 'not_' the switch is turned off. However,
boolean options may optionally be followed by an equals sign and one of the
words 'true', 'false', 'yes', or 'no', as an alternative syntax. For example,
the following two settings have exactly the same effect:

  queue_only
  queue_only = true

The following two lines also have the same (opposite) effect:

  no_queue_only
  queue_only = false

You can use whichever syntax you prefer.


6.8 Integer values

If an integer data item starts with the characters '0x', the remainder of it
is interpreted as a hexadecimal number. Otherwise, it is treated as octal if
it starts with the digit 0, and decimal if not. If an integer value is
followed by the letter K, it is multiplied by 1024; if it is followed by the
letter M, it is multiplied by 1024x1024.

When the values of integer option settings are output, values which are an
exact multiple of 1024 or 1024x1024 are sometimes, but not always, printed
using the letters K and M. The printing style is independent of the actual
input format that was used.


6.9 Octal integer values

The value of an option specified as an octal integer is always interpreted in
octal, whether or not it starts with the digit zero. Such options are always
output in octal.


6.10 Fixed point number values

A fixed point number consists of a decimal integer, optionally followed by a
decimal point and up to three further digits.


6.11 Time interval values

A time interval is specified as a sequence of numbers, each followed by one of
the following letters, with no intervening white space:

  "s" seconds
  "m" minutes
  "h" hours
  "d" days
  "w" weeks

For example, '3h50m' specifies 3 hours and 50 minutes. The values of time
intervals are output in the same format. Exim does not restrict the values; it
is perfectly acceptable, for example, to specify '90m' instead of '1h30m'.


6.12 String values

If a string data item does not start with a double-quote character, it is
taken as consisting of the remainder of the line plus any continuation lines,
starting at the first character after any leading white space, with trailing
white space characters removed, and with no interpretation of the characters
in the string. Because Exim removes comment lines (those beginning with #) at
an early stage, they can appear in the middle of a multi-line string. The
following settings are therefore equivalent:

  trusted_users = uucp:mail

  trusted_users = uucp:\
                  # This comment line is ignored
                  mail

If a string does start with a double-quote, it must end with a closing double-
quote, and any backslash characters other than those used for line continu-
ation are interpreted as escape characters, as follows:

  \\               single backslash
  \n               newline
  \r               carriage return
  \t               tab
  \<octal digits>  up to 3 octal digits specify one character
  \x<hex digits>   up to 2 hexadecimal digits specify one character

If a backslash is followed by some other character, including a double-quote
character, that character replaces the pair.

Quoting is necessary only if you want to make use of the backslash escapes to
insert special characters, or if you need to specify a value with leading or
trailing spaces. These cases are rare, so quoting is almost never needed in
current versions of Exim. In versions of Exim before 3.14, quoting was
required in order to continue lines, so you may come across older configur-
ation files and examples that apparently quote unnecessarily.


6.13 Expanded strings

Some strings in the configuration file are subjected to "string expansion", by
which means various parts of the string may be changed according to the
circumstances (see chapter 11). The input syntax for such strings is as just
described; in particular, the handling of backslashes in quoted strings is
done as part of the input process, before expansion takes place. However,
backslash is also an escape character for the expander, so any backslashes
that are required for that reason must be doubled if they are within a quoted
configuration string.


6.14 User and group names

User and group names are specified as strings, using the syntax described
above, but the strings are interpreted specially. A user or group name must
either consist entirely of digits, or be a name that can be looked up using
the "getpwnam()" or "getgrnam()" function, as appropriate.


6.15 List construction

The data for some configuration options is a colon-separated list of items.    |
Many of these options are shown with type 'string list' in the descriptions    |
later in this document. Others are listed as 'domain list', 'host list',       |
'address list', or 'local part list'. Syntactically, they are all the same;    |
however, those other than 'string list' are subject to particular kinds of     |
interpretation, as described in chapter 10.                                    |

In all these cases, the entire list is treated as a single string as far as
the input syntax is concerned. The "trusted_users" setting in section 6.12
above is an example. If a colon is actually needed in an item in a list, it
must be entered as two colons. Leading and trailing white space on each item
in a list is ignored. This makes it possible to include items that start with
a colon, and in particular, certain forms of IPv6 address. For example, the
list

  local_interfaces = 127.0.0.1 : ::::1

contains two IP addresses, the IPv4 address 127.0.0.1 and the IPv6 address
::1. IPv6 addresses are going to become more and more common as the new
protocol gets more widely deployed. Doubling their colons is an unwelcome
chore, so a mechanism was introduced to allow the separator character to be
changed. If a list begins with a left angle bracket, followed by any
punctuation character, that character is used instead of colon as the list
separator. For example, the list above can be rewritten to use a semicolon
separator like this:

  local_interfaces = <; 127.0.0.1 ; ::1

This facility applies to all lists, with the exception of the list in
"log_file_path". It is recommended that the use of non-colon separators be
confined to circumstances where they really are needed.


6.16 Format of driver configurations

There are separate parts in the configuration for defining routers, trans-
ports, and authenticators. In each part, you are defining a number of driver
instances, each with its own set of options. Each driver instance is defined
by a sequence of lines like this:

  <instance name>:
    <option>
    ...
    <option>

In the following example, the instance name is localuser, and it is followed
by three options settings:

  localuser:
    driver = accept
    check_local_user
    transport = local_delivery

For each driver instance, you specify which Exim code module it uses - by the
setting of the "driver" option - and (optionally) some configuration settings.
For example, in the case of transports, if you want a transport to deliver
with SMTP you would use the smtp driver; if you want to deliver to a local
file you would use the appendfile driver. Each of the drivers is described in
detail in its own separate chapter later in this manual.

You can have several routers, transports, or authenticators that are based on
the same underlying driver (each must have a different name).

The order in which routers are defined is important, because addresses are
passed to individual routers one by one, in order. The order in which
transports are defined does not matter at all. The order in which
authenticators are defined is used only when Exim, as a client, is searching
them to find one that matches an authentication mechanism offered by the
server.

Within a driver instance definition, there are two kinds of option: generic
and private. The generic options are those that apply to all drivers of the
same type (that is, all routers, all transports or all authenticators). The
"driver" option is a generic option that must appear in every definition. The
private options are special for each driver, and none need appear, because
they all have default values.

The options may appear in any order, except that the "driver" option must
precede any private options, since these depend on the particular driver. For
this reason, it is recommended that "driver" always be the first option.

Driver instance names, which are used for reference in log entries and
elsewhere, can be any sequence of letters, digits, and underscores (starting
with a letter) and must be unique among drivers of the same type. A router and
a transport (for example) can each have the same name, but no two router
instances can have the same name. The name of a driver instance should not be
confused with the name of the underlying driver module. For example, the
configuration lines:

  remote_smtp:
    driver = smtp

create an instance of the smtp transport driver whose name is remote_smtp. The
same driver code can be used more than once, with different instance names and
different option settings each time. A second instance of the smtp transport,
with different options, might be defined thus:

  special_smtp:
    driver = smtp
    port = 1234
    command_timeout = 10s

The names remote_smtp and special_smtp would be used to reference these
transport instances from routers, and these names would appear in log lines.

Comment lines may be present in the middle of driver specifications. The full
list of option settings for any particular driver instance, including all the
defaulted values, can be extracted by making use of the -bP command line
option.



                      7. THE DEFAULT CONFIGURATION FILE


The default configuration file supplied with Exim as src/configure.default is
sufficient for a host with simple mail requirements. As an introduction to the
way Exim is configured, this chapter 'walks through' the default configur-
ation, giving brief explanations of the settings. Detailed descriptions of the
options are given in subsequent chapters. The default configuration file
itself contains extensive comments about ways you might want to modify the
initial settings. However, note that there are many options that are not
mentioned at all in the default configuration.


7.1 Main configuration settings

The main (global) configuration option settings must always come first in the
file. The first thing you'll see in the file, after some initial comments, is
the line

  # primary_hostname =

This is a commented-out setting of the "primary_hostname" option. Exim needs
to know the official, fully qualified name of your host, and this is where you
can specify it. However, in most cases you do not need to set this option.
When it is unset, Exim uses the "uname()" system function to obtain the host
name.

The first three non-comment configuration lines are as follows:

  domainlist local_domains = @
  domainlist relay_to_domains =
  hostlist   relay_from_hosts = 127.0.0.1

These are not, in fact, option settings. They are definitions of two named
domain lists and one named host list. Exim allows you to give names to lists
of domains, hosts, and email addresses, in order to make it easier to manage
the configuration file (see section 10.5).

The first line defines a domain list called "local_domains"; this is used
later in the configuration to identify domains that are to be delivered on the
local host. There is just one item in this list, the string '@'. This is a
special form of entry which means 'the name of the local host'. Thus, if the
local host is called "a.host.example", mail to "any.user@a.host.example" is
expected to be delivered locally. Because the local host's name is referenced
indirectly, the same configuration file can be used on different hosts.

The second line defines a domain list called "relay_to_domains", but the list
itself is empty. Later in the configuration we will come to the part that
controls mail relaying through the local host; it allows relaying to any
domains in this list. By default, therefore, no relaying on the basis of a
mail domain is permitted.

The third line defines a host list called "relay_from_hosts". This list is
used later in the configuration to permit relaying from any host or IP address
that matches the list. The default contains just the IP address of the IPv4
loopback interface, which means that processes on the local host are able to
submit mail for relaying by sending it over TCP/IP to that interface. No other
hosts are permitted to submit messages for relaying.

Just to be sure there's no misunderstanding: at this point in the configur-
ation we aren't actually setting up any controls. We are just defining some
domains and hosts that will be used in the controls that are specified later.

The next configuration line is a genuine option setting:

  acl_smtp_rcpt = acl_check_rcpt

This option specifies an "Access Control List" (ACL) which is to be used
during an incoming SMTP session for every recipient of a message (every RCPT
command). The name of the list is "acl_check_rcpt", and we will come to its
definition below, in the ACL section of the configuration. ACLs control which
recipients are accepted for an incoming message - if a configuration does not
provide an ACL to check recipients, no SMTP mail can be accepted.

Two commented-out options settings are next:

  # qualify_domain =
  # qualify_recipient =

The first of these specifies a domain that Exim uses when it constructs a
complete email address from a local login name. This is often needed when Exim
receives a message from a local process. If you do not set "qualify_domain",
the value of "primary_hostname" is used. If you set both of these options, you
can have different qualification domains for sender and recipient addresses.
If you set only the first one, its value is used in both cases.

The following line must be uncommented if you want Exim to recognize addresses
of the form "user@[10.11.12.13]" that is, with a 'domain literal' (an IP
address) instead of a named domain.

  # allow_domain_literals

The RFCs still require this form, but in the modern Internet it makes little
sense to permit mail to be sent to specific hosts by quoting their IP
addresses. This ancient format has been used by those seeking to abuse hosts
by using them for unwanted relaying.

The next configuration line is a kind of trigger guard:

  never_users = root

It specifies that no delivery must ever be run as the root user. The normal
convention is to set up "root" as an alias for the system administrator. This
setting is a guard against slips in the configuration. The list of users       |
specified by "never_users" is not, however, the complete list; the build-time  |
configuration in Local/Makefile has an option called FIXED_NEVER_USERS speci-  |
fying a list that cannot be overridden. The contents of "never_users" are      |
added to this list. By default FIXED_NEVER_USERS also specifies root.          |

When a remote host connects to Exim in order to send mail, the only
information Exim has about the host's identity is its IP address. The next
configuration line,

  host_lookup = *

specifies that Exim should do a reverse DNS lookup on all incoming connec-
tions, in order to get a host name. This improves the quality of the logging
information, but if you feel it is too expensive, you can remove it entirely,
or restrict the lookup to hosts on 'nearby' networks. Note that it is not
always possible to find a host name from an IP address, because not all DNS
reverse zones are maintained, and sometimes DNS servers are unreachable.

The next two lines are concerned with "ident" callbacks, as defined by RFC
1413 (hence their names):

  rfc1413_hosts = *
  rfc1413_query_timeout = 30s

These settings cause Exim to make ident callbacks for all incoming SMTP calls.
You can limit the hosts to which these calls are made, or change the timeout
that is used. If you set the timeout to zero, all ident calls are disabled.
Although they are cheap and can provide useful information for tracing problem
messages, some hosts and firewalls have problems with ident calls. This can
result in a timeout instead of an immediate refused connection, leading to
delays on starting up an incoming SMTP session.

When Exim receives messages over SMTP connections, it expects all addresses to
be fully qualified with a domain, as required by the SMTP definition. However,
if you are running a server to which simple clients submit messages, you may
find that they send unqualified addresses. The two commented-out options:

  # sender_unqualified_hosts =
  # recipient_unqualified_hosts =

show how you can specify hosts that are permitted to send unqualified sender
and recipient addresses, respectively.

The "percent_hack_domains" option is also commented out:

  # percent_hack_domains =

It provides a list of domains for which the 'percent hack' is to operate. This
is an almost obsolete form of explicit email routing. If you do not know
anything about it, you can safely ignore this topic.

The last two settings in the main part of the default configuration are
concerned with messages that have been 'frozen' on Exim's queue. When a
message is frozen, Exim no longer continues to try to deliver it. Freezing
occurs when a bounce message encounters a permanent failure because the sender
address of the original message that caused the bounce is invalid, so the
bounce cannot be delivered. This is probably the most common case, but there
are also other conditions that cause freezing, and frozen messages are not
always bounce messages.

  ignore_bounce_errors_after = 2d
  timeout_frozen_after = 7d

The first of these options specifies that failing bounce messages are to be
discarded after 2 days on the queue. The second specifies that any frozen
message (whether a bounce message or not) is to be timed out (and discarded)
after a week. In this configuration, the first setting ensures that no failing
bounce message ever lasts a week.


7.2 ACL configuration

In the default configuration, the ACL section follows the main configuration.
It starts with the line

  begin acl

and it contains the definition of one ACL called "acl_check_rcpt" that was
referenced in the setting of "acl_smtp_rcpt" above. This ACL is used for every
RCPT command in an incoming SMTP message. Each RCPT command specifies one of
the message's recipients. The ACL statements are considered in order, until
the recipient address is either accepted or rejected. The RCPT command is then
accepted or rejected, according to the result of the ACL processing.

  acl_check_rcpt:

This line, consisting of a name terminated by a colon, marks the start of the
ACL, and names it.

  accept  hosts = :

This ACL statement accepts the recipient if the sending host matches the list.
But what does that strange list mean? It doesn't actually contain any host
names or IP addresses. The presence of the colon puts an empty item in the
list; Exim matches this only if the incoming message didn't come from a remote
host. The colon is important. Without it, the list itself is empty, and can
never match anything.

What this statement is doing is to accept unconditionally all recipients in
messages that are submitted by SMTP from local processes using the standard
input and output (that is, not using TCP/IP). A number of MUAs operate in this
manner.                                                                        |
                                                                               |
  deny    domains       = +local_domains                                       |
          local_parts   = ^[.] : ^.*[@%!/|]                                    |
                                                                               |
  deny    domains       = !+local_domains                                      |
          local_parts   = ^[./|] : ^.*[@%!] : ^.*/\\.\\./                      |
                                                                               |
These statements are concerned with local parts that contain any of the        |
characters '@', '%', '!', '/', '|', or dots in unusual places. Although these  |
characters are entirely legal in local parts (in the case of '@' and leading   |
dots, only if correctly quoted), they do not commonly occur in Internet mail   |
addresses.                                                                     |
                                                                               |
The first three have in the past been associated with explicitly routed        |
addresses (percent is still sometimes used - see the "percent_hack_domains"    |
option). Addresses containing these characters are regularly tried by spammers |
in an attempt to bypass relaying restrictions, and also by open relay testing  |
programs. Unless you really need them it is safest to reject these characters  |
at this early stage. This configuration is heavy-handed in rejecting these     |
characters for all messages it accepts from remote hosts. This is a deliberate |
policy of being as safe as possible.                                           |
                                                                               |
The first rule above is stricter, and is applied to messages that are          |
addressed to one of the local domains handled by this host. This is            |
implemented by the first condition, which restricts it to domains that are     |
listed in the "local_domains" domain list. The '+' character is used to        |
indicate a reference to a named list. In this configuration, there is just one |
domain in "local_domains", but in general there may be many.                   |
                                                                               |
The second condition on the first statement uses two regular expressions to    |
block local parts that begin with a dot or contain '@', '%', '!', '/', or '|'. |
If you have local accounts that include these characters, you will have to     |
modify this rule.                                                              |
                                                                               |
Empty components (two dots in a row) are not valid in RFC 2822, but Exim       |
allows them because they have been encountered in practice. (Consider local    |
parts constructed as 'first-initial.second-initial.family-name' when applied   |
to someone like the author of Exim, who has no second initial.) However, a     |
local part starting with a dot or containing '/../' can cause trouble if it is |
used as part of a file name (for example, for a mailing list). This is also    |
true for local parts that contain slashes. A pipe symbol can also be           |
troublesome if the local part is incorporated unthinkingly into a shell        |
command line.                                                                  |
                                                                               |
The second rule above applies to all other domains, and is less strict. This   |
allows your own users to send outgoing messages to sites that use slashes and  |
vertical bars in their local parts. It blocks local parts that begin with a    |
dot, slash, or vertical bar, but allows these characters within the local      |
part. However, the sequence '/../' is barred. The use of '@', '%', and '!' is  |
blocked, as before. The motivation here is to prevent your users (or your      |
users' viruses) from mounting certain kinds of attack on remote sites.         |

  accept  local_parts   = postmaster
          domains       = +local_domains

This statement, which has two conditions, accepts an incoming address if the
local part is "postmaster" and the domain is one of those listed in the
"local_domains" domain list. The '+' character is used to indicate a reference
to a named list. In this configuration, there is just one domain in
"local_domains", but in general there may be many.

The presence of this statement means that mail to postmaster is never blocked
by any of the subsequent tests. This can be helpful while sorting out problems
in cases where the subsequent tests are incorrectly denying access.

  require verify        = sender

This statement requires the sender address to be verified before any subse-
quent ACL statement can be used. If verification fails, the incoming recipient
address is refused. Verification consists of trying to route the address, to
see if a bounce message could be delivered to it. In the case of remote
addresses, basic verification checks only the domain, but "callouts" can be
used for more verification if required. Section 38.20 discusses the details of
address verification.

  # deny    message       = rejected because $sender_host_address is \
  #                         in a black list at $dnslist_domain\n\
  #                         $dnslist_text
  #         dnslists      = black.list.example
  #
  # warn    message       = X-Warning: $sender_host_address is \
  #                         in a black list at $dnslist_domain
  #         log_message   = found in $dnslist_domain
  #         dnslists      = black.list.example

These commented-out lines are examples of how you could configure Exim to
check sending hosts against a DNS black list. The first statement rejects
messages from blacklisted hosts, whereas the second merely inserts a warning
header line.

  accept  domains       = +local_domains
          endpass
          message       = unknown user
          verify        = recipient

This statement accepts the incoming recipient address if its domain is one of
the local domains, but only if the address can be verified. Verification of
local addresses normally checks both the local part and the domain. The
"endpass" line needs some explanation: if the condition above "endpass" fails,
that is, if the address is not in a local domain, control is passed to the
next ACL statement. However, if the condition below "endpass" fails, that is,
if a recipient in a local domain cannot be verified, access is denied and the
recipient is rejected. The "message" modifier provides a customized error
message for the failure.

  accept  domains       = +relay_to_domains
          endpass
          message       = unrouteable address
          verify        = recipient

This statement accepts the incoming recipient address if its domain is one of
the domains for which this host is a relay, but again, only if the address can
be verified.

  accept  hosts         = +relay_from_hosts

Control reaches this statement only if the recipient's domain is neither a
local domain, nor a relay domain. The statement accepts the address if the
message is coming from one of the hosts that are defined as being allowed to
relay through this host. Recipient verification is omitted here, because in
many cases the clients are dumb MUAs that do not cope well with SMTP error
responses. If you are actually relaying out from MTAs, you should probably add
recipient verification here.

  accept  authenticated = *

Control reaches here for attempts to relay to arbitrary domains from arbitrary
hosts. The statement accepts the address only if the client host has
authenticated itself. The default configuration does not define any
authenticators, which means that no client can in fact authenticate. You will
need to add authenticator definitions if you want to make use of this ACL
statement.

  deny    message       = relay not permitted

The final statement denies access, giving a specific error message. Reaching
the end of the ACL also causes access to be denied, but with the generic
message 'administrative prohibition'.


7.3 Router configuration

The router configuration comes next in the default configuration, introduced
by the line

  begin routers

Routers are the modules in Exim that make decisions about where to send
messages. An address is passed to each router in turn, until it is either
accepted, or failed. This means that the order in which you define the routers
matters. Each router is fully described in its own chapter later in this
manual. Here we give only brief overviews.

  # domain_literal:
  #   driver = ipliteral
  #   domains = !+local_domains
  #   transport = remote_smtp

This router is commented out because the vast majority of sites do not want to
support domain literal addresses (those of the form "user@[10.9.8.7]"). If you
uncomment this router, you will also need to uncomment the setting of
"allow_domain_literals" in the main part of the configuration.

  dnslookup:
    driver = dnslookup
    domains = ! +local_domains
    transport = remote_smtp
    ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
    no_more

The first uncommented router handles addresses that do not involve any local
domains. This is specified by the line

  domains = ! +local_domains

The "domains" option lists the domains to which this router applies, but the
exclamation mark is a negation sign, so the router is used only for domains
that are not in the domain list called "local_domains" (which was defined at
the start of the configuration). The plus sign before "local_domains" indi-
cates that it is referring to a named list. Addresses in other domains are
passed on to the following routers.

The name of the router driver is dnslookup, and is specified by the "driver"
option. Do not be confused by the fact that the name of this router instance
is the same as the name of the driver. The instance name is arbitrary, but the
name set in the "driver" option must be one of the driver modules that is in
the Exim binary.

The dnslookup router routes addresses by looking up their domains in the DNS
in order to obtain a list of hosts to which the address is routed. If the
router succeeds, the address is queued for the remote_smtp transport, as
specified by the "transport" option. If the router does not find the domain in
the DNS, no further routers are tried because of the "no_more" setting, so the
address fails and is bounced.

The "ignore_target_hosts" option specifies a list of IP addresses that are to
be entirely ignored. This option is present because a number of cases have
been encountered where MX records in the DNS point to host names whose IP
addresses are 0.0.0.0 or are in the 127 subnet (typically 127.0.0.1).
Completely ignoring these IP addresses causes Exim to fail to route the email
address, so it bounces. Otherwise, Exim would log a routing problem, and
continue to try to deliver the message periodically until the address timed
out.

  system_aliases:
    driver = redirect
    allow_fail
    allow_defer
    data = ${lookup{$local_part}lsearch{/etc/aliases}}
  # user = exim
    file_transport = address_file
    pipe_transport = address_pipe

Control reaches this and subsequent routers only for addresses in the local
domains. This router checks to see whether the local part is defined as an
alias in the /etc/aliases file, and if so, redirects it according to the data
that it looks up from that file. If no data is found for the local part, the
value of the "data" option is empty, causing the address to be passed to the
next router.

/etc/aliases is a conventional name for the system aliases file that is often
used. That is why it is referenced by from the default configuration file.
However, you can change this by setting SYSTEM_ALIASES_FILE in Local/Makefile
before building Exim.

  userforward:
    driver = redirect
    check_local_user
    file = $home/.forward
    no_verify
    no_expn
    check_ancestor
  # allow_filter
    file_transport = address_file
    pipe_transport = address_pipe
    reply_transport = address_reply

This is the most complicated router in the default configuration. It is
another redirection router, but this time it is looking for forwarding data
set up by individual users. The "check_local_user" setting means that the
first thing it does is to check that the local part of the address is the
login name of a local user. If it is not, the router is skipped. When a local
user is found, the file called .forward in the user's home directory is
consulted. If it does not exist, or is empty, the router declines. Otherwise,
the contents of .forward are interpreted as redirection data (see chapter 22
for more details).

Traditional .forward files contain just a list of addresses, pipes, or files.
Exim supports this by default. However, if "allow_filter" is set (it is
commented out by default), the contents of the file are interpreted as a set
of Exim or Sieve filtering instructions, provided the file begins with '#Exim  |
filter' or '#Sieve filter', respectively. User filtering is discussed in the   |
separate document entitled "Exim's interfaces to mail filtering".

The "no_verify" and "no_expn" options mean that this router is skipped when
verifying addresses, or when running as a consequence of an SMTP EXPN command.
There are two reasons for doing this:

(1)  Whether or not a local user has a .forward file is not really relevant
     when checking an address for validity; it makes sense not to waste
     resources doing unnecessary work.

(2)  More importantly, when Exim is verifying addresses or handling an EXPN
     command during an SMTP session, it is running as the Exim user, not as
     root. The group is the Exim group, and no additional groups are set up.   |
     It may therefore not be possible for Exim to read users' .forward files
     at this time.

The setting of "check_ancestor" prevents the router from generating a new
address that is the same as any previous address that was redirected. (This
works round a problem concerning a bad interaction between aliasing and
forwarding - see section 22.5).

The final three option settings specify the transports that are to be used
when forwarding generates a direct delivery to a file, or to a pipe, or sets
up an auto-reply, respectively. For example, if a .forward file contains

  a.nother@elsewhere.example, /home/spqr/archive

the delivery to /home/spqr/archive is done by running the "address_file"
transport.

  localuser:
    driver = accept
    check_local_user
    transport = local_delivery

The final router sets up delivery into local mailboxes, provided that the
local part is the name of a local login, by accepting the address and queuing
it for the local_delivery transport. Otherwise, we have reached the end of the
routers, so the address is bounced.


7.4 Transport configuration

Transports define mechanisms for actually delivering messages. They operate
only when referenced from routers, so the order in which they are defined does
not matter. The transports section of the configuration starts with

  begin transports

One remote transport and four local transports are defined.

  remote_smtp:
    driver = smtp

This transport is used for delivering messages over SMTP connections. All its
options are defaulted. The list of remote hosts comes from the router.

  local_delivery:
    driver = appendfile
    file = /var/mail/$local_part
    delivery_date_add
    envelope_to_add
    return_path_add
  # group = mail
  # mode = 0660

This appendfile transport is used for local delivery to user mailboxes in
traditional BSD mailbox format. By default it runs under the uid and gid of
the local user, which requires the sticky bit to be set on the /var/mail
directory. Some systems use the alternative approach of running mail deliver-
ies under a particular group instead of using the sticky bit. The commented
options show how this can be done.

Exim adds three headers to the message as it delivers it: Delivery-date:,
Envelope-to: and Return-path:. This action is requested by the three simi-
larly-named options above.

  address_pipe:
    driver = pipe
    return_output

This transport is used for handling deliveries to pipes that are generated by
redirection (aliasing or users' .forward files). The "return_output" option
specifies that any output generated by the pipe is to be returned to the
sender.

  address_file:
    driver = appendfile
    delivery_date_add
    envelope_to_add
    return_path_add

This transport is used for handling deliveries to files that are generated by
redirection. The name of the file is not specified in this instance of
appendfile, because it comes from the redirect router.

  address_reply:
    driver = autoreply

This transport is used for handling automatic replies generated by users'
filter files.


7.5 Default retry rule

The retry section of the configuration file contains rules which affect the
way Exim retries deliveries that cannot be completed at the first attempt. It
is introduced by the line

  begin retry

In the default configuration, there is just one rule, which applies to all
errors:

  *   *   F,2h,15m; G,16h,1h,1.5; F,4d,6h

This causes any temporarily failing address to be retried every 15 minutes for
2 hours, then at intervals starting at one hour and increasing by a factor of
1.5 until 16 hours have passed, then every 6 hours up to 4 days. If an address
is not delivered after 4 days of failure, it is bounced.


7.6 Rewriting configuration

The rewriting section of the configuration, introduced by

  begin rewrite

contains rules for rewriting addresses in messages as they arrive. There are
no rewriting rules in the default configuration file.


7.7 Authenticators configuration

The authenticators section of the configuration, introduced by

  begin authenticators

defines mechanisms for the use of the SMTP AUTH command. No authenticators are
specified in the default configuration file.



                            8. REGULAR EXPRESSIONS


Exim supports the use of regular expressions in many of its options. It uses
the PCRE regular expression library; this provides regular expression matching
that is compatible with Perl 5. The syntax and semantics of regular
expressions is discussed in many Perl reference books, and also in Jeffrey
Friedl's Mastering Regular Expressions (O'Reilly, ISBN 0-596-00289-0).

The documentation for the syntax and semantics of the regular expressions
that are supported by PCRE is included in plain text in the file
doc/pcrepattern.txt in the Exim distribution, and also in the HTML tarbundle
of Exim documentation, and as an appendix to the Exim book. It describes in
detail the features of the regular expressions that PCRE supports, so no
further description is included here. The PCRE functions are called from Exim
using the default option settings (that is, with no PCRE options set), except
that the PCRE_CASELESS option is set when the matching is required to be case-
insensitive.


8.1 Testing regular expressions

A program called "pcretest" forms part of the PCRE distribution and is built
with PCRE during the process of building Exim. It is primarily intended for
testing PCRE itself, but it can also be used for experimenting with regular
expressions. After building Exim, the binary can be found in the build
directory (it is not installed anywhere automatically). There is documentation
of various options in doc/pcretest.txt, but for simple testing, none are
needed. This is the output of a sample run of "pcretest":

    re> /^([^@]+)@.+\.(ac|edu)\.(?!kr)[a-z]{2}$/
  data> x@y.ac.uk
   0: x@y.ac.uk
   1: x
   2: ac
  data> x@y.ac.kr
  No match
  data> x@y.edu.com
  No match
  data> x@y.edu.co
   0: x@y.edu.co
   1: x
   2: edu

After the 're>' prompt, a regular expression enclosed in delimiters is
expected. If this compiles without error, 'data>' prompts are given for
strings against which the expression is matched. An empty data line causes a
new regular expression to be read. If the match is successful, the captured
substring values (that is, what would be in the variables $0, $1, $2, etc.)
are shown. The above example tests for an email address whose domain ends with
either 'ac' or 'edu' followed by a two-character top-level domain that is not
'kr'. The local part is captured in $1 and the 'ac' or 'edu' in $2.



                         9. FILE AND DATABASE LOOKUPS


Exim can be configured to look up data in files or databases as it processes
messages. Two different kinds of syntax are used:

(1)  A string that is to be expanded may contain explicit lookup requests.
     These cause parts of the string to be replaced by data that is obtained
     from the lookup.

(2)  Lists of domains, hosts, and email addresses can contain lookup requests
     as a way of avoiding excessively long linear lists. In this case, the
     data that is returned by the lookup is often (but not always) discarded;
     whether the lookup succeeds or fails is what really counts. These kinds
     of list are described in chapter 10.

It is easy to confuse the two different kinds of lookup, especially as the
lists that may contain the second kind are always expanded before being
processed as lists. Therefore, they may also contain lookups of the first
kind. Be careful to distinguish between the following two examples:

  domains = ${lookup{$sender_host_address}lsearch{/some/file}}
  domains = lsearch;/some/file

The first uses a string expansion, the result of which must be a domain list.
String expansions are described in detail in chapter 11. The expansion takes
place first, and the file that is searched could contain lines like this:

  192.168.3.4: domain1 : domain2 : ...
  192.168.1.9: domain3 : domain4 : ...

Thus, the result of the expansion is a list of domains (and possibly other
types of item that are allowed in domain lists).

In the second case, the lookup is a single item in a domain list. It causes
Exim to use a lookup to see if the domain that is being processed can be found
in the file. The file could contains lines like this:

  domain1:
  domain2:

Any data that follows the keys is not relevant when checking that the domain
matches the list item.

It is possible to use both kinds of lookup at once. Consider a file containing
lines like this:

  192.168.5.6: lsearch;/another/file

If the value of $sender_host_address is 192.168.5.6, expansion of the first
"domains" setting above generates the second setting, which therefore causes a
second lookup to occur.

The rest of this chapter describes the different lookup types that are
available. Any of them can be used in either of the circumstances described
above. The syntax requirements for the two cases are described in chapters 11
and 10, respectively.


9.1 Lookup types

Two different styles of data lookup are implemented:

 .   The "single-key" style requires the specification of a file in which to
     look, and a single key to search for. The lookup type determines how the
     file is searched.

 .   The "query" style accepts a generalized database query. No particular key
     value is assumed by Exim for query-style lookups. You can use whichever
     Exim variable(s) you need to construct the database query.

The code for each lookup type is in a separate source file that is included in
the binary of Exim only if the corresponding compile-time option is set. The
default settings in src/EDITME are:

  LOOKUP_DBM=yes
  LOOKUP_LSEARCH=yes

which means that only linear searching and DBM lookups are included by
default. For some types of lookup (e.g. SQL databases), you need to install
appropriate libraries and header files before building Exim.


9.2 Single-key lookup types

The following single-key lookup types are implemented:

 .   cdb: The given file is searched as a Constant DataBase file, using the
     key string without a terminating binary zero. The cdb format is designed
     for indexed files that are read frequently and never updated, except by
     total re-creation. As such, it is particulary suitable for large files
     containing aliases or other indexed data referenced by an MTA.
     Information about cdb can be found in several places:

       http://www.pobox.com/~djb/cdb.html
       ftp://ftp.corpit.ru/pub/tinycdb/
       http://packages.debian.org/stable/utils/freecdb.html

     A cdb distribution is not needed in order to build Exim with cdb support,
     because the code for reading cdb files is included directly in Exim
     itself. However, no means of building or testing cdb files is provided
     with Exim, so you need to obtain a cdb distribution in order to do this.

 .   dbm: Calls to DBM library functions are used to extract data from the
     given DBM file by looking up the record with the given key. A terminating
     binary zero is included in the key that is passed to the DBM library. See
     section 4.3 for a discussion of DBM libraries. For all versions of
     Berkeley DB, Exim uses the DB_HASH style of database when building DBM
     files using the "exim_dbmbuild" utility. However, when using Berkeley DB
     versions 3 or 4, it opens existing databases for reading with the
     DB_UNKNOWN option. This enables it to handle any of the types of database
     that the library supports, and can be useful for accessing DBM files
     created by other applications. (For earlier DB versions, DB_HASH is
     always used.)

 .   dbmnz: This is the same as dbm, except that a terminating binary zero is
     not included in the key that is passed to the DBM library. You may need
     this if you want to look up data in files that are created by or shared
     with some other application that does not use terminating zeros. For
     example, you need to use dbmnz rather than dbm if you want to
     authenticate incoming SMTP calls using the passwords from Courier's
     /etc/userdbshadow.dat file. Exim's utility program for creating DBM files
     ("exim_dbmbuild") includes the zeros by default, but has an option to
     omit them (see section 46.8).

 .   dsearch: The given file must be a directory; this is searched for a file
     whose name is the key. The key may not contain any forward slash
     characters. The result of a successful lookup is the name of the file. An
     example of how this lookup can be used to support virtual domains is
     given in section 42.6.

 .   lsearch: The given file is a text file that is searched linearly for a
     line beginning with the key, terminated by a colon or white space or the
     end of the line. The first occurrence that is found in the file is used.
     White space between the key and the colon is permitted. The remainder of
     the line, with leading and trailing white space removed, is the data.
     This can be continued onto subsequent lines by starting them with any
     amount of white space, but only a single space character is included in
     the data at such a junction. If the data begins with a colon, the key
     must be terminated by a colon, for example:

       baduser:  :fail:

     Empty lines and lines beginning with # are ignored, even if they occur in
     the middle of an item. This is the traditional textual format of alias
     files. Note that the keys in an lsearch file are literal strings. There
     is no wildcarding of any kind.

     In most lsearch files, keys are not required to contain colons and
     whitespace. However, if you need this feature, it is available. If a key
     begins with a doublequote character, it is terminated only by a matching
     quote (or end of line), and the normal escaping rules apply to its
     contents (see section 6.12). An optional colon is permitted after quoted
     keys (exactly as for unquoted keys). There is no special handling of
     quotes for the data part of an lsearch line.

 .   nis: The given file is the name of a NIS map, and a NIS lookup is done
     with the given key, without a terminating binary zero. There is a variant
     called nis0 which does include the terminating binary zero in the key.
     This is reportedly needed for Sun-style alias files. Exim does not
     recognize NIS aliases; the full map names must be used.

 .   wildlsearch or nwildlsearch: These search a file linearly, like lsearch,  |
     but instead of being interpreted as a literal string, each key may be     |
     wildcarded. The difference between these two lookup types is that for     |
     wildlsearch, each key in the file is string-expanded before being used,   |
     whereas for nwildlsearch, no expansion takes place.                       |
                                                                               |
     Like lsearch, the testing is done case-insensitively. The following forms |
     of wildcard are recognized:                                               |
                                                                               |
      *   The string may begin with an asterisk to mean 'begins with'. For     |
          example:                                                             |
                                                                               |
            *.a.b.c       data for anything.a.b.c                              |
            *fish         data for anythingfish                                |
                                                                               |
      *   The string may begin with a circumflex to indicate a regular         |
          expression. For example, for wildlsearch:                            |
                                                                               |
            ^\N\d+\.a\.b\N    data for <digits>.a.b                            |
                                                                               |
          Note the use of "\N" to disable expansion of the contents of the     |
          regular expression. If you are using nwildlsearch, where the keys    |
          are not string-expanded, the equivalent entry is:                    |
                                                                               |
            ^\d+\.a\.b        data for <digits>.a.b                            |
                                                                               |
          If the regular expression contains white space or colon characters,
          you must either quote it (see lsearch above), or represent these
          characters in other ways. For example, "\s" can be used for white
          space and "\x3A" for a colon. This may be easier than quoting,
          because if you quote, you have to escape all the backslashes inside
          the quotes.

      *   Although I cannot see it being of much use, the general matching
          function that is used to implement (n)wildlsearch means that the     |
          string may begin with a lookup name terminated by a semicolon, and
          followed by lookup data. For example:

            cdb;/some/file  data for keys that match the file

          The data that is obtained from the nested lookup is discarded.       |

     Keys that do not match any of these patterns are interpreted literally.   |
     The continuation rules for the data are the same as for lsearch, and keys |
     may be followed by optional colons.                                       |
                                                                               |
     Warning: Unlike other single-key lookup types, a file of data for         |
     (n)wildlsearch can "not" be turned into a DBM or cdb file, because those  |
     lookup types support only literal keys.


9.3 Query-style lookup types

The supported query-style lookup types are listed below. Further details about
many of them are given in later sections.

 .   dnsdb: This does a DNS search for a record whose domain name is the
     supplied query. The resulting data is the contents of the record. See
     section 9.9.

 .   ibase: This does a lookup in an Interbase database.                       |

 .   ldap: This does an LDAP lookup using a query in the form of a URL, and
     returns attributes from a single entry. There is a variant called ldapm
     that permits values from multiple entries to be returned. A third variant
     called ldapdn returns the Distinguished Name of a single entry instead of
     any attribute values. See section 9.10.

 .   mysql: The format of the query is an SQL statement that is passed to a
     MySQL database. See section 9.17.

 .   nisplus: This does a NIS+ lookup using a query that can specify the name
     of the field to be returned. See section 9.16.

 .   oracle: The format of the query is an SQL statement that is passed to an
     Oracle database. See section 9.17.

 .   passwd is a query-style lookup with queries that are just user names. The
     lookup calls "getpwnam()" to interrogate the system password data, and on
     success, the result string is the same as you would get from an lsearch
     lookup on a traditional /etc/passwd file, though with "*" for the
     password value. For example:

       *:42:42:King Rat:/home/kr:/bin/bash

 .   pgsql: The format of the query is an SQL statement that is passed to a
     PostgreSQL database. See section 9.17.

 .   testdb: This is a lookup type that is used for testing Exim. It is not
     likely to be useful in normal operation.

 .   whoson: "Whoson" (http://whoson.sourceforge.net) is a proposed Internet
     protocol that allows Internet server programs to check whether a particu-
     lar (dynamically allocated) IP address is currently allocated to a known
     (trusted) user and, optionally, to obtain the identity of the said user.
     In Exim, this can be used to implement 'POP before SMTP' checking using
     ACL statements such as

       require condition = \
         ${lookup whoson {$sender_host_address}{yes}{no}}

     The query consists of a single IP address. The value returned is the name
     of the authenticated user.


9.4 Temporary errors in lookups

Lookup functions can return temporary error codes if the lookup cannot be
completed. For example, a NIS or LDAP database might be unavailable. For this
reason, it is not advisable to use a lookup that might do this for critical
options such as a list of local domains.

When a lookup cannot be completed in a router or transport, delivery of the
message (to the relevant address) is deferred, as for any other temporary
error. In other circumstances Exim may assume the lookup has failed, or may
give up altogether.


9.5 Default values in single-key lookups

In this context, a 'default value' is a value specified by the administrator
that is to be used if a lookup fails.

If '*' is added to a single-key lookup type (for example, "lsearch*") and the
initial lookup fails, the key '*' is looked up in the file to provide a
default value. See also the section on partial matching below.

Alternatively, if '*@' is added to a single-key lookup type (for example
"dbm*@") then, if the initial lookup fails and the key contains an @
character, a second lookup is done with everything before the last @ replaced
by *. This makes it possible to provide per-domain defaults in alias files
that include the domains in the keys. If the second lookup fails (or doesn't
take place because there is no @ in the key), '*' is looked up. For example, a
redirect router might contain:

  data = ${lookup{$local_part@$domain}lsearch*@{/etc/mixed-aliases}}

Suppose the address that is being processed is "jane@eyre.example". Exim looks
up these keys, in this order:

  jane@eyre.example
  *@eyre.example
  *

The data is taken from whichever key it finds first. Note: in an lsearch file,
this does not mean the first of these keys in the file. A complete scan is
done for each key, and only if it is not found at all does Exim move on to try
the next key.


9.6 Partial matching in single-key lookups

The normal operation of a single-key lookup is to search the file for an exact
match with the given key. However, in a number of situations where domains are
being looked up, it is useful to be able to do partial matching. In this case,
information in the file that has a key starting with '*.' is matched by any
domain that ends with the components that follow the full stop. For example,
if a key in a DBM file is

  *.dates.fict.example

then when partial matching is enabled this is matched by (amongst others)
"2001.dates.fict.example" and "1984.dates.fict.example". It is also matched by
"dates.fict.example", if that does not appear as a separate key in the file.

Note: Partial matching is not available for query-style lookups. It is also
not available for any lookup items in address lists (see section 10.16).

Partial matching is implemented by doing a series of separate lookups using
keys constructed by modifying the original subject key. This means that it can
be used with any of the single-key lookup types, provided that partial
matching keys beginning with a special prefix (default '*.') are included in
the data file. Keys in the file that do not begin with the prefix are matched
only by unmodified subject keys when partial matching is in use.

Partial matching is requested by adding the string 'partial-' to the front of
the name of a single-key lookup type, for example, "partial-dbm". When this is
done, the subject key is first looked up unmodified; if that fails, '*.' is
added at the start of the subject key, and it is looked up again. If that
fails, further lookups are tried with dot-separated components removed from
the start of the subject key, one-by-one, and '*.' added on the front of what
remains.

A minimum number of two non-* components are required. This can be adjusted by
including a number before the hyphen in the search type. For example,
"partial3-lsearch" specifies a minimum of three non-* components in the
modified keys. Omitting the number is equivalent to 'partial2-'. If the
subject key is "2250.dates.fict.example" then the following keys are looked up
when the minimum number of non-* components is two:

  2250.dates.fict.example
  *.2250.dates.fict.example
  *.dates.fict.example
  *.fict.example

As soon as one key in the sequence is successfully looked up, the lookup
finishes.

The use of '*.' as the partial matching prefix is a default that can be
changed. The motivation for this feature is to allow Exim to operate with file
formats that are used by other MTAs. A different prefix can be supplied in
parentheses instead of the hyphen after 'partial'. For example:

  domains = partial(.)lsearch;/some/file

In this example, if the domain is "a.b.c", the sequence of lookups is "a.b.c",
".a.b.c", and ".b.c" (the default minimum of 2 non-wild components is
unchanged). The prefix may consist of any punctuation characters other than a
closing parenthesis. It may be empty, for example:

  domains = partial1()cdb;/some/file

For this example, if the domain is "a.b.c", the sequence of lookups is
"a.b.c", "b.c", and "c".

If 'partial0' is specified, what happens at the end (when the lookup with just
one non-wild component has failed, and the original key is shortened right
down to the null string) depends on the prefix:

 .   If the prefix has zero length, the whole lookup fails.

 .   If the prefix has length 1, a lookup for just the prefix is done. For
     example, the final lookup for 'partial0(.)' is for "." alone.

 .   Otherwise, if the prefix ends in a dot, the dot is removed, and the
     remainder is looked up. With the default prefix, therefore, the final
     lookup is for '*' on its own.

 .   Otherwise, the whole prefix is looked up.

If the search type ends in '*' or '*@' (see section 9.5 above), the search for
an ultimate default that this implies happens after all partial lookups have
failed. If 'partial0' is specified, adding '*' to the search type has no
effect with the default prefix, because the '*' key is already included in the
sequence of partial lookups. However, there might be a use for lookup types
such as 'partial0(.)lsearch*'.

The use of '*' in lookup partial matching differs from its use as a wildcard
in domain lists and the like. Partial matching works only in terms of dot-
separated components; a key such as "*fict.example" in a database file is
useless, because the asterisk in a partial matching subject key is always
followed by a dot.


9.7 Lookup caching

An Exim process caches the most recent lookup result on a per-file basis for
single-key lookup types, and keeps the relevant files open. In some types of
configuration this can lead to many files being kept open for messages with
many recipients. To avoid hitting the operating system limit on the number of
simultaneously open files, Exim closes the least recently used file when it
needs to open more files than its own internal limit, which can be changed via
the "lookup_open_max" option.

For query-style lookups, a single data cache per lookup type is kept. The
files are closed and the caches flushed at strategic points during delivery -
for example, after all routing is complete.


9.8 Quoting lookup data

When data from an incoming message is included in a query-style lookup, there
is the possibility of special characters in the data messing up the syntax of
the query. For example, a NIS+ query that contains

  [name=$local_part]

will be broken if the local part happens to contain a closing square bracket.
For NIS+, data can be enclosed in double quotes like this:

  [name="$local_part"]

but this still leaves the problem of a double quote in the data. The rule for
NIS+ is that double quotes must be doubled. Other lookup types have different
rules, and to cope with the differing requirements, an expansion operator of
the following form is provided:

  ${quote_<lookup-type>:<string>}

For example, the safest way to write the NIS+ query is

  [name="${quote_nisplus:$local_part}"]

See chapter 11 for full coverage of string expansions. The quote operator can
be used for all lookup types, but has no effect for single-key lookups, since
no quoting is ever needed in their key strings.


9.9 More about dnsdb

The dnsdb lookup type uses the DNS as its database. A query consists of a
record type and a domain name, separated by an equals sign. For example, an
expansion string could contain:

  ${lookup dnsdb{mx=a.b.example}{$value}fail}

The supported record types are A, CNAME, MX, NS, PTR, and TXT, and, when Exim
is compiled with IPv6 support, AAAA (and A6 if that is also configured). If no
type is given, TXT is assumed. When the type is PTR, the address should be
given as normal; it is converted to the necessary inverted format internally.
For example:

  ${lookup dnsdb{ptr=192.168.4.5}{$value}fail}

For MX records, both the preference value and the host name are returned,
separated by a space. If multiple records are found (or, for A6 lookups, if a
single record leads to multiple addresses), the data is returned as a
concatenation, separated by newlines. The order, of course, depends on the DNS
resolver.


9.10 More about LDAP

The original LDAP implementation came from the University of Michigan; this
has become 'Open LDAP', and there are now two different releases. Another
implementation comes from Netscape, and Solaris 7 and subsequent releases
contain inbuilt LDAP support. Unfortunately, though these are all compatible
at the lookup function level, their error handling is different. For this
reason it is necessary to set a compile-time variable when building Exim with
LDAP, to indicate which LDAP library is in use. One of the following should
appear in your Local/Makefile:

  LDAP_LIB_TYPE=UMICHIGAN
  LDAP_LIB_TYPE=OPENLDAP1
  LDAP_LIB_TYPE=OPENLDAP2
  LDAP_LIB_TYPE=NETSCAPE
  LDAP_LIB_TYPE=SOLARIS

If LDAP_LIB_TYPE is not set, Exim assumes "OPENLDAP1", which has the same
interface as the University of Michigan version.

There are three LDAP lookup types in Exim. These behave slightly differently
in the way they handle the results of a query:

 .   ldap requires the result to contain just one entry; if there are more, it
     gives an error.

 .   ldapdn also requires the result to contain just one entry, but it is the
     Distinguished Name that is returned rather than any attribute values.

 .   ldapm permits the result to contain more than one entry; the attributes
     from all of them are returned.

For ldap and ldapm, if a query finds only entries with no attributes, Exim
behaves as if the entry did not exist, and the lookup fails. The format of the
data returned by a successful lookup is described in the next section. First
we explain how LDAP queries are coded.


9.11 Format of LDAP queries

An LDAP query takes the form of a URL as defined in RFC 2255. For example, in
the configuration of a redirect router one might have this setting:

  data = ${lookup ldap \
    {ldap:///cn=$local_part,o=University%20of%20Cambridge,\
    c=UK?mailbox?base?}}

The URL may begin with "ldap" or "ldaps" if your LDAP library supports secure
(encrypted) LDAP connections. The second of these ensures that an encrypted
TLS connection is used.


9.12 LDAP quoting

Two levels of quoting are required in LDAP queries, the first for LDAP itself
and the second because the LDAP query is represented as a URL. Furthermore,
within an LDAP query, two different kinds of quoting are required. For this
reason, there are two different LDAP-specific quoting operators.

The "quote_ldap" operator is designed for use on strings that are part of
filter specifications. Conceptually, it first does the following conversions
on the string:

  *   =>   \2A
  (   =>   \28
  )   =>   \29
  \   =>   \5C

in accordance with RFC 2254. The resulting string is then quoted according to
the rules for URLs, that is, all characters except

  ! $ ' - . _ ( ) * +

are converted to their hex values, preceded by a percent sign. For example:

  ${quote_ldap: a(bc)*, a<yz>; }

yields

  %20a%5C28bc%5C29%5C2A%2C%20a%3Cyz%3E%3B%20

Removing the URL quoting, this is (with a leading and a trailing space):

  a\28bc\29\2A, a<yz>;

The "quote_ldap_dn" operator is designed for use on strings that are part of
base DN specifications in queries. Conceptually, it first converts the string
by inserting a backslash in front of any of the following characters:

  , + " \ < > ;

It also inserts a backslash before any leading spaces or # characters, and
before any trailing spaces. (These rules are in RFC 2253.) The resulting
string is then quoted according to the rules for URLs. For example:

  ${quote_ldap_dn: a(bc)*, a<yz>; }

yields

  %5C%20a(bc)*%5C%2C%20a%5C%3Cyz%5C%3E%5C%3B%5C%20

Removing the URL quoting, this is (with a trailing space):

  \ a(bc)*\, a\<yz\>\;\

There are some further comments about quoting in the section on LDAP
authentication below.


9.13 LDAP connections

The connection to an LDAP server may either be over TCP/IP, or, when OpenLDAP
is in use, via a Unix domain socket. The example given above does not specify
an LDAP server. A server that is reached by TCP/IP can be specified in a query
by starting it with

  ldap://<hostname>:<port>/...

If the port (and preceding colon) are omitted, the standard LDAP port (389) is
used. When no server is specified in a query, a list of default servers is
taken from the "ldap_default_servers" configuration option. This supplies a
colon-separated list of servers which are tried in turn until one successfully
handles a query, or there is a serious error. Successful handling either
returns the requested data, or indicates that it does not exist. Serious
errors are syntactical, or multiple values when only a single value is
expected. Errors which cause the next server to be tried are connection
failures, bind failures, and timeouts.

For each server name in the list, a port number can be given. The standard way
of specifing a host and port is to use a colon separator (RFC 1738). Because
"ldap_default_servers" is a colon-separated list, such colons have to be
doubled. For example

  ldap_default_servers = ldap1.example.com::145:ldap2.example.com

If "ldap_default_servers" is unset, a URL with no server name is passed to the
LDAP library with no server name, and the library's default (normally the
local host) is used.

If you are using the OpenLDAP library, you can connect to an LDAP server using
a Unix domain socket instead of a TCP/IP connection. This is specified by
using "ldapi" instead of "ldap" in LDAP queries. What follows here applies
only to OpenLDAP. If Exim is compiled with a different LDAP library, this
feature is not available.

For this type of connection, instead of a host name for the server, a pathname
for the socket is required, and the port number is not relevant. The pathname
can be specified either as an item in "ldap_default_servers", or inline in the
query. In the former case, you can have settings such as

  ldap_default_servers = /tmp/ldap.sock : backup.ldap.your.domain

When the pathname is given in the query, you have to escape the slashes as
"%2F" to fit in with the LDAP URL syntax. For example:

  ${lookup ldap {ldapi://%2Ftmp%2Fldap.sock/o=...

When Exim processes an LDAP lookup and finds that the 'hostname' is really a
pathname, it uses the Unix domain socket code, even if the query actually
specifies "ldap" or "ldaps". In particular, no encryption is used for a socket
connection. This behaviour means that you can use a setting of
"ldap_default_servers" such as in the example above with traditional "ldap" or
"ldaps" queries, and it will work. First, Exim tries a connection via the Unix
domain socket; if that fails, it tries a TCP/IP connection to the backup host.

If an explicit "ldapi" type is given in a query when a host name is specified,
an error is diagnosed. However, if there are more items in
"ldap_default_servers", they are tried. In other words:

 .   Using a pathname with "ldap" or "ldaps" forces the use of the Unix domain
     interface.

 .   Using "ldapi" with a host name causes an error.

Using "ldapi" with no host or path in the query, and no setting of
"ldap_default_servers", does whatever the library does by default.


9.14 LDAP authentication and control information

The LDAP URL syntax provides no way of passing authentication and other
control information to the server. To make this possible, the URL in an LDAP
query may be preceded by any number of '<name>=<value>' settings, separated by
spaces. If a value contains spaces it must be enclosed in double quotes, and
when double quotes are used, backslash is interpreted in the usual way inside
them.

The following names are recognized:

  CONNECT     set a connection timeout
  DEREFERENCE set the dereferencing parameter                                  |
  USER        set the DN, for authenticating the LDAP bind
  PASS        set the password, likewise
  SIZE        set the limit for the number of entries returned
  TIME        set the maximum waiting time for a query

The value of the DEREFERENCE parameter must be one of the words 'never',       |
'searching', 'finding', or 'always'.                                           |

Here is an example of an LDAP query in an Exim lookup that uses some of these
values. This is a single line, folded for ease of reading:

${lookup ldap
  {user="cn=manager,o=University of Cambridge,c=UK" pass=secret
  ldap:///o=University%20of%20Cambridge,c=UK?sn?sub?(cn=foo)}
  {$value}fail}

The encoding of spaces as %20 is a URL thing which should not be done for any
of the auxiliary data. Exim configuration settings that include lookups which
contain password information should be preceded by 'hide' to prevent non-admin
users from using the -bP option to see their values.

The auxiliary data items may be given in any order. The default is no
connection timeout (the system timeout is used), no user or password, no limit
on the number of entries returned, and no time limit on queries.

The time limit for connection is given in seconds; zero means use the default.
This facility is available in Netscape SDK 4.1; it may not be available in
other LDAP implementations. Exim uses the given value if
LDAP_X_OPT_CONNECT_TIMEOUT is defined in the LDAP headers.

When a DN is quoted in the USER= setting for LDAP authentication, Exim removes
any URL quoting that it may contain before passing it LDAP. Apparently some
libraries do this for themselves, but some do not. Removing the URL quoting
has two advantages:

 .   It makes it possible to use the same "quote_ldap_dn" expansion for USER=
     DNs as with DNs inside actual queries.

 .   It permits spaces inside USER= DNs.

For example, a setting such as

  USER=cn=${quote_ldap_dn:$1}

should work even if $1 contains spaces.

Expanded data for the PASS= value should be quoted using the "quote" expansion
operator, rather than the LDAP quote operators. The only reason this field
needs quoting is to ensure that it conforms to the Exim syntax, which does not
allow unquoted spaces. For example:

  PASS=${quote:$3}

The LDAP authentication mechanism can be used to check passwords as part of
SMTP authentication. See the "ldapauth" expansion string condition in chapter
11.


9.15 Format of data returned by LDAP

The ldapdn lookup type returns the Distinguished Name from a single entry as a
sequence of values, for example

  cn=manager, o=University of Cambridge, c=UK

The ldap lookup type generates an error if more than one entry matches the
search filter, whereas ldapm permits this case, and inserts a newline in the
result between the data from different entries. It is possible for multiple
values to be returned for both ldap and ldapm, but in the former case you know
that whatever values are returned all came from a single entry in the
directory.

In the common case where you specify a single attribute in your LDAP query,
the result is not quoted, and does not contain the attribute name. If the
attribute has multiple values, they are separated by commas.

If you specify multiple attributes, the result contains space-separated,
quoted strings, each preceded by the attribute name and an equals sign. Within
the quotes, the quote character, backslash, and newline are escaped with
backslashes, and commas are used to separate multiple values for the
attribute. Apart from the escaping, the string within quotes takes the same
form as the output when a single attribute is requested. Specifying no
attributes is the same as specifying all of an entry's attributes.

Here are some examples of the output format. The first line of each pair is an
LDAP query, and the second is the data that is returned. The attribute called
"attr1" has two values, whereas "attr2" has only one value:

  ldap:///o=base?attr1?sub?(uid=fred)
  value1.1, value1.2

  ldap:///o=base?attr2?sub?(uid=fred)
  value two

  ldap:///o=base?attr1,attr2?sub?(uid=fred)
  attr1="value1.1, value1.2" attr2="value two"

  ldap:///o=base??sub?(uid=fred)
  objectClass="top" attr1="value1.1, value1.2" attr2="value two"

The "extract" operator in string expansions can be used to pick out individual
fields from data that consists of key=value pairs. You can make use of Exim's
-be option to run expansion tests and thereby check the results of LDAP
lookups.


9.16 More about NIS+

NIS+ queries consist of a NIS+ "indexed name" followed by an optional colon
and field name. If this is given, the result of a successful query is the
contents of the named field; otherwise the result consists of a concatenation
of "field-name=field-value" pairs, separated by spaces. Empty values and
values containing spaces are quoted. For example, the query

  [name=mg1456],passwd.org_dir

might return the string

  name=mg1456 passwd="" uid=999 gid=999 gcos="Martin Guerre"
  home=/home/mg1456 shell=/bin/bash shadow=""

(split over two lines here to fit on the page), whereas

  [name=mg1456],passwd.org_dir:gcos

would just return

  Martin Guerre

with no quotes. A NIS+ lookup fails if NIS+ returns more than one table entry
for the given indexed key. The effect of the "quote_nisplus" expansion
operator is to double any quote characters within the text.


9.17 More about MySQL, PostgreSQL, Oracle, and Interbase

If any MySQL, PostgreSQL, Oracle, or Interbase lookups are used, the           |
"mysql_servers", "pgsql_servers", "oracle_servers", or "ibase_servers" option  |
(as appropriate) must be set to a colon-separated list of server information.
Each item in the list is a slash-separated list of four items: host name,
database name, user name, and password. In the case of Oracle, the host name
field is used for the 'service name', and the database name field is not used
and should be empty. For example:

  hide oracle_servers = oracle.plc.example//ph10/abcdwxyz

Because password data is sensitive, you should always precede the setting with
'hide', to prevent non-admin users from obtaining the setting via the -bP
option. Here is an example where two MySQL servers are listed:

  hide mysql_servers = localhost/users/root/secret:\
                       otherhost/users/root/othersecret

For MySQL and PostgreSQL, a host may be specified as <name>:<port> but because
this is a colon-separated list, the colon has to be doubled.

For each query, these parameter groups are tried in order until a connection
and a query succeeds. Queries for these databases are SQL statements, so an
example might be

${lookup mysql{select mailbox from users where id='ph10'}{$value}fail}

If the result of the query contains more than one field, the data for each
field in the row is returned, preceded by its name, so the result of

${lookup pgsql{select home,name from users where id='ph10'}{$value}}

might be

  home=/home/ph10 name="Philip Hazel"

Values containing spaces and empty values are double quoted, with embedded
quotes escaped by a backslash.

If the result of the query contains just one field, the value is passed back
verbatim, without a field name, for example:

  Philip Hazel

If the result of the query yields more than one row, it is all concatenated,
with a newline between the data for each row.

The "quote_mysql", "quote_pgsql", and "quote_oracle" expansion operators con-
vert newline, tab, carriage return, and backspace to \n, \t, \r, and \b
respectively, and the characters single-quote, double-quote, and backslash
itself are escaped with backslashes. The "quote_pgsql" expansion operator, in
addition, escapes the percent and underscore characters. This cannot be done
for MySQL because these escapes are not recognized in contexts where these
characters are not special.


9.18 Special MySQL features

For MySQL, an empty host name or the use of 'localhost' in "mysql_servers"
causes a connection to the server on the local host by means of a Unix domain
socket. An alternate socket can be specified in parentheses. The full syntax
of each item in "mysql_servers" is:

  <hostname>::<port>(<socket name>)/<database>/<user>/<password>

Any of the three sub-parts of the first field can be omitted. For normal use
on the local host it can be left blank or set to just 'localhost'.

No database need be supplied - but if it is absent here, it must be given in
the queries.

If a MySQL query is issued that does not request any data (an insert, update,
or delete command), the result of the lookup is the number of rows affected.


9.19 Special PostgreSQL features

PostgreSQL lookups can also use Unix domain socket connections to the
database. This is usually faster and costs less CPU time than a TCP/IP
connection. However it can be used only if the mail server runs on the same
machine as the database server. A configuration line for PostgreSQL via Unix
domain sockets looks like this:

  hide pgsql_servers = (/tmp/.s.PGSQL.5432)/db/user/password : ...

In other words, instead of supplying a host name, a path to the socket is
given. The path name is enclosed in parentheses so that its slashes aren't
visually confused with the delimiters for the other server parameters.

If a PostgreSQL query is issued that does not request any data (an insert,
update, or delete command), the result of the lookup is the number of rows
affected.



               10. DOMAIN, HOST, ADDRESS, AND LOCAL PART LISTS


A number of Exim configuration options contain lists of domains, hosts, email
addresses, or local parts. For example, the "hold_domains" option contains a
list of domains whose delivery is currently suspended. These lists are also
used as data in ACL statements (see chapter 38).

Each item in one of these lists is a pattern to be matched against a domain,
host, email address, or local part, respectively. In the sections below, the
different types of pattern for each case are described, but first we cover
some general facilities that apply to all four kinds of list.


10.1 Expansion of lists

Each list is expanded as a single string before it is used. If the expansion
is forced to fail, Exim behaves as if the item it is testing (domain, host,
address, or local part) is not in the list. Other expansion failures cause
temporary errors.

If an item in a list is a regular expression, backslashes, dollars and
possibly other special characters in the expression must be protected against
misinterpretation by the string expander. The easiest way to do this is to use
the "\N" expansion feature to indicate that the contents of the regular
expression should not be expanded. For example, in an ACL you might have:

  deny senders = \N^\d{8}\w@.*\.baddomain\.example$\N :
                 ${lookup{$domain}lsearch{/badsenders/bydomain}}

The first item is a regular expression that is protected from expansion by
"\N", whereas the second uses the expansion to obtain a list of unwanted
senders based on the receiving domain.

After expansion, the list is split up into separate items for matching.
Normally, colon is used as the separator character, but this can be varied if
necessary, as described in section 6.15.


10.2 Negated items in lists

Items in a list may be positive or negative. Negative items are indicated by a
leading exclamation mark, which may be followed by optional white space. A
list defines a set of items (domains, etc). When Exim processes one of these
lists, it is trying to find out whether a domain, host, address, or local part
(respectively) is in the set that is defined by the list. It works like this:

The list is scanned from left to right. If a positive item is matched, the
subject that is being checked is in the set; if a negative item is matched,
the subject is not in the set. If the end of the list is reached without the
subject having matched any of the patterns, it is in the set if the last item
was a negative one, but not if it was a positive one. For example, the list in

  domainlist relay_domains = !a.b.c : *.b.c

matches any domain ending in ".b.c" except for "a.b.c". Domains that match
neither "a.b.c" nor "*.b.c" do not match, because the last item in the list is
positive. However, if the setting were

  domainlist relay_domains = !a.b.c

then all domains other than "a.b.c" would match because the last item in the
list is negative. In other words, a list that ends with a negative item
behaves as if it had an extra item ":*" on the end.

Another way of thinking about positive and negative items in lists is to read
the connector as 'or' after a positive item and as 'and' after a negative
item.


10.3 File names in lists

If an item in a domain, host, address, or local part list is an absolute file
name (beginning with a slash character), each line of the file is read and
processed as if it were an independent item in the list, except that further
file names are not allowed, and no expansion of the data from the file takes
place. Empty lines in the file are ignored, and the file may also contain
comment lines:

 .   For domain and host lists, if a # character appears anywhere in a line of
     the file, it and all following characters are ignored.

 .   Because local parts may legitimately contain # characters, a comment in
     an address list or local part list file is recognized only if # is
     preceded by white space or the start of the line. For example:

       not#comment@x.y.z   # but this is a comment

Putting a file name in a list has the same effect as inserting each line of
the file as an item in the list (blank lines and comments excepted). However,
there is one important difference: the file is read each time the list is
processed, so if its contents vary over time, Exim's behaviour changes.

If a file name is preceded by an exclamation mark, the sense of any match
within the file is inverted. For example, if

  hold_domains = !/etc/nohold-domains

and the file contains the lines

  !a.b.c
  *.b.c

then "a.b.c" is in the set of domains defined by "hold_domains", whereas any
domain matching "*.b.c" is not.


10.4 An lsearch file is not an out-of-line list

As will be described in the sections that follow, lookups can be used in lists
to provide indexed methods of checking list membership. There has been some
confusion about the way lsearch lookups work in lists. Because an lsearch file
contains plain text and is scanned sequentially, it is sometimes thought that
it is allowed to contain wild cards and other kinds of non-constant pattern.
This is not the case. The keys in an lsearch file are always fixed strings,
just as for any other single-key lookup type.

If you want to use a file to contain wild-card patterns that form part of a
list, just give the file name on its own, without a search type, as described
in the previous section.


10.5 Named lists

A list of domains, hosts, email addresses, or local parts can be given a name
which is then used to refer to the list elsewhere in the configuration. This
is particularly convenient if the same list is required in several different
places. It also allows lists to be given meaningful names, which can improve
the readability of the configuration. For example, it is conventional to
define a domain list called "local_domains" for all the domains that are
handled locally on a host, using a configuration line such as

  domainlist local_domains = localhost:my.dom.example

Named lists are referenced by giving their name preceded by a plus sign, so,
for example, a router that is intended to handle local domains would be
configured with the line

  domains = +local_domains

The first router in a configuration is often one that handles all domains
except the local ones, using a configuration with a negated item like this:

  dnslookup:
    driver = dnslookup
    domains = ! +local_domains
    transport = remote_smtp
    no_more

The four kinds of named list are created by configuration lines starting
with the words "domainlist", "hostlist", "addresslist", or "localpartlist",
respectively. Then there follows the name that you are defining, followed by
an equals sign and the list itself. For example:

  hostlist    relay_hosts = 192.168.23.0/24 : my.friend.example
  addresslist bad_senders = cdb;/etc/badsenders

A named list may refer to other named lists:

  domainlist  dom1 = first.example : second.example
  domainlist  dom2 = +dom1 : third.example
  domainlist  dom3 = fourth.example : +dom2 : fifth.example

Warning: If the last item in a referenced list is a negative one, the effect
may not be what you intended, because the negation does not propagate out to
the higher level. For example, consider:

  domainlist  dom1 = !a.b
  domainlist  dom2 = +dom1 : *.b

The second list specifies 'either in the "dom1" list or "*.b"'. The first list
specifies just 'not "a.b"', so the domain "x.y" matches it. That means it
matches the second list as well. The effect is not the same as

  domainlist  dom2 = !a.b : *.b

where "x.y" does not match. It's best to avoid negation altogether in
referenced lists if you can.

Named lists may have a performance advantage. When Exim is routing an address
or checking an incoming message, it caches the result of tests on named lists.
So, if you have a setting such as

  domains = +local_domains

on several of your routers or in several ACL statements, the actual test is
done only for the first one. However, the caching works only if there are no
expansions within the list itself or any sublists that it references. In other
words, caching happens only for lists that are known to be the same each time
they are referenced.

By default, there may be up to 16 named lists of each type. This limit can be
extended by changing a compile-time variable. The use of domain and host lists
is recommended for concepts such as local domains, relay domains, and relay
hosts. The default configuration is set up like this.


10.6 Named lists compared with macros

At first sight, named lists might seem to be no different from macros in the
configuration file. However, macros are just textual substitutions. If you
write

  ALIST = host1 : host2
  auth_advertise_hosts = !ALIST

it probably won't do what you want, because that is exactly the same as

  auth_advertise_hosts = !host1 : host2

Notice that the second host name is not negated. However, if you use a host
list, and write

  hostlist alist = host1 : host2
  auth_advertise_hosts = ! +alist

the negation applies to the whole list, and so that is equivalent to

  auth_advertise_hosts = !host1 : !host2


10.7 Domain lists

Domain lists contain patterns that are to be matched against a mail domain.
The following types of item may appear in domain lists:

 .   If a pattern consists of a single @ character, it matches the local host
     name, as set by the "primary_hostname" option (or defaulted). This makes
     it possible to use the same configuration file on several different hosts
     that differ only in their names.

 .   If a pattern consists of the string "@[]" it matches any local IP
     interface address, enclosed in square brackets, as in an email address
     that contains a domain literal. The use of domain literals is dying out
     in today's Internet.

 .   If a pattern consists of the string "@mx_any" it matches any domain that
     has an MX record pointing to the local host or to any host that is listed
     in "hosts_treat_as_local". The items "@mx_primary" and "@mx_secondary"
     are similar, except that the first matches only when a primary MX target
     is the local host, and the second only when no primary MX target is the
     local host, but a secondary MX target is. 'Primary' means an MX record
     with the lowest preference value - there may of course be more than one
     of them.

 .   If a pattern starts with an asterisk, the remaining characters of the
     pattern are compared with the terminating characters of the domain. The
     use of '*' in domain lists differs from its use in partial matching
     lookups. In a domain list, the character following the asterisk need not
     be a dot, whereas partial matching works only in terms of dot-separated
     components. For example, a domain list item such as "*key.ex" matches
     "donkey.ex" as well as "cipher.key.ex".

 .   If a pattern starts with a circumflex character, it is treated as a
     regular expression, and matched against the domain using a regular
     expression matching function. The circumflex is treated as part of the
     regular expression. References to descriptions of the syntax of regular
     expressions are given in chapter 8.

     Warning: Because domain lists are expanded before being processed, you
     must escape any backslash and dollar characters in the regular
     expression, or use the special "\N" sequence (see chapter 11) to specify
     that it is not to be expanded (unless you really do want to build a
     regular expression by expansion, of course).

 .   If a pattern starts with the name of a single-key lookup type followed by
     a semicolon (for example, 'dbm;' or 'lsearch;'), the remainder of the
     pattern must be a file name in a suitable format for the lookup type. For
     example, for 'cdb;' it must be an absolute path:

       domains = cdb;/etc/mail/local_domains.cdb

     The appropriate type of lookup is done on the file using the domain name
     as the key. In most cases, the data that is looked up is not used; Exim
     is interested only in whether or not the key is present in the file.
     However, when a lookup is used for the "domains" option on a router or a
     "domains" condition in an ACL statement, the data is preserved in the
     $domain_data variable and can be referred to in other router options or
     other statements in the same ACL.

 .   Any of the single-key lookup type names may be preceded by 'partial<n>-',
     where the <n> is optional, for example,

       domains = partial-dbm;/partial/domains

     This causes partial matching logic to be invoked; a description of how
     this works is given in section 9.6.

 .   Any of the single-key lookup types may be followed by an asterisk. This
     causes a default lookup for a key consisting of a single asterisk to be
     done if the original lookup fails. This is not a useful feature when
     using a domain list to select particular domains (because any domain
     would match), but it might have value if the result of the lookup is
     being used via the $domain_data expansion variable.

 .   If the pattern starts with the name of a query-style lookup type followed
     by a semicolon (for example, 'nisplus;' or 'ldap;'), the remainder of the
     pattern must be an appropriate query for the lookup type, as described in
     chapter 9. For example:

       hold_domains = mysql;select domain from holdlist \
         where domain = '$domain';

     In most cases, the data that is looked up is not used (so for an SQL
     query, for example, it doesn't matter what field you select). Exim is
     interested only in whether or not the query succeeds. However, when a
     lookup is used for the "domains" option on a router, the data is
     preserved in the $domain_data variable and can be referred to in other
     options.

 .   If none of the above cases apply, a caseless textual comparison is made
     between the pattern and the domain.

Here is an example that uses several different kinds of pattern:

  domainlist funny_domains = \
    @ : \
    lib.unseen.edu : \
    *.foundation.fict.example : \
    \N^[1-2]\d{3}\.fict\.example$\N : \
    partial-dbm;/opt/data/penguin/book : \
    nis;domains.byname : \
    nisplus;[name=$domain,status=local],domains.org_dir

There are obvious processing trade-offs among the various matching modes.
Using an asterisk is faster than a regular expression, and listing a few names
explicitly probably is too. The use of a file or database lookup is expensive,
but may be the only option if hundreds of names are required. Because the
patterns are tested in order, it makes sense to put the most commonly matched
patterns earlier.


10.8 Host lists

Host lists are used to control what remote hosts are allowed to do. For
example, some hosts may be allowed to use the local host as a relay, and some
may be permitted to use the SMTP ETRN command. Hosts can be identified in two
different ways, by name or by IP address. In a host list, some types of
pattern are matched to a host name, and some are matched to an IP address. You
need to be particularly careful with this when single-key lookups are
involved, to ensure that the right value is being used as the key.


10.9 Special host list patterns

If a host list item is the empty string, it matches only when no remote host
is involved. This is the case when a message is being received from a local
process using SMTP on the standard input, that is, when a TCP/IP connection is
not used.

The special pattern '*' in a host list matches any host or no host. Neither
the IP address nor the name is actually inspected.


10.10 Host list patterns that match by IP address

If an IPv4 host calls an IPv6 host and the call is accepted on an IPv6
socket, the incoming address actually appears in the IPv6 host as
'::ffff:<v4address>'. When such an address is tested against a host list, it
is converted into a traditional IPv4 address first. (Not all operating systems
accept IPv4 calls on IPv6 sockets, as there have been some security concerns.)

The following types of pattern in a host list check the remote host by
inspecting its IP address:

 .   If the pattern is a plain domain name (not a regular expression, not
     starting with *, not a lookup of any kind), Exim calls the operating      |
     system function to find the associated IP address(es). Exim uses the
     newer "getipnodebyname()" function when available, otherwise
     "gethostbyname()". This typically causes a forward DNS lookup of the
     name. The result is compared with the IP address of the subject host.

 .   If the pattern is '@', the primary host name is substituted and used as a
     domain name, as just described.

 .   If the pattern is an IP address, it is matched against the IP address of
     the subject host. IPv4 addresses are given in the normal 'dotted-quad'
     notation. IPv6 addresses can be given in colon-separated format, but the
     colons have to be doubled so as not to be taken as item separators when
     the default list separator is used. IPv6 addresses are recognized even
     when Exim is compiled without IPv6 support. This means that if they
     appear in a host list on an IPv4-only host, Exim will not treat them as
     host names. They are just addresses that can never match a client host.

 .   If the pattern is '@[]', it matches the IP address of any IP interface on
     the local host. For example, if the local host is an IPv4 host with one
     interface address 10.45.23.56, these two ACL statements have the same
     effect:

       accept hosts = 127.0.0.1 : 10.45.23.56
       accept hosts = @[]

 .   If the pattern is an IP address followed by a slash and a mask length
     (for example 10.11.42.0/24), it is matched against the IP address of the
     subject host under the given mask. This allows, an entire network of
     hosts to be included (or excluded) by a single item. The mask uses CIDR
     notation; it specifies the number of address bits that must match,
     starting from the most significant end of the address.

     Note: the mask is "not" a count of addresses, nor is it the high number
     of a range of addresses. It is the number of bits in the network portion
     of the address. The above example specifies a 24-bit netmask, so it
     matches all 256 addresses in the 10.11.42.0 network. An item such as

       192.168.23.236/31

     matches just two addresses, 192.168.23.236 and 192.168.23.237. A mask
     value of 32 for an IPv4 address is the same as no mask at all; just a
     single address matches.

     Here is another example which shows an IPv4 and an IPv6 network:

       recipient_unqualified_hosts = 192.168.0.0/16: \
                                     3ffe::ffff::836f::::/48

     The doubling of list separator characters applies only when these items
     appear inline in a host list. It is not required when indirecting via a
     file. For example,

       recipient_unqualified_hosts = /opt/exim/unqualnets

     could make use of a file containing

       172.16.0.0/12
       3ffe:ffff:836f::/48

     to have exactly the same effect as the previous example. When listing
     IPv6 addresses inline, it is usually more convenient to use the facility
     for changing separator characters. This list contains the same two
     networks:

       recipient_unqualified_hosts = <; 172.16.0.0/12; \
                                        3ffe:ffff:836f::/48

     The separator is changed to semicolon by the leading '<;' at the start of
     the list.

                                                                               |
10.11 Host list patterns for single-key lookups by host address                |
                                                                               |
When a host is to be identified by a single-key lookup of its complete IP      |
address, the pattern takes this form:                                          |
                                                                               |
  net-<single-key-search-type>;<search-data>                                   |
                                                                               |
For example:                                                                   |
                                                                               |
  hosts_lookup = net-cdb;/hosts-by-ip.db                                       |
                                                                               |
The text form of the IP address of the subject host is used as the lookup key. |
IPv6 addresses are converted to an unabbreviated form, using lower case        |
letters, with dots as separators because colon is the key terminator in        |
lsearch files. [Colons can in fact be used in keys in lsearch files by quoting |
the keys, but this is a facility that was added later.] The data returned by   |
the lookup is not used.                                                        |
                                                                               |
Single-key lookups can also be performed using masked IP addresses, using      |
patterns of this form:                                                         |
                                                                               |
  net<number>-<single-key-search-type>;<search-data>                           |
                                                                               |
For example:                                                                   |
                                                                               |
  net24-dbm;/networks.db                                                       |
                                                                               |
The IP address of the subject host is masked using <number> as the mask        |
length. A textual string is constructed from the masked value, followed by the |
mask, and this is used as the lookup key. For example, if the host's IP        |
address is 192.168.34.6, the key that is looked up for the above example is    |
'192.168.34.0/24'. IPv6 addresses are converted to a text value using lower    |
case letters and dots as separators instead of the more usual colon, because   |
colon is the key terminator in lsearch files. Full, unabbreviated IPv6         |
addresses are always used.                                                     |
                                                                               |
Warning: Specifing "net32-" (for an IPv4 address) or "net128-" (for an IPv6    |
address) is not the same as specifing just "net-" without a number. In the     |
former case the key strings include the mask value, whereas in the latter case |
the IP address is used on its own.                                             |
                                                                               |
                                                                               |
10.12 Host list patterns that match by host name                               |
                                                                               |
There are several types of pattern that require Exim to know the name of the   |
remote host. These are either wildcard patterns or lookups by name. (If a      |
complete hostname is given without any wildcarding, it is used to find an IP   |
address to match against, as described in the section 10.10 above.)            |
                                                                               |
If the remote host name is not already known when Exim encounters one of these |
patterns, it has to be found from the IP address. By default, Exim first does  |
a reverse DNS lookup; if no name is found in the DNS, the system function      |
("gethostbyaddr()" or "getipnodebyaddr()" if available) is tried. The order in |
which these lookups are done can be changed by setting the "host_lookup_order" |
option.                                                                        |
                                                                               |
Although many sites on the Internet are conscientious about maintaining        |
reverse DNS data for their hosts, there are also many that do not do this.     |
Consequently, a name cannot always be found, and this may lead to unwanted     |
effects. For this reason, matching against host names is not as common as      |
matching against IP addresses.                                                 |
                                                                               |
If no name can be found for the IP address, Exim behaves as if the host does   |
not match the list. This may not always be what you want to happen. To change  |
Exim's behaviour, the special item '+include_unknown' may appear in the list   |
(at top level - it is not recognized in an indirected file). If any subsequent |
items require a host name, but no name can be found, Exim behaves as if the    |
host does match the list. For example,                                         |
                                                                               |
  host_reject_connection = +include_unknown:*.enemy.ex                         |
                                                                               |
rejects connections from any host whose name matches "*.enemy.ex", and also    |
any hosts whose name it cannot find.                                           |
                                                                               |
Warning: Take care when configuring host lists with wildcarded name patterns.  |
Consider what will happen if a name cannot be found.                           |
                                                                               |
As a result of aliasing, hosts may have more than one name. When processing    |
any of the following types of pattern, all the host's names are checked:       |
                                                                               |
 .   If a pattern starts with '*' the remainder of the item must match the end |
     of the host name. For example, "*.b.c" matches all hosts whose names end  |
     in ".b.c". This special simple form is provided because this is a very    |
     common requirement. Other kinds of wildcarding require the use of a       |
     regular expression.                                                       |
                                                                               |
 .   If the item starts with '^' it is taken to be a regular expression which  |
     is matched against the host name. For example,                            |
                                                                               |
       ^(a|b)\.c\.d$                                                           |
                                                                               |
     is a regular expression that matches either of the two hosts "a.c.d" or   |
     "b.c.d". When a regular expression is used in a host list, you must take  |
     care that backslash and dollar characters are not misinterpreted as part  |
     of the string expansion. The simplest way to do this is to use "\N" to    |
     mark that part of the string as non-expandable. For example:              |
                                                                               |
       sender_unqualified_hosts = \N^(a|b)\.c\.d$\N : ....                     |
                                                                               |
                                                                               |
10.13 Host list patterns for single-key lookups by host name                   |
                                                                               |
If a pattern is of the form                                                    |
                                                                               |
  <single-key-search-type>;<search-data>                                       |
                                                                               |
for example                                                                    |
                                                                               |
  dbm;/host/accept/list                                                        |
                                                                               |
a single-key lookup is performend, using the host name as its key. If the      |
lookup succeeds, the host matches the item. The actual data that is looked up  |
is not used.                                                                   |
                                                                               |
Reminder: With this kind of pattern, you must have host names as keys in the   |
file, not IP addresses. If you want to do lookups based on IP addresses, you   |
must precede the search type with 'net-' (see section 10.11). There is,        |
however, no reason why you could not use two items in the same list, one doing |
an address lookup and one doing a name lookup, both using the same file.       |
                                                                               |
                                                                               |
10.14 Host list patterns for query-style lookups                               |
                                                                               |
If a pattern is of the form                                                    |
                                                                               |
  <query-style-search-type>;<query>                                            |
                                                                               |
the query is obeyed, and if it succeeds, the host matches the item. The actual |
data that is looked up is not used. The variables $sender_host_address and     |
$sender_host_name can be used in the query. For example:                       |
                                                                               |
  hosts_lookup = pgsql;\                                                       |
    select ip from hostlist where ip='$sender_host_address'                    |
                                                                               |
The value of $sender_host_address for an IPv6 address uses colon separators.   |
You can use the "sg" expansion item to change this if you need to. If you want |
to use masked IP addresses in database queries, you can use the "mask"         |
expansion operator.                                                            |
                                                                               |
If the query contains a reference to $sender_host_name, Exim automatically     |
looks up the host name if has not already done so. (See section 10.12 for      |
comments on finding host names.)                                               |
                                                                               |
Historical note: prior to release 4.30, Exim would always attempt to find a    |
host name before running the query, unless the search type was preceded by     |
"net-". This is no longer the case. For backwards compatibility, "net-" is     |
still recognized for query-style lookups, but its presence or absence has no   |
effect. (Of course, for single-key lookups, "net-" is important.)              |


10.15 Mixing wildcarded host names and addresses in host lists

If you have name lookups or wildcarded host names and IP addresses in the same
host list, you should normally put the IP addresses first. For example, in an
ACL you could have:

  accept hosts = 10.9.8.7 : *.friend.example

The reason for this lies in the left-to-right way that Exim processes lists.
It can test IP addresses without doing any DNS lookups, but when it reaches an
item that requires a host name, it fails if it cannot find a host name to
compare with the pattern. If the above list is given in the opposite order,
the "accept" statement fails for a host whose name cannot be found, even if
its IP address is 10.9.8.7.

If you really do want to do the name check first, and still recognize the IP
address, you can rewrite the ACL like this:

  accept hosts = *.friend.example
  accept hosts = 10.9.8.7

If the first "accept" fails, Exim goes on to try the second one. See chapter
38 for details of ACLs.


10.16 Address lists

Address lists contain patterns that are matched against mail addresses. There
is one special case to be considered: the sender address of a bounce message
is always empty. You can test for this by providing an empty item in an
address list. For example, you can set up a router to process bounce messages
by using this option setting:

  senders = :

The presence of the colon creates an empty item. If you do not provide any
data, the list is empty and matches nothing. The empty sender can also be
detected by a regular expression that matches an empty string.

The following kinds of pattern are supported in address lists:

 .   If (after expansion) a pattern starts with '^', a regular expression
     match is done against the complete address, with the pattern as the
     regular expression. You must take care that backslash and dollar charac-
     ters are not misinterpreted as part of the string expansion. The simplest
     way to do this is to use "\N" to mark that part of the string as non-
     expandable. For example:

       deny senders = \N^\d{8}.+@spamhaus.example$\N : ...

     The "\N" sequences are removed by the expansion, so the item does start
     with '^' by the time it is being interpreted as an address pattern.

 .   If a pattern starts with '@@' followed by a single-key lookup item (for
     example, "@@lsearch;/some/file"), the address that is being checked is
     split into a local part and a domain. The domain is looked up in the
     file. If it is not found, there is no match. If it is found, the data
     that is looked up from the file is treated as a colon-separated list of
     local part patterns, each of which is matched against the subject local
     part in turn.

     The lookup may be a partial one, and/or one involving a search for a
     default keyed by '*' (see section 9.5). The local part patterns that are
     looked up can be regular expressions or begin with '*', or even be
     further lookups. They may also be independently negated. For example,
     with

       deny senders = @@dbm;/etc/reject-by-domain

     the data from which the DBM file is built could contain lines like

       baddomain.com:  !postmaster : *

     to reject all senders except "postmaster" from that domain. If a local
     part that actually begins with an exclamation mark is required, it has to
     be specified using a regular expression. In lsearch files, an entry may
     be split over several lines by indenting the second and subsequent lines,
     but the separating colon must still be included at line breaks. White
     space surrounding the colons is ignored. For example:

       aol.com:  spammer1 : spammer2 : ^[0-9]+$ :
                 spammer3 : spammer4

     As in all colon-separated lists in Exim, a colon can be included in an
     item by doubling.

     If the last item in the list starts with a right angle-bracket, the
     remainder of the item is taken as a new key to look up in order to obtain
     a continuation list of local parts. The new key can be any sequence of
     characters. Thus one might have entries like

       aol.com: spammer1 : spammer 2 : >*
       xyz.com: spammer3 : >*
       *:       ^\d{8}$

     in a file that was searched with "@@dbm*", to specify a match for 8-digit
     local parts for all domains, in addition to the specific local parts
     listed for each domain. Of course, using this feature costs another
     lookup each time a chain is followed, but the effort needed to maintain
     the data is reduced. It is possible to construct loops using this
     facility, and in order to catch them, the chains may be no more than
     fifty items long.

 .   The @@<lookup> style of item can also be used with a query-style lookup,
     but in this case, the chaining facility is not available. The lookup can
     only return a single list of local parts.

 .   Complete addresses can be looked up by using a pattern that starts with a
     lookup type terminated by a semicolon, follwed by the data for the
     lookup. For example:

       deny senders = cdb;/etc/blocked.senders : \
         mysql;select address from blocked where \
         address='${quote_mysql:$sender_address}'

     For a single-key lookup type, Exim uses the complete address as the key.
     Partial matching (section 9.6) cannot be used, and is ignored if
     specified, with an entry being written to the panic log.

     You can configure lookup defaults, as described in section 9.5, but this
     is useful only for the '*@' type of default. For example, with this
     lookup:

       accept senders = lsearch*@;/some/file

     the file could contains lines like this:

       user1@domain1.example
       *@domain2.example

     and for the sender address "nimrod@jaeger.example", the sequence of keys
     that are tried is:

       nimrod@jaeger.example
       *@jaeger.example
       *

     Warning 1: Do not include a line keyed by '*' in the file, because that
     would mean that every address matches, thus rendering the test useless.

     Warning 2: Do not confuse these two kinds of item:

       deny recipients = dbm*@;/some/file
       deny recipients = *@dbm;/some/file

     The first does a whole address lookup, with defaulting, as just
     described, because it starts with a lookup type. The second matches the
     local part and domain independently, as described in the next paragraph.

 .   If a pattern contains an @ character, but is not a regular expression and
     does not begin with a lookup type as described above, the local part of
     the subject address is compared with the local part of the pattern, which
     may start with an asterisk. If the local parts match, the domain is
     checked in exactly the same way as for a pattern in a domain list. For
     example, the domain can be wildcarded, refer to a named list, be be or a
     lookup:

       deny senders = *@*.spamming.site:\
                      *@+hostile_domains:\
                      bozo@partial-lsearch;/list/of/dodgy/sites:\
                      *@dbm;/bad/domains.db

     If a local part that begins with an exclamation mark is required, it has
     to be specified using a regular expression, because otherwise the
     exclamation mark is treated as a sign of negation.

 .   If a pattern is not one of the above syntax forms, that is, if a pattern
     which is not a regular expression or a lookup does not contain an @
     character, it is matched against the domain part of the subject address.
     The only two formats that are recognized this way are a literal domain,
     or a domain pattern that starts with *. In both these cases, the effect
     is the same as if "*@" preceded the pattern.

Warning: there is an important difference between the address list items in
these two examples:

  senders = +my_list
  senders = *@+my_list

In the first one, "my_list" is a named address list, whereas in the second
example it is a named domain list.


10.17 Case of letters in address lists

Domains in email addresses are always handled caselessly, but for local parts
case may be significant on some systems (see "caseful_local_part" for how
Exim deals with this when routing addresses). However, RFC 2505 (Anti-Spam
Recommendations for SMTP MTAs) suggests that matching of addresses to blocking
lists should be done in a case-independent manner. Since most address lists in
Exim are used for this kind of control, Exim attempts to do this by default.

The domain portion of an address is always lowercased before matching it to an
address list. The local part is lowercased by default, and any string
comparisons that take place are done caselessly. This means that the data in
the address list itself, in files included as plain file names, and in any
file that is looked up using the '@@' mechanism, can be in any case. However,
the keys in files that are looked up by a search type other than lsearch
(which works caselessly) must be in lower case, because these lookups are not
case-independent.

To allow for the possibility of caseful address list matching, if an item in
an address list is the string '+caseful', the original case of the local part
is restored for any comparisons that follow, and string comparisons are no
longer case-independent. This does not affect the domain, which remains in
lower case. However, although independent matches on the domain alone are
still performed caselessly, regular expressions that match against an entire
address become case-sensitive after '+caseful' has been seen.


10.18 Local part lists

Case-sensitivity in local part lists is handled in the same way as for address
lists, as just described. The '+caseful' item can be used if required. In a
setting of the "local_parts" option in a router with "caseful_local_part"
set false, the subject is lowercased and the matching is initially case-
insensitive. In this case, '+caseful' will restore case-sensitive matching in
the local part list, but not elsewhere in the router. If "caseful_local_part"
is set true in a router, matching in the "local_parts" option is case-
sensitive from the start.

If a local part list is indirected to a file (see section 10.3), comments are
handled in the same way as address lists - they are recognized only if the #
is preceded by white space or the start of the line. Otherwise, local part
lists are matched in the same way as domain lists, except that the special
items that refer to the local host ("@", "@[]", "@mx_any", "@mx_primary", and
"@mx_secondary") are not recognized. Refer to section 10.7 for details of the
other available item types.



                            11. STRING EXPANSIONS


Many strings in Exim's run time configuration are expanded before use. Some of
them are expanded every time they are used; others are expanded only once.

When a string is being expanded it is copied verbatim from left to right
except when a dollar or backslash character is encountered. A dollar specifies
the start of a portion of the string which is interpreted and replaced as
described below in section 11.4 onwards. Backslash is used as an escape
character, as described in the following section.


11.1 Literal text in expanded strings

An uninterpreted dollar can be included in an expanded string by putting a
backslash in front of it. A backslash can be used to prevent any special
character being treated specially in an expansion, including itself. If the
string appears in quotes in the configuration file, two backslashes are
required because the quotes themselves cause interpretation of backslashes
when the string is read in (see section 6.12).

A portion of the string can specified as non-expandable by placing it between
two occurrences of "\N". This is particularly useful for protecting regular
expressions, which often contain backslashes and dollar signs. For example:

  deny senders = \N^\d{8}[a-z]@some\.site\.example$\N

On encountering the first "\N", the expander copies subsequent characters
without interpretation until it reaches the next "\N" or the end of the
string.


11.2 Character escape sequences in expanded strings

A backslash followed by one of the letters 'n', 'r', or 't' in an expanded
string is recognized as an escape sequence for the character newline, carriage
return, or tab, respectively. A backslash followed by up to three octal digits
is recognized as an octal encoding for a single character, and a backslash
followed by 'x' and up to two hexadecimal digits is a hexadecimal encoding.

These escape sequences are also recognized in quoted strings when they are
read in. Their interpretation in expansions as well is useful for unquoted
strings, and for other cases such as looked-up strings that are then expanded.


11.3 Testing string expansions

Many expansions can be tested by calling Exim with the -be option. This takes
the command arguments, or lines from the standard input if there are no
arguments, runs them through the string expansion code, and writes the results
to the standard output. Variables based on configuration values are set up,
but since no message is being processed, variables such as $local_part have no
value. Nevertheless the -be option can be useful for checking out file and
database lookups, and the use of expansion operators such as "sg", "substr"
and "nhash".

Exim gives up its root privilege when it is called with the -be option, and
instead runs under the uid and gid it was called with, to prevent users from
using -be for reading files to which they do not have access.


11.4 Expansion items

The following items are recognized in expanded strings. White space may be
used between sub-items that are keywords or substrings enclosed in braces
inside an outer set of braces, to improve readability. Warning: Within braces,
white space is significant.

$<variable name> or ${<variable name>}

   Substitute the contents of the named variable, for example

     $local_part
     ${domain}

   The second form can be used to separate the name from subsequent
   alphanumeric characters. This form (using curly brackets) is available only
   for variables; it does not apply to message headers. The names of the
   variables are given in section 11.8 below. If the name of a non-existent
   variable is given, the expansion fails.

${<op>:<string>}

   The string is first itself expanded, and then the operation specified by
   <op> is applied to it. For example,

     ${lc:$local_part}

   The string starts with the first character after the colon, which may be
   leading white space. A list of operators is given in section 11.5 below.
   The operator notation is used for simple expansion items that have just one
   argument, because it reduces the number of braces and therefore makes the
   string easier to understand.

${extract{<key>}{<string1>}{<string2>}{<string3>}}

   The key and <string1> are first expanded separately. Leading and trailing   |
   whitespace is removed from the key (but not from any of the strings). The   |
   key must not consist entirely of digits. The expanded <string1> must be of
   the form:

     <key1> = <value1>  <key2> = <value2> ...

   where the equals signs and spaces (but not both) are optional. If any of
   the values contain white space, they must be enclosed in double quotes, and
   any values that are enclosed in double quotes are subject to escape
   processing as described in section 6.12. The expanded <string1> is searched
   for the value that corresponds to the key. The search is case-insensitive.
   If the key is found, <string2> is expanded, and replaces the whole item;
   otherwise <string3> is used. During the expansion of <string2> the variable
   $value contains the value that has been extracted. Afterwards, it is
   restored to any previous value it might have had.

   If {<string3>} is omitted, the item is replaced by an empty string if the
   key is not found. If {<string2>} is also omitted, the value that was
   extracted is used. Thus, for example, these two expansions are identical,
   and yield '2001':

     ${extract{gid}{uid=1984 gid=2001}}
     ${extract{gid}{uid=1984 gid=2001}{$value}}

   Instead of {<string3>} the word 'fail' (not in curly brackets) can appear,
   for example:

     ${extract{Z}{A=... B=...}{$value} fail }

   {<string2>} must be present for 'fail' to be recognized. When this syntax
   is used, if the extraction fails, the entire string expansion fails in a
   way that can be detected by the code in Exim which requested the expansion.
   This is called 'forced expansion failure', and its consequences depend on
   the circumstances. In some cases it is no different from any other
   expansion failure, but in others a different action may be taken. Such
   variations are mentioned in the documentation of the option which is
   expanded.

${extract{<number>}{<separators>}{<string1>}{<string2>}{<string3>}}

   The <number> argument must consist entirely of decimal digits, apart from   |
   leading and trailing whitespace, which is ignored. This is what dis-        |
   tinguishes this form of "extract" from the previous kind. It behaves in the
   same way, except that, instead of extracting a named field, it extracts
   from <string1> the field whose number is given as the first argument. You
   can use $value in <string2> or "fail" instead of <string3> as before.

   The fields in the string are separated by any one of the characters in the  |
   separator string. These may include space or tab characters. The first      |
   field is numbered one. If the number is negative, the fields are counted
   from the end of the string, with the rightmost one numbered -1. If the
   number given is zero, the entire string is returned. If the modulus of the
   number is greater than the number of fields in the string, the result is
   the expansion of <string3>, or the empty string if <string3> is not
   provided. For example:

     ${extract{2}{:}{x:42:99:& Mailer::/bin/bash}}

   yields '42', and

     ${extract{-4}{:}{x:42:99:& Mailer::/bin/bash}}

   yields '99'. Two successive separators mean that the field between them is
   empty (for example, the fifth field above).

${hash{<string1>}{<string2>}{<string3>}}

   This is a textual hashing function, and was the first to be implemented in
   early versions of Exim. In current releases, there are other hashing
   functions (numeric, MD5, and SHA-1), which are described below.

   The first two strings, after expansion, must be numbers. Call them <m> and
   <n>. If you are using fixed values for these numbers, that is, if <string1>
   and <string2> do not change when they are expanded, you can use the simpler
   operator notation that avoids some of the braces:

     ${hash_<n>_<m>:<string>}

   The second number is optional (in both notations).

   If <n> is greater than or equal to the length of the string, the expansion
   item returns the string. Otherwise it computes a new string of length <n>
   by applying a hashing function to the string. The new string consists of
   characters taken from the first <m> characters of the string

     abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQWRSTUVWXYZ0123456789

   If <m> is not present the value 26 is used, so that only lower case letters
   appear. For example:

     ${hash{3}{monty}}              yields  "jmg"
     ${hash{5}{monty}}              yields  "monty"
     ${hash{4}{62}{monty python}}   yields  "fbWx"

$header_<header name>: or $h_<header name>:                                    |
                                                                               |
$bheader_<header name>: or $bh_<header name>:                                  |
                                                                               |
$rheader_<header name>: or $rh_<header name>:                                  |
                                                                               |
   Substitute the contents of the named message header line, for example

     $header_reply-to:

   The newline that terminates a header line is not included in the expansion,
   but internal newlines (caused by splitting the header line over several
   physical lines) may be present.

   The difference between "rheader", "bheader", and "header" is in the way the |
   data in the header line is interpreted.                                     |
                                                                               |
    .   "rheader" gives the original 'raw' content of the header line, with no |
        processing at all, and without the removal of leading and trailing     |
        whitespace.                                                            |
                                                                               |
    .   "bheader" removes leading and trailing whitespace, and then decodes    |
        base64 or quoted-printable MIME 'words' within the header text, but    |
        does no character set translation. If decoding of what looks super-    |
        ficially like a MIME 'word' fails, the raw string is returned. If      |
        decoding produces a binary zero character, it is replaced by a         |
        question mark - this is what Exim does for binary zeros that are       |
        actually received in header lines.                                     |
                                                                               |
    .   "header" tries to translate the string as decoded by "bheader" to a    |
        standard character set. This is an attempt to produce the same string  |
        as would be displayed on a user's MUA. If translation fails, the       |
        "bheader" string is returned. Translation is attempted only on operat- |
        ing systems that support the "iconv()" function. This is indicated     |
        by the compile-time macro HAVE_ICONV in a system Makefile or in        |
        Local/Makefile.                                                        |
                                                                               |
   In a filter file, the target character set for "header" can be specified by |
   a command of the following form:                                            |
                                                                               |
     headers charset "UTF-8"                                                   |
                                                                               |
   This command affects all references to $h_ (or $header_) expansions in      |
   subsequently obeyed filter commands. In the absence of this command, the    |
   target character set in a filter is taken from the setting of the           |
   "headers_charset" option in the runtime configuration. The value of this    |
   option defaults to the value of HEADERS_CHARSET in Local/Makefile. The      |
   ultimate default is ISO-8859-1.                                             |

   Header names follow the syntax of RFC 2822, which states that they may
   contain any printing characters except space and colon. Consequently, curly
   brackets do not terminate header names, and should not be used to enclose
   them as if they were variables. Attempting to do so causes a syntax error.

   Only header lines that are common to all copies of a message are visible to
   this mechanism. These are the original header lines that are received with
   the message, and any that are added by an ACL "warn" statement or by a
   system filter. Header lines that are added to a particular copy of a
   message by a router or transport are not accessible.

   For incoming SMTP messages, no header lines are visible in ACLs that are
   obeyed before the DATA ACL, because the header structure is not set up
   until the message is received. Header lines that are added by "warn"
   statements in a RCPT ACL (for example) are saved until the message's
   incoming header lines are available, at which point they are added. When a
   DATA ACL is running, however, header lines added by earlier ACLs are
   visible.

   Upper case and lower case letters are synonymous in header names. If the
   following character is white space, the terminating colon may be omitted,
   but this is not recommended, because you may then forget it when it is
   needed. When white space terminates the header name, it is included in the
   expanded string. If the message does not contain the given header, the
   expansion item is replaced by an empty string. (See the "def" condition in
   section 11.6 for a means of testing for the existence of a header.)

   If there is more than one header with the same name, they are all
   concatenated to form the substitution string, up to a maximum length of
   64K. A newline character is inserted between each line. For the "header"
   expansion, for those headers that contain lists of addresses, a comma is
   also inserted at the junctions between lines. This does not happen for the
   "rheader" expansion.

${hmac{<hashname>}{<secret>}{<string>}}

   This function uses cryptographic hashing (either MD5 or SHA-1) to convert a
   shared secret and some text into a message authentication code, as
   specified in RFC 2104. This differs from "${md5:secret_text...}" or         |
   "${sha1:secret_text...}" in that the hmac step adds a signature to the      |
   cryptographic hash, allowing for authentication that is not possible with   |
   MD5 or SHA-1 alone. The hash name must expand to either "md5" or "sha1" at  |
   present. For example:

     ${hmac{md5}{somesecret}{$primary_hostname $tod_log}}

   For the hostname "mail.example.com" and time 2002-10-17 11:30:59, this
   produces:

     dd97e3ba5d1a61b5006108f8c8252953

   As an example of how this might be used, you might put in the main part of
   an Exim configuration:

     SPAMSCAN_SECRET=cohgheeLei2thahw

   In a router or a transport you could then have:

     headers_add = \
       X-Spam-Scanned: ${primary_hostname} ${message_id} \
       ${hmac{md5}{SPAMSCAN_SECRET}\
       {${primary_hostname},${message_id},$h_message-id:}}

   Then given a message, you can check where it was scanned by looking at the
   X-Spam-Scanned: header line. If you know the secret, you can check that
   this header line is authentic by recomputing the authentication code from
   the host name, message ID and the Message-id: header line. This can be done
   using Exim's -be option, or by other means, for example by using the
   "hmac_md5_hex()" function in Perl.

${if <condition> {<string1>}{<string2>}}

   If <condition> is true, <string1> is expanded and replaces the whole item;
   otherwise <string2> is used. For example,

     ${if eq {$local_part}{postmaster} {yes}{no} }

   The second string need not be present; if it is not and the condition is
   not true, the item is replaced with nothing. Alternatively, the word 'fail'
   may be present instead of the second string (without any curly brackets).
   In this case, the expansion is forced to fail if the condition is not true.
   The available conditions are described in section 11.6 below.

${length{<string1>}{<string2>}}

   The "length" item is used to extract the initial portion of a string. Both
   strings are expanded, and the first one must yield a number, <n>, say. If
   you are using a fixed value for the number, that is, if <string1> does not
   change when expanded, you can use the simpler operator notation that avoids
   some of the braces:

     ${length_<n>:<string>}

   The result of this item is either the first <n> characters or the whole of
   <string2>, whichever is the shorter. Do not confuse "length" with "strlen",
   which gives the length of a string.

${lookup{<key>} <search type> {<file>} {<string1>} {<string2>}}

${lookup <search type> {<query>} {<string1>} {<string2>}}

   These items specify data lookups in files and databases, as discussed in
   chapter 9. The first form is used for single-key lookups, and the second is
   used for query-style lookups. The <key>, <file>, and <query> strings are
   expanded before use.

   If there is any white space in a lookup item which is part of a filter
   command, a retry or rewrite rule, a routing rule for the manualroute
   router, or any other place where white space is significant, the lookup
   item must be enclosed in double quotes. The use of data lookups in users'
   filter files may be locked out by the system administrator.

   If the lookup succeeds, <string1> is expanded and replaces the entire item.
   During its expansion, the variable $value contains the data returned by the
   lookup. Afterwards it reverts to the value it had previously (at the outer
   level it is empty). If the lookup fails, <string2> is expanded and replaces
   the entire item. If {<string2>} is omitted, the replacement is null on
   failure. Alternatively, <string2> can itself be a nested lookup, thus
   providing a mechanism for looking up a default value when the original
   lookup fails.

   If a nested lookup is used as part of <string1>, $value contains the data
   for the outer lookup while the parameters of the second lookup are
   expanded, and also while <string2> of the second lookup is expanded, should
   the second lookup fail.

   Instead of {<string2>} the word 'fail' can appear, and in this case, if the
   lookup fails, the entire expansion is forced to fail. If both {<string1>}
   and {<string2>} are omitted, the result is the looked up value in the case
   of a successful lookup, and nothing in the case of failure.

   For single-key lookups, the string 'partial' is permitted to precede the
   search type in order to do partial matching, and * or *@ may follow a
   search type to request default lookups if the key does not match (see
   sections 9.5 and 9.6 for details).

   If a partial search is used, the variables $1 and $2 contain the wild and
   non-wild parts of the key during the expansion of the replacement text.
   They return to their previous values at the end of the lookup item.

   This example looks up the postmaster alias in the conventional alias file:

     ${lookup {postmaster} lsearch {/etc/aliases} {$value}}

   This example uses NIS+ to look up the full name of the user corresponding
   to the local part of an address, forcing the expansion to fail if it is not
   found:

     ${lookup nisplus {[name=$local_part],passwd.org_dir:gcos} \
       {$value}fail}

${nhash{<string1>}{<string2>}{<string3>}}

   The three strings are expanded; the first two must yield numbers. Call them
   <n> and <m>. If you are using fixed values for these numbers, that is, if
   <string1> and <string2> do not change when they are expanded, you can use
   the simpler operator notation that avoids some of the braces:

     ${nhash_<n>_<m>:<string>}

   The second number is optional (in both notations). If there is only one
   number, the result is a number in the range 0-<n>-1. Otherwise, the string
   is processed by a div/mod hash function that returns two numbers, separated
   by a slash, in the ranges 0 to <n>-1 and 0 to <m>-1, respectively. For
   example,

     ${nhash{8}{64}{supercalifragilisticexpialidocious}}

   returns the string '6/33'.

${perl{<subroutine>}{<arg>}{<arg>}...}

   This item is available only if Exim has been built to include an embedded
   Perl interpreter. The subroutine name and the arguments are first separ-
   ately expanded, and then the Perl subroutine is called with those argu-
   ments. No additional arguments need be given; the maximum number permitted,
   including the name of the subroutine, is nine.

   The return value of the subroutine is inserted into the expanded string,
   unless the return value is "undef". In that case, the expansion fails in
   the same way as an explicit 'fail' on a lookup item. The return value is a
   scalar. Whatever you return is evaluated in a scalar context. For example,
   if you return the name of a Perl vector, the return value is the size of
   the vector, not its contents.

   If the subroutine exits by calling Perl's "die" function, the expansion
   fails with the error message that was passed to "die". More details of the
   embedded Perl facility are given in chapter 12.

   The redirect router has an option called "forbid_filter_perl" which locks
   out the use of this expansion item in filter files.

${readfile{<file name>}{<eol string>}}

   The file name and end-of-line string are first expanded separately. The
   file is then read, and its contents replace the entire item. All newline
   characters in the file are replaced by the end-of-line string if it is
   present. Otherwise, newlines are left in the string. String expansion is
   not applied to the contents of the file. If you want this, you must wrap
   the item in an "expand" operator. If the file cannot be read, the string
   expansion fails.

   The redirect router has an option called "forbid_filter_readfile" which
   locks out the use of this expansion item in filter files.

${readsocket{<name>}{<request>}{<timeout>}{<eol string>}{<fail string>}}

   This item inserts data that is read from a Unix domain socket into the
   expanded string. The minimal way of using it uses just two arguments:

     ${readsocket{/socket/name}{request string}}

   Exim connects to the socket, writes the request string (unless it is an
   empty string) and reads from the socket until an end-of-file is read. A
   timeout of 5 seconds is applied. Additional, optional arguments extend what
   can be done. Firstly, you can vary the timeout. For example:

     ${readsocket{/socket/name}{request-string}{3s}}

   A fourth argument allows you to change any newlines that are in the data
   that is read, in the same way as for "readfile" (see above). This example
   turns them into spaces:

     ${readsocket{/socket/name}{request-string}{3s}{ }}

   As with all expansions, the substrings are expanded before the processing
   happens. Errors in these sub-expansions cause the expansion to fail. In
   addition, the following errors can occur:

    .   Failure to create a socket file descriptor;

    .   Failure to connect the socket;

    .   Failure to write the request-string;

    .   Timeout on reading from the socket.

   By default, any of these errors causes the expansion to fail. However, if
   you supply a fifth substring, it is expanded and used when any of the above
   errors occurs. For example:

     ${readsocket{/socket/name}{request-string}{3s}{\n}\
       {socket failure}}

   You can test for the existence of the socket by wrapping this expansion in
   "${if exists", but there is a race condition between that test and the
   actual opening of the socket, so it is safer to use the fifth argument if
   you want to be absolutely sure of avoiding an expansion error for a non-
   existent socket.

   The redirect router has an option called "forbid_filter_readsocket" which
   locks out the use of this expansion item in filter files.

$rheader_<header name>: or $rh_<header name>:

   This item inserts 'raw' header lines. It is described with the "header"
   expansion item above.

${run{<command> <args>}{<string1>}{<string2>}}

   The command and its arguments are first expanded separately, and then the
   command is run in a separate process, but under the same uid and gid. As in
   other command executions from Exim, a shell is not used by default. If you
   want a shell, you must explicitly code it. If the command succeeds (gives a
   zero return code) <string1> is expanded and replaces the entire item;
   during this expansion, the standard output from the command is in the
   variable $value. If the command fails, <string2>, if present, is expanded.
   If it is absent, the result is empty. Alternatively, <string2> can be the
   word 'fail' (not in braces) to force expansion failure if the command does
   not succeed. If both strings are omitted, the result is the standard output
   on success, and nothing on failure.

   The return code from the command is put in the variable $runrc, and this
   remains set afterwards, so in a filter file you can do things like this:

     if "${run{x y z}{}}$runrc" is 1 then ...
       elif $runrc is 2 then ...
       ...
     endif

   If execution of the command fails (for example, the command does not        |
   exist), the return code is 127 - the same code that shells use for non-     |
   existent commands.                                                          |

   Warning: In a router or transport, you cannot assume the order in which
   option values are expanded, except for those pre-conditions whose order of
   testing is documented. Therefore, you cannot reliably expect to set $runrc
   by the expansion of one option, and use it in another.

   The redirect router has an option called "forbid_filter_run" which locks
   out the use of this expansion item in filter files.

${sg{<subject>}{<regex>}{<replacement>}}

   This item works like Perl's substitution operator (s) with the global (/g)
   option; hence its name. However, unlike the Perl equivalent, Exim does not
   modify the subject string; instead it returns the modified string for
   insertion into the overall expansion. The item takes three arguments: the
   subject string, a regular expression, and a substitution string. For
   example

     ${sg{abcdefabcdef}{abc}{xyz}}

   yields 'xyzdefxyzdef'. Because all three arguments are expanded before use,
   if any $ or \ characters are required in the regular expression or in the
   substitution string, they have to be escaped. For example

     ${sg{abcdef}{^(...)(...)\$}{\$2\$1}}

   yields 'defabc', and

     ${sg{1=A 4=D 3=C}{\N(\d+)=\N}{K\$1=}}

   yields 'K1=A K4=D K3=C'. Note the use of "\N" to protect the contents of
   the regular expression from string expansion.

${substr{<string1>}{<string2>}{<string3>}}

   The three strings are expanded; the first two must yield numbers. Call them
   <n> and <m>. If you are using fixed values for these numbers, that is, if
   <string1> and <string2> do not change when they are expanded, you can use
   the simpler operator notation that avoids some of the braces:

     ${substr_<n>_<m>:<string>}

   The second number is optional (in both notations).

   The "substr" item can be used to extract more general substrings than
   "length". The first number, <n>, is a starting offset, and <m> is the
   length required. For example

     ${substr{3}{2}{$local_part}}

   If the starting offset is greater than the string length the result is the
   null string; if the length plus starting offset is greater than the string
   length, the result is the right-hand part of the string, starting from the
   given offset. The first character in the string has offset zero.

   The "substr" expansion item can take negative offset values to count from
   the right-hand end of its operand. The last character is offset -1, the
   second-last is offset -2, and so on. Thus, for example,

     ${substr{-5}{2}{1234567}}

   yields '34'. If the absolute value of a negative offset is greater than the
   length of the string, the substring starts at the beginning of the string,
   and the length is reduced by the amount of overshoot. Thus, for example,

     ${substr{-5}{2}{12}}

   yields an empty string, but

     ${substr{-3}{2}{12}}

   yields '1'.

   If the second number is omitted from "substr", the remainder of the string
   is taken if the offset was positive. If it was negative, all characters in
   the string preceding the offset point are taken. For example, an offset of
   -1 and no length yields all but the last character of the string.

${tr{<subject>}{<characters>}{<replacements>}}

   This item does single-character translation on its subject string. The
   second argument is a list of characters to be translated in the subject
   string. Each matching character is replaced by the corresponding character
   from the replacement list. For example

     ${tr{abcdea}{ac}{13}}

   yields '1b3de1'. If there are duplicates in the second character string,
   the last occurrence is used. If the third string is shorter than the
   second, its last character is replicated. However, if it is empty, no
   translation takes place.


11.5 Expansion operators

For expansion items that perform transformations on a single argument string,
the 'operator' notation is used because it is simpler and uses fewer braces.
The substring is first expanded before the operation is applied to it. The
following operations can be performed:

${address:<string>}

   The string is interpreted as an RFC 2822 address, as it might appear in a
   header line, and the effective address is extracted from it. If the string
   does not parse successfully, the result is empty.

${base62:<digits>}

   The string must consist entirely of decimal digits. The number is converted
   to base 62 (sic) and output as a string of six characters, including
   leading zeros. Note: Just to be absolutely clear: this is "not" base64
   encoding.

${domain:<string>}

   The string is interpreted as an RFC 2822 address and the domain is
   extracted from it. If the string does not parse successfully, the result is
   empty.

${escape:<string>}

   If the string contains any non-printing characters, they are converted to
   escape sequences starting with a backslash. Whether characters with the
   most significant bit set (so-called '8-bit characters') count as printing
   or not is controlled by the "print_topbitchars" option.

${eval:<string>}

   This item supports simple arithmetic in expansion strings. The string
   (after expansion) must be a conventional arithmetic expression, but it is
   limited to the four basic operators (plus, minus, times, divide) and
   parentheses. All operations are carried out using integer arithmetic. Plus
   and minus have a lower priority than times and divide; operators with the
   same priority are evaluated from left to right. Numbers may be decimal,
   octal (starting with '0') or hexadecimal (starting with '0x'). A number may
   be followed by 'K' or 'M' to multiply it by 1024 or 1024*1024, respect-
   ively. Negative numbers are supported. The result of the computation is a
   decimal representation of the answer (without 'K' or 'M'). For example:

       ${eval:1+1}       yields 2
       ${eval:1+2*3}     yields 7
       ${eval:(1+2)*3}   yields 9

   As a more realistic example, in an ACL you might have

     deny   message = Too many bad recipients
            condition =                    \
              ${if and {                   \
                {>{$rcpt_count}{10}}       \
                {                          \
                <                          \
                  {$recipients_count}      \
                  {${eval:$rcpt_count/2}}  \
                }                          \
              }{yes}{no}}

   The condition is true if there have been more than 10 RCPT commands and
   fewer than half of them have resulted in a valid recipient.

${expand:<string>}

   The "expand" operator causes a string to be expanded for a second time. For
   example,

     ${expand:${lookup{$domain}dbm{/some/file}{$value}}}

   first looks up a string in a file while expanding the operand for "expand",
   and then re-expands what it has found.

${from_utf8:<string>}

   The world is slowly moving towards Unicode, although there are no standards
   for email yet. However, other applications (including some databases) are
   starting to store data in Unicode, using UTF-8 encoding. This operator
   converts from a UTF-8 string to an ISO-8859-1 string. UTF-8 code values
   greater than 255 are converted to underscores. The input must be a valid
   UTF-8 string. If it is not, the result is an undefined sequence of bytes.

   Unicode code points with values less than 256 are compatible with ASCII and
   ISO-8859-1 (also known as Latin-1). For example, character 169 is the       |
   copyright symbol in both cases, though the way it is encoded is different.  |
   In UTF-8, more than one byte is needed for characters with code values      |
   greater than 127, whereas ISO-8859-1 is a single-byte encoding (but thereby |
   limited to 256 characters). This makes translation from UTF-8 to ISO-8859-1 |
   straightforward.                                                            |

${hash_<n>_<m>:<string>}

   The "hash" operator is a simpler interface to the hashing function that can
   be used when the two parameters are fixed numbers (as opposed to strings
   that change when expanded). The effect is the same as

     ${hash{<n>}{<m>}{<string>}}

   See the description of the general "hash" item above for details. The
   abbreviation "h" can be used when "hash" is used as an operator.

${hex2b64:<hexstring>}

   This operator converts a hex string into one that is base64 encoded. This
   can be useful for processing the output of the MD5 and SHA-1 hashing
   functions.

${lc:<string>}

   This forces the letters in the string into lower-case, for example:

     ${lc:$local_part}

${length_<number>:<string>}

   The "length" operator is a simpler interface to the "length" function that
   can be used when the parameter is a fixed number (as opposed to a string
   that changes when expanded). The effect is the same as

     ${length{<number>}{<string>}}

   See the description of the general "length" item above for details. Note
   that "length" is not the same as "strlen". The abbreviation "l" can be used
   when "length" is used as an operator.

${local_part:<string>}

   The string is interpreted as an RFC 2822 address and the local part is
   extracted from it. If the string does not parse successfully, the result is
   empty.

${mask:<IP address>/<bit count>}

   If the form of the string to be operated on is not an IP address followed
   by a slash and an integer (that is, a network address in CIDR notation),
   the expansion fails. Otherwise, this operator converts the IP address to
   binary, masks off the least significant bits according to the bit count,
   and converts the result back to text, with mask appended. For example,

     ${mask:10.111.131.206/28}

   returns the string '10.111.131.192/28'. Since this operation is expected to
   be mostly used for looking up masked addresses in files, the result for an
   IPv6 address uses dots to separate components instead of colons, because
   colon terminates a key string in lsearch files. So, for example,

     ${mask:3ffe:ffff:836f:0a00:000a:0800:200a:c031/99}

   returns the string

     3ffe.ffff.836f.0a00.000a.0800.2000.0000/99

   Letters in IPv6 addresses are always output in lower case.

${md5:<string>}

   The "md5" operator computes the MD5 hash value of the string, and returns
   it as a 32-digit hexadecimal number, in which any letters are in lower
   case.

${nhash_<n>_<m>:<string>}

   The "nhash" operator is a simpler interface to the numeric hashing function
   that can be used when the two parameters are fixed numbers (as opposed to
   strings that change when expanded). The effect is the same as

     ${nhash{<n>}{<m>}{<string>}}

   See the description of the general "nhash" item above for details.

${quote:<string>}

   The "quote" operator puts its argument into double quotes if it is an empty |
   string or contains anything other than letters, digits, underscores, dots,  |
   and hyphens. Any occurrences of double quotes and backslashes are escaped
   with a backslash. Newlines and carriage returns are converted to "\n" and
   "\r", respectively For example,

     ${quote:ab"*"cd}

   becomes

     "ab\"*\"cd"

   The place where this is useful is when the argument is a substitution from
   a variable or a message header.

${quote_local_part:<string>}                                                   |
                                                                               |
   This operator is like "quote", except that it quotes the string only if     |
   required to do so by the rules of RFC 2822 for quoting local parts. For     |
   example, a plus sign would not cause quoting (but it would for "quote"). If |
   you are creating a new email address from the contents of $local_part (or   |
   any other unknown data), you should always use this operator.               |

${quote_<lookup-type>:<string>}

   This operator applies lookup-specific quoting rules to the string. Each
   query-style lookup type has its own quoting rules which are described with
   the lookups in chapter 9. For example,

     ${quote_ldap:two * two}

   returns

     two%20%5C2A%20two

   For single-key lookup types, no quoting is ever necessary and this operator
   yields an unchanged string.

${rxquote:<string>}

   The "rxquote" operator inserts a backslash before any non-alphanumeric
   characters in its argument. This is useful when substituting the values of
   variables or headers inside regular expressions.

${rfc2047:<string>}

   This operator encodes text according to the rules of RFC 2047. This is an   |
   encoding that is used in header lines to encode non-ASCII characters. It is |
   assumed that the input string is in the encoding specified by the           |
   "headers_charset" option, which defaults to ISO-8859-1. If the string       |
   contains only characters in the range 33-126, and no instances of the
   characters

     ? = ( ) < > @ , ; : \ " . [ ] _

   it is not modified. Otherwise, the result is the RFC 2047 encoding, as a
   single 'coded word'.

${sha1:<string>}

   The "sha1" operator computes the SHA-1 hash value of the string, and
   returns it as a 40-digit hexadecimal number, in which any letters are in
   upper case.

${stat:<string>}

   The string, after expansion, must be a file path. A call to the "stat()"
   function is made for this path. If "stat()" fails, an error occurs and the
   expansion fails. If it succeeds, the data from the stat replaces the item,
   as a series of <name>=<value> pairs, where the values are all numerical,
   except for the value of 'smode'. The names are: 'mode' (giving the mode as
   a 4-digit octal number), 'smode' (giving the mode in symbolic format as a
   10-character string, as for the "ls" command), 'inode', 'device', 'links',
   'uid', 'gid', 'size', 'atime', 'mtime', and 'ctime'. You can extract
   individual fields using the "extract" expansion item. Warning: The file
   size may be incorrect on 32-bit systems for files larger than 2GB.

${strlen:<string>}

   The item is replace by the length of the expanded string, expressed as a
   decimal number. Note: Do not confuse "strlen" with "length".

${substr_<start>_<length>:<string>}

   The "substr" operator is a simpler interface to the "substr" function that
   can be used when the two parameters are fixed numbers (as opposed to
   strings that change when expanded). The effect is the same as

     ${substr{<start>}{<length>}{<string>}}

   See the description of the general "substr" item above for details. The
   abbreviation "s" can be used when "substr" is used as an operator.

${uc:<string>}

   This forces the letters in the string into upper-case.


11.6 Expansion conditions

The following conditions are available for testing by the "${if" construct
while expanding strings:

!<condition>

   Preceding any condition with an exclamation mark negates the result of the
   condition.

<symbolic operator> {<string1>}{<string2>}

   There are a number of symbolic operators for doing numeric comparisons.
   They are:

     =    equal
     ==   equal
     >    greater
     >=   greater or equal
     <    less
     <=   less or equal

   For example,

     ${if >{$message_size}{10M} ...

   Note that the general negation operator provides for inequality testing.
   The two strings must take the form of optionally signed decimal integers,
   optionally followed by one of the letters 'K' or 'M' (in either upper or
   lower case), signifying multiplication by 1024 or 1024*1024, respectively.

crypteq {<string1>}{<string2>}

   This condition is included in the Exim binary if it is built to support any
   authentication mechanisms (see chapter 33). Otherwise, it is necessary to
   define SUPPORT_CRYPTEQ in Local/Makefile to get "crypteq" included in the
   binary.

   The "crypteq" condition has two arguments. The first is encrypted and
   compared against the second, which is already encrypted. The second string
   may be in the LDAP form for storing encrypted strings, which starts with
   the encryption type in curly brackets, followed by the data. If the second
   string does not begin with '{' it is assumed to be encrypted with "crypt()"
   or "crypt16()" (see below), since such strings cannot begin with '{'.
   Typically this will be a field from a password file.

   An example of an encrypted string in LDAP form is:

     {md5}CY9rzUYh03PK3k6DJie09g==

   If such a string appears directly in an expansion, the curly brackets have
   to be quoted, because they are part of the expansion syntax. For example:

     ${if crypteq {test}{\{md5\}CY9rzUYh03PK3k6DJie09g==}{yes}{no}}

   The following encryption types (whose names are matched case-independently) |
   are supported:

    .   "{md5}" computes the MD5 digest of the first string, and expresses
        this as printable characters to compare with the remainder of the
        second string. If the length of the comparison string is 24, Exim
        assumes that it is base64 encoded (as in the above example). If the
        length is 32, Exim assumes that it is a hexadecimal encoding of the
        MD5 digest. If the length not 24 or 32, the comparison fails.

    .   "{sha1}" computes the SHA-1 digest of the first string, and expresses
        this as printable characters to compare with the remainder of the
        second string. If the length of the comparison string is 28, Exim
        assumes that it is base64 encoded. If the length is 40, Exim assumes
        that it is a hexadecimal encoding of the SHA-1 digest. If the length
        is not 28 or 40, the comparison fails.

    .   "{crypt}" calls the "crypt()" function, which uses only the first
        eight characters of the password.

    .   "{crypt16}" calls the "crypt16()" function (also known as
        "bigcrypt()"), which uses up to 16 characters of the password.

   Exim has its own version of "crypt16()" (which is just a double call to
   "crypt()"). For operating systems that have their own version, setting
   HAVE_CRYPT16 in Local/Makefile when building Exim causes it to use the
   operating system version instead of its own. This option is set by default
   in the OS-dependent Makefile for those operating systems that are known to
   support "crypt16()".

   If you do not put any curly bracket encryption type in a "crypteq"
   comparison, the default is either "{crypt}" or "{crypt16}", as determined
   by the setting of DEFAULT_CRYPT in Local/Makefile. The default default is
   "{crypt}". Whatever the default, you can always use either function by
   specifying it explicitly in curly brackets.

   Note that if a password is no longer than 8 characters, the results of
   encrypting it with "crypt()" and "crypt16()" are identical. That means that
   "crypt16()" is backwards compatible, as long as nobody feeds it a password
   longer than 8 characters.

def:<variable name>

   The "def" condition must be followed by the name of one of the expansion
   variables defined in section 11.8. The condition is true if the named
   expansion variable does not contain the empty string, for example

     ${if def:sender_ident {from $sender_ident}}

   Note that the variable name is given without a leading "$" character. If
   the variable does not exist, the expansion fails.

def:header_<header name>:  or  def:h_<header name>:

   This condition is true if a message is being processed and the named header
   exists in the message. For example,

     ${if def:header_reply-to:{$h_reply-to:}{$h_from:}}

   Note that no "$" appears before "header_" or "h_" in the condition, and
   that header names must be terminated by colons if white space does not
   follow.

eq {<string1>}{<string2>}                                                      |
                                                                               |
eqi {<string1>}{<string2>}                                                     |
                                                                               |
   The two substrings are first expanded. The condition is true if the two     |
   resulting strings are identical: for "eq" the comparison includes the case  |
   of letters, whereas for "eqi" the comparison is case-independent.           |

exists {<file name>}

   The substring is first expanded and then interpreted as an absolute path.
   The condition is true if the named file (or directory) exists. The
   existence test is done by calling the "stat()" function. The use of the
   "exists" test in users' filter files may be locked out by the system
   administrator.

first_delivery

   This condition, which has no data, is true during a message's first
   delivery attempt. It is false during any subsequent delivery attempts.

isip {<string>}                                                                |
                                                                               |
isip4 {<string>}                                                               |
                                                                               |
isip6 {<string>}                                                               |
                                                                               |
   The substring is first expanded, and then tested to see if it has the form  |
   of an IP address. Both IPv4 and IPv6 addresses are valid for "isip",        |
   whereas "isip4" and "isip6" test just for IPv4 or IPv6 addresses, respect-  |
   ively. For example, you could use                                           |
                                                                               |
     ${if isip4{$sender_host_address}...                                       |
                                                                               |
   to test which version of IP an incoming SMTP connection is using.           |

ldapauth {<ldap query>}

   This condition supports user authentication using LDAP. See section 9.10
   for details of how to use LDAP in lookups and the syntax of queries. For
   this use, the query must contain a user name and password. The query itself
   is not used, and can be empty. The condition is true if the password is not
   empty, and the user name and password are accepted by the LDAP server. An
   empty password is rejected without calling LDAP because LDAP binds with an
   empty password are considered anonymous regardless of the username, and
   will succeed in most configurations. See chapter 33 for details of SMTP
   authentication, and chapter 34 for an example of how this can be used.

match {<string1>}{<string2>}

   The two substrings are first expanded. The second is then treated as a
   regular expression and applied to the first. Because of the pre-expansion,
   if the regular expression contains dollar, or backslash characters, they
   must be escaped. Care must also be taken if the regular expression contains
   braces (curly brackets). A closing brace must be escaped so that it is not
   taken as a premature termination of <string2>. The easiest approach is to
   use the "\N" feature to disable expansion of the regular expression. For
   example,

     ${if match {$local_part}{\N^\d{3}\N} ...

   If the whole expansion string is in double quotes, further escaping of
   backslashes is also required.

   The condition is true if the regular expression match succeeds. At the
   start of an "if" expansion the values of the numeric variable substitutions
   $1 etc. are remembered. Obeying a "match" condition that succeeds causes
   them to be reset to the substrings of that condition and they will have
   these values during the expansion of the success string. At the end of the
   "if" expansion, the previous values are restored. After testing a combi-
   nation of conditions using "or", the subsequent values of the numeric
   variables are those of the condition that succeeded.

pam {<string1>:<string2>:...}

   "Pluggable Authentication Modules"
   (http://www.kernel.org/pub/linux/libs/pam/) are a facility which is avail-
   able in the latest releases of Solaris and in some GNU/Linux distributions.
   The Exim support, which is intended for use in conjunction with the SMTP
   AUTH command, is available only if Exim is compiled with

     SUPPORT_PAM=yes

   in Local/Makefile. You probably need to add -lpam to EXTRALIBS, and in some
   releases of GNU/Linux -ldl is also needed.

   The argument string is first expanded, and the result must be a colon-
   separated list of strings. Leading and trailing whitespace is ignored. The  |
   PAM module is initialized with the service name 'exim' and the user name
   taken from the first item in the colon-separated data string (<string1>).
   The remaining items in the data string are passed over in response to
   requests from the authentication function. In the simple case there will
   only be one request, for a password, so the data consists of just two
   strings.

   There can be problems if any of the strings are permitted to contain colon
   characters. In the usual way, these have to be doubled to avoid being taken
   as separators. If the data is being inserted from a variable, the "sg"
   expansion item can be used to double any existing colons. For example, the
   configuration of a LOGIN authenticator might contain this setting:

     server_condition = ${if pam{$1:${sg{$2}{:}{::}}}{yes}{no}}

   For a PLAIN authenticator you could use:

     server_condition = ${if pam{$2:${sg{$3}{:}{::}}}{yes}{no}}

   In some operating systems, PAM authentication can be done only from a
   process running as root. Since Exim is running as the Exim user when
   receiving messages, this means that PAM cannot be used directly in those
   systems. A patched version of the "pam_unix" module that comes with the
   Linux PAM package is available from http://www.e-admin.de/pam_exim/. The
   patched module allows one special uid/gid combination, in addition to root,
   to authenticate. If you build the patched module to allow the Exim user and
   group, PAM can then be used from an Exim authenticator.

pwcheck {<string1>:<string2>}

   This condition supports user authentication using the Cyrus "pwcheck"
   daemon. This is one way of making it possible for passwords to be checked
   by a process that is not running as root. Note: The use of "pwcheck" is now |
   deprecated. Its replacement is "saslauthd" (see below).                     |

   The pwcheck support is not included in Exim by default. You need to specify
   the location of the pwcheck daemon's socket in Local/Makefile before
   building Exim. For example:

     CYRUS_PWCHECK_SOCKET=/var/pwcheck/pwcheck

   You do not need to install the full Cyrus software suite in order to use
   the pwcheck daemon. You can compile and install just the daemon alone from
   the Cyrus SASL library. Ensure that "exim" is the only user that has access
   to the /var/pwcheck directory.

   The "pwcheck" condition takes one argument, which must be the user name and
   password, separated by a colon. For example, in a LOGIN authenticator
   configuration, you might have this:

     server_condition = ${if pwcheck{$1:$2}{1}{0}}

queue_running

   This condition, which has no data, is true during delivery attempts that
   are initiated by queue runner processes, and false otherwise.

radius {<authentication string>}

   Radius authentication (RFC 2865) is supported in a similar way to PAM. You
   must set RADIUS_CONFIG_FILE in Local/Makefile to specify the location of
   the Radius client configuration file in order to build Exim with Radius
   support. You may also have to supply a suitable setting in EXTRALIBS so
   that the Radius library can be found when Exim is linked. The string
   specified by RADIUS_CONFIG_FILE is expanded and passed to the Radius client
   library, which calls the Radius server. The condition is true if the
   authentication is successful. For example

     server_condition = ${if radius{<arguments>}{yes}{no}}

saslauthd {{<user>}{<password>}{<service>}{<realm>}}                           |
                                                                               |
   This condition supports user authentication using the Cyrus "saslauthd"     |
   daemon. This replaces the older "pwcheck" daemon, which is now deprecated.  |
   Using this daemon is one way of making it possible for passwords to be      |
   checked by a process that is not running as root.                           |
                                                                               |
   The saslauthd support is not included in Exim by default. You need to       |
   specify the location of the saslauthd daemon's socket in Local/Makefile     |
   before building Exim. For example:                                          |
                                                                               |
     CYRUS_SASLAUTHD_SOCKET=/var/state/saslauthd/mux                           |
                                                                               |
   You do not need to install the full Cyrus software suite in order to use    |
   the saslauthd daemon. You can compile and install just the daemon alone     |
   from the Cyrus SASL library.                                                |
                                                                               |
   Up to four arguments can be supplied to the "saslauthd" condition, but only |
   two are mandatory. For example:                                             |
                                                                               |
     server_condition = ${if saslauthd{{$1}{$2}}{1}{0}}                        |
                                                                               |
   The service and the realm are optional (which is why the arguments are      |
   enclosed in their own set of braces). For details of the meaning of the     |
   service and realm, and how to run the daemon, consult the Cyrus             |
   documentation.                                                              |


11.7 Combining expansion conditions

Several conditions can be tested at once by combining them using the "and" and
"or" combination conditions. Note that "and" and "or" are complete conditions
on their own, and precede their lists of sub-conditions. Each sub-condition
must be enclosed in braces within the overall braces that contain the list. No
repetition of "if" is used.

or {{<cond1>}{<cond2>}...}

   The sub-conditions are evaluated from left to right. The condition is true
   if any one of the sub-conditions is true. For example,

     ${if or {{eq{$local_part}{spqr}}{eq{$domain}{testing.com}}}...

   When a true sub-condition is found, the following ones are parsed but not
   evaluated. If there are several 'match' sub-conditions the values of the
   numeric variables afterwards are taken from the first one that succeeds.

and {{<cond1>}{<cond2>}...}

   The sub-conditions are evaluated from left to right. The condition is true
   if all of the sub-conditions are true. If there are several 'match' sub-
   conditions, the values of the numeric variables afterwards are taken from
   the last one. When a false sub-condition is found, the following ones are
   parsed but not evaluated.


11.8 Expansion variables

The variables that are available for use in expansion strings are:

$0, $1, etc: When a "match" expansion condition succeeds, these variables
  contain the captured substrings identified by the regular expression during
  subsequent processing of the success string of the containing "if" expansion
  item. They may also be set externally by some other matching process which
  precedes the expansion of the string. For example, the commands available in
  Exim filter files include an "if" command with its own regular expression
  matching condition.

$acl_c0 - $acl_c9: Values can be placed in these variables by the "set"
  modifier in an ACL. The values persist throughout the lifetime of an SMTP
  connection. They can be used to pass information between ACLs and different
  invocations of the same ACL. When a message is received, the values of these |
  variables are saved with the message, and can be accessed by filters,        |
  routers, and transports during subsequent delivery.                          |

$acl_m0 - $acl_m9: Values can be placed in these variables by the "set"
  modifier in an ACL. They retain their values while a message is being
  received, but are reset afterwards. They are also reset by MAIL, RSET, EHLO,
  HELO, and after starting a TLS session. When a message is received, the      |
  values of these variables are saved with the message, and can be accessed by |
  filters, routers, and transports during subsequent delivery.                 |

$acl_verify_message: During the expansion of the "message" and "log_message"   |
  modifiers in an ACL statement after an address verification has failed, this |
  variable contains the original failure message that will be overridden by    |
  the expanded string.                                                         |

$address_data: This variable is set by means of the "address_data" option in
  routers. The value then remains with the address while it is processed by
  subsequent routers and eventually a transport. If the transport is handling
  multiple addresses, the value from the first address is used. See chapter 15
  for more details. Note: the contents of $address_data are visible in user
  filter files.

  If $address_data is set when the routers are called to verify an address
  from an ACL, the final value remains available in subsequent conditions in
  the ACL statement. If routing the address caused it to be redirected to a
  single address, the child address is also routed as part of the verifi-
  cation, and in this case the final value of $address_data is from the
  child's routing.

$address_file: When, as a result of aliasing, forwarding, or filtering, a
  message is directed to a specific file, this variable holds the name of the
  file when the transport is running. At other times, the variable is empty.
  For example, using the default configuration, if user "r2d2" has a .forward
  file containing

    /home/r2d2/savemail

  then when the address_file transport is running, $address_file contains
  '/home/r2d2/savemail'. For Sieve filters, the value may be 'inbox' or a      |
  relative folder name. It is then up to the transport configuration to        |
  generate an appropriate absolute path to the relevant file.                  |

$address_pipe: When, as a result of aliasing or forwarding, a message is
  directed to a pipe, this variable holds the pipe command when the transport
  is running.

$authenticated_id: When a server successfully authenticates a client it may be
  configured to preserve some of the authentication information in the
  variable $authenticated_id (see chapter 33). For example, a user/password
  authenticator configuration might preserve the user name for use in the
  routers. When a message is submitted locally (that is, not over a TCP
  connection), the value of $authenticated_id is the login name of the calling
  process.

$authenticated_sender: Exim takes note of the AUTH= parameter on an SMTP MAIL  |
  command if it believes the sender is sufficiently trusted, as described in   |
  section 33.2. Unless the data is the string '<>', it is set as the           |
  authenticated sender of the message, and the value is available during       |
  delivery in the $authenticated_sender variable. If the sender is not         |
  trusted, Exim accepts the syntax of AUTH=, but ignores the data.             |

  When a message is submitted locally (that is, not over a TCP connection),
  the value of $authenticated_sender is an address constructed from the login
  name of the calling process and $qualify_domain.

$authentication_failed: This variable is set to '1' in an Exim server if a
  client issues an AUTH command that does not succeed. Otherwise it is set to
  '0'. This makes it possible to distinguish between 'did not try to
  authenticate' ($sender_host_authenticated is empty and $authentication_
  failed is set to '0') and 'tried to authenticate but failed'
  ($sender_host_authenticated is empty and $authentication_failed is set to
  '1'). Failure includes any negative response to an AUTH command, including
  (for example) an attempt to use an undefined mechanism.

$body_linecount: When a message is being received or delivered, this variable
  contains the number of lines in the message's body.

$bounce_recipient: This is set to the recipient address of a bounce message
  while Exim is creating it. It is useful if a customized bounce message text
  file is in use (see chapter 41).

$bounce_return_size_limit: This contains the value set in the                  |
  "bounce_return_size_limit" option, rounded up to a multiple of 1000. It is   |
  useful when a customized error message text file is in use (see chapter 41). |

$caller_gid: The group id under which the process that called Exim was
  running. This is not the same as the group id of the originator of a message
  (see $originator_gid). If Exim re-execs itself, this variable in the new
  incarnation normally contains the Exim gid.

$caller_uid: The user id under which the process that called Exim was running.
  This is not the same as the user id of the originator of a message (see
  $originator_uid). If Exim re-execs itself, this variable in the new incar-
  nation normally contains the Exim uid.

$compile_date: The date on which the Exim binary was compiled.

$compile_number: The building process for Exim keeps a count of the number of
  times it has been compiled. This serves to distinguish different compi-
  lations of the same version of the program.

$dnslist_domain: When a client host is found to be on a DNS (black) list, the
  list's domain name is put into this variable so that it can be included in
  the rejection message.

$dnslist_text: When a client host is found to be on a DNS (black) list, the
  contents of any associated TXT record are placed in this variable.

$dnslist_value: When a client host is found to be on a DNS (black) list, the
  IP address from the resource record is placed in this variable. If there are |
  multiple records, all the addresses are included, comma-space separated.     |

$domain: When an address is being routed, or delivered on its own, this
  variable contains the domain. Global address rewriting happens when a
  message is received, so the value of $domain during routing and delivery is
  the value after rewriting. $domain is set during user filtering, but not
  during system filtering, because a message may have many recipients and the
  system filter is called just once.

  When more than one address is being delivered at once (for example, several
  RCPT commands in one SMTP delivery), $domain is set only if they all have
  the same domain. Transports can be restricted to handling only one domain at
  a time if the value of $domain is required at transport time - this is the
  default for local transports. For further details of the environment in
  which local transports are run, see chapter 23.

  At the end of a delivery, if all deferred addresses have the same domain, it
  is set in $domain during the expansion of "delay_warning_condition".

  The $domain variable is also used in some other circumstances:

   .   When an ACL is running for a RCPT command, $domain contains the domain
       of the recipient address. Note: the domain of the sender address is in  |
       $sender_address_domain at MAIL time and at RCPT time. $domain is not    |
       set for the MAIL ACL.                                                   |

   .   When a rewrite item is being processed (see chapter 31), $domain
       contains the domain portion of the address that is being rewritten; it
       can be used in the expansion of the replacement address, for example,
       to rewrite domains by file lookup.

   .   With one important exception, whenever a domain list is being scanned,  |
       $domain contains the subject domain. Exception: When a domain list in a |
       "sender_domains" condition in an ACL is being processed, the subject    |
       domain is in $sender_address_domain and not in $domain. It works this   |
       way so that, in a RCPT ACL, the sender domain list can be dependent on  |
       the recipient domain (which is what is in $domain at this time).        |

   .   When the "smtp_etrn_command" option is being expanded, $domain contains
       the complete argument of the ETRN command (see section 43.9).

$domain_data: When the "domains" option on a router matches a domain by means
  of a lookup, the data read by the lookup is available during the running of
  the router as $domain_data. In addition, if the driver routes the address to
  a transport, the value is available in that transport. If the transport is
  handling multiple addresses, the value from the first address is used.

  $domain_data is also set when the "domains" condition in an ACL matches a
  domain by means of a lookup. The data read by the lookup is available during
  the rest of the ACL statement. In all other situations, this variable
  expands to nothing.

$header_<name>: This is not strictly an expansion variable. It is expansion
  syntax for inserting the message header line with the given name. Note that
  the name must be terminated by colon or white space, because it may contain
  a wide variety of characters. Note also that braces must "not" be used.

$home: When the "check_local_user" option is set for a router, the user's home
  directory is placed in $home when the check succeeds. In particular, this
  means it is set during the running of users' filter files. A router may also
  explicitly set a home directory for use by a transport; this can be
  overridden by a setting on the transport itself.

  When running a filter test via the -bf option, $home is set to the value of
  the environment variable HOME.

$host: When the smtp transport is expanding its options for encryption using
  TLS, $host contains the name of the host to which it is connected. Likewise,
  when used in the client part of an authenticator configuration (see chapter
  33), $host contains the name of the server to which the client is connected.
  When used in a transport filter (see chapter 24) $host refers to the host
  involved in the current connection. When a local transport is run as a
  result of a router that sets up a host list, $host contains the name of the
  first host.

$host_address: This variable is set to the remote host's IP address whenever
  $host is set for a remote connection.

$host_data: If a "hosts" condition in an ACL is satisfied by means of a
  lookup, the result of the lookup is made available in the $host_data
  variable. This allows you, for example, to do things like this:

    deny  hosts = net-lsearch;/some/file
          message = $host_data

$host_lookup_failed: This variable contains '1' if the message came from a
  remote host and there was an attempt to look up the host's name from its IP
  address, but the attempt failed. Otherwise the value of the variable is '0'.

$inode: The only time this variable is set is while expanding the
  "directory_file" option in the appendfile transport. The variable contains
  the inode number of the temporary file which is about to be renamed. It can
  be used to construct a unique name for the file.

$interface_address: When a message is received over a TCP/IP connection, this
  variable contains the address of the local IP interface. See also the -oMi
  command line option. This variable can be used in ACLs and also, for         |
  example, to make the file name for a TLS certificate depend on which         |
  interface is being used.                                                     |

$interface_port: When a message is received over a TCP/IP connection, this
  variable contains the local port number. See also the -oMi command line
  option. This variable can be used in ACLs and also, for example, to make the |
  file name for a TLS certificate depend on which port is being used.          |

$ldap_dn: This variable, which is available only when Exim is compiled with
  LDAP support, contains the DN from the last entry in the most recently
  successful LDAP lookup.

$load_average: This variable contains the system load average, multiplied by
  1000 to that it is an integer. For example, if the load average is 0.21, the
  value of the variable is 210. The value is recomputed every time the
  variable is referenced.

$local_part: When an address is being routed, or delivered on its own, this
  variable contains the local part. When a number of addresses are being
  delivered together (for example, multiple RCPT commands in an SMTP session),
  $local_part is not set.

  Global address rewriting happens when a message is received, so the value of
  $local_part during routing and delivery is the value after rewriting.
  $local_part is set during user filtering, but not during system filtering,
  because a message may have many recipients and the system filter is called
  just once.

  If a local part prefix or suffix has been recognized, it is not included in
  the value of $local_part during routing and subsequent delivery. The values
  of any prefix or suffix are in $local_part_prefix and $local_part_suffix,
  respectively.

  When a message is being delivered to a file, pipe, or autoreply transport as
  a result of aliasing or forwarding, $local_part is set to the local part of
  the parent address, not to the file name or command (see $address_file and
  $address_pipe).

  When an ACL is running for a RCPT command, $local_part contains the local
  part of the recipient address.

  When a rewrite item is being processed (see chapter 31), $local_part
  contains the local part of the address that is being rewritten; it can be
  used in the expansion of the replacement address, for example.

  In all cases, all quoting is removed from the local part. For example, for
  both the addresses

    "abc:xyz"@test.example
    abc\:xyz@test.example

  the value of $local_part is

    abc:xyz

  If you use $local_part to create another address, you should always wrap it
  inside a quoting operator. For example, in a redirect router you could have:

    data = ${quote_local_part:$local_part}@new.domain.example                  |

$local_part_data: When the "local_parts" option on a router matches a local
  part by means of a lookup, the data read by the lookup is available during
  the running of the router as $local_part_data. In addition, if the driver
  routes the address to a transport, the value is available in that transport.
  If the transport is handling multiple addresses, the value from the first
  address is used.

  $local_part_data is also set when the "local_parts" condition in an ACL
  matches a local part by means of a lookup. The data read by the lookup is
  available during the rest of the ACL statement. In all other situations,
  this variable expands to nothing.

$local_part_prefix: When an address is being routed or delivered, and a
  specific prefix for the local part was recognized, it is available in this
  variable, having been removed from $local_part.

$local_part_suffix: When an address is being routed or delivered, and a
  specific suffix for the local part was recognized, it is available in this
  variable, having been removed from $local_part.

$local_scan_data: This variable contains the text returned by the
  "local_scan()" function when a message is received. See chapter 39 for more
  details.

$local_user_uid: See $local_user_uid.                                          |
                                                                               |
$local_user_uid: This variable and $local_user_gid are set to the uid and gid  |
  after the "check_local_user" router precondition succeeds. This means that   |
  their values are available for the remaining preconditions ("senders",       |
  "require_files", and "condition"), for the "address_data" expansion, and for |
  any router-specific expansions. At all other times, the values in these      |
  variables are "(uid_t)(-1)" and "(gid_t)(-1)", respectively.                 |

$localhost_number: This contains the expanded value of the "localhost_number"
  option. The expansion happens after the main options have been read.

$mailstore_basename: This variable is set only when doing deliveries in        |
  'mailstore' format in the appendfile transport. During the expansion of      |
  the "mailstore_prefix", "mailstore_suffix", "message_prefix", and            |
  "message_suffix" options, it contains the basename of the files that are     |
  being written, that is, the name without the '.tmp', '.env', or '.msg'       |
  suffix. At all other times, this variable is empty.                          |

$message_age: This variable is set at the start of a delivery attempt to
  contain the number of seconds since the message was received. It does not
  change during a single delivery attempt.

$message_body: This variable contains the initial portion of a message's body
  while it is being delivered, and is intended mainly for use in filter files.
  The maximum number of characters of the body that are put into the variable
  is set by the "message_body_visible" configuration option; the default is
  500. Newlines are converted into spaces to make it easier to search for
  phrases that might be split over a line break. Binary zeros are also
  converted into spaces.

$message_body_end: This variable contains the final portion of a message's
  body while it is being delivered. The format and maximum size are as for
  $message_body.

$message_body_size: When a message is being processed, this variable contains
  the size of the body in bytes. The count starts from the character after the
  blank line that separates the body from the header. Newlines are included in
  the count. See also $message_size and $body_linecount.

$message_headers: This variable contains a concatenation of all the header
  lines when a message is being processed, except for lines added by routers
  or transports. The header lines are separated by newline characters.

$message_id: When a message is being received or delivered, this variable
  contains the unique message id that is used by Exim to identify the message.
  An id is not allocated to a message until after its header has been
  successfully received.

$message_size: When a message is being processed, this variable contains its
  size in bytes. In most cases, the size includes those headers that were
  received with the message, but not those (such as Envelope-to:) that are
  added to individual deliveries as they are written. However, there is one
  special case: during the expansion of the "maildir_tag" option in the
  appendfile transport while doing a delivery in maildir format, the value of
  $message_size is the precise size of the file that has been written. See
  also $message_body_size and $body_linecount.

  While running an ACL at the time of an SMTP RCPT command, $message_size
  contains the size supplied on the MAIL command, or -1 if no size was given.  |
  The value may not, of course, be truthful.

$n0 - $n9: These variables are counters that can be incremented by means of
  the "add" command in filter files.

$original_domain: When a top-level address is being processed for delivery,
  this contains the same value as $domain. However, if a 'child' address (for
  example, generated by an alias, forward, or filter file) is being processed,
  this variable contains the domain of the original address. This differs from
  $parent_domain only when there is more than one level of aliasing or
  forwarding. When more than one address is being delivered in a single
  transport run, $original_domain is not set.

  If new an address is created by means of a "deliver" command in a system
  filter, it is set up with an artificial 'parent' address. This has the local
  part "system-filter" and the default qualify domain.

$original_local_part: When a top-level address is being processed for
  delivery, this contains the same value as $local_part, unless a prefix or
  suffix was removed from the local part, in which case $original_local_part
  contains the full local part. When a 'child' address (for example, generated
  by an alias, forward, or filter file) is being processed, this variable
  contains the full local part of the original address. If the router that did
  the redirection processed the local part case-insensitively, the value in
  $original_local_part is in lower case. This variable differs from
  $parent_local_part only when there is more than one level of aliasing or
  forwarding. When more than one address is being delivered in a single
  transport run, $original_local_part is not set.

  If new an address is created by means of a "deliver" command in a system
  filter, it is set up with an artificial 'parent' address. This has the local
  part "system-filter" and the default qualify domain.

$originator_gid: The value of $caller_gid that was set when the message was
  received. For messages received via the command line, this is the gid of the
  sending user. For messages received by SMTP over TCP/IP, this is normally
  the gid of the Exim user.

$originator_uid: The value of $caller_uid that was set when the message was
  received. For messages received via the command line, this is the uid of the
  sending user. For messages received by SMTP over TCP/IP, this is normally
  the uid of the Exim user.

$parent_domain: This variable is similar to $original_domain (see above),
  except that it refers to the immediately preceding parent address.

$parent_local_part: This variable is similar to $original_local_part (see
  above), except that it refers to the immediately preceding parent address.

$pid: This variable contains the current process id.

$pipe_addresses: This is not an expansion variable, but is mentioned here
  because the string '$pipe_addresses' is handled specially in the command
  specification for the pipe transport (chapter 29) and in transport filters
  (described under "transport_filter" in chapter 24). It cannot be used in
  general expansion strings, and provokes an 'unknown variable' error if
  encountered.

$primary_hostname: The value set in the configuration file, or read by the
  "uname()" function. If "uname()" returns a single-component name, Exim calls
  "gethostbyname()" (or "getipnodebyname()" where available) in an attempt to
  acquire a fully qualified host name.

$qualify_domain: The value set for this option in the configuration file.

$qualify_recipient: The value set for this option in the configuration file,
  or if not set, the value of $qualify_domain.

$rcpt_count: When a message is being received by SMTP, this variable contains
  the number of RCPT commands received for the current message. If this
  variable is used in a RCPT ACL, its value includes the current command.

$rcpt_defer_count: When a message is being received by SMTP, this variable
  contains the number of RCPT commands in the current message that have
  previously been rejected with a temporary (4"xx") response.

$rcpt_fail_count: When a message is being received by SMTP, this variable
  contains the number of RCPT commands in the current message that have
  previously been rejected with a permanent (5"xx") response.

$received_count: This variable contains the number of Received: header lines   |
  in the message, including the one added by Exim (so its value is always      |
  greater than zero). It is available in the DATA ACL, the non-SMTP ACL, and   |
  while routing and delivering.                                                |

$received_for: If there is only a single recipient address in an incoming
  message, this variable contains that address when the Received: header line
  is being built.

$received_protocol: When a message is being processed, this variable contains
  the name of the protocol by which it was received. See also the -oMr option.

$recipients: This variable contains a list of envelope recipients for a
  message. A comma and a space separate the addresses in the replacement text.
  However, the variable is not generally available, to prevent exposure of Bcc
  recipients in unprivileged users' filter files. You can use $recipients only

  (1)  In a system filter file.

  (2)  In the DATA or non-SMTP ACL, that is, in the final ACL for accepting a
       message.

$recipients_count: When a message is being processed, this variable contains
  the number of envelope recipients that came with the message. Duplicates are
  not excluded from the count. While a message is being received over SMTP,
  the number increases for each accepted recipient. It can be referenced in an
  ACL.

$reply_address: When a message is being processed, this variable contains the
  contents of the Reply-To: header line if one exists and it is not empty, or
  otherwise the contents of the From: header line.

$return_path: When a message is being delivered, this variable contains the
  return path - the sender field that will be sent as part of the envelope. It
  is not enclosed in <> characters. At the start of routing an address,        |
  $return_path has the same value as $sender_address, but if, for example, an
  incoming message to a mailing list has been expanded by a router which
  specifies a different address for bounce messages, $return_path subsequently
  contains the new bounce address, whereas $sender_address always contains the
  original sender address that was received with the message. In other words,  |
  $sender_address contains the incoming envelope sender, and $return_path      |
  contains the outgoing envelope sender.                                       |

$return_size_limit: This is an obsolete name for $bounce_return_size_limit.    |

$runrc: This variable contains the return code from a command that is run by
  the "${run...}" expansion item. Warning: In a router or transport, you
  cannot assume the order in which option values are expanded, except for
  those pre-conditions whose order of testing is documented. Therefore, you
  cannot reliably expect to set $runrc by the expansion of one option, and use
  it in another.

$self_hostname: When an address is routed to a supposedly remote host that
  turns out to be the local host, what happens is controlled by the "self"
  generic router option. One of its values causes the address to be passed to
  another router. When this happens, $self_hostname is set to the name of the
  local host that the original router encountered. In other circumstances its
  contents are null.

$sender_address: When a message is being processed, this variable contains the
  sender's address that was received in the message's envelope. For bounce
  messages, the value of this variable is the empty string. See also           |
  $return_path.                                                                |

$sender_address_domain: The domain portion of $sender_address.

$sender_address_local_part: The local part portion of $sender_address.

$sender_fullhost: When a message is received from a remote host, this variable
  contains the host name and IP address in a single string. It ends with the
  IP address in square brackets, followed by a colon and a port number if the
  logging of ports is enabled. The format of the rest of the string depends on
  whether the host issued a HELO or EHLO SMTP command, and whether the host
  name was verified by looking up its IP address. (Looking up the IP address
  can be forced by the "host_lookup" option, independent of verification.) A
  plain host name at the start of the string is a verified host name; if this
  is not present, verification either failed or was not requested. A host name
  in parentheses is the argument of a HELO or EHLO command. This is omitted if
  it is identical to the verified host name or to the host's IP address in
  square brackets.

$sender_helo_name: When a message is received from a remote host that has
  issued a HELO or EHLO command, the argument of that command is placed in
  this variable. It is also set if HELO or EHLO is used when a message is
  received using SMTP locally via the -bs or -bS options.

$sender_host_address: When a message is received from a remote host, this
  variable contains that host's IP address. For locally submitted messages, it
  is empty.

$sender_host_authenticated: This variable contains the name (not the public
  name) of the authenticator driver which successfully authenticated the
  client from which the message was received. It is empty if there was no
  successful authentication.

$sender_host_name: When a message is received from a remote host, this
  variable contains the host's name as obtained by looking up its IP address.  |
  For messages received by other means, this variable is empty.                |
                                                                               |
  If the host name has not previously been looked up, a reference to           |
  $sender_host_name triggers a lookup (for messages from remote hosts). If the |
  lookup fails, $sender_host_name remains empty, and $host_lookup_failed is    |
  set to '1'.                                                                  |
                                                                               |
  Exim does not automatically look up every calling host's name. If you want   |
  maximum efficiency, you should arrange your configuration so that it avoids  |
  these lookups altogether. The lookup happens only if one or more of the      |
  following are true:                                                          |

  (1)  A string containing $sender_host_name is expanded.                      |

  (2)  The calling host matches the list in "host_lookup". In the default      |
       configuration, this option is set to *, so it must be changed if        |
       lookups are to be avoided. (In the code, the default for "host_lookup"  |
       is unset.)                                                              |

  (3)  Exim needs the host name in order to test an item in a host list. The   |
       items that require this are described in sections 10.12 and 10.13.      |

  (4)  The calling host matches "helo_try_verify_hosts" or
       "helo_verify_hosts". In this case, the host name is required to compare
       with the name quoted in any EHLO or HELO commands that the client
       issues.

  (5)  The remote host issues a EHLO or HELO command that quotes one of the
       domains in "helo_lookup_domains". The default value of this option is

         helo_lookup_domains = @ : @[]

       which causes a lookup if a remote host (incorrectly) gives the server's
       name or IP address in an EHLO or HELO command.

$sender_host_port: When a message is received from a remote host, this
  variable contains the port number that was used on the remote host.

$sender_ident: When a message is received from a remote host, this variable
  contains the identification received in response to an RFC 1413 request.
  When a message has been received locally, this variable contains the login
  name of the user that called Exim.

$sender_rcvhost: This is provided specifically for use in Received: headers.
  It starts with either the verified host name (as obtained from a reverse DNS
  lookup) or, if there is no verified host name, the IP address in square
  brackets. After that there may be text in parentheses. When the first item
  is a verified host name, the first thing in the parentheses is the IP
  address in square brackets, followed by a colon and a port number if port
  logging is enabled. When the first item is an IP address, the port is
  recorded as 'port=xxxx' inside the parentheses.

  There may also be items of the form 'helo=xxxx' if HELO or EHLO was used and
  its argument was not identical to the real host name or IP address, and
  'ident=xxxx' if an RFC 1413 ident string is available. If all three items
  are present in the parentheses, a newline and tab are inserted into the
  string, to improve the formatting of the Received: header.

$smtp_command_argument: While an ACL is running to check an AUTH, EHLO, EXPN,
  ETRN, HELO, or VRFY command, this variable contains the argument for the
  SMTP command.

$sn0 - $sn9: These variables are copies of the values of the $n0 - $n9
  accumulators that were current at the end of the system filter file. This
  allows a system filter file to set values that can be tested in users'
  filter files. For example, a system filter could set a value indicating how
  likely it is that a message is junk mail.

$spool_directory: The name of Exim's spool directory.

$thisaddress: This variable is set only during the processing of the
  "foranyaddress" command in a filter file. Its use is explained in the
  description of that command.

$tls_certificate_verified: This variable is set to '1' if a TLS certificate
  was verified when the message was received, and '0' otherwise.

$tls_cipher: When a message is received from a remote host over an encrypted
  SMTP connection, this variable is set to the cipher suite that was
  negotiated, for example DES-CBC3-SHA. In other circumstances, in particular,
  for message received over unencrypted connections, the variable is empty.
  See chapter 37 for details of TLS support.

$tls_peerdn: When a message is received from a remote host over an encrypted
  SMTP connection, and Exim is configured to request a certificate from the    |
  client, the value of the Distinguished Name of the certificate is made       |
  available in the $tls_peerdn during subsequent processing.

$tod_bsdinbox: The time of day and date, in the format required for BSD-style
  mailbox files, for example: Thu Oct 17 17:14:09 1995.

$tod_epoch: The time and date as a number of seconds since the start of the
  Unix epoch.

$tod_full: A full version of the time and date, for example: Wed, 16 Oct 1995
  09:51:40 +0100. The timezone is always given as a numerical offset from UTC,
  with positive values used for timezones that are ahead (east) of UTC, and
  negative values for those that are behind (west).

$tod_log: The time and date in the format used for writing Exim's log files,
  for example: 1995-10-12 15:32:29, but without a timezone.

$tod_logfile: This variable contains the date in the format yyyymmdd. This is
  the format that is used for datestamping log files when "log_file_path"
  contains the "%D" flag.

$tod_zone: This variable contains the numerical value of the local timezone,
  for example: -0500.

$tod_zulu: This variable contains the UTC date and time in 'Zulu' format, as
  specified by ISO 8601, for example: 20030221154023Z.

$value: This variable contains the result of an expansion lookup, extraction
  operation, or external command, as described above.

$version_number: The version number of Exim.

$warn_message_delay: This variable is set only during the creation of a
  message warning about a delivery delay. Details of its use are explained in
  section 41.2.

$warn_message_recipients: This variable is set only during the creation of a
message warning about a delivery delay. Details of its use are explained in
section 41.2.



                              12. EMBEDDED PERL


Exim can be built to include an embedded Perl interpreter. When this is done,
Perl subroutines can be called as part of the string expansion process. To
make use of the Perl support, you need version 5.004 or later of Perl
installed on your system. To include the embedded interpreter in the Exim
binary, include the line

  EXIM_PERL = perl.o

in your Local/Makefile and then build Exim in the normal way.

Access to Perl subroutines is via a global configuration option called
"perl_startup" and an expansion string operator "${perl ...}". If there is no
"perl_startup" option in the Exim configuration file then no Perl interpreter
is started and there is almost no overhead for Exim (since none of the Perl
library will be paged in unless used). If there is a "perl_startup" option
then the associated value is taken to be Perl code which is executed in a
newly created Perl interpreter.

The value of "perl_startup" is not expanded in the Exim sense, so you do not
need backslashes before any characters to escape special meanings. The option
should usually be something like

  perl_startup = do '/etc/exim.pl'

where /etc/exim.pl is Perl code which defines any subroutines you want to use
from Exim. Exim can be configured either to start up a Perl interpreter as
soon as it is entered, or to wait until the first time it is needed. Starting
the interpreter at the beginning ensures that it is done while Exim still has
its setuid privilege, but can impose an unnecessary overhead if Perl is not in
fact used in a particular run. Also, note that this does not mean that Exim is
necessarily running as root when Perl is called at a later time. By default,
the interpreter is started only when it is needed, but this can be changed in
two ways:

 .   Setting "perl_at_start" (a boolean option) in the configuration requests
     a startup when Exim is entered.

 .   The command line option -ps also requests a startup when Exim is entered,
     overriding the setting of "perl_at_start".

There is also a command line option -pd (for delay) which suppresses the
initial startup, even if "perl_at_start" is set.

When the configuration file includes a "perl_startup" option you can make use
of the string expansion item to call the Perl subroutines that are defined by
the "perl_startup" code. The operator is used in any of the following forms:

  ${perl{foo}}
  ${perl{foo}{argument}}
  ${perl{foo}{argument1}{argument2} ... }

which calls the subroutine "foo" with the given arguments. A maximum of eight
arguments may be passed. Passing more than this results in an expansion
failure with an error message of the form

  Too many arguments passed to Perl subroutine "foo" (max is 8)

The return value of the Perl subroutine is evaluated in a scalar context
before it is passed back to Exim to be inserted into the expanded string. If
the return value is "undef", the expansion fails in the same way as an
explicit 'fail' on an "${if ...}" or "${lookup...}" item. If the subroutine
aborts by obeying Perl's "die" function, the expansion fails with the error
message that was passed to "die".

Within any Perl code called from Exim, the function "Exim::expand_string" is
available to call back into Exim's string expansion function. For example, the
Perl code

  my $lp = Exim::expand_string('$local_part');

makes the current Exim $local_part available in the Perl variable $lp. Note
those are single quotes and not double quotes to protect against $local_part
being interpolated as a Perl variable.

If the string expansion is forced to fail by a 'fail' item, the result of
"Exim::expand_string" is "undef". If there is a syntax error in the expansion
string, the Perl call from the original expansion string fails with an
appropriate error message, in the same way as if "die" were used.

Two other Exim functions are available for use from within Perl code.          |
"Exim::debug_write(<string>)" writes the string to the standard error stream   |
if Exim's debugging is enabled. If you want a newline at the end, you must     |
supply it. "Exim::log_write(<string>)" writes the string to Exim's main log,   |
adding a leading timestamp. In this case, you should not supply a terminating  |
newline.                                                                       |



          13. STARTING THE DAEMON AND THE USE OF NETWORK INTERFACES


A host that is connected to a TCP/IP network may have one or more physical     |
hardware network interfaces. Each of these interfaces may be configured as one |
or more 'logical' interfaces, which are the entities that a program actually   |
works with. Each of these logical interfaces is associated with an IP address. |
In addition, TCP/IP software supports 'loopback' interfaces (127.0.0.1 in IPv4 |
and ::1 in IPv6), which do not use any physical hardware. Exim requires        |
knowledge about the host's interfaces for use in three different               |
circumstances:                                                                 |
                                                                               |
(1)  When a listening daemon is started, Exim needs to know which interfaces   |
     and ports to listen on.                                                   |
                                                                               |
(2)  When Exim is routing an address, it needs to know which IP addresses are  |
     associated with local interfaces. This is required for the correct        |
     processing of MX lists by removing the local host and others with the     |
     same or higher priority values. Also, Exim needs to detect cases when an  |
     address is routed to an IP address that in fact belongs to the local      |
     host. Unless the "self" router option or the "allow_localhost" option of  |
     the smtp transport is set (as appropriate), this is treated as an error   |
     situation.                                                                |
                                                                               |
(3)  When Exim connects to a remote host, it may need to know which interface  |
     to use for the outgoing connection.                                       |
                                                                               |
Exim's default behaviour is likely to be appropriate in the vast majority of   |
cases. If your host has only one interface, and you want all its IP addresses  |
to be treated in the same way, and you are using only the standard SMTP port,  |
you should not need to take any special action. The rest of this chapter does  |
not apply to you.                                                              |
                                                                               |
In a more complicated situation you may want to listen only on certain         |
interfaces, or on different ports, and for this reason there are a number of   |
options that can be used to influence Exim's behaviour. The rest of this       |
chapter describes how they operate.                                            |
                                                                               |
When a message is received over TCP/IP, the interface and port that were       |
actually used are set in $interface_address and $interface_port.               |
                                                                               |
                                                                               |
13.1 Starting a listening daemon                                               |
                                                                               |
When a listening daemon is started (by means of the -bd command line option),  |
the interfaces and ports on which it listens are controlled by the following   |
options:                                                                       |
                                                                               |
 .   "daemon_smtp_ports" contains a list of default ports. (For backward       |
     compatibility, this option can also be specified in the singular.)        |
                                                                               |
 .   "local_interfaces" contains list of interface IP addresses on which to    |
     listen. Each item may optionally also specify a port.                     |
                                                                               |
The default list separator in both cases is a colon, but this can be changed   |
as described in section 6.15. When IPv6 addresses are involved, it is usually  |
best to change the separator to avoid having to double all the colons. For     |
example:                                                                       |
                                                                               |
  local_interfaces = <; 127.0.0.1 ; \                                          |
                        192.168.23.65 ; \                                      |
                        ::1 ; \                                                |
                        3ffe:ffff:836f::fe86:a061                              |
                                                                               |
There are two different formats for specifying a port along with an IP address |
in "local_interfaces":                                                         |
                                                                               |
(1)  The port is added onto the address with a dot separator. For example, to  |
     listen on port 1234 on two different IP addresses:                        |
                                                                               |
       local_interfaces = <; 192.168.23.65.1234 ; \                            |
                             3ffe:ffff:836f::fe86:a061.1234                    |
                                                                               |
(2)  The IP address is enclosed in square brackets, and the port is added with |
     a colon separator, for example:                                           |
                                                                               |
       local_interfaces = <; [192.168.23.65]:1234 ; \                          |
                             [3ffe:ffff:836f::fe86:a061]:1234                  |
                                                                               |
When a port is not specified, the value of "daemon_smtp_ports" is used. The    |
default setting contains just one port:                                        |
                                                                               |
  daemon_smtp_ports = smtp                                                     |
                                                                               |
If more than one port is listed, each interface that does not have its own     |
port specified listens on all of them. Ports that are listed in                |
"daemon_smtp_ports" can be identified either by name (defined in               |
/etc/services) or by number. However, when ports are given with individual IP  |
addresses in "local_interfaces", only numbers (not names) can be used.         |
                                                                               |
                                                                               |
13.2 Special IP listening addresses                                            |
                                                                               |
The addresses 0.0.0.0 and ::0 are treated specially. They are interpreted as   |
'all IPv4 interfaces' and 'all IPv6 interfaces', respectively. In each case,   |
Exim tells the TCP/IP stack to 'listen on all IPv"x" interfaces' instead of    |
setting up separate listening sockets for each interface. The default value of |
"local_interfaces" is                                                          |
                                                                               |
  local_interfaces = 0.0.0.0                                                   |
                                                                               |
when Exim is built without IPv6 support; otherwise it is:                      |
                                                                               |
  local_interfaces = <; ::0 ; 0.0.0.0                                          |
                                                                               |
Thus, by default, Exim listens on all available interfaces, on the SMTP port.  |
                                                                               |
                                                                               |
13.3 Overriding local_interfaces and daemon_smtp_ports                         |
                                                                               |
The -oX command line option can be used to override the values of              |
"daemon_smtp_ports" and/or "local_interfaces" for a particular daemon          |
instance. Another way of doing this would be to use macros and the -D option.  |
However, -oX can be used by any admin user, whereas modification of the        |
runtime configuration by -D is allowed only when the caller is root or exim.   |
                                                                               |
The value of -oX is a list of items. The default colon separator can be        |
changed in the usual way if required. If there are any items that do not       |
contain dots or colons (that is, are not IP addresses), the value of           |
"daemon_smtp_ports" is replaced by the list of those items. If there are any   |
items that do contain dots or colons, the value of "local_interfaces" is       |
replaced by those items. Thus, for example,                                    |
                                                                               |
  -oX 1225                                                                     |
                                                                               |
overrides "daemon_smtp_ports", but leaves "local_interfaces" unchanged,        |
whereas                                                                        |
                                                                               |
  -oX 192.168.34.5.1125                                                        |
                                                                               |
overrides "local_interfaces", leaving "daemon_smtp_ports" unchanged. (However, |
since "local_interfaces" now contains no items without ports, the value of     |
"daemon_smtp_ports" is no longer relevant in this example.)                    |
                                                                               |
                                                                               |
13.4 IPv6 address scopes                                                       |
                                                                               |
IPv6 addresses have 'scopes', and a host with multiple hardware interfaces     |
can, in principle, have the same link-local IPv6 address on different          |
interfaces. Thus, additional information is needed, over and above the IP      |
address, to distinguish individual interfaces. A convention of using a percent |
sign followed by something (often the interface name) has been adopted in some |
cases, leading to addresses like this:                                         |
                                                                               |
  3ffe:2101:12:1:a00:20ff:fe86:a061%eth0                                       |
                                                                               |
To accommodate this usage, a percent sign followed by an arbitrary string is   |
allowed at the end of an IPv6 address. By default, Exim calls "getaddrinfo()"  |
to convert a textual IPv6 address for actual use. This function recognizes the |
percent convention in operating systems that support it, and it processes the  |
address appropriately. Unfortunately, some older libraries have problems with  |
"getaddrinfo()". If                                                            |
                                                                               |
  IPV6_USE_INET_PTON=yes                                                       |
                                                                               |
is set in Local/Makefile (or an OS-dependent Makefile) when Exim is built,     |
Exim uses "inet_pton()" to convert a textual IPv6 address for actual use,      |
instead of "getaddrinfo()". (Before version 4.14, it always used this func-    |
tion.) Of course, this means that the additional functionality of              |
"getaddrinfo()" - recognizing scoped addresses - is lost.                      |
                                                                               |
                                                                               |
13.5 Examples of starting a listening daemon                                   |
                                                                               |
The default case in an IPv6 environment is                                     |
                                                                               |
  daemon_smtp_port = smtp                                                      |
  local_interfaces = <; ::0 ; 0.0.0.0                                          |
                                                                               |
This specifies listening on the smtp port on all IPv6 and IPv4 interfaces.     |
Either one or two sockets may be used, depending on the characteristics of the |
TCP/IP stack. (This is complicated and messy; for more information, read the   |
comments in the daemon.c source file.)                                         |
                                                                               |
To specify listening on ports 25 and 26 on all interfaces:                     |
                                                                               |
  daemon_smtp_ports = 25 : 26                                                  |
                                                                               |
(leaving "local_interfaces" at the default setting) or, more explicitly:       |
                                                                               |
  local_interfaces = <; ::0.25     ; ::0.26 \                                  |
                        0.0.0.0.25 ; 0.0.0.0.26                                |
                                                                               |
To listen on the default port on all IPv4 interfaces, and on port 26 on the    |
IPv4 loopback address only:                                                    |
                                                                               |
  local_interfaces = 0.0.0.0 : 127.0.0.1.26                                    |
                                                                               |
To specify listening on the default port on specific interfaces only:          |
                                                                               |
  local_interfaces = 192.168.34.67 : 192.168.34.67                             |
                                                                               |
Note: such a setting excludes listening on the loopback interfaces.            |
                                                                               |
                                                                               |
13.6 Recognising the local host                                                |
                                                                               |
The "local_interfaces" option is also used when Exim needs to determine        |
whether or not an IP address refers to the local host. That is, the IP         |
addresses of all the interfaces on which a daemon is listening are always      |
treated as local.                                                              |
                                                                               |
For this usage, port numbers in "local_interfaces" are ignored. If either of   |
the items 0.0.0.0 or ::0 are encountered, Exim gets a complete list of         |
available interfaces from the operating system, and extracts the relevant      |
(that is, IPv4 or IPv6) addresses to use for checking.                         |
                                                                               |
Some systems set up large numbers of virtual interfaces in order to provide    |
many virtual web servers. In this situation, you may want to listen for email  |
on only a few of the available interfaces, but nevertheless treat all          |
interfaces as local when routing. You can do this by setting                   |
"extra_local_interfaces" to a list of IP addresses, possibly including the     |
'all' wildcard values. These addresses are recognized as local, but are not    |
used for listening. Consider this example:                                     |
                                                                               |
  local_interfaces = <; 127.0.0.1 ; ::1 ; \                                    |
                        192.168.53.235 ; \                                     |
                        3ffe:2101:12:1:a00:20ff:fe86:a061                      |
                                                                               |
  extra_local_interfaces = <; ::0 ; 0.0.0.0                                    |
                                                                               |
The daemon listens on the loopback interfaces and just one IPv4 and one IPv6   |
address, but all available interface addresses are treated as local when Exim  |
is routing.                                                                    |
                                                                               |
In some environments the local host name may be in an MX list, but with an IP  |
address that is not assigned to any local interface. In other cases it may be  |
desirable to treat other host names as if they referred to the local host.     |
Both these cases can be handled by setting the "hosts_treat_as_local" option.  |
This contains host names rather than IP addresses. When a host is referenced   |
during routing, either via an MX record or directly, it is treated as the      |
local host if its name matches "hosts_treat_as_local", or if any of its IP     |
addresses match "local_interfaces" or "extra_local_interfaces".                |
                                                                               |
                                                                               |
13.7 Delivering to a remote host                                               |
                                                                               |
Delivery to a remote host is handled by the smtp transport. By default, it     |
allows the system's TCP/IP functions to choose which interface to use (if      |
there is more than one) when connecting to a remote host. However, the         |
"interface" option can be set to specify which interface is used. See the      |
description of the smtp transport in chapter 30 for more details.              |
                                                                               |


                            14. MAIN CONFIGURATION


The first part of the run time configuration file contains three types of
item:

 .   Macro definitions: These lines start with an upper case letter. See
     section 6.4 for details of macro processing.

 .   Named list definitions: These lines start with one of the words
     'domainlist', 'hostlist', 'addresslist', or 'localpartlist'. Their use is
     described in section 10.5.

 .   Main configuration settings: Each setting occupies one line of the file
     (with possible continuations). If any setting is preceded by the word
     'hide', the -bP command line option displays its value to admin users
     only. See section 6.6 for a description of the syntax of these option
     settings.

This chapter specifies all the main configuration options, along with their
types and default values. For ease of finding a particular option, they appear
in alphabetical order in section 14.23 below. However, because there are now
so many options, they are first listed briefly in functional groups, as an aid
to finding the name of the option you are looking for. Some options are listed |
in more than one group.                                                        |


14.1 Miscellaneous

  "bi_command"                         to run for -bi command line option
  "keep_malformed"                     for broken files - should not happen
  "localhost_number"                   for unique message ids in clusters
  "message_body_visible"               how much to show in $message_body
  "print_topbitchars"                  top-bit characters are printing
  "timezone"                           force time zone


14.2 Exim parameters

  "exim_group"                         override compiled-in value
  "exim_path"                          override compiled-in value
  "exim_user"                          override compiled-in value
  "primary_hostname"                   default from "uname()"
  "split_spool_directory"              use multiple directories
  "spool_directory"                    override compiled-in value


14.3 Privilege controls

  "admin_groups"                       groups that are Exim admin users
  "deliver_drop_privilege"             drop root for delivery processes
  "local_from_check"                   insert Sender: if necessary
  "local_from_prefix"                  for testing From: for local sender
  "local_from_suffix"                  for testing From: for local sender
  "local_sender_retain"                keep Sender: from untrusted user        |
  "never_users"                        do not run deliveries as these
  "prod_requires_admin"                forced delivery requires admin user
  "queue_list_requires_admin"          queue listing requires admin user
  "trusted_groups"                     groups that are trusted
  "trusted_users"                      users that are trusted


14.4 Logging

  "log_file_path"                      override compiled-in value
  "log_selector"                       set/unset optional logging
  "log_timezone"                       add timezone to log lines
  "message_logs"                       keep per-message logs
  "preserve_message_logs"              in another directory
  "process_log_path"                   for SIGUSR1 and "exiwhat"               |
  "syslog_duplication"                 controls duplicate log lines on syslog  |
  "syslog_facility"                    set syslog 'facility' field
  "syslog_processname"                 set syslog 'ident' field
  "syslog_timestamp"                   timestamp syslog lines


14.5 Frozen messages

  "auto_thaw"                          sets time for retrying frozen messages
  "freeze_tell"                        send message when freezing
  "move_frozen_messages"               to another directory
  "timeout_frozen_after"               keep frozen messages only so long


14.6 Data lookups

  "ldap_default_servers"               used if no server in query
  "ldap_version"                       set protocol version
  "lookup_open_max"                    lookup files held open
  "mysql_servers"                      as it says
  "oracle_servers"                     as it says
  "pgsql_servers"                      as it says


14.7 Message ids

  "message_id_header_domain"           used to build Message-ID: header
  "message_id_header_text"             ditto


14.8 Embedded Perl Startup

  "perl_at_start"                      always start the interpreter
  "perl_startup"                       code to obey when starting Perl


14.9 Daemon

  "daemon_smtp_ports"                  default ports
  "extra_local_interfaces"             not necessarily listened on             |
  "local_interfaces"                   on which to listen, with optional ports
  "pid_file_path"                      override compiled-in value
  "queue_run_max"                      maximum number of simultaneous queue runners


14.10 Resource control

  "check_log_inodes"                   before accepting a message
  "check_log_space"                    before accepting a message
  "check_spool_inodes"                 before accepting a message
  "check_spool_space"                  before accepting a message
  "deliver_queue_load_max"             no queue deliveries if load high
  "queue_only_load"                    queue incoming if load high
  "queue_run_max"                      maximum number of simultaneous queue runners
  "remote_max_parallel"                parallel SMTP delivery per message
  "smtp_accept_max"                    simultaneous incoming connections
  "smtp_accept_max_nommail"            non-mail commands
  "smtp_accept_max_nonmail_hosts"      hosts to which the limit applies
  "smtp_accept_max_per_connection"     messages per connection
  "smtp_accept_max_per_host"           connections from one host
  "smtp_accept_queue"                  queue mail if more connections
  "smtp_accept_queue_per_connection"   queue if more messages per connection
  "smtp_accept_reserve"                only reserve hosts if more connections
  "smtp_check_spool_space"             from SIZE on MAIL command
  "smtp_connect_backlog"               passed to TCP/IP stack
  "smtp_load_reserve"                  SMTP from reserved hosts if load high
  "smtp_reserve_hosts"                 these are the reserve hosts


14.11 Policy controls

  "acl_not_smtp"                       set ACL for non-SMTP messages
  "acl_smtp_auth"                      set ACL for AUTH
  "acl_smtp_connect"                   set ACL for connection
  "acl_smtp_data"                      set ACL for DATA
  "acl_smtp_etrn"                      set ACL for ETRN
  "acl_smtp_expn"                      set ACL for EXPN
  "acl_smtp_helo"                      set ACL for EHLO or HELO
  "acl_smtp_mail"                      set ACL for MAIL
  "acl_smtp_mailauth"                  set ACL for AUTH on MAIL command        |
  "acl_smtp_rcpt"                      set ACL for RCPT
  "acl_smtp_starttls"                  set ACL for STARTTLS
  "acl_smtp_vrfy"                      set ACL for VRFY
  "header_maxsize"                     total size of message header
  "header_line_maxsize"                individual header line limit
  "helo_accept_junk_hosts"             allow syntactic junk from these hosts
  "helo_allow_chars"                   allow illegal chars in HELO names
  "helo_lookup_domains"                lookup hostname for these HELO names
  "helo_try_verify_hosts"              HELO soft-checked for these hosts
  "helo_verify_hosts"                  HELO hard-checked for these hosts
  "host_lookup"                        host name looked up for these hosts
  "host_lookup_order"                  order of DNS and local name lookups     |
  "host_reject_connection"             reject connection from these hosts
  "hosts_treat_as_local"               useful in some cluster configurations
  "local_scan_timeout"                 timeout for "local_scan()"
  "message_size_limit"                 for all messages
  "percent_hack_domains"               recognize %-hack for these domains


14.12 Callout cache

  "callout_domain_negative_expire"     timeout for negative domain cache item
  "callout_domain_positive_expire"     timeout for positive domain cache item
  "callout_negative_expire"            timeout for negative address cache item
  "callout_positive_expire"            timeout for positive address cache item
  "callout_random_local_part"          string to use for 'random' testing


14.13 TLS

  "tls_advertise_hosts"                advertise TLS to these hosts
  "tls_certificate"                    location of server certificate
  "tls_dhparam"                        DH parameters for server
  "tls_privatekey"                     location of server private key
  "tls_remember_esmtp"                 don't reset after starting TLS          |
  "tls_try_verify_hosts"               try to verify client certificate
  "tls_verify_certificates"            expected client certificates
  "tls_verify_hosts"                   insist on client certificate verify


14.14 Local user handling

  "finduser_retries"                   useful in NIS environments
  "gecos_name"                         used when creating Sender:
  "gecos_pattern"                      ditto
  "max_username_length"                for systems that truncate
  "unknown_login"                      used when no login name found
  "unknown_username"                   ditto
  "uucp_from_pattern"                  for recognizing 'From ' lines
  "uucp_from_sender"                   ditto


14.15 All incoming messages (SMTP and non-SMTP)

  "header_maxsize"                     total size of message header
  "header_line_maxsize"                individual header line limit
  "message_size_limit"                 applies to all messages
  "percent_hack_domains"               recognize %-hack for these domains
  "received_header_text"               expanded to make Received:
  "received_headers_max"               for mail loop detection
  "recipients_max"                     limit per message
  "recipients_max_reject"              permanently reject excess


14.16 Non-SMTP incoming messages

  "receive_timeout"                    for non-SMTP messages


14.17 Incoming SMTP messages

See also the "Policy controls" section above.

  "host_lookup"                        host name looked up for these hosts
  "host_lookup_order"                  order of DNS and local name lookups
  "recipient_unqualified_hosts"        may send unqualified recipients
  "rfc1413_hosts"                      make ident calls to these hosts
  "rfc1413_query_timeout"              zero disables ident calls
  "sender_unqualified_hosts"           may send unqualified senders
  "smtp_accept_keepalive"              some TCP/IP magic
  "smtp_accept_max"                    simultaneous incoming connections
  "smtp_accept_max_nommail"            non-mail commands
  "smtp_accept_max_nonmail_hosts"      hosts to which the limit applies
  "smtp_accept_max_per_connection"     messages per connection
  "smtp_accept_max_per_host"           connections from one host
  "smtp_accept_queue"                  queue mail if more connections
  "smtp_accept_queue_per_connection"   queue if more messages per connection
  "smtp_accept_reserve"                only reserve hosts if more connections
  "smtp_banner"                        text for welcome banner
  "smtp_check_spool_space"             from SIZE on MAIL command
  "smtp_connect_backlog"               passed to TCP/IP stack
  "smtp_enforce_sync"                  of SMTP command/responses
  "smtp_etrn_command"                  what to run for ETRN
  "smtp_etrn_serialize"                only one at once
  "smtp_load_reserve"                  only reserve hosts if this load
  "smtp_max_unknown_commands"          before dropping connection
  "smtp_ratelimit_hosts"               apply ratelimiting to these hosts
  "smtp_ratelimit_mail"                ratelimit for MAIL commands
  "smtp_ratelimit_rcpt"                ratelimit for RCPT commands
  "smtp_receive_timeout"               per command or data line
  "smtp_reserve_hosts"                 these are the reserve hosts
  "smtp_return_error_details"          give detail on rejections


14.18 SMTP extensions

  "accept_8bitmime"                    advertise 8BITMIME
  "auth_advertise_hosts"               advertise AUTH to these hosts
  "ignore_fromline_hosts"              allow 'From ' from these hosts
  "ignore_fromline_local"              allow 'From ' from local SMTP
  "pipelining_advertise_hosts"         advertise pipelining to these hosts
  "tls_advertise_hosts"                advertise TLS to these hosts


14.19 Processing messages

  "allow_domain_literals"              recognize domain literal syntax
  "allow_mx_to_ip"                     allow MX to point to IP address
  "allow_utf8_domains"                 in addresses
  "delivery_date_remove"               from incoming messages
  "envelope_to_remote"                 from incoming messages
  "extract_addresses_remove_arguments" affects -t processing
  "headers_charset"                    default for translations                |
  "qualify_domain"                     default for senders
  "qualify_recipient"                  default for recipients
  "return_path_remove"                 from incoming messages
  "strip_excess_angle_brackets"        in addresses
  "strip_trailing_dot"                 at end of addresses
  "untrusted_set_sender"               untrusted can set envelope sender


14.20 System filter

  "system_filter"                      locate system filter
  "system_filter_directory_transport"  transport for delivery to a directory
  "system_filter_file_transport"       transport for delivery to a file
  "system_filter_group"                group for filter running
  "system_filter_pipe_transport"       transport for delivery to a pipe
  "system_filter_reply_transport"      transport for autoreply delivery
  "system_filter_user"                 user for filter running


14.21 Routing and delivery

  "dns_again_means_nonexist"           for broken domains
  "dns_check_names_pattern"            pre-DNS syntax check
  "dns_ipv4_lookup"                    only v4 lookup for these domains
  "dns_retrans"                        parameter for resolver
  "dns_retry"                          parameter for resolver
  "hold_domains"                       hold delivery for these domains
  "local_interfaces"                   for routing checks
  "queue_domains"                      no immediate delivery for these
  "queue_only"                         no immediate delivery at all
  "queue_only_file"                    no immediate deliveryif file exists
  "queue_only_load"                    no immediate delivery if load is high
  "queue_only_override"                allow command line to override          |
  "queue_run_in_order"                 order of arrival
  "queue_run_max"                      of simultaneous queue runners
  "queue_smtp_domains"                 no immediate SMTP delivery for these
  "remote_max_parallel"                parallel SMTP delivery per message
  "remote_sort_domains"                order of remote deliveries
  "retry_data_expire"                  timeout for retry data
  "retry_interval_max"                 safety net for retry rules


14.22 Bounce and warning messages

  "bounce_message_file"                content of bounce
  "bounce_message_text"                content of bounce
  "bounce_return_body"                 include body if returning message       |
  "bounce_return_message"              include original message in bounce
  "bounce_return_size_limit"           limit on returned message
  "bounce_sender_authentication"       send authenticated sender with bounce
  "errors_copy"                        copy bounce messages
  "errors_reply_to"                    Reply-to: in bounces
  "delay_warning"                      time schedule
  "delay_warning_condition"            condition for warning messages
  "ignore_bounce_errors_after"         discard undeliverable bounces
  "warn_message_file"                  content of warning message


14.23 Alphabetical list of main options

Those options that undergo string expansion before use are marked with *.


accept_8bitmime                 Type: boolean                  Default: false

    This option causes Exim to send 8BITMIME in its response to an SMTP EHLO
    command, and to accept the BODY= parameter on MAIL commands. However,
    though Exim is 8-bit clean, it is not a protocol converter, and it takes
    no steps to do anything special with messages received by this route.
    Consequently, this option is turned off by default.


acl_not_smtp                    Type: string*                  Default: unset

    This option defines the ACL that is run when a non-SMTP message is on the
    point of being accepted. See chapter 38 for further details.


acl_smtp_connect                Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP connection is
    received. See chapter 38 for further details.


acl_smtp_auth                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP AUTH command is
    received. See chapter 38 for further details.


acl_smtp_data                   Type: string*                  Default: unset

    This option defines the ACL that is run after an SMTP DATA command has
    been processed and the message itself has been received, but before the
    final acknowledgement is sent. See chapter 38 for further details.


acl_smtp_etrn                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP ETRN command is
    received. See chapter 38 for further details.


acl_smtp_expn                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP EXPN command is
    received. See chapter 38 for further details.


acl_smtp_helo                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP EHLO or HELO command
    is received. See chapter 38 for further details.


acl_smtp_mail                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP MAIL command is
    received. See chapter 38 for further details.

                                                                               |
acl_smtp_mailauth               Type: string*                  Default: unset  |
                                                                               |
    This option defines the ACL that is run when there is an AUTH parameter on |
    a MAIL command. See chapter 38 for details of ACLs, and chapter 33 for     |
    details of authentication.                                                 |


acl_smtp_rcpt                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP RCPT command is
    received. See chapter 38 for further details.


acl_smtp_starttls               Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP STARTTLS command is
    received. See chapter 38 for further details.


acl_smtp_vrfy                   Type: string*                  Default: unset

    This option defines the ACL that is run when an SMTP VRFY command is
    received. See chapter 38 for further details.

admin_groups                  Type: string list                Default: unset

    If the current group or any of the supplementary groups of the caller is
    in this colon-separated list, the caller has admin privileges. If all your
    system programmers are in a specific group, for example, you can give them
    all Exim admin privileges by putting that group in "admin_groups".
    However, this does not permit them to read Exim's spool files (whose group
    owner is the Exim gid). To permit this, you have to add individuals to the
    Exim group.

allow_domain_literals           Type: boolean                  Default: false

    If this option is set, the RFC 2822 domain literal format is permitted in
    email addresses. The option is not set by default, because the domain
    literal format is not normally required these days, and few people know
    about it. It has, however, been exploited by mail abusers.

    Unfortunately, it seems that some DNS black list maintainers are using
    this format to report black listing to postmasters. If you want to accept
    messages addressed to your hosts by IP address, you need to set
    "allow_domain_literals" true, and also to add "@[]" to the list of local
    domains (defined in the named domain list "local_domains" in the default
    configuration). This 'magic string' matches the domain literal form of all
    the local host's IP addresses.

allow_mx_to_ip                  Type: boolean                  Default: false

    It appears that more and more DNS zone administrators are breaking the
    rules and putting domain names that look like IP addresses on the right
    hand side of MX records. Exim follows the rules and rejects this, giving
    an error message that explains the mis-configuration. However, some other
    MTAs support this practice, so to avoid 'Why can't Exim do this?'
    complaints, "allow_mx_to_ip" exists, in order to enable this heinous
    activity. It is not recommended, except when you have no other choice.


allow_utf8_domains              Type: boolean                  Default: false

    Lots of discussion is going on about internationalized domain names. One
    camp is strongly in favour of just using UTF-8 characters, and it seems
    that at least two other MTAs permit this. This option allows Exim users to
    experiment if they wish.

    If it is set true, Exim's domain parsing function allows valid UTF-8
    multicharacters to appear in domain name components, in addition to
    letters, digits, and hyphens. However, just setting this option is not
    enough; if you want to look up these domain names in the DNS, you must
    also adjust the value of "dns_check_names_pattern" to match the extended
    form. A suitable setting is:

      dns_check_names_pattern = (?i)^(?>(?(1)\.|())[a-z0-9\xc0-\xff]\
        (?>[-a-z0-9\x80-\xff]*[a-z0-9\x80-\xbf])?)+$

    Alternatively, you can just disable this feature by setting

      dns_check_names_pattern =

    That is, set the option to an empty string so that no check is done.

auth_advertise_hosts           Type: host list*                     Default: *

    If any server authentication mechanisms are configured, Exim advertises
    them in response to an EHLO command only if the calling host matches this
    list. Otherwise, Exim does not advertise AUTH. Exim does not accept AUTH
    commands from clients to which it has not advertised the availability of
    AUTH. The advertising of individual authentication mechanisms can be
    controlled by the use of the "server_advertise_condition" generic
    authenticator option on the individual authenticators. See chapter 33 for
    further details.

    Certain mail clients (for example, Netscape) require the user to provide a
    name and password for authentication if AUTH is advertised, even though it
    may not be needed (the host may accept messages from hosts on its local
    LAN without authentication, for example). The "auth_advertise_hosts"
    option can be used to make these clients more friendly by excluding them
    from the set of hosts to which Exim advertises AUTH.

    If you want to advertise the availability of AUTH only when the connection
    is encrypted using TLS, you can make use of the fact that the value of
    this option is expanded, with a setting like this:

      auth_advertise_hosts = ${if eq{$tls_cipher}{}{}{*}}

    If $tls_cipher is empty, the session is not encrypted, and the result of
    the expansion is empty, thus matching no hosts. Otherwise, the result of
    the expansion is *, which matches all hosts.

auto_thaw                         Type: time                       Default: 0s

    If this option is set to a time greater than zero, a queue runner will try
    a new delivery attempt on any frozen message if this much time has passed
    since it was frozen. This may result in the message being re-frozen if
    nothing has changed since the last attempt. It is a way of saying 'keep
    on trying, even though there are big problems'. See also
    "timeout_frozen_after" and "ignore_bounce_errors_after".

bi_command                       Type: string                   Default: unset

    This option supplies the name of a command that is run when Exim is called
    with the -bi option (see chapter 5). The string value is just the command
    name, it is not a complete command line. If an argument is required, it
    must come from the -oA command line option.

bounce_message_file              Type: string                   Default: unset

    This option defines a template file containing paragraphs of text to be
    used for constructing bounce messages. Details of the file's contents are
    given in chapter 41. See also "warn_message_file".

bounce_message_text              Type: string                   Default: unset

    When this option is set, its contents are included in the default bounce
    message immediately after 'This message was created automatically by mail
    delivery software.' It is not used if "bounce_message_file" is set.

                                                                               |
bounce_return_body              Type: boolean                   Default: true  |
                                                                               |
    This option controls whether the body of an incoming message is included   |
    in a bounce message when "bounce_return_message" is true. If it is not     |
    set, only the message header is included.                                  |


bounce_return_message           Type: boolean                   Default: true

    If this option is set false, the original message is not included in
    bounce messages generated by Exim. See also "bounce_return_size_limit".

bounce_return_size_limit        Type: integer                   Default: 100K  |
                                                                               |
    This option sets a limit in bytes on the size of messages that are         |
    returned to senders as part of bounce messages when                        |
    "bounce_return_message" is true. The limit should be less than the value   |
    of the global "message_size_limit" and of any "message_size_limit" set-    |
    tings on transports, to allow for the bounce text that Exim generates. If  |
    this option is set to zero there is no limit.                              |
                                                                               |
    When the body of any message that is to be included in a bounce message is |
    greater than the limit, it is truncated, and a comment pointing this out   |
    is added at the top. The actual cutoff may be greater than the value       |
    given, owing to the use of buffering for transferring the message in       |
    chunks (typically 8K in size). The idea is to save bandwidth on those      |
    undeliverable 15-megabyte messages.                                        |


bounce_sender_authentication     Type: string                   Default: unset

    This option provides an authenticated sender address that is sent with any
    bounce messages generated by Exim that are sent over an authenticated SMTP
    connection. A typical setting might be:

      bounce_sender_authentication = mailer-daemon@my.domain.example

    which would cause bounce messages to be sent using the SMTP command:

      MAIL FROM:<> AUTH=mailer-daemon@my.domain.example

    The value of "bounce_sender_authentication" must always be a complete
    email address.


callout_domain_negative_expire    Type: time                       Default: 3h

    This option specifies the expiry time for negative callout cache data for
    a domain. See section 38.21 for details of callout verification, and
    section 38.23 for details of the caching.

callout_domain_positive_expire    Type: time                       Default: 7d

    This option specifies the expiry time for positive callout cache data for
    a domain. See section 38.21 for details of callout verification, and
    section 38.23 for details of the caching.

callout_negative_expire           Type: time                       Default: 2h

    This option specifies the expiry time for negative callout cache data for
    an address. See section 38.21 for details of callout verification, and
    section 38.23 for details of the caching.

callout_positive_expire           Type: time                      Default: 24h

    This option specifies the expiry time for positive callout cache data for
    an address. See section 38.21 for details of callout verification, and
    section 38.23 for details of the caching.

callout_random_local_part       Type: string*              Default: see below  |
                                                                               |
    This option defines the 'random' local part that can be used as part of    |
    callout verification. The default value is                                 |
                                                                               |
      $primary_host_name-$tod_epoch-testing                                    |
                                                                               |
    See section 38.22 for details of how this value is used.                   |

check_log_inodes                Type: integer                      Default: 0

    See "check_spool_space" below.

check_log_space                 Type: integer                      Default: 0

    See "check_spool_space" below.

check_spool_inodes              Type: integer                      Default: 0

    See "check_spool_space" below.

check_spool_space               Type: integer                      Default: 0

    The four "check_..." options allow for checking of disk resources before a
    message is accepted. "check_spool_space" and "check_spool_inodes" check
    the spool partition if either value is greater than zero, for example:

      check_spool_space = 10M
      check_spool_inodes = 100

    The spool partition is the one which contains the directory defined by
    SPOOL_DIRECTORY in Local/Makefile. It is used for holding messages in
    transit.

    "check_log_space" and "check_log_inodes" check the partition in which log
    files are written if either is greater than zero. These should be set only
    if "log_file_path" and "spool_directory" refer to different partitions.

    If there is less space or fewer inodes than requested, Exim refuses to
    accept incoming mail. In the case of SMTP input this is done by giving a
    452 temporary error response to the MAIL command. If ESMTP is in use and
    there was a SIZE parameter on the MAIL command, its value is added to
    the "check_spool_space" value, and the check is performed even if
    "check_spool_space" is zero, unless "no_smtp_check_spool_space" is set.

    The values for "check_spool_space" and "check_log_space" are held as a     |
    number of kilobytes. If a non-multiple of 1024 is specified, it is rounded |
    up.                                                                        |

    For non-SMTP input and for batched SMTP input, the test is done at start-
    up; on failure a message is written to stderr and Exim exits with a non-
    zero code, as it obviously cannot send an error message of any kind.

                                                                               |
daemon_smtp_ports                Type: string                    Default: smtp |
                                                                               |
    This option specifies one or more default SMTP ports on which the Exim     |
    daemon listens. See chapter 13 for details of how it is used. For backward |
    compatibility, "daemon_smtp_port" (singular) is a synonym.                 |

delay_warning                  Type: time list                   Default: 24h

    When a message is delayed, Exim sends a warning message to the sender at
    intervals specified by this option. If it is set to a zero, no warnings
    are sent. The data is a colon-separated list of times after which to send
    warning messages. Up to 10 times may be given. If a message has been on
    the queue for longer than the last time, the last interval between the
    times is used to compute subsequent warning times. For example, with

      delay_warning = 4h:8h:24h

    the first message is sent after 4 hours, the second after 8 hours, and the
    third one after 24 hours. After that, messages are sent every 16 hours,
    because that is the interval between the last two times on the list. If
    you set just one time, it specifies the repeat interval. For example,
    with:

      delay_warning = 6h

    messages are repeated every six hours. To stop warnings after a given
    time, set a very large time at the the end of the list. For example:

      delay_warning = 2h:12h:99d

delay_warning_condition         Type: string*              Default: see below

    The string is expanded at the time a warning message might be sent. If all
    the deferred addresses have the same domain, it is set in $domain during
    the expansion. Otherwise $domain is empty. If the result of the expansion
    is a forced failure, an empty string, or a string matching any of '0',
    'no' or 'false' (the comparison being done caselessly) then the warning
    message is not sent. The default is

      delay_warning_condition = \
        ${if match{$h_precedence:}{(?i)bulk|list|junk}{no}{yes}}

    which suppresses the sending of warnings about messages that have 'bulk',
    'list' or 'junk' in a Precedence: header.


deliver_drop_privilege          Type: boolean                  Default: false

    If this option is set true, Exim drops its root privilege at the start of
    a delivery process, and runs as the Exim user throughout. This severely
    restricts the kinds of local delivery that are possible, but is viable in
    certain types of configuration. There is a discussion about the use of
    root privilege in chapter 48.


deliver_queue_load_max        Type: fixed-point                Default: unset

    When this option is set, a queue run is abandoned if the system load
    average becomes greater than the value of the option. The option has no
    effect on ancient operating systems on which Exim cannot determine the
    load average. See also "queue_only_load" and "smtp_load_reserve".

delivery_date_remove            Type: boolean                   Default: true

    Exim's transports have an option for adding a Delivery-date: header to a
    message when it is delivered - in exactly the same way as Return-path: is
    handled. Delivery-date: records the actual time of delivery. Such headers
    should not be present in incoming messages, and this option causes them to
    be removed at the time the message is received, to avoid any problems that
    might occur when a delivered message is subsequently sent on to some other
    recipient.


dns_again_means_nonexist      Type: domain list*                Default: unset

    DNS lookups give a 'try again' response for the DNS errors 'non-
    authoritative host not found' and 'SERVERFAIL'. This can cause Exim to
    keep trying to deliver a message, or to give repeated temporary errors to
    incoming mail. Sometimes the effect is caused by a badly set up name
    server and may persist for a long time. If a domain which exhibits this
    problem matches anything in "dns_again_means_nonexist", it is treated as
    if it did not exist. This option should be used with care.


dns_check_names_pattern          Type: string               Default: see below

    When this option is set to a non-empty string, it causes Exim to check
    domain names for illegal characters before handing them to the DNS
    resolver, because some resolvers give temporary errors for malformed
    names. If a domain name contains any illegal characters, a 'not found'
    result is forced, and the resolver is not called. The check is done by
    matching the domain name against a regular expression, which is the value
    of this option. The default pattern is

      dns_check_names_pattern = \
        (?i)^(?>(?(1)\.|())[^\W_](?>[a-z0-9-]*[^\W_])?)+$

    which permits only letters, digits, and hyphens in components, but they
    may not start or end with a hyphen. If you set "allow_utf8_domains", you
    must modify this pattern, or set the option to an empty string.

dns_ipv4_lookup               Type: domain list*                Default: unset

    When Exim is compiled with IPv6 support, it looks for IPv6 address records
    (AAAA and, if configured, A6) as well as IPv4 address records when trying
    to find IP addresses for hosts, unless the host's domain matches this
    list.

    This is a fudge to help with name servers that give big delays or
    otherwise do not work for the new IPv6 record types. If Exim is handed an
    IPv6 address record as a result of an MX lookup, it always recognizes it,
    and may as a result make an outgoing IPv6 connection. All this option does
    is to make Exim look only for IPv4-style A records when it needs to find
    an IP address for a host name. In due course, when the world's name
    servers have all been upgraded, there should be no need for this option.

dns_retrans                       Type: time                       Default: 0s

    The options "dns_retrans" and "dns_retry" can be used to set the
    retransmission and retry parameters for DNS lookups. Values of zero (the
    defaults) leave the system default settings unchanged. The first value is
    the time between retries, and the second is the number of retries. It
    isn't totally clear exactly how these settings affect the total time a DNS
    lookup may take. I haven't found any documentation about timeouts on DNS
    lookups; these parameter values are available in the external resolver
    interface structure, but nowhere does it seem to describe how they are
    used or what you might want to set in them.

dns_retry                       Type: integer                      Default: 0

    See "dns_retrans" above.

drop_cr                         Type: boolean                  Default: false  |
                                                                               |
    This is an obsolete option that is now a no-op. It used to affect the way  |
    Exim handled CR and LF characters in incoming messages. What happens now   |
    is described in section 44.1.                                              |

envelope_to_remove              Type: boolean                   Default: true

    Exim's transports have an option for adding an Envelope-to: header to a
    message when it is delivered - in exactly the same way as Return-path: is
    handled. Envelope-to: records the original recipient address from the
    messages's envelope that caused the delivery to happen. Such headers
    should not be present in incoming messages, and this option causes them to
    be removed at the time the message is received, to avoid any problems that
    might occur when a delivered message is subsequently sent on to some other
    recipient.

errors_copy                   Type: string list*                Default: unset

    Setting this option causes Exim to send bcc copies of bounce messages that
    it generates to other addresses. Note: this does not apply to bounce
    messages coming from elsewhere. The value of the option is a colon-
    separated list of items. Each item consists of a pattern, terminated by
    white space, followed by a comma-separated list of email addresses. If a
    pattern contains spaces, it must be enclosed in double quotes.

    Each pattern is processed in the same way as a single item in an address
    list (see section 10.16). When a pattern matches the recipient of the
    bounce message, the message is copied to the addresses on the list. The
    items are scanned in order, and once a matching one is found, no further
    items are examined. For example:

      errors_copy = spqr@mydomain   postmaster@mydomain.example :\
                    rqps@mydomain   hostmaster@mydomain.example,\
                                    postmaster@mydomain.example

    The address list is expanded before use. The expansion variables
    $local_part and $domain are set from the original recipient of the error
    message, and if there was any wildcard matching in the pattern, the
    expansion variables $0, $1, etc. are set in the normal way.

errors_reply_to                  Type: string                   Default: unset

    Exim's bounce and delivery warning messages contain the header line        |
                                                                               |
      From: Mail Delivery System <Mailer-Daemon@<qualify-domain>>              |
                                                                               |
    where <qualify-domain> is the value of the "qualify_domain" option.        |
    Experience shows that people reply to bounce messages. If the
    "errors_reply_to" option is set, a Reply-To: header is added to bounce and
    warning messages. For example:

      errors_reply_to = postmaster@my.domain.example

    The value of the option is not expanded. It must specify a valid RFC 2822
    address.

exim_group                       Type: string Default: compile-time configured

    This option changes the gid under which Exim runs when it gives up root
    privilege. The default value is compiled into the binary. The value of
    this option is used only when "exim_user" is also set. Unless it consists
    entirely of digits, the string is looked up using "getgrnam()", and
    failure causes a configuration error. See chapter 48 for a discussion of
    security issues.

exim_path                        Type: string               Default: see below

    This option specifies the path name of the Exim binary, which is used when
    Exim needs to re-exec itself. The default is set up to point to the file
    "exim" in the directory configured at compile time by the BIN_DIRECTORY
    setting. It is necessary to change "exim_path" if, exceptionally, Exim is
    run from some other place. Warning: Do not use a macro to define the value
    of this option, because you will break those Exim utilities that scan the
    configuration file to find where the binary is. (They then use the -bP
    option to extract option settings such as the value of "spool_directory".)

exim_user                        Type: string Default: compile-time configured

    This option changes the uid under which Exim runs when it gives up root
    privilege. The default value is compiled into the binary. Ownership of the
    run time configuration file and the use of the -C and -D command line
    options is checked against the values in the binary, not what is set here.

    Unless it consists entirely of digits, the string is looked up using
    "getpwnam()", and failure causes a configuration error. If "exim_group" is
    not also supplied, the gid is taken from the result of "getpwnam()" if it
    is used. See chapter 48 for a discussion of security issues.

extra_local_interfaces        Type: string list                Default: unset  |
                                                                               |
    This option defines network interfaces that are to be considered local     |
    when routing, but which are not used for listening by the daemon. See      |
    section 13.6 for details.                                                  |

extract_addresses_remove_arguments
                                  Type: boolean                 Default: true

    According to some Sendmail documentation (Sun, IRIX, HP-UX), if any
    addresses are present on the command line when the -t option is used to
    build an envelope from a message's To:, Cc: and Bcc: headers, the command
    line addresses are removed from the recipients list. This is also how
    Smail behaves. However, other Sendmail documentation (the O'Reilly book)
    states that command line addresses are added to those obtained from the
    header lines. When "extract_addresses_remove_arguments" is true (the
    default), Exim subtracts argument headers. If it is set false, Exim adds
    rather than removes argument addresses.

finduser_retries                Type: integer                      Default: 0

    On systems running NIS or other schemes in which user and group infor-
    mation is distributed from a remote system, there can be times when
    "getpwnam()" and related functions fail, even when given valid data,
    because things time out. Unfortunately these failures cannot be dis-
    tinguished from genuine 'not found' errors. If "finduser_retries" is set
    greater than zero, Exim will try that many extra times to find a user or a
    group, waiting for one second between retries.

freeze_tell           Type: string list, comma separated        Default: unset

    On encountering certain errors, or when configured to do so in a system
    filter, or in an ACL, Exim freezes a message. This means that no further   |
    delivery attempts take place until an administrator (or the "auto_thaw"
    feature) thaws the message. If "freeze_tell" is set, Exim generates a
    warning message whenever it freezes something, unless the message it is
    freezing is a locally-generated bounce message. (Without this exception    |
    there is the possibility of looping.) The warning message is sent to the
    addresses supplied as the comma-separated value of this option. If several
    of the message's addresses cause freezing, only a single message is sent.  |
    If the freezing was automatic, the reason(s) for freezing can be found in  |
    the message log. If you configure freezing in a filter or ACL, you must    |
    arrange for any logging that you require.                                  |

gecos_name                      Type: string*                  Default: unset

    Some operating systems, notably HP-UX, use the 'gecos' field in the system
    password file to hold other information in addition to users' real names.
    Exim looks up this field for use when it is creating Sender: or From:
    headers. If either "gecos_pattern" or "gecos_name" are unset, the contents
    of the field are used unchanged, except that, if an ampersand is
    encountered, it is replaced by the user's login name with the first
    character forced to upper case, since this is a convention that is
    observed on many systems.

    When these options are set, "gecos_pattern" is treated as a regular
    expression that is to be applied to the field (again with & replaced by
    the login name), and if it matches, "gecos_name" is expanded and used as
    the user's name. Numeric variables such as $1, $2, etc. can be used in the
    expansion to pick up sub-fields that were matched by the pattern. In HP-
    UX, where the user's name terminates at the first comma, the following can
    be used:

      gecos_pattern = ([^,]*)
      gecos_name = $1

gecos_pattern                    Type: string                   Default: unset

    See "gecos_name" above.

headers_charset                  Type: string               Default: see below |
                                                                               |
    This option sets a default character set for translating from encoded MIME |
    'words' in header lines, when referenced by an $h_xxx expansion item. The  |
    default is the value of HEADERS_CHARSET in Local/Makefile. The ultimate    |
    default is ISO-8859-1. For more details see the description of header      |
    insertions in section 11.4.                                                |

header_maxsize                  Type: integer              Default: see below

    This option controls the overall maximum size of a message's header
    section. The default is the value of HEADER_MAXSIZE in Local/Makefile; the
    default for that is 1M. Messages with larger header sections are rejected.

header_line_maxsize             Type: integer                      Default: 0

    This option limits the length of any individual header line in a message,
    after all the continuations have been joined together. Messages with
    individual header lines that are longer than the limit are rejected. The
    default value of zero means 'no limit'.

helo_accept_junk_hosts         Type: host list*                 Default: unset

    Exim checks the syntax of HELO and EHLO commands for incoming SMTP mail,
    and gives an error response for invalid data. Unfortunately, there are
    some SMTP clients that send syntactic junk. They can be accommodated
    by setting this option. Note that this is a syntax check only. See
    "helo_verify_hosts" if you want to do semantic checking. See also
    "helo_allow_chars" for a way of extending the permitted character set.

helo_allow_chars                 Type: string                   Default: unset

    This option can be set to a string of rogue characters that are permitted
    in all EHLO and HELO names in addition to the standard letters, digits,
    hyphens, and dots. If you really must allow underscores, you can set

      helo_allow_chars = _

    Note that the value is one string, not a list.

helo_lookup_domains           Type: domain list*                Default: @:@[]

    If the domain given by a client in a HELO or EHLO command matches this
    list, a reverse lookup is done in order to establish the host's true name.
    The default forces a lookup if the client host gives the server's name or
    any of its IP addresses (in brackets), something that broken clients have
    been seen to do.

helo_try_verify_hosts          Type: host list*                 Default: unset

    The RFCs mandate that a server must not reject a message because it
    doesn't like the HELO or EHLO command. By default, Exim just checks the
    syntax of these commands (see "helo_accept_junk_hosts" and
    "helo_allow_chars" above). However, some sites like to be stricter. If the
    calling host matches "helo_try_verify_hosts", Exim checks that the host
    name given in the HELO or EHLO command either:

     .   is an IP literal matching the calling address of the host (the RFCs
         specifically allow this), or

     .   matches the host name that Exim obtains by doing a reverse lookup of
         the calling host address, or

     .   when looked up using "gethostbyname()" (or "getipnodebyname()" when
         available) yields the calling host address.

    However, the EHLO or HELO command is not rejected if any of the checks
    fail. Processing continues, but the result of the check is remembered, and
    can be detected later in an ACL by the "verify = helo" condition. If you
    want verification failure to cause rejection of EHLO or HELO, use
    "helo_verify_hosts" instead.

helo_verify_hosts              Type: host list*                 Default: unset

    For hosts that match this option, Exim checks the host name given in the
    HELO or EHLO in the same way as for "helo_try_verify_hosts". If the check
    fails, the HELO or EHLO command is rejected with a 550 error, and entries
    are written to the main and reject logs. If a MAIL command is received
    before EHLO or HELO, it is rejected with a 503 error.

hold_domains                  Type: domain list*                Default: unset

    This option allows mail for particular domains to be held on the queue
    manually. The option is overridden if a message delivery is forced with
    the -M, -qf, -Rf or -Sf options, and also while testing or verifying
    addresses using -bt or -bv. Otherwise, if a domain matches an item in
    "hold_domains", no routing or delivery for that address is done, and it is
    deferred every time the message is looked at.

    This option is intended as a temporary operational measure for delaying
    the delivery of mail while some problem is being sorted out, or some new
    configuration tested. If you just want to delay the processing of some
    domains until a queue run occurs, you should use "queue_domains" or
    "queue_smtp_domains", not "hold_domains".

    A setting of "hold_domains" does not override Exim's code for removing
    messages from the queue if they have been there longer than the longest
    retry time in any retry rule. If you want to hold messages for longer than
    the normal retry times, insert a dummy retry rule with a long retry time.

host_lookup                    Type: host list*                 Default: unset

    Exim does not look up the name of a calling host from its IP address
    unless it is required to compare against some host list, or the host
    matches "helo_try_verify_hosts" or "helo_verify_hosts", or the host
    matches this option (which normally contains IP addresses rather than host
    names). The default configuration file contains

      host_lookup = *

    which causes a lookup to happen for all hosts. If the expense of these
    lookups is felt to be too great, the setting can be changed or removed.

    After a successful reverse lookup, Exim does a forward lookup on the name
    it has obtained, to verify that it yields the IP address that it started
    with. If this check fails, Exim behaves as if the name lookup failed.

    After any kind of failure, the host name (in $sender_host_name) remains
    unset, and $host_lookup_failed is set to the string '1'. See also
    "helo_lookup_domains" and "verify = reverse_host_lookup" in ACLs.

host_lookup_order             Type: string list         Default: bydns:byaddr  |
                                                                               |
    This option specifies the order of different lookup methods when Exim is   |
    trying to find a host name from an IP address. The default is to do a DNS  |
    lookup first, and then to try a local lookup (using "gethostbyaddr()" or   |
    equivalent) if that fails. You can change the order of these lookups, or   |
    omit one entirely, if you want.                                            |
                                                                               |
    Warning: the 'byaddr' method does not always yield aliases when there are  |
    multiple PTR records in the DNS and the IP address is not listed in        |
    /etc/hosts. Different operating systems give different results in this     |
    case. That is why the default tries a DNS lookup first.                    |

host_reject_connection         Type: host list*                 Default: unset

    If this option is set, incoming SMTP calls from the hosts listed are
    rejected as soon as the connection is made. This option is obsolete, and   |
    retained only for backward compatibility, because nowadays the ACL speci-  |
    fied by "acl_smtp_connect" can also reject incoming connections            |
    immediately.                                                               |
                                                                               |
    The ability to give an immediate rejection (either by this option or using |
    an ACL) is provided for use in unusual cases. Many hosts will just try     |
    again, sometimes without much delay. Normally, it is better to use an ACL
    to reject incoming messages at a later stage, such as after RCPT commands.
    See chapter 38.

hosts_treat_as_local          Type: domain list*                Default: unset

    If this option is set, any host names that match the domain list are
    treated as if they were the local host when Exim is scanning host lists
    obtained from MX records or other sources. Note that the value of this     |
    option is a domain list, not a host list, because it is always used to     |
    check host names, not IP addresses.                                        |

    This option also applies when Exim is matching the special items
    "@mx_any", "@mx_primary", and "@mx_secondary" in a domain list (see
    section 10.7), and when checking the "hosts" option in the smtp transport
    for the local host (see the "allow_localhost" option in that transport).   |
    See also "local_interfaces", "extra_local_interfaces", and chapter 13,     |
    which contains a discussion about local network interfaces and recognising |
    the local host.                                                            |

ignore_bounce_errors_after        Type: time                      Default: 10w

    This option affects the processing of bounce messages that cannot be
    delivered, that is, those that suffer a permanent delivery failure.        |
    (Bounce messages that suffer temporary delivery failures are of course     |
    retried in the usual way.)                                                 |
                                                                               |
    After a permanent delivery failure, bounce messages are frozen, because    |
    there is no sender to whom they can be returned. When a frozen bounce
    message has been on the queue for more than the given time, it is unfrozen
    at the next queue run, and a further delivery is attempted. If delivery
    fails again, the bounce message is discarded. This makes it possible to
    keep failed bounce messages around for a shorter time than the normal
    maximum retry time for frozen messages. For example,

      ignore_bounce_errors_after = 12h

    retries failed bounce message deliveries after 12 hours, discarding any
    further failures. If the value of this option is set to a zero time
    period, bounce failures are discarded immediately. Setting a very long
    time (as in the default value) has the effect of disabling this option.
    For ways of automatically dealing with other kinds of frozen message, see
    "auto_thaw" and "timeout_frozen_after".

ignore_fromline_hosts          Type: host list*                 Default: unset

    Some broken SMTP clients insist on sending a UUCP-like 'From' line before
    the headers of a message. By default this is treated as the start of the
    message's body, which means that any following headers are not recognized
    as such. Exim can be made to ignore it by setting "ignore_fromline_hosts"
    to match those hosts that insist on sending it. If the sender is actually
    a local process rather than a remote host, and is using -bs to inject the
    messages, "ignore_fromline_local" must be set to achieve this effect.

ignore_fromline_local           Type: boolean                  Default: false

    See "ignore_fromline_hosts" above.

keep_malformed                    Type: time                       Default: 4d

    This option specifies the length of time to keep messages whose spool
    files have been corrupted in some way. This should, of course, never
    happen. At the next attempt to deliver such a message, it gets removed.
    The incident is logged.

ldap_default_servers          Type: string list                Default: unset

    This option provides a list of LDAP servers which are tried in turn when
    an LDAP query does not contain a server. See section 9.11 for details of
    LDAP queries. This option is available only when Exim has been built with
    LDAP support.

ldap_version                    Type: integer                  Default: unset

    This option can be used to force Exim to set a specific protocol version
    for LDAP. If it option is unset, it is shown by the -bP command line
    option as -1. When this is the case, the default is 3 if LDAP_VERSION3 is
    defined in the LDAP headers; otherwise it is 2. This option is available
    only when Exim has been built with LDAP support.

local_from_check                Type: boolean                   Default: true

    When a message is submitted locally (that is, not over a TCP/IP connec-
    tion) by an untrusted user, Exim removes any existing Sender: header line,
    and checks that the From: header line matches the login of the calling
    user. You can use "local_from_prefix" and "local_from_suffix" to permit
    affixes on the local part. If the From: header line does not match, Exim
    adds a Sender: header with an address constructed from the calling user's
    login and the default qualify domain.

    If "local_from_check" is set false, the From: header check is disabled,
    and no Sender: header is ever added. If, in addition, you want to retain
    Sender: header lines supplied by untrusted users, you must also set
    "local_sender_retain" to be true.

    These options affect only the header lines in the message. The envelope
    sender is still forced to be the login id at the qualify domain unless
    "untrusted_set_sender" permits the user to supply an envelope sender.
    Section 44.14 has more details about Sender: processing.

local_from_prefix                Type: string                   Default: unset

    When Exim checks the From: header line of locally submitted messages for
    matching the login id (see "local_from_check" above), it can be configured
    to ignore certain prefixes and suffixes in the local part of the address.
    This is done by setting "local_from_prefix" and/or "local_from_suffix" to
    appropriate lists, in the same form as the "local_part_prefix" and
    "local_part_suffix" router options (see chapter 15). For example, if

      local_from_prefix = *-

    is set, a From: line containing

      From: anything-user@your.domain.example

    will not cause a Sender: header to be added if "user@your.domain.example"
    matches the actual sender address that is constructed from the login name
    and qualify domain.

local_from_suffix                Type: string                   Default: unset

    See "local_from_prefix" above.

local_interfaces              Type: string list            Default: see below  |
                                                                               |
    This option controls which network interfaces are used by the daemon for   |
    listening; they are also used to identify the local host when routing.     |
    Chapter 13 contains a full description of this option and the related      |
    options "extra_local_interfaces" and "hosts_treat_as_local". The default   |
    value for "local_interfaces" is                                            |
                                                                               |
      local_interfaces = 0.0.0.0                                               |
                                                                               |
    when Exim is built without IPv6 support; otherwise it is                   |
                                                                               |
      local_interfaces = <; ::0 ; 0.0.0.0                                      |
                                                                               |
local_scan_timeout                Type: time                       Default: 5m

    This timeout applies to the "local_scan()" function (see chapter 39). Zero
    means 'no timeout'. If the timeout is exceeded, the incoming message is
    rejected with a temporary error if it is an SMTP message. For a non-SMTP
    message, the message is dropped and Exim ends with a non-zero code. The
    incident is logged on the main and reject logs.

local_sender_retain             Type: boolean                  Default: false

    When a message is submitted locally (that is, not over a TCP/IP connec-
    tion) by an untrusted user, Exim removes any existing Sender: header line.
    If you do not want this to happen, you must set "local_sender_retain", and
    you must also set "local_from_check" to be false (Exim will complain if
    you do not). Section 44.14 has more details about Sender: processing.

localhost_number                Type: string*                  Default: unset

    Exim's message ids are normally unique only within the local host. If
    uniqueness among a set of hosts is required, each host must set a
    different value for the "localhost_number" option. The string is expanded
    immediately after reading the configuration file (so that a number can be
    computed from the host name, for example) and the result of the expansion
    must be a number in the range 0-16 (or 0-10 on operating systems with
    case-insensitive file systems). This is available in subsequent string
    expansions via the variable $localhost_number. When "localhost_number is
    set", the final two characters of the message id, instead of just being a
    fractional part of the time, are computed from the time and the local host
    number as described in section 3.4.

log_file_path                 Type: string list*  Default: set at compile time

    This option sets the path which is used to determine the names of Exim's
    log files, or indicates that logging is to be to syslog, or both. It is
    expanded when Exim is entered, so it can, for example, contain a reference
    to the host name. If no specific path is set for the log files at compile
    or run time, they are written in a sub-directory called log in Exim's
    spool directory. Chapter 45 contains further details about Exim's logging,
    and section 45.1 describes how the contents of "log_file_path" are used.
    If this string is fixed at your installation (contains no expansion
    variables) it is recommended that you do not set this option in the
    configuration file, but instead supply the path using LOG_FILE_PATH in
    Local/Makefile so that it is available to Exim for logging errors detected
    early on - in particular, failure to read the configuration file.

log_selector                     Type: string                   Default: unset

    This option can be used to reduce or increase the number of things that
    Exim writes to its log files. Its argument is made up of names preceded by
    plus or minus characters. For example:

      log_selector = +arguments -retry_defer

    A list of possible names and what they control is given in the chapter on
    logging, in section 45.15.

log_timezone                    Type: boolean                  Default: false

    By default, the timestamps on log lines are in local time without the
    timezone. This means that if your timezone changes twice a year, the
    timestamps in log lines are ambiguous for an hour when the clocks go back.
    One way of avoiding this problem is to set the timezone to UTC. An
    alternative is to set "log_timezone" true. This turns on the addition of
    the timezone offset to timestamps in log lines. Turning on this option can
    add quite a lot to the size of log files because each line is extended by
    6 characters. Note that the $tod_log variable contains the log timestamp
    without the zone, but there is another variable called $tod_zone that
    contains just the timezone offset.

lookup_open_max                 Type: integer                     Default: 25

    This option limits the number of simultaneously open files for single-key
    lookups that use regular files (that is, lsearch, dbm, and cdb). Exim
    normally keeps these files open during routing, because often the same
    file is required several times. If the limit is reached, Exim closes the
    least recently used file. Note that if you are using the "ndbm" library,
    it actually opens two files for each logical DBM database, though it still
    counts as one for the purposes of "lookup_open_max". If you are getting
    'too many open files' errors with NDBM, you need to reduce the value of
    "lookup_open_max".

max_username_length             Type: integer                      Default: 0

    Some operating systems are broken in that they truncate long arguments to
    "getpwnam()" to eight characters, instead of returning 'no such user'. If
    this option is set greater than zero, any attempt to call "getpwnam()"
    with an argument that is longer behaves as if "getpwnam()" failed.

message_body_visible            Type: integer                    Default: 500

    This option specifies how much of a message's body is to be included in
    the $message_body and $message_body_end expansion variables.

message_id_header_domain        Type: string*                  Default: unset

    If this option is set, the string is expanded and used as the right hand
    side (domain) of the Message-ID: header that Exim creates if a locally-    |
    originated incoming message does not have one. 'Locally-originated' means  |
    'not received over TCP/IP.' Otherwise, the primary host name is used. Only |
    letters, digits, dot and hyphen are accepted; any other characters are
    replaced by hyphens. If the expansion is forced to fail, or if the result
    is an empty string, the option is ignored.

message_id_header_text          Type: string*                  Default: unset

    If this variable is set, the string is expanded and used to augment the
    text of the Message-id: header that Exim creates if a locally-originated   |
    incoming message does not have one. The text of this header is required by
    RFC 2822 to take the form of an address. By default, Exim uses its
    internal message id as the local part, and the primary host name as the
    domain. If this option is set, it is expanded, and provided the expansion
    is not forced to fail, and does not yield an empty string, the result is
    inserted into the header immediately before the @, separated from the
    internal message id by a dot. Any characters that are illegal in an
    address are automatically converted into hyphens. This means that vari-
    ables such as $tod_log can be used, because the spaces and colons will
    become hyphens.

message_logs                    Type: boolean                   Default: true

    If this option is turned off, per-message log files are not created in the
    msglog spool sub-directory. This reduces the amount of disk I/O required
    by Exim, by reducing the number of files involved in handling a message
    from a minimum of four (header spool file, body spool file, delivery
    journal, and per-message log) to three. The other major I/O activity is
    Exim's main log, which is not affected by this option.

message_size_limit              Type: string*                    Default: 50M

    This option limits the maximum size of message that Exim will process. The
    value is expanded for each incoming connection so, for example, it can be
    made to depend on the IP address of the remote host for messages arriving
    via TCP/IP. Note: This limit cannot be made to depend on a message's
    sender or any other properties of an individual message, because it has to
    be advertised in the server's response to EHLO. String expansion failure
    causes a temporary error. A value of zero means no limit, but its use is
    not recommended. See also "bounce_return_size_limit".

    Incoming SMTP messages are failed with a 552 error if the limit is
    exceeded; locally-generated messages either get a stderr message or a
    delivery failure message to the sender, depending on the -oe setting.
    Rejection of an oversized message is logged in both the main and the
    reject logs. See also the generic transport option "message_size_limit",
    which limits the size of message that an individual transport can process.

move_frozen_messages            Type: boolean                  Default: false

    This option, which is available only if Exim has been built with the
    setting

      SUPPORT_MOVE_FROZEN_MESSAGES=yes

    in Local/Makefile, causes frozen messages and their message logs to be
    moved from the input and msglog directories on the spool to Finput and
    Fmsglog, respectively. There is currently no support in Exim or the
    standard utilities for handling such moved messages, and they do not show
    up in lists generated by -bp or by the Exim monitor.

mysql_servers                 Type: string list                Default: unset

    This option provides a list of MySQL servers and associated connection
    data, to be used in conjunction with mysql lookups (see section 9.17). The
    option is available only if Exim has been built with MySQL support.

never_users                   Type: string list                Default: unset

    Local message deliveries are normally run in processes that are setuid to
    the recipient, and remote deliveries are normally run under Exim's own uid
    and gid. It is usually desirable to prevent any deliveries from running as
    root, as a safety precaution.

    When Exim is built, an option called FIXED_NEVER_USERS can be set to a     |
    list of users that must not be used for local deliveries. This list is     |
    fixed in the binary and cannot be overridden by the configuration file. By |
    default, it contains just the single user name 'root'. The "never_users"   |
    runtime option can be used to add more users to the fixed list.            |
                                                                               |
    If a message is to be delivered as one of the users on the fixed list or   |
    the "never_users" list, an error occurs, and delivery is deferred. A       |
    common example is                                                          |
                                                                               |
      never_users = root:daemon:bin                                            |
                                                                               |
    Including root is redundant if it is also on the fixed list, but it does   |
    no harm. This option overrides the "pipe_as_creator" option of the pipe    |
    transport driver.

oracle_servers                Type: string list                Default: unset

    This option provides a list of Oracle servers and associated connection
    data, to be used in conjunction with oracle lookups (see section 9.17).
    The option is available only if Exim has been built with Oracle support.

percent_hack_domains          Type: domain list*                Default: unset

    The 'percent hack' is the convention whereby a local part containing a
    percent sign is re-interpreted as a new email address, with the percent
    replaced by @. This is sometimes called 'source routing', though that term
    is also applied to RFC 2822 addresses that begin with an @ character. If
    this option is set, Exim implements the percent facility for those domains
    listed, but no others. This happens before an incoming SMTP address is
    tested against an ACL.

    Warning: The 'percent hack' has often been abused by people who are trying
    to get round relaying restrictions. For this reason, it is best avoided if
    at all possible. Unfortunately, a number of less security-conscious MTAs
    implement it unconditionally. If you are running Exim on a gateway host,
    and routing mail through to internal MTAs without processing the local
    parts, it is a good idea to reject recipient addresses with percent
    characters in their local parts. Exim's default configuration does this.

perl_at_start                   Type: boolean                  Default: false

    This option is available only when Exim is built with an embedded Perl
    interpreter. See chapter 12 for details of its use.

perl_startup                     Type: string                   Default: unset

    This option is available only when Exim is built with an embedded Perl
    interpreter. See chapter 12 for details of its use.

pgsql_servers                 Type: string list                Default: unset

    This option provides a list of PostgreSQL servers and associated connec-
    tion data, to be used in conjunction with pgsql lookups (see section
    9.17). The option is available only if Exim has been built with PostgreSQL
    support.

pid_file_path                   Type: string*    Default: set at compile time

    This option sets the name of the file to which the Exim daemon writes its
    process id. The string is expanded, so it can contain, for example,
    references to the host name:

      pid_file_path = /var/log/$primary_hostname/exim.pid

    If no path is set, the pid is written to the file exim-daemon.pid in
    Exim's spool directory. The value set by the option can be overridden by
    the -oP command line option. A pid file is not written if a 'non-standard'
    daemon is run by means of the -oX option, unless a path is explicitly
    supplied by -oP.

pipelining_advertise_hosts     Type: host list*                     Default: *

    This option can be used to suppress the advertisement of the SMTP
    PIPELINING extension to specific hosts. When PIPELINING is not advertised
    and "smtp_enforce_sync" is true, an Exim server enforces strict
    synchronization for each SMTP command and response.

preserve_message_logs           Type: boolean                  Default: false

    If this option is set, message log files are not deleted when messages are
    completed. Instead, they are moved to a sub-directory of the spool
    directory called msglog.OLD, where they remain available for statistical
    or debugging purposes. This is a dangerous option to set on systems with
    any appreciable volume of mail. Use with care!

primary_hostname                 Type: string               Default: see below

    This specifies the name of the current host. This is used in the EHLO
    command for outgoing SMTP messages, and as the default for
    "qualify_domain". If it is not set, Exim calls "uname()" to find it. If
    this fails, Exim panics and dies. If the name returned by "uname()"
    contains only one component, Exim passes it to "gethostbyname()" (or
    "getipnodebyname()" when available) in order to obtain the fully qualified
    version.

print_topbitchars               Type: boolean                  Default: false

    By default, Exim considers only those characters whose codes lie in the
    range 32-126 to be printing characters. In a number of circumstances (for
    example, when writing log entries) non-printing characters are converted
    into escape sequences, primarily to avoid messing up the layout. If
    "print_topbitchars" is set, code values of 128 and above are also
    considered to be printing characters.

process_log_path                 Type: string                   Default: unset |
                                                                               |
    This option sets the name of the file to which an Exim process writes its  |
    'process log' when sent a USR1 signal. This is used by the "exiwhat"       |
    utility script. If this option is unset, the file called exim-process.info |
    in Exim's spool directory is used. The ability to specify the name         |
    explicitly can be useful in environments where two different Exims are     |
    running, using different spool directories.                                |

prod_requires_admin             Type: boolean                   Default: true

    The -M, -R, and -q command-line options require the caller to be an admin
    user unless "prod_requires_admin" is set false. See also
    "queue_list_requires_admin".

qualify_domain                   Type: string               Default: see below

    This option specifies the domain name that is added to any sender
    addresses that do not have a domain qualification. It also applies to
    recipient addresses if "qualify_recipient" is not set. Such addresses are
    accepted by default only for locally-generated messages. Messages from
    external sources must always contain fully qualified addresses, unless
    the sending host matches "sender_unqualified_hosts" or
    "recipient_unqualified_hosts" (as appropriate), in which case incoming
    addresses are qualified with "qualify_domain" or "qualify_recipient" as
    necessary. Internally, Exim always works with fully qualified addresses.
    If "qualify_domain" is not set, it defaults to the "primary_hostname"
    value.

qualify_recipient                Type: string               Default: see below

    This specifies the domain name that is added to any recipient addresses
    that do not have a domain qualification. Such addresses are accepted
    by default only for locally-generated messages. Messages from external
    sources must always contain fully qualified recipient addresses, unless
    the sending host matches "recipient_unqualified_hosts", in which case
    incoming recipient addresses are qualified with "qualify_recipient". If
    "qualify_recipient" is not set, it defaults to the "qualify_domain" value.

queue_domains                 Type: domain list*                Default: unset

    This option lists domains for which immediate delivery is not required. A
    delivery process is started whenever a message is received, but only those
    domains that do not match are processed. All other deliveries wait until
    the next queue run. See also "hold_domains" and "queue_smtp_domains".

queue_list_requires_admin       Type: boolean                   Default: true

    The -bp command-line option, which lists the messages that are on the
    queue, requires the caller to be an admin user unless "queue_list_
    requires_admin" is set false. See also "prod_requires_admin".

queue_only                      Type: boolean                  Default: false

    If "queue_only" is set, a delivery process is not automatically started
    whenever a message is received. Instead, the message waits on the queue
    for the next queue run. Even if "queue_only" is false, incoming messages
    may not get delivered immediately when certain conditions (such as heavy
    load) occur.

    The -odq command line has the same effect as "queue_only". The -odb and    |
    -odi command line options override "queue_only" unless                     |
    "queue_only_override" is set false. See also "queue_only_file",            |
    "queue_only_load", and "smtp_accept_queue".                                |

queue_only_file                  Type: string                   Default: unset

    This option can be set to a colon-separated list of absolute path names,
    each one optionally preceded by 'smtp'. When Exim is receiving a message,
    it tests for the existence of each listed path using a call to "stat()".
    For each path that exists, the corresponding queuing option is set. For
    paths with no prefix, "queue_only" is set; for paths prefixed by 'smtp',
    "queue_smtp_domains" is set to match all domains. So, for example,

      queue_only_file = smtp/some/file

    causes Exim to behave as if "queue_smtp_domains" were set to '*' whenever
    /some/file exists.

queue_only_load               Type: fixed-point                Default: unset

    If the system load average is higher than this value, incoming messages
    from all sources are queued, and no automatic deliveries are started. If
    this happens during local or remote SMTP input, all subsequent messages on
    the same connection are queued. Deliveries will subsequently be performed
    by queue runner processes. This option has no effect on ancient operating
    systems on which Exim cannot determine the load average. See also
    "deliver_queue_load_max" and "smtp_load_reserve".

queue_only_override             Type: boolean                   Default: true  |
                                                                               |
    When this option is true, the -od"x" command line options override the     |
    setting of "queue_only" or "queue_only_file" in the configuration file. If |
    "queue_only_override" is set false, the -od"x" options cannot be used to   |
    override; they are accepted, but ignored.                                  |

queue_run_in_order              Type: boolean                  Default: false

    If this option is set, queue runs happen in order of message arrival
    instead of in an arbitrary order. For this to happen, a complete list of
    the entire queue must be set up before the deliveries start. When the
    queue is all in a single directory (the default), this happens anyway, but
    if "split_spool_directory" is set it does not - for delivery in random
    order, the sub-directories are processed one at a time (in random order),
    to avoid setting up one huge list. Thus, setting "queue_run_in_order" with
    "split_spool_directory" may degrade performance when the queue is large.
    In most situations, "queue_run_in_order" should not be set.

queue_run_max                   Type: integer                      Default: 5

    This controls the maximum number of queue runner processes that an Exim
    daemon can run simultaneously. This does not mean that it starts them all
    at once, but rather that if the maximum number are still running when the
    time comes to start another one, it refrains from starting another one.
    This can happen with very large queues and/or very sluggish deliveries.
    This option does not, however, interlock with other processes, so
    additional queue runners can be started by other means, or by killing and
    restarting the daemon.

queue_smtp_domains            Type: domain list*                Default: unset

    When this option is set, a delivery process is started whenever a message
    is received, routing is performed, and local deliveries take place.
    However, if any SMTP deliveries are required for domains that match
    "queue_smtp_domains", they are not immediately delivered, but instead the
    message waits on the queue for the next queue run. Since routing of the
    message has taken place, Exim knows to which remote hosts it must be
    delivered, and so when the queue run happens, multiple messages for the
    same host are delivered over a single SMTP connection. The -odqs command
    line option causes all SMTP deliveries to be queued in this way, and is
    equivalent to setting "queue_smtp_domains" to '*'. See also "hold_domains"
    and "queue_domains".

receive_timeout                   Type: time                       Default: 0s

    This option sets the timeout for accepting a non-SMTP message, that is,
    the maximum time that Exim waits when reading a message on the standard
    input. If the value is zero, it will wait for ever. This setting is
    overridden by the -or command line option. The timeout for incoming SMTP
    messages is controlled by "smtp_receive_timeout".


received_header_text            Type: string*              Default: see below

    This string defines the contents of the Received: message header that is
    added to each message, except for the timestamp, which is automatically
    added on at the end (preceded by a semicolon). The string is expanded each
    time it is used. If the expansion yields an empty string, no Received:     |
    header line is added to the message. Otherwise, the string should start    |
    with the text 'Received:' and conform to the RFC 2822 specification for    |
    Received: header lines. The default setting is:                            |

      received_header_text = Received: \
          ${if def:sender_rcvhost {from $sender_rcvhost\n\t}\
          {${if def:sender_ident {from $sender_ident }}\
          ${if def:sender_helo_name {(helo=$sender_helo_name)\n\t}}}}\
          by $primary_hostname \
          ${if def:received_protocol {with $received_protocol}} \
          ${if def:tls_cipher {($tls_cipher)\n\t}}\
          (Exim $version_number)\n\t\
          id $message_id\
          ${if def:received_for {\n\tfor $received_for}}

    Note the use of quotes, to allow the sequences "\n" and "\t" to be used
    for newlines and tabs, respectively. The reference to the TLS cipher is
    omitted when Exim is built without TLS support. The use of conditional
    expansions ensures that this works for both locally generated messages and
    messages received from remote hosts, giving header lines such as the
    following:

      Received: from scrooge.carol.example ([192.168.12.25] ident=root)
              by marley.carol.example with esmtp (Exim 4.00)
              id 16IOWa-00019l-00
              for chas@dickens.example; Tue, 25 Dec 2001 14:43:44 +0000
      Received: by scrooge.carol.example with local (Exim 4.00)
              id 16IOWW-000083-00; Tue, 25 Dec 2001 14:43:41 +0000

received_headers_max            Type: integer                     Default: 30

    When a message is to be delivered, the number of Received: headers is
    counted, and if it is greater than this parameter, a mail loop is assumed
    to have occurred, the delivery is abandoned, and an error message is
    generated. This applies to both local and remote deliveries.

recipient_unqualified_hosts    Type: host list*                 Default: unset

    This option lists those hosts from which Exim is prepared to accept
    unqualified recipient addresses in message envelopes. The addresses are
    made fully qualified by the addition of the "qualify_recipient" value.
    This option also affects message header lines. Exim does not reject
    unqualified recipient addresses in headers, but it qualifies them only if
    the message came from a host that matches "recipient_unqualified_hosts".

recipients_max                  Type: integer                      Default: 0

    If this option is set greater than zero, it specifies the maximum number
    of original recipients for any message. Additional recipients that are
    generated by aliasing or forwarding do not count. SMTP messages get a 452
    response for all recipients over the limit; earlier recipients are
    delivered as normal. Non-SMTP messages with too many recipients are
    failed, and no deliveries are done. Note that the RFCs specify that an
    SMTP server should accept at least 100 RCPT commands in a single message.

recipients_max_reject           Type: boolean                  Default: false

    If this option is set true, Exim rejects SMTP messages containing too many
    recipients by giving 552 errors to the surplus RCPT commands, and a 554
    error to the eventual DATA command. Otherwise (the default) it gives a 452
    error to the surplus RCPT commands and accepts the message on behalf of
    the initial set of recipients. The remote server should then re-send the
    message for the remaining recipients at a later time.

remote_max_parallel             Type: integer                      Default: 2

    This option controls parallel delivery of one message to a number of
    remote hosts. If the value is less than 2, parallel delivery is disabled,
    and Exim does all the remote deliveries for a message one by one.
    Otherwise, if a single message has to be delivered to more than one remote
    host, or if several copies have to be sent to the same remote host, up to
    "remote_max_parallel" deliveries are done simultaneously. If more than
    "remote_max_parallel" deliveries are required, the maximum number of
    processes are started, and as each one finishes, another is begun. The
    order of starting processes is the same as if sequential delivery were
    being done, and can be controlled by the "remote_sort_domains" option. If
    parallel delivery takes place while running with debugging turned on, the
    debugging output from each delivery process is tagged with its process id.

    This option controls only the maximum number of parallel deliveries for
    one message in one Exim delivery process. Because Exim has no central
    queue manager, there is no way of controlling the total number of
    simultaneous deliveries if the configuration allows a delivery attempt as
    soon as a message is received. If you want to control the total number of
    deliveries on the system, you need to set the "queue_only" option. This
    ensures that all incoming messages are added to the queue without starting
    a delivery process. Then set up an Exim daemon to start queue runner
    processes at appropriate intervals (probably fairly often, for example,
    every minute), and limit the total number of queue runners by setting the
    "queue_run_max" parameter. Because each queue runner delivers only one
    message at a time, the maximum number of deliveries that can then take
    place at once is "queue_run_max" multiplied by "remote_max_parallel".

    If it is purely remote deliveries you want to control, use "queue_smtp"
    instead of "queue_only". This has the added benefit of doing the SMTP
    routing before queuing, so that several messages for the same host will
    eventually get delivered down the same connection.

remote_sort_domains           Type: domain list*                Default: unset

    When there are a number of remote deliveries for a message, they are
    sorted by domain into the order given by this list. For example,

      remote_sort_domains = *.cam.ac.uk:*.uk

    would attempt to deliver to all addresses in the "cam.ac.uk" domain first,
    then to those in the "uk" domain, then to any others.

retry_data_expire                 Type: time                       Default: 7d

    This option sets a 'use before' time on retry information in Exim's hints
    database. Any older retry data is ignored. This means that, for example,
    once a host has not been tried for 7 days, Exim behaves as if it has no
    knowledge of past failures.

retry_interval_max                Type: time                      Default: 24h

    Chapter 32 describes Exim's mechanisms for controlling the intervals
    between delivery attempts for messages that cannot be delivered straight
    away. This option sets an overall limit to the length of time between
    retries.

return_path_remove              Type: boolean                   Default: true

    RFC 2821, section 4.4, states that an SMTP server must insert a Return-
    path: header line into a message when it makes a 'final delivery'. The
    Return-path: header preserves the sender address as received in the MAIL
    command. This description implies that this header should not be present
    in an incoming message. If "return_path_remove" is true, any existing
    Return-path: headers are removed from messages at the time they are
    received. Exim's transports have options for adding Return-path: headers
    at the time of delivery. They are normally used only for final local
    deliveries.

return_size_limit               Type: integer                   Default: 100K  |
                                                                               |
    This option is an obsolete synonym for "bounce_return_size_limit".         |

rfc1413_hosts                  Type: host list*                     Default: *

    RFC 1413 identification calls are made to any client host which matches an
    item in the list.

rfc1413_query_timeout             Type: time                      Default: 30s

    This sets the timeout on RFC 1413 identification calls. If it is set to
    zero, no RFC 1413 calls are ever made.

sender_unqualified_hosts       Type: host list*                 Default: unset

    This option lists those hosts from which Exim is prepared to accept
    unqualified sender addresses. The addresses are made fully qualified by
    the addition of "qualify_domain". This option also affects message header
    lines. Exim does not reject unqualified addresses in headers that contain
    sender addresses, but it qualifies them only if the message came from a
    host that matches "sender_unqualified_hosts".

smtp_accept_keepalive           Type: boolean                   Default: true

    This option controls the setting of the SO_KEEPALIVE option on incoming
    TCP/IP socket connections. When set, it causes the kernel to probe idle
    connections periodically, by sending packets with 'old' sequence numbers.
    The other end of the connection should send an acknowledgement if the
    connection is still okay or a reset if the connection has been aborted.
    The reason for doing this is that it has the beneficial effect of freeing
    up certain types of connection that can get stuck when the remote host is
    disconnected without tidying up the TCP/IP call properly. The keepalive
    mechanism takes several hours to detect unreachable hosts.

smtp_accept_max                 Type: integer                     Default: 20

    This option specifies the maximum number of simultaneous incoming SMTP
    calls that Exim will accept. It applies only to the listening daemon;
    there is no control (in Exim) when incoming SMTP is being handled by
    "inetd". If the value is set to zero, no limit is applied. However,
    it is required to be non-zero if either "smtp_accept_max_per_host" or
    "smtp_accept_queue" is set. See also "smtp_accept_reserve".

smtp_accept_max_nonmail         Type: integer                     Default: 10

    Exim counts the number of 'non-mail' commands in an SMTP session, and
    drops the connection if there are too many. This option defines 'too
    many'. The check catches some denial-of-service attacks. repeated failing
    AUTHs, or a mad client looping sending EHLO, for example. The check is
    applied only if the client host matches "smtp_accept_max_nonmail_hosts".

    When a new message is expected, one occurrence of RSET is not counted.
    This allows a client to send one RSET between messages (this is not
    necessary, but some clients do it). Exim also allows one uncounted
    occurence of HELO or EHLO, and one occurrence of STARTTLS between
    messages. After starting up a TLS session, another EHLO is expected, and
    so it too is not counted. The first occurrence of AUTH in a connection, or
    immediately following STARTTLS is not counted. Otherwise, all commands
    other than MAIL, RCPT, DATA, and QUIT are counted.

smtp_accept_max_nonmail_hosts  Type: host list*                     Default: *

    You can control which hosts are subject to the "smtp_max_nonmail" check by
    setting this option. The default value makes it apply to all hosts. By
    changing the value, you can exclude any badly-behaved hosts that you have
    to live with.

smtp_accept_max_per_connection  Type: integer                   Default: 1000

    The value of this option limits the number of MAIL commands that Exim is
    prepared to accept over a single SMTP connection, whether or not each
    command results in the transfer of a message. After the limit is reached,
    a 421 response is given to subsequent MAIL commands. This limit is a
    safety precaution against a client that goes mad (incidents of this type
    have been seen).

smtp_accept_max_per_host        Type: string*                  Default: unset

    This option restricts the number of simultaneous IP connections from a
    single host (strictly, from a single IP address) to the Exim daemon. The
    option is expanded, to enable different limits to be applied to different
    hosts by reference to $sender_host_address. Once the limit is reached,
    additional connection attempts from the same host are rejected with error
    code 421. The default value of zero imposes no limit. If this option is
    set, it is required that "smtp_accept_max" be non-zero.

    Warning: When setting this option you should not use any expansion
    constructions that take an appreciable amount of time. The expansion and
    test happen in the main daemon loop, in order to reject additional
    connections without forking additional processes (otherwise a denial-of-
    service attack could cause a vast number or processes to be created).
    While the daemon is doing this processing, it cannot accept any other
    incoming connections.

smtp_accept_queue               Type: integer                      Default: 0

    If the number of simultaneous incoming SMTP calls handled via the
    listening daemon exceeds this value, messages received by SMTP are just
    placed on the queue; no delivery processes are started automatically. A
    value of zero implies no limit, and clearly any non-zero value is useful
    only if it is less than the "smtp_accept_max" value (unless that is zero).
    See also "queue_only", "queue_only_load", "queue_smtp_domains", and the
    various -od command line options.

smtp_accept_queue_per_connectionType: integer                     Default: 10

    This option limits the number of delivery processes that Exim starts
    automatically when receiving messages via SMTP, whether via the daemon or
    by the use of -bs or -bS. If the value of the option is greater than zero,
    and the number of messages received in a single SMTP session exceeds this
    number, subsequent messages are placed on the queue, but no delivery
    processes are started. This helps to limit the number of Exim processes
    when a server restarts after downtime and there is a lot of mail waiting
    for it on other systems. On large systems, the default should probably be
    increased, and on dial-in client systems it should probably be set to zero
    (that is, disabled).

smtp_accept_reserve             Type: integer                      Default: 0

    When "smtp_accept_max" is set greater than zero, this option specifies a
    number of SMTP connections that are reserved for connections from the
    hosts that are specified in "smtp_reserve_hosts". The value set in
    "smtp_accept_max" includes this reserve pool. The specified hosts are not
    restricted to this number of connections; the option specifies a minimum
    number of connection slots for them, not a maximum. It is a guarantee
    that that group of hosts can always get at least "smtp_accept_reserve"
    connections.

    For example, if "smtp_accept_max" is set to 50 and "smtp_accept_reserve"
    is set to 5, once there are 45 active connections (from any hosts), new
    connections are accepted only from hosts listed in "smtp_reserve_hosts".
    See also "smtp_accept_max_per_host".

smtp_banner                     Type: string*              Default: see below

    This string, which is expanded every time it is used, is output as the
    initial positive response to an SMTP connection. The default setting is:

      smtp_banner = $primary_hostname ESMTP Exim $version_number \
        $tod_full

    Failure to expand the string causes a panic error. If you want to create a
    multiline response to the initial SMTP connection, use '\n' in the string
    at appropriate points, but not at the end. Note that the 220 code is not
    included in this string. Exim adds it automatically (several times in the
    case of a multiline response).

smtp_check_spool_space          Type: boolean                   Default: true

    When this option is set, if an incoming SMTP session encounters the SIZE
    option on a MAIL command, it checks that there is enough space in the
    spool directory's partition to accept a message of that size, while still
    leaving free the amount specified by "check_spool_space" (even if that
    value is zero). If there isn't enough space, a temporary error code is
    returned.

smtp_connect_backlog            Type: integer                     Default: 20

    This option specifies a maximum number of waiting SMTP connections. Exim
    passes this value to the TCP/IP system when it sets up its listener. Once
    this number of connections are waiting for the daemon's attention,
    subsequent connection attempts are refused at the TCP/IP level. At least,
    that is what the manuals say; in some circumstances such connection
    attempts have been observed to time out instead. For large systems it is
    probably a good idea to increase the value (to 50, say). It also gives
    some protection against denial-of-service attacks by SYN flooding.

smtp_enforce_sync               Type: boolean                   Default: true

    The SMTP protocol specification requires the client to wait for a response
    from the server at certain points in the dialogue. Without PIPELINING
    these synchronization points are after every command; with PIPELINING they
    are fewer, but they still exist. Some spamming sites send out a complete
    set of SMTP commands without waiting for any response. Exim protects
    against this by rejecting a message if the client has sent further input
    when it should not have. The error response '554 SMTP synchronization
    error' is sent, and the connection is dropped. Testing for this error      |
    cannot be perfect because of transmission delays (unexpected input may be  |
    on its way but not yet received when Exim checks). However, it does detect |
    many instances. The check can be disabled by setting "smtp_enforce_sync"   |
    false.                                                                     |

smtp_etrn_command               Type: string*                  Default: unset

    If this option is set, the given command is run whenever an SMTP ETRN
    command is received from a host that is permitted to issue such commands
    (see chapter 38). The string is split up into separate arguments which are
    independently expanded. The expansion variable $domain is set to the
    argument of the ETRN command, and no syntax checking is done on it. For
    example:

      smtp_etrn_command = /etc/etrn_command $domain $sender_host_address

    A new process is created to run the command, but Exim does not wait for it
    to complete. Consequently, its status cannot be checked. If the command
    cannot be run, a line is written to the panic log, but the ETRN caller
    still receives a 250 success response. Exim is normally running under its
    own uid when receiving SMTP, so it is not possible for it to change the
    uid before running the command.

smtp_etrn_serialize             Type: boolean                   Default: true

    When this option is set, it prevents the simultaneous execution of more
    than one identical command as a result of ETRN in an SMTP connection. See
    section 43.9 for details.

smtp_load_reserve             Type: fixed-point                Default: unset

    If the system load average ever gets higher than this, incoming SMTP
    calls are accepted only from those hosts that match an entry in
    "smtp_reserve_hosts". If "smtp_reserve_hosts" is not set, no incoming SMTP
    calls are accepted when the load is over the limit. The option has no
    effect on ancient operating systems on which Exim cannot determine the
    load average. See also "deliver_queue_load_max" and "queue_only_load".

smtp_max_synprot_errors         Type: integer                      Default: 3  |
                                                                               |
    Exim rejects SMTP commands that contain syntax or protocol errors. In      |
    particular, a syntactically invalid email address, as in this command:     |
                                                                               |
      RCPT TO:<abc xyz@a.b.c>                                                  |
                                                                               |
    causes immediate rejection of the command, before any other tests are      |
    done. (The ACL cannot be run if there is no valid address to set up for    |
    it.) An example of a protocol error is receiving RCPT before MAIL. If      |
    there are too many syntax or protocol errors in one SMTP session, the      |
    connection is dropped. The limit is set by this option.                    |

smtp_max_unknown_commands       Type: integer                      Default: 3

    If there are too many unrecognized commands in an incoming SMTP session,
    an Exim server drops the connection. This is a defence against some kinds
    of abuse that subvert web servers into making connections to SMTP ports;
    in these circumstances, a number of non-SMTP command lines are sent first.

smtp_ratelimit_hosts           Type: host list*                 Default: unset

    Some sites find it helpful to be able to limit the rate at which certain
    hosts can send them messages, and the rate at which an individual message
    can specify recipients. When a host matches "smtp_ratelimit_hosts", the
    values of "smtp_ratelimit_mail" and "smtp_ratelimit_rcpt" are used to
    control the rate of acceptance of MAIL and RCPT commands in a single SMTP
    session, respectively. Each option, if set, must contain a set of four
    comma-separated values:

     .   A threshold, before which there is no rate limiting.

     .   An initial time delay. Unlike other times in Exim, numbers with
         decimal fractional parts are allowed here.

     .   A factor by which to increase the delay each time.

     .   A maximum value for the delay. This should normally be less than 5
         minutes, because after that time, the client is liable to timeout the
         SMTP command.

    For example, these settings have been used successfully at the site which
    first suggested this feature, for controlling mail from their customers:

      smtp_ratelimit_mail = 2,0.5s,1.05,4m
      smtp_ratelimit_rcpt = 4,0.25s,1.015,4m

    The first setting specifies delays that are applied to MAIL commands after
    two have been received over a single connection. The initial delay is 0.5
    seconds, increasing by a factor of 1.05 each time. The second setting
    applies delays to RCPT commands when more than four occur in a single
    message.

    It is also possible to configure delays explicitly in ACLs. See section
    38.11 for details.

smtp_ratelimit_mail              Type: string                   Default: unset

    See "smtp_ratelimit_hosts" above.

smtp_ratelimit_rcpt              Type: string                   Default: unset

    See "smtp_ratelimit_hosts" above.

smtp_receive_timeout              Type: time                       Default: 5m

    This sets a timeout value for SMTP reception. It applies to all forms of
    SMTP input, including batch SMTP. If a line of input (either an SMTP
    command or a data line) is not received within this time, the SMTP
    connection is dropped and the message is abandoned. A line is written to
    the log containing one of the following messages:

      SMTP command timeout on connection from...
      SMTP data timeout on connection from...

    The former means that Exim was expecting to read an SMTP command; the
    latter means that it was in the DATA phase, reading the contents of a
    message.

    The value set by this option can be overridden by the -os command-line
    option. A setting of zero time disables the timeout, but this should never
    be used for SMTP over TCP/IP. (It can be useful in some cases of local
    input using -bs or -bS.) For non-SMTP input, the reception timeout is
    controlled by "receive_timeout" and -or.

smtp_reserve_hosts             Type: host list*                 Default: unset

    This option defines hosts for which SMTP connections are reserved; see
    "smtp_accept_reserve" and "smtp_load_reserve" above.

smtp_return_error_details       Type: boolean                  Default: false

    In the default state, Exim uses bland messages such as 'Administrative
    prohibition' when it rejects SMTP commands for policy reasons. Many
    sysadmins like this because it gives away little information to spammers.
    However, some other syadmins who are applying strict checking policies
    want to give out much fuller information about failures. Setting
    "smtp_return_error_details" true causes Exim to be more forthcoming. For
    example, instead of 'Administrative prohibition', it might give:

      550-Rejected after DATA: '>' missing at end of address:
      550 failing address in "From" header is: <user@dom.ain

split_spool_directory           Type: boolean                  Default: false

    If this option is set, it causes Exim to split its input directory into 62
    subdirectories, each with a single alphanumeric character as its name. The
    sixth character of the message id is used to allocate messages to
    subdirectories; this is the least significant base-62 digit of the time of
    arrival of the message.

    Splitting up the spool in this way may provide better performance on
    systems where there are long mail queues, by reducing the number of files
    in any one directory. The msglog directory is also split up in a similar
    way to the input directory; however, if "preserve_message_logs" is set,
    all old msglog files are still placed in the single directory msglog.OLD.

    It is not necessary to take any special action for existing messages when
    changing "split_spool_directory". Exim notices messages that are in the
    'wrong' place, and continues to process them. If the option is turned off
    after a period of being on, the subdirectories will eventually empty and
    be automatically deleted.

    When "split_spool_directory" is set, the behaviour of queue runner pro-
    cesses changes. Instead of creating a list of all messages in the queue,
    and then trying to deliver each one in turn, it constructs a list of those
    in one sub-directory and tries to deliver them, before moving on to the
    next sub-directory. The sub-directories are processed in a random order.
    This spreads out the scanning of the input directories, and uses less
    memory. It is particularly beneficial when there are lots of messages on
    the queue. However, if "queue_run_in_order" is set, none of this new
    processing happens. The entire queue has to be scanned and sorted before
    any deliveries can start.

spool_directory                 Type: string*    Default: set at compile time

    This defines the directory in which Exim keeps its spool, that is, the
    messages it is waiting to deliver. The default value is taken from the
    compile-time configuration setting, if there is one. If not, this option
    must be set. The string is expanded, so it can contain, for example, a
    reference to $primary_hostname.

    If the spool directory name is fixed on your installation, it is
    recommended that you set it at build time rather than from this option,
    particularly if the log files are being written to the spool directory
    (see "log_file_path"). Otherwise log files cannot be used for errors that
    are detected early on, such as failures in the configuration file.

    By using this option to override the compiled-in path, it is possible to
    run tests of Exim without using the standard spool.

strip_excess_angle_brackets     Type: boolean                  Default: false

    If this option is set, redundant pairs of angle brackets round 'route-
    addr' items in addresses are stripped. For example, "<<xxx@a.b.c.d>>" is
    treated as "<xxx@a.b.c.d>". If this is in the envelope and the message is
    passed on to another MTA, the excess angle brackets are not passed on. If
    this option is not set, multiple pairs of angle brackets cause a syntax
    error.

strip_trailing_dot              Type: boolean                  Default: false

    If this option is set, a trailing dot at the end of a domain in an address
    is ignored. If this is in the envelope and the message is passed on to
    another MTA, the dot is not passed on. If this option is not set, a dot at
    the end of a domain causes a syntax error.

syslog_duplication              Type: boolean                   Default: true  |
                                                                               |
    When Exim is logging to syslog, it writes the log lines for its three      |
    separate logs at different syslog priorities so that they can in principle |
    be separated on the logging hosts. Some installations do not require this  |
    separation, and in those cases, the duplication of certain log lines is a  |
    nuisance. If "syslog_duplication" is set false, only one copy of any       |
    particular log line is written to syslog. For lines that normally go to    |
    both the main log and the reject log, the reject log version (possibly     |
    containing message header lines) is written, at LOG_NOTICE priority. Lines |
    that normally go to both the main and the panic log are written at the     |
    LOG_ALERT priority.                                                        |

syslog_facility                  Type: string                   Default: unset

    This option sets the syslog 'facility' name, used when Exim is logging to
    syslog. The value must be one of the strings 'mail', 'user', 'news',
    'uucp', 'daemon', or 'local"x"' where "x" is a digit between 0 and 7. If
    this option is unset, 'mail' is used. See chapter 45 for details of Exim's
    logging.

syslog_processname               Type: string                    Default: exim

    This option sets the syslog 'ident' name, used when Exim is logging to
    syslog. The value must be no longer than 32 characters. See chapter 45 for
    details of Exim's logging.

syslog_timestamp                Type: boolean                   Default: true

    If "syslog_timestamp" is set false, the timestamps on Exim's log lines are
    omitted when these lines are sent to syslog. See chapter 45 for details of
    Exim's logging.

system_filter                   Type: string*                  Default: unset

    This option specifies an Exim filter file that is applied to all messages  |
    at the start of each delivery attempt, before any routing is done. System  |
    filters must be Exim filters; they cannot be Sieve filters. If the system  |
    filter generates any deliveries to files or pipes, or any new mail
    messages, the appropriate "system_filter_..._transport" option(s) must be
    set, to define which transports are to be used. Details of this facility
    are given in chapter 40.

system_filter_directory_transport
                                  Type: string*                Default: unset

    This sets the name of the transport driver that is to be used when the
    "save" command in a system message filter specifies a path ending in '/',
    implying delivery of each message into a separate file in some directory.
    During the delivery, the variable $address_file contains the path name.

system_filter_file_transport    Type: string*                  Default: unset

    This sets the name of the transport driver that is to be used when the
    "save" command in a system message filter specifies a path not ending in
    '/'. During the delivery, the variable $address_file contains the path
    name.


system_filter_group              Type: string                   Default: unset

    This option is used only when "system_filter_user" is also set. It sets
    the gid under which the system filter is run, overriding any gid that is
    associated with the user. The value may be numerical or symbolic.

system_filter_pipe_transport    Type: string*                  Default: unset

    This specifies the transport driver that is to be used when a "pipe"
    command is used in a system filter. During the delivery, the variable
    $address_pipe contains the pipe command.

system_filter_reply_transport   Type: string*                  Default: unset

    This specifies the transport driver that is to be used when a "mail"
    command is used in a system filter.


system_filter_user               Type: string                   Default: unset

    If this option is not set, the system filter is run in the main Exim
    delivery process, as root. When the option is set, the system filter runs
    in a separate process, as the given user. Unless the string consists
    entirely of digits, it is looked up in the password data. Failure to find
    the named user causes a configuration error. The gid is either taken from
    the password data, or specified by "system_filter_group". When the uid is
    specified numerically, "system_filter_group" is required to be set.

    If the system filter generates any pipe, file, or reply deliveries, the
    uid under which the filter is run is used when transporting them, unless a
    transport option overrides. Normally you should set "system_filter_user"
    if your system filter generates these kinds of delivery.

tcp_nodelay                     Type: boolean                   Default: true  |
                                                                               |
    If this option is set false, it stops the Exim daemon setting the          |
    TCP_NODELAY option on its listening sockets. Setting TCP_NODELAY turns off |
    the 'Nagle algorithm', which is a way of improving network performance in  |
    interactive (character-by-character) situations. Turning it off should     |
    improve Exim's performance a bit, so that is what happens by default.      |
    However, it appears that some broken clients cannot cope, and time out.    |
    Hence this option. It affects only those sockets that are set up for       |
    listening by the daemon. Sockets created by the smtp transport for         |
    delivering mail always set TCP_NODELAY.                                    |

timeout_frozen_after              Type: time                       Default: 0s

    If "timeout_frozen_after" is set to a time greater than zero, a frozen
    message of any kind that has been on the queue for longer than the given
    time is automatically cancelled at the next queue run. If it is a bounce
    message, it is just discarded; otherwise, a bounce is sent to the sender,
    in a similar manner to cancellation by the -Mg command line option. If you
    want to timeout frozen bounce messages earlier than other kinds of frozen
    message, see "ignore_bounce_errors_after".

timezone                         Type: string                   Default: unset

    The value of "timezone" is used to set the environment variable TZ while
    running Exim (if it is different on entry). This ensures that all
    timestamps created by Exim are in the required timezone. If you want all
    your timestamps to be in UTC (aka GMT) you should set

      timezone = UTC

    The default value is taken from TIMEZONE_DEFAULT in Local/Makefile, or, if
    that is not set, from the value of the TZ environment variable when Exim
    is built. If "timezone" is set to the empty string, either at build or run
    time, any existing TZ variable is removed from the environment when Exim
    runs. This is appropriate behaviour for obtaining wall-clock time on some,
    but unfortunately not all, operating systems.

tls_advertise_hosts            Type: host list*                 Default: unset

    When Exim is built with support for TLS encrypted connections, the
    availability of the STARTTLS command to set up an encrypted session is
    advertised in response to EHLO only to those client hosts that match this
    option. See chapter 37 for details of Exim's support for TLS.

tls_certificate                 Type: string*                  Default: unset

    The value of this option is expanded, and must then be the absolute path
    to a file which contains the server's certificates. The server's private
    key is also assumed to be in this file if "tls_privatekey" is unset. See
    chapter 37 for further details.

    Note: The certificates defined by this option are used only when Exim is   |
    receiving incoming messages as a server. If you want to supply certifi-    |
    cates for use when sending messages as a client, you must set the          |
    "tls_certificate" option in the relevant smtp transport.                   |

tls_dhparam                     Type: string*                  Default: unset

    The value of this option is expanded, and must then be the absolute path
    to a file which contains the server's DH parameter values. This is used
    only for OpenSSL. When Exim is linked with GnuTLS, this option is ignored.
    See section 37.1 for further details.

tls_privatekey                  Type: string*                  Default: unset

    The value of this option is expanded, and must then be the absolute path
    to a file which contains the server's private key. If this option is
    unset, the private key is assumed to be in the same file as the server's
    certificates. See chapter 37 for further details.

tls_remember_esmtp              Type: boolean                  Default: false  |
                                                                               |
    If this option is set true, Exim violates the RFCs by remembering that it  |
    is in 'esmtp' state after successfully negotiating a TLS session. This     |
    provides support for broken clients that fail to send a new EHLO after     |
    starting a TLS session.                                                    |

tls_try_verify_hosts           Type: host list*                 Default: unset

    See "tls_verify_hosts" below.

tls_verify_certificates         Type: string*                  Default: unset

    The value of this option is expanded, and must then be the absolute path   |
    to a file containing permitted certificates for clients that match         |
    "tls_verify_hosts" or "tls_try_verify_hosts". Alternatively, if you are    |
    using OpenSSL, you can set "tls_verify_certificates" to the name of a      |
    directory containing certificate files. This does not work with GnuTLS;    |
    the option must be set to the name of a single file if you are using       |
    GnuTLS.                                                                    |

tls_verify_hosts               Type: host list*                 Default: unset

    This option, along with "tls_try_verify_hosts", controls the checking of
    certificates from clients. The expected certificates are defined by
    "tls_verify_certificates", which must be set. A configuration error occurs
    if either "tls_verify_hosts" or "tls_try_verify_hosts" is set and
    "tls_verify_certificates" is not set.

    Any client that matches "tls_verify_hosts" is constrained by
    "tls_verify_certificates". The client must present one of the listed
    certificates. If it does not, the connection is aborted.

    A weaker form of checking is provided by "tls_try_verify_hosts". If a
    client matches this option (but not "tls_verify_hosts"), Exim requests a
    certificate and checks it against "tls_verify_certificates", but does not
    abort the connection if there is no certificate or if it does not match.
    This state can be detected in an ACL, which makes it possible to implement
    policies such as 'accept for relay only if a verified certificate has been
    received, but accept for local delivery if encrypted, even without a
    verified certificate'.

    Client hosts that match neither of these lists are not asked to present
    certificates.

trusted_groups                Type: string list                Default: unset

    If this option is set, any process that is running in one of the listed
    groups, or which has one of them as a supplementary group, is trusted. The |
    groups can be specified numerically or by name. See section 5.2 for        |
    details of what trusted callers are permitted to do. If neither
    "trusted_groups" nor "trusted_users" is set, only root and the Exim user
    are trusted.

trusted_users                 Type: string list                Default: unset

    If this option is set, any process that is running as one of the listed
    users is trusted. The users can be specified numerically or by name. See   |
    section 5.2 for details of what trusted callers are permitted to do. If
    neither "trusted_groups" nor "trusted_users" is set, only root and the
    Exim user are trusted.


unknown_login                   Type: string*                  Default: unset

    This is a specialized feature for use in unusual configurations. By
    default, if the uid of the caller of Exim cannot be looked up using
    "getpwuid()", Exim gives up. The "unknown_login" option can be used to set
    a login name to be used in this circumstance. It is expanded, so values
    like "user$caller_uid" can be set. When "unknown_login" is used, the value
    of "unknown_username" is used for the user's real name (gecos field),
    unless this has been set by the -F option.

unknown_username                 Type: string                   Default: unset

    See "unknown_login".

untrusted_set_sender         Type: address list*               Default: unset

    When an untrusted user submits a message to Exim using the standard input,
    Exim normally creates an envelope sender address from the user's login and
    the default qualification domain. Data from the -f option (for setting
    envelope senders on non-SMTP messages) or the SMTP MAIL command (if -bs or
    -bS is used) is ignored.

    However, untrusted users are permitted to set an empty envelope sender
    address, to declare that a message should never generate any bounces. For
    example:

      exim -f '<>' user@domain.example

    The "untrusted_set_sender" option allows you to permit untrusted users to
    set other envelope sender addresses in a controlled way. When it is set,
    untrusted users are allowed to set envelope sender addresses that match
    any of the patterns in the list. Like all address lists, the string is
    expanded. The identity of the user is in $sender_ident, so you can, for
    example, restrict users to setting senders that start with their login ids |
    followed by a hyphen by a setting like this:                               |

      untrusted_set_sender = ^$sender_ident-

    If you want to allow untrusted users to set envelope sender addresses
    without restriction, you can use

      untrusted_set_sender = *

    The "untrusted_set_sender" option applies to all forms of local input, but
    only to the setting of the envelope sender. It does not permit untrusted
    users to use the other options which trusted user can use to override
    message parameters. Furthermore, it does not stop Exim from removing an
    existing Sender: header in the message, or from adding a Sender: header if
    necessary. See "local_sender_retain" and "local_from_check" for ways of
    overriding these actions. The handling of the Sender: header is also
    described in section 44.14.

    The log line for a message's arrival shows the envelope sender following
    '<='. For local messages, the user's login always follows, after 'U='. In
    -bp displays, and in the Exim monitor, if an untrusted user sets an
    envelope sender address, the user's login is shown in parentheses after
    the sender address.

uucp_from_pattern                Type: string               Default: see below

    Some applications that pass messages to an MTA via a command line
    interface use an initial line starting with 'From' to pass the envelope
    sender. In particular, this is used by UUCP software. Exim recognizes
    such a line by means of a regular expression that is set in
    "uucp_from_pattern". When the pattern matches, the sender address is
    constructed by expanding the contents of "uucp_from_sender", provided that
    the caller of Exim is a trusted user. The default pattern recognizes lines
    in the following two forms:

         From ph10 Fri Jan  5 12:35 GMT 1996
         From ph10 Fri, 7 Jan 97 14:00:00 GMT

    The pattern can be seen by running

      exim -bP uucp_from_pattern

    It checks only up to the hours and minutes, and allows for a 2-digit or
    4-digit year in the second case. The first word after 'From' is matched in
    the regular expression by a parenthesized subpattern. The default value
    for "uucp_from_sender" is '$1', which therefore just uses this first word
    ('ph10' in the example above) as the message's sender. See also
    "ignore_fromline_hosts".

uucp_from_sender                Type: string*                     Default: $1

    See "uucp_from_pattern" above.

warn_message_file                Type: string                   Default: unset

    This option defines a template file containing paragraphs of text to be
    used for constructing the warning message which is sent by Exim when a
    message has been on the queue for a specified amount of time, as specified
    by "delay_warning". Details of the file's contents are given in chapter
    41. See also "bounce_message_file".



                       15. GENERIC OPTIONS FOR ROUTERS


This chapter describes the generic options that apply to all routers,
identifying those that are preconditions. For a general description of how a
router operates, see sections 3.10 and 3.11. The second of these sections
specifies the order in which the preconditions are tested. The order of
expansion of the options that provide data for a transport is: "errors_to",
"headers_add", "headers_remove", "transport".

address_data                    Type: string*                  Default: unset

    The string is expanded just before the router is run, that is, after all
    the precondition tests have succeeded. If the expansion is forced to fail,
    the router declines. Other expansion failures cause delivery of the
    address to be deferred.

    When the expansion succeeds, the value is retained with the address, and
    can be accessed using the variable $address_data in the current router,
    subsequent routers, and the eventual transport.

    Warning: if the current or any subsequent router is a redirect router that
    runs a user's filter file, the contents of $address_data are accessible in
    the filter. This is not normally a problem, because such data is usually
    either not confidential or it 'belongs' to the current user, but if you do
    put confidential data into $address_data you need to remember this point.

    Even if the router declines or passes, the value of $address_data remains
    with the address, though it can be changed by another "address_data"
    setting on a subsequent router. If a router generates child addresses, the
    value of $address_data propagates to them. This also applies to the
    special kind of 'child' that is generated by a router with the "unseen"
    option.

    The idea of "address_data" is that you can use it to look up a lot of data
    for the address once, and then pick out parts of the data later. For
    example, you could use a single LDAP lookup to return a string of the form

      uid=1234 gid=5678 mailbox=/mail/xyz forward=/home/xyz/.forward

    In the transport you could pick out the mailbox by a setting such as

      file = ${extract{mailbox}{$address_data}}

    This makes the configuration file less messy, and also reduces the number
    of lookups. (Exim does cache the most recent lookup, but there may be
    several addresses in a message which cause lookups to occur.)

    The "address_data" facility is also useful as a means of passing infor-
    mation from one router to another, and from a router to a transport. In
    addition, if "address_data" is set by a router when verifying an address
    from an ACL, its value is available for use in the rest of the ACL
    statement.

address_test             Type: boolean (precondition)            Default: true

    If this option is set false, the router is skipped when routing is being
    tested by means of the -bt command line option. This can be a convenience
    when your first router sends messages to to an external scanner, because
    it saves you having to set the 'already scanned' indicator when testing
    real address routing.

cannot_route_message            Type: string*                  Default: unset

    This option specifies a text message that is used when an address cannot
    be routed because Exim has run out of routers. The default message is
    'Unrouteable address'. This option is useful only on routers that have
    "more" set false, or on the very last router in a configuration, because
    the value that is used is taken from the last router that inspects an
    address. For example, using the default configuration, you could put:

      cannot_route_message = Remote domain not found in DNS

    on the first (dnslookup) router, and

      cannot_route_message = Unknown local user

    on the final router that checks for local users. If string expansion
    fails, the default message is used. Unless the expansion failure was       |
    explicitly forced, a message about the failure is written to the main and  |
    panic logs, in addition to the normal message about the routing failure.   |

caseful_local_part              Type: boolean                  Default: false

    By default, routers handle the local parts of addresses in a case-
    insensitive manner, though the actual case is preserved for transmission
    with the message. If you want the case of letters to be significant in a
    router, you must set this option true. For individual router options that
    contain address or local part lists (for example, "local_parts"), case-
    sensitive matching can be turned on by '+caseful' as a list item. See
    section 10.17 for more details.

check_local_user         Type: boolean (precondition)           Default: false

    When this option is true, Exim checks that the local part of the recipient
    address (with affixes removed if relevant) is the name of an account on
    the local system. The check is done by calling the "getpwnam()" function
    rather than trying to read /etc/passwd directly. This means that other
    methods of holding password data (such as NIS) are supported. If the local
    part is a local user, $home is set from the password data, and can be
    tested in other preconditions that are evaluated after this one (the order
    of evaluation is given in section 3.11). However, the value of $home can
    be overridden by "router_home_directory". If the local part is not a local
    user, the router is skipped.

    If you want to check that the local part is either the name of a local
    user or matches something else, you cannot combine "check_local_user" with
    a setting of "local_parts", because that specifies the logical "and" of
    the two conditions. However, you can use a passwd lookup in a
    "local_parts" setting to achieve this. For example:

      local_parts = passwd;$local_part : lsearch;/etc/other/users

    Note, however, that the side effects of "check_local_user" (such as
    setting up a home directory) do not occur when a passwd lookup is used in
    a "local_parts" (or any other) precondition.

condition                Type: string* (precondition)           Default: unset

    This option specifies a general precondition test that has to succeed for
    the router to be called. The string is expanded, and if the result is a
    forced failure or an empty string or one of the strings '0' or 'no' or
    'false' (checked without regard to the case of the letters), the router is
    skipped, and the address is offered to the next one. This provides a means
    of applying special-purpose conditions to the running of routers.

    If the expansion fails (other than forced failure) delivery is deferred.
    Some of the other options below are common special cases that could in
    fact be specified using "condition". Note that "condition" is the last
    precondition to be evaluated (see section 3.11).

debug_print                     Type: string*                  Default: unset

    If this option is set and debugging is enabled (see the -d command line
    option), the string is expanded and included in the debugging output. If   |
    expansion of the string fails, the error message is written to the         |
    debugging output, and Exim carries on processing. This option is provided  |
    to help with checking out the values of variables and so on when debugging
    router configurations. For example, if a "condition" option appears not to
    be working, "debug_print" can be used to output the variables it refer-
    ences. The output happens after checks for "domains", "local_parts", and
    "check_local_user" but before any other preconditions are tested. A
    newline is added to the text if it does not end with one.

disable_logging                 Type: boolean                  Default: false

    If this option is set true, nothing is logged for any routing errors. You
    should not set this option unless you really, really know what you are
    doing.

domains               Type: domain list* (precondition)        Default: unset

    If this option is set, the router is skipped unless the current domain
    matches the list. If the match is achieved by means of a file lookup, the
    data that the lookup returned for the domain is placed in $domain_data for
    use in string expansions of the driver's private options. See section 3.11
    for a list of the order in which preconditions are evaluated.

driver                           Type: string                   Default: unset

    This option must always be set. It specifies which of the available
    routers is to be used.

errors_to                       Type: string*                  Default: unset

    If a router successfully handles an address, it may queue the address for
    delivery or it may generate child addresses. In both cases, if there is a
    delivery problem during later processing, the resulting bounce message is
    sent to the address that results from expanding this string, provided
    that the address verifies successfully. "errors_to" is expanded before
    "headers_add", "headers_remove", and "transport".

    If the option is unset, of the expansion is forced to fail, or the result
    of the expansion fails to verify, the errors address associated with the
    incoming address is used. At top level, this is the envelope sender. A
    non-forced expansion failure causes delivery to be deferred.

    If an address for which "errors_to" has been set ends up being delivered
    over SMTP, the envelope sender for that delivery is the "errors_to" value,
    so that any bounces that are generated by other MTAs on the delivery route
    are also sent there. The most common use of "errors_to" is probably to
    direct mailing list bounces to the manager of the list, as described in
    section 42.2.

    The "errors_to" setting associated with an address can be overridden if it
    subsequently passes through other routers that have their own "errors_to"
    settings, or if it is delivered by a transport with a "return_path"
    setting.

    You can set "errors_to" to the empty string by either of these settings:

      errors_to =
      errors_to = ""

    An expansion item that yields an empty string has the same effect. If you
    do this, a locally detected delivery error for addresses processed by this
    router no longer gives rise to a bounce message; the error is discarded.
    If the address is delivered to a remote host, the return path is set to
    "<>", unless overridden by the "return_path" option on the transport.

    If for some reason you want to discard local errors, but use a non-empty
    MAIL command for remote delivery, you can preserve the original return
    path in $address_data in the router, and reinstate it in the transport by
    setting "return_path".

expn                     Type: boolean (precondition)            Default: true

    If this option is turned off, the router is skipped when testing an
    address as a result of processing an SMTP EXPN command. You might, for
    example, want to turn it off on a router for users' .forward files, while
    leaving it on for the system alias file. See section 3.11 for a list of
    the order in which preconditions are evaluated.

    The use of the SMTP EXPN command is controlled by an ACL (see chapter 38).
    When Exim is running an EXPN command, it is similar to testing an address
    with -bt. Compare VRFY, whose counterpart is -bv.

fail_verify                     Type: boolean                  Default: false

    Setting this option has the effect of setting both "fail_verify_sender"
    and "fail_verify_recipient" to the same value.

fail_verify_recipient           Type: boolean                  Default: false

    If this option is true and an address is accepted by this router when
    verifying a recipient, verification fails.

fail_verify_sender              Type: boolean                  Default: false

    If this option is true and an address is accepted by this router when
    verifying a sender, verification fails.

fallback_hosts                Type: string list                Default: unset

    String expansion is not applied to this option. The argument must be a
    colon-separated list of host names or IP addresses. If a router queues an
    address for a remote transport, this host list is associated with the
    address, and used instead of the transport's fallback host list. If
    "hosts_randomize" is set on the transport, the order of the list is
    randomized for each use. See the "fallback_hosts" option of the smtp
    transport for further details.

group                           Type: string*              Default: see below

    When a router queues an address for a transport, and the transport does
    not specify a group, the group given here is used when running the
    delivery process. The group may be specified numerically or by name. If    |
    expansion fails, the error is logged and delivery is deferred. The default |
    is unset, unless "check_local_user" is set, when the default is taken from
    the password information. See also "initgroups" and "user" and the
    discussion in chapter 23.

headers_add                     Type: string*                  Default: unset

    This option specifies a string of text that is expanded at routing time,
    and associated with any addresses that are processed by the router when
    delivering a message. This option has no effect when an address is just
    being verified.

    The "headers_add" option is expanded after "errors_to", but before
    "headers_remove" and "transport". If the expanded string is empty, or if
    the expansion is forced to fail, the option has no effect. Other expansion
    failures are treated as configuration errors. The expanded string must be
    in the form of one or more RFC 2822 header lines, separated by newlines
    (coded as '\n'). For example:

      headers_add = X-added-header: added by $primary_hostname\n\
                    X-added-second: another added header line

    Exim does not check the syntax of these added header lines. If an address
    passes through several routers as a result of aliasing or forwarding
    operations, any "headers_add" or "headers_remove" specifications are cumu-
    lative. This does not apply for multiple routers that result from the use
    of 'unseen'.

    At transport time, all the original headers listed in "headers_remove" are
    removed. If there are multiple instances of any listed header, they are
    all removed. Then the new headers specified by "headers_add" are added, in
    the order in which they were attached to the address. Finally, any
    additional headers specified by the transport are added. It is not
    possible to remove headers added to an address by "headers_add".

    Because the addition does not happen until transport time, header lines
    that are added by "headers_add" are not accessible by means of the
    $header_xxx expansion syntax. Conversely, header lines that are removed by
    "headers_remove" remain visible.

    Addresses with different "headers_add" or "headers_remove" settings cannot
    be delivered together in a batch. The "headers_add" option cannot be used
    for a redirect router that has the "one_time" option set.

headers_remove                  Type: string*                  Default: unset

    The string is expanded at routing time and is then associated with any
    addresses that are processed by the router when delivering a message. This
    option has no effect when an address is just being verified. The
    "headers_remove" option is expanded after "errors_to" and "headers_add",
    but before "transport". If the expansion is forced to fail, the option has
    no effect. Other expansion failures are treated as configuration errors.
    After expansion, the string must consist of a colon-separated list of
    header names, not including the terminating colon, for example:

      headers_remove = return-receipt-to:acknowledge-to

    It is used at transport time as described under "headers_add" above. The
    "headers_remove" option cannot be used for a redirect router that has the
    "one_time" option set.

ignore_target_hosts            Type: host list*                 Default: unset

    Although this option is a host list, it should normally contain IP address
    entries rather than names. If any host that is looked up by the router has
    an IP address that matches an item in this list, Exim behaves as if that
    IP address did not exist. This option allows you to cope with rogue DNS
    entries like

      remote.domain.example  A  127.0.0.1

    by setting

      ignore_target_hosts = 127.0.0.1

    on the relevant router. An attempt to mail to such a domain then provokes
    the 'unrouteable domain' error, and an attempt to verify an address in the
    domain fails. This option may also be useful for ignoring link-local and
    site-local IPv6 addresses. Because, like all host lists, the value of
    "ignore_target_hosts" is expanded before use as a list, it is possible to
    make it dependent on the domain that is being routed.


initgroups                      Type: boolean                  Default: false

    If the router queues an address for a transport, and this option is true,
    and the uid supplied by the router is not overridden by the transport, the
    "initgroups()" function is called when running the transport to ensure
    that any additional groups associated with the uid are set up. See also
    "group" and "user" and the discussion in chapter 23.

local_part_prefix      Type: string list (precondition)         Default: unset

    If this option is set, the router is skipped unless the local part starts
    with one of the given strings, or "local_part_prefix_optional" is true.
    See section 3.11 for a list of the order in which preconditions are
    evaluated.

    The list is scanned from left to right, and the first prefix that matches
    is used. A limited form of wildcard is available; if the prefix begins
    with an asterisk, it matches the longest possible sequence of arbitrary
    characters at the start of the local part. An asterisk should therefore
    always be followed by some character that does not occur in normal local
    parts. Wildcarding can be used to set up multiple user mailboxes, as
    described in section 42.7.

    During the testing of the "local_parts" option, and while the router is
    running, the prefix is removed from the local part, and is available in
    the expansion variable $local_part_prefix. If the router accepts the
    address, this remains true during subsequent delivery. In particular, the  |
    local part that is transmitted in the RCPT command for LMTP, SMTP, and     |
    BSMTP deliveries has the prefix removed by default. This behaviour can     |
    be overridden by setting "rcpt_include_affixes" true on the relevant       |
    transport.                                                                 |

    The prefix facility is commonly used to handle local parts of the form
    "owner-something". Another common use is to support local parts of the
    form "real-username" to bypass a user's .forward file - helpful when
    trying to tell a user their forwarding is broken - by placing a router
    like this one immediately before the router that handles .forward files:

      real_localuser:
        driver = accept
        local_part_prefix = real-
        check_local_user
        transport = local_delivery

    If both "local_part_prefix" and "local_part_suffix" are set for a router,
    both conditions must be met if not optional. Care must be taken if
    wildcards are used in both a prefix and a suffix on the same router.
    Different separator characters must be used to avoid ambiguity.

local_part_prefix_optional      Type: boolean                  Default: false

    See "local_part_prefix" above.

local_part_suffix      Type: string list (precondition)         Default: unset

    This option operates in the same way as "local_part_prefix", except that
    the local part must end (rather than start) with the given string, the
    "local_part_suffix_optional" option determines whether the suffix is
    mandatory, and the wildcard * character, if present, must be the last
    character of the suffix. This option facility is commonly used to handle
    local parts of the form "something-request" and multiple user mailboxes of
    the form "username-foo".

local_part_suffix_optional      Type: boolean                  Default: false

    See "local_part_suffix" above.

local_parts         Type: local part list* (precondition)      Default: unset

    The router is run only if the local part of the address matches the list.
    See section 3.11 for a list of the order in which preconditions are
    evaluated, and section 10.18 for a discussion of local part lists. Because
    the string is expanded, it is possible to make it depend on the domain,
    for example:

      local_parts = dbm;/usr/local/specials/$domain

    If the match is achieved by a lookup, the data that the lookup returned
    for the local part is placed in the variable $local_part_data for use in
    expansions of the router's private options. You might use this option, for
    example, if you have a large number of local virtual domains, and you want
    to send all postmaster mail to the same place without having to set up an
    alias in each virtual domain:

      postmaster:
        driver = redirect
        local_parts = postmaster
        data = postmaster@real.domain.example

log_as_local                    Type: boolean              Default: see below

    Exim has two logging styles for delivery, the idea being to make local
    deliveries stand out more visibly from remote ones. In the 'local' style,
    the recipient address is given just as the local part, without a domain.
    The use of this style is controlled by this option. It defaults to true
    for the accept router, and false for all the others.

more                            Type: boolean*                   Default: true

    The result of string expansion for this option must be a valid boolean
    value, that is, one of the strings 'yes', 'no', 'true', or 'false'. Any
    other result causes an error, and delivery is deferred. If the expansion
    is forced to fail, the default value for the option (true) is used. Other
    failures cause delivery to be deferred.

    If this option is set false, and the router is run, but declines to handle
    the address, no further routers are tried, routing fails, and the address
    is bounced. However, if the router explicitly passes an address to the
    following router by means of the setting

      self = pass

    or otherwise, the setting of "more" is ignored. Also, the setting of
    "more" does not affect the behaviour if one of the precondition tests
    fails. In that case, the address is always passed to the next router.

pass_on_timeout                 Type: boolean                  Default: false

    If a router times out during a host lookup, it normally causes deferral of
    the address. If "pass_on_timeout" is set, the address is passed on to the
    next router, overriding "no_more". This may be helpful for systems that
    are intermittently connected to the Internet, or those that want to pass
    to a smart host any messages that cannot immediately be delivered.

    There are occasional other temporary errors that can occur while doing DNS
    lookups. They are treated in the same way as a timeout, and this option
    applies to all of them.

pass_router                      Type: string                   Default: unset

    When a router returns 'pass', the address is normally handed on to the
    next router in sequence. This can be changed by setting "pass_router" to
    the name of another router. However (unlike "redirect_router") the named
    router must be below the current router, to avoid loops. Note that this
    option applies only to the special case of 'pass'. It does not apply when
    a router returns 'decline'.

redirect_router                  Type: string                   Default: unset

    Sometimes an administrator knows that it is pointless to reprocess
    addresses generated from alias or forward files with the same router
    again. For example, if an alias file translates real names into login ids
    there is no point searching the alias file a second time, especially if it
    is a large file.

    The "redirect_router" option can be set to the name of any router
    instance. It causes the routing of any generated addresses to start at the
    named router instead of at the first router. This option has no effect if
    the router in which it is set does not generate new addresses.

require_files         Type: string list* (precondition)        Default: unset

    This option provides a general mechanism for predicating the running of a
    router on the existence or non-existence of certain files or directories.
    Before running a router, as one of its precondition tests, Exim works its
    way through the "require_files" list, expanding each item separately.

    Because the list is split before expansion, any colons in expansion items
    must be doubled, or the facility for using a different list separator must
    be used. If any expansion is forced to fail, the item is ignored. Other
    expansion failures cause routing of the address to be deferred.

    If any expanded string is empty, it is ignored. Otherwise, except as
    described below, each string must be a fully qualified file path,
    optionally preceded by '!'. The paths are passed to the "stat()" function
    to test for the existence of the files or directories. The router is
    skipped if any paths not preceded by '!' do not exist, or if any paths
    preceded by '!' do exist.

    If "stat()" cannot determine whether a file exists or not, delivery of the
    message is deferred. This can happen when NFS-mounted filesystems are
    unavailable.

    This option is checked after the "domains", "local_parts", and "senders"
    options, so you cannot use it to check for the existence of a file in
    which to look up up a domain, local part, or sender. (See section 3.11 for
    a full list of the order in which preconditions are evaluated.) However,
    as these options are all expanded, you can use the "exists" expansion
    condition to make such tests. The "require_files" option is intended for
    checking files that the router may be going to use internally, or which
    are needed by a transport (for example .procmailrc).

    During delivery, the "stat()" function is run as root, but there is a      |
    facility for some checking of the accessibility of a file by another user. |
    This is not a proper permissions check, but just a 'rough' check that      |
    operates as follows:                                                       |

    If an item in a "require_files" list does not contain any forward slash
    characters, it is taken to be the user (and optional group, separated by a
    comma) to be checked for subsequent files in the list. If no group is
    specified but the user is specified symbolically, the gid associated with
    the uid is used. For example:

      require_files = mail:/some/file
      require_files = $local_part:$home/.procmailrc

    If a user or group name in a "require_files" list does not exist, the
    "require_files" condition fails.

    Exim performs the check by scanning along the components of the file path,
    and checking the access for the given uid and gid. It checks for 'x'
    access on directories, and 'r' access on the final file. Note that this
    means that file access control lists, if the operating system has them,
    are ignored.

    Warning 1: When the router is being run to verify addresses for an
    incoming SMTP message, Exim is not running as root, but under its own uid.
    This may affect the result of a "require_files" check. In particular,
    "stat()" may yield the error EACCES ('Permission denied'). This means that
    the Exim user is not permitted to read one of the directories on the
    file's path.

    Warning 2: Even when Exim is running as root while delivering a message,   |
    "stat()" can yield EACCES for a file on an NFS directory that is mounted   |
    without root access.                                                       |
                                                                               |
    In both cases, the default action is to consider this a configuration      |
    error, and routing is deferred because the existence or non-existence of
    the file cannot be determined. However, in some circumstances it may be
    desirable to treat this condition as if the file did not exist. If the
    file name (or the exclamation mark that precedes the file name for non-
    existence) is preceded by a plus sign, the EACCES error is treated as if
    the file did not exist. For example:

      require_files = +/some/file

    If the router is not an essential part of verification (for example, it
    handles users' .forward files), another solution is to set the "verify"
    option false so that the router is skipped when verifying.

retry_use_local_part            Type: boolean              Default: see below

    When a delivery suffers a temporary routing failure, a retry record is
    created in Exim's hints database. For addresses whose routing depends only
    on the domain, the key for the retry record should not involve the local
    part, but for other addresses, both the domain and the local part should
    be included. Usually, remote routing is of the former kind, and local
    routing is of the latter kind.

    This option controls whether the local part is used to form the key for
    retry hints for addresses that suffer temporary errors while being handled
    by this router. The default value is true for any router that has
    "check_local_user" set, and false otherwise. Note that this option does
    not apply to hints keys for transport delays; they are controlled by a
    generic transport option of the same name.

router_home_directory           Type: string*                  Default: unset

    This option sets a home directory for use while the router is running.
    (Compare "transport_home_directory", which sets a home directory for later
    transporting.) In particular, if used on a redirect router, this option
    sets a value for $home while a filter is running. The value is expanded;
    forced expansion failure causes the option to be ignored - other failures
    cause the router to defer.

    Expansion of "router_home_directory" happens immediately after the
    "check_local_user" test (if configured), before any further expansions
    take place. (See section 3.11 for a list of the order in which precon-
    ditions are evaluated.) While the router is running, "router_
    home_directory" overrides the value of $home that came from
    "check_local_user".

    When a router accepts an address and routes it to a transport (including
    the cases when a redirect router generates a pipe, file, or autoreply
    delivery), the home directory setting for the transport is taken from the
    first of these values that is set:

     .   The "home_directory" option on the transport;

     .   The "transport_home_directory" option on the router;

     .   The password data if "check_local_user" is set on the router;

     .   The "router_home_directory" option on the router.

    In other words, "router_home_directory" overrides the password data for
    the router, but not for the transport.

self                             Type: string                  Default: freeze

    This option applies to those routers that use a recipient address to find
    a list of remote hosts. Currently, these are the dnslookup, ipliteral, and
    manualroute routers. Certain configurations of the queryprogram router can |
    also specify a list of remote hosts. Usually such routers are configured   |
    to send the message to a remote host via an smtp transport. The "self"
    option specifies what happens when the first host on the list turns out to
    be the local host. The way in which Exim checks for the local host is      |
    described in section 13.6.                                                 |

    Normally this situation indicates either an error in Exim's configuration
    (for example, the router should be configured not to process this domain),
    or an error in the DNS (for example, the MX should not point to this
    host). For this reason, the default action is to log the incident, defer
    the address, and freeze the message. The following alternatives are
    provided for use in special cases:

     .   "defer"
         Delivery of the message is tried again later, but the message is not
         frozen.

     .   "reroute: <domain>"
         The domain is changed to the given domain, and the address is passed
         back to be reprocessed by the routers. No rewriting of headers takes
         place. This behaviour is essentially a redirection.

     .   "reroute: rewrite: <domain>"
         The domain is changed to the given domain, and the address is passed
         back to be reprocessed by the routers. Any headers that contain the
         original domain are rewritten.

     .   "pass"
         The router passes the address to the next router, or to the router
         named in the "pass_router" option if it is set. This overrides
         "no_more".

         During subsequent routing and delivery, the variable $self_hostname
         contains the name of the local host that the router encountered. This
         can be used to distinguish between different cases for hosts with
         multiple names. The combination

           self = pass
           no_more

         ensures that only those addresses that routed to the local host are
         passed on. Without "no_more", addresses that were declined for other
         reasons would also be passed to the next router.

     .   "fail"
         Delivery fails and an error report is generated.

     .   "send"
         The anomaly is ignored and the address is queued for the transport.
         This setting should be used with extreme caution. For an smtp
         transport, it makes sense only in cases where the program that is
         listening on the SMTP port is not this version of Exim. That is, it
         must be some other MTA, or Exim with a different configuration file
         that handles the domain in another way.

senders               Type: address list* (precondition)        Default: unset

    If this option is set, the router is skipped unless the message's sender
    address matches something on the list. See section 3.11 for a list of the
    order in which preconditions are evaluated.

    There are issues concerning verification when the running of routers is
    dependent on the sender. When Exim is verifying the address in an
    "errors_to" setting, it sets the sender to the null string. When using the
    -bt option to check a configuration file, it is necessary also to use the
    -f option to set an appropriate sender. For incoming mail, the sender is
    unset when verifying the sender, but is available when verifying any
    recipients. If the SMTP VRFY command is enabled, it must be used after
    MAIL if the sender address matters.

translate_ip_address            Type: string*                  Default: unset

    There exist some rare networking situations (for example, packet radio)
    where it is helpful to be able to translate IP addresses generated by
    normal routing mechanisms into other IP addresses, thus performing a kind
    of manual IP routing. This should be done only if the normal IP routing of
    the TCP/IP stack is inadequate or broken. Because this is an extremely
    uncommon requirement, the code to support this option is not included
    in the Exim binary unless SUPPORT_TRANSLATE_IP_ADDRESS=yes is set in
    Local/Makefile.

    The "translate_ip_address" string is expanded for every IP address gener-
    ated by the router, with the generated address set in $host_address. If
    the expansion is forced to fail, no action is taken. For any other         |
    expansion error, delivery of the message is deferred. If the result of the |
    expansion is an IP address, that replaces the original address; otherwise
    the result is assumed to be a host name - this is looked up using
    "gethostbyname()" (or "getipnodebyname()" when available) to produce one
    or more replacement IP addresses. For example, to subvert all IP addresses
    in some specific networks, this could be added to a router:

      translate_ip_address = \
        ${lookup{${mask:$host_address/26}}lsearch{/some/file}{$value}fail}

    The file would contain lines like

      10.2.3.128/26    some.host
      10.8.4.34/26     10.44.8.15

    You should not make use of this facility unless you really understand what
    you are doing.

transport                       Type: string*                  Default: unset

    This option specifies the transport to be used when a router accepts an
    address and sets it up for delivery. A transport is never needed if a
    router is used only for verification. The value of the option is expanded
    at routing time, after the expansion of "errors_to", "headers_add", and
    "headers_remove", and result must be the name of one of the configured
    transports. If it is not, delivery is deferred.

    The "transport" option is not used by the redirect router, but it does
    have some private options that set up transports for pipe and file
    deliveries (see chapter 22).

transport_current_directory     Type: string*                  Default: unset

    This option associates a current directory with any address that is routed
    to a local transport. This can happen either because a transport is
    explicitly configured for the router, or because it generates a delivery
    to a file or a pipe. During the delivery process (that is, at transport
    time), this option string is expanded and is set as the current directory,
    unless overridden by a setting on the transport. If the expansion fails    |
    for any reason, including forced failure, an error is logged, and delivery |
    is deferred. See chapter 23 for details of the local delivery environment. |

transport_home_directory        Type: string*              Default: see below

    This option associates a home directory with any address that is routed
    to a local transport. This can happen either because a transport is
    explicitly configured for the router, or because it generates a delivery
    to a file or a pipe. During the delivery process (that is, at transport
    time), the option string is expanded and is set as the home directory,
    unless overridden by a setting of "home_directory" on the transport. If    |
    the expansion fails for any reason, including forced failure, an error is  |
    logged, and delivery is deferred.                                          |

    If the transport does not specify a home directory, and
    "transport_home_directory" is not set for the router, the home directory
    for the tranport is taken from the password data if "check_local_user" is
    set for the router. Otherwise it is taken from "router_home_directory" if
    that option is set; if not, no home directory is set for the transport.

    See chapter 23 for further details of the local delivery environment.

unseen                          Type: boolean*                  Default: false

    The result of string expansion for this option must be a valid boolean
    value, that is, one of the strings 'yes', 'no', 'true', or 'false'. Any
    other result causes an error, and delivery is deferred. If the expansion
    is forced to fail, the default value for the option (false) is used. Other
    failures cause delivery to be deferred.

    When this option is set true, routing does not cease if the router accepts
    the address. Instead, a copy of the incoming address is passed to the next
    router, overriding a false setting of "more". There is little point in
    setting "more" false if "unseen" is always true, but it may be useful in
    cases when the value of "unseen" contains expansion items (and therefore,
    presumably, is sometimes true and sometimes false).

    The "unseen" option can be used to cause copies of messages to be          |
    delivered to some other destination, while also carrying out a normal      |
    delivery. In effect, the current address is made into a 'parent' that has  |
    two children - one that is delivered as specified by this router, and a    |
    clone that goes on to be routed further.                                   |
                                                                               |
    Header lines added to the address (or specified for removal) by this       |
    router or by previous routers affect the 'unseen' copy of the message      |
    only. The clone that continues to be processed by further routers starts   |
    with no added headers and none specified for removal.                      |

    However, any data that was set by the "address_data" option in the current
    or previous routers is passed on. Setting this option has a similar effect
    to the "unseen" command qualifier in filter files.

user                            Type: string*              Default: see below

    When a router queues an address for a transport, and the transport does
    not specify a user, the user given here is used when running the delivery
    process. The user may be specified numerically or by name. If expansion    |
    fails, the error is logged and delivery is deferred. This user is also     |
    used by the redirect router when running a filter file. The default is
    unset, except when "check_local_user" is set. In this case, the default is
    taken from the password information. If the user is specified as a name,
    and "group" is not set, the group associated with the user is used. See
    also "initgroups" and "group" and the discussion in chapter 23.

verify                   Type: boolean (precondition)            Default: true

    Setting this option has the effect of setting "verify_sender" and
    "verify_recipient" to the same value.

verify_only              Type: boolean (precondition)           Default: false

    If this option is set, the router is used only when verifying an address
    or testing with the -bv option, not when actually doing a delivery,
    testing with the -bt option, or running the SMTP EXPN command. It can be
    further restricted to verifying only senders or recipients by means of
    "verify_sender" and "verify_recipient".

    Warning: When the router is being run to verify addresses for an incoming
    SMTP message, Exim is not running as root, but under its own uid. If the
    router accesses any files, you need to make sure that they are accessible
    to the Exim user or group.

verify_recipient         Type: boolean (precondition)            Default: true

    If this option is false, the router is skipped when verifying recipient
    addresses or testing recipient verification using -bv. See section 3.11    |
    for a list of the order in which preconditions are evaluated.

verify_sender            Type: boolean (precondition)            Default: true

    If this option is false, the router is skipped when verifying sender
    addresses or testing sender verification using -bvs. See section 3.11 for  |
    a list of the order in which preconditions are evaluated.



                            16. THE ACCEPT ROUTER


The accept router has no private options of its own. Unless it is being used
purely for verification (see "verify_only") a transport is required to be
defined by the generic "transport" option. If the preconditions that are
specified by generic options are met, the router accepts the address and
queues it for the given transport. The most common use of this router is for
setting up deliveries to local mailboxes. For example:

  localusers:
    driver = accept
    domains = mydomain.example
    check_local_user
    transport = local_delivery

The "domains" condition in this example checks the domain of the address, and
"check_local_user" checks that the local part is the login of a local user.
When both preconditions are met, the accept router runs, and queues the
address for the local_delivery transport.



                           17. THE DNSLOOKUP ROUTER


The dnslookup router looks up the hosts that handle mail for the given domain
in the DNS. A transport must always be set for this router, unless
"verify_only" is set.

MX records are looked up first, followed by address records if no MX records
are found, unless the domain matches "mx_domains". MX records of equal
priority are sorted by Exim into a random order. Unless they have the highest
priority (lowest MX value), MX records that point to the local host, or to any
host name that matches "hosts_treat_as_local", are discarded, together with
any other MX records of equal or lower priority.

If the host pointed to by the highest priority MX record, or looked up as an
address record, is the local host, or matches "hosts_treat_as_local", what
happens is controlled by the generic "self" option.

There are a number of private options that can be used to vary the way the DNS
lookup is handled.


check_secondary_mx              Type: boolean                  Default: false

    If this option is set, the router declines unless the local host is found
    in (and removed from) the list of hosts obtained by MX lookup. This can be
    used to process domains for which the local host is a secondary mail
    exchanger differently to other domains. The way in which Exim decides      |
    whether a host is the local host is described in section 13.6.             |

mx_domains                    Type: domain list*                Default: unset

    A domain which matches "mx_domains" is required to have an MX record
    in order to be recognised. For example, if all the mail hosts in
    "fict.example" are known to have MX records, except for those in
    "discworld.fict.example", you could use this setting:

      mx_domains = ! *.discworld.fict.example : *.fict.example

    This specifies that messages addressed to a domain that matches the list
    but has no MX record should be bounced immediately instead of being routed
    using the address record.

qualify_single                  Type: boolean                   Default: true

    When this option is true, the resolver option RES_DEFNAMES is set for DNS
    lookups. Typically, but not standardly, this causes the resolver to
    qualify single-component names with the default domain. For example, on a
    machine called "dictionary.ref.example", the domain "thesaurus" would be
    changed to "thesaurus.ref.example" inside the resolver. For details of
    what your resolver actually does, consult your man pages for "resolver"
    and "resolv.conf".

rewrite_headers                 Type: boolean                   Default: true

    If the domain name in the address that is being processed is not fully
    qualified, it may be expanded to its full form by a DNS lookup. For
    example, if an address is specified as "dormouse@teaparty", the domain
    might be expanded to "teaparty.wonderland.fict.example". Domain expansion
    can also occur as a result of setting the "widen_domains" option. If
    "rewrite_headers" is true, all occurrences of the abbreviated domain name
    in any Bcc:, Cc:, From:, Reply-to:, Sender:, and To: header lines of the
    message are rewritten with the full domain name.

    This option should be turned off only when it is known that no message is
    ever going to be sent outside an environment where the abbreviation makes
    sense.

    When an MX record is looked up in the DNS and matches a wildcard record,
    name servers normally return a record containing the name that has been
    looked up, making it impossible to detect whether a wildcard was present
    or not. However, some name servers have recently been seen to return the
    wildcard entry. If the name returned by a DNS lookup begins with an
    asterisk, it is not used for header rewriting.

same_domain_copy_routing        Type: boolean                  Default: false

    Addresses with the same domain are normally routed by the dnslookup router
    to the same list of hosts. However, this cannot be presumed, because the
    router options and preconditions may refer to the local part of the
    address. By default, therefore, Exim routes each address in a message
    independently. DNS servers run caches, so repeated DNS lookups are not
    normally expensive, and in any case, personal messages rarely have more
    than a few recipients.

    If you are running mailing lists with large numbers of subscribers at the
    same domain, and you are using a dnslookup router which is independent of
    the local part, you can set "same_domain_copy_routing" to bypass repeated
    DNS lookups for identical domains in one message. In this case, when
    dnslookup routes an address to a remote transport, any other unrouted
    addresses in the message that have the same domain are automatically given
    the same routing without processing them independently, provided the       |
    following conditions are met:                                              |
                                                                               |
     .   No router that processed the address specified "headers_add" or       |
         "headers_remove".                                                     |
                                                                               |
     .   The router did not change the address in any way, for example, by     |
         'widening' the domain.                                                |
                                                                               |
search_parents                  Type: boolean                  Default: false

    When this option is true, the resolver option RES_DNSRCH is set for DNS
    lookups. This is different from the "qualify_single" option in that it
    applies to domains containing dots. Typically, but not standardly, it
    causes the resolver to search for the name in the current domain and in
    parent domains. For example, on a machine in the "fict.example" domain,
    if looking up "teaparty.wonderland" failed, the resolver would try
    "teaparty.wonderland.fict.example". For details of what your resolver
    actually does, consult your man pages for "resolver" and "resolv.conf".

    Setting this option true can cause problems in domains that have a
    wildcard MX record, because any domain that does not have its own MX
    record matches the local wildcard.

widen_domains                 Type: string list                Default: unset

    If a DNS lookup fails and this option is set, each of its strings in turn
    is added onto the end of the domain, and the lookup is tried again. For
    example, if

      widen_domains = fict.example:ref.example

    is set and a lookup of "klingon.dictionary" fails,
    "klingon.dictionary.fict.example" is looked up, and if this fails,
    "klingon.dictionary.ref.example" is tried. Note that the "qualify_single"
    and "search_parents" options can cause some widening to be undertaken
    inside the DNS resolver.



                           18. THE IPLITERAL ROUTER


This router has no private options. Unless it is being used purely for
verification (see "verify_only") a transport is required to be defined by the
generic "transport" option. The router accepts the address if its domain part
takes the form of an RFC 2822 domain literal, that is, an IP address enclosed
in square brackets. For example, this router handles the address

  root@[192.168.1.1]

by setting up delivery to the host with that IP address. If an IP literal
turns out to refer to the local host, the generic "self" option determines
what happens. The RFCs require support for domain literals, though it seems
anachronistic in today's Internet.

If you want to use this router, you must also set the main configuration
option "allow_domain_literals". Otherwise, Exim will not recognize the domain
literal syntax in addresses.



                           19. THE IPLOOKUP ROUTER


The iplookup router was written to fulfil a specific requirement in Cambridge
University. For this reason, it is not included in the binary of Exim by
default. If you want to include it, you must set

  ROUTER_IPLOOKUP=yes

in your Local/Makefile configuration file.

The iplookup router routes an address by sending it over a TCP or UDP
connection to one or more specific hosts. The host can then return the same or
a different address - in effect rewriting the recipient address in the
message's envelope. The new address is then passed on to subsequent routers.

If this process fails, the address can be passed on to other routers, or
delivery can be deferred.

Background, for those that are interested: We have an Oracle database of all
Cambridge users, and one of the items of data it maintains for each user
is where to send mail addressed to "user@cam.ac.uk". The MX records for
"cam.ac.uk" point to a central machine that has a large alias list that is
abstracted from the database. Mail from outside is switched by this system,
and originally internal mail was also done this way. However, this resulted in
a fair number of messages travelling from some of our larger systems to the
switch and back again. The Oracle machine now runs a UDP service that can be
called by the iplookup router in Exim to find out where "user@cam.ac.uk"
addresses really have to go; this saves passing through the central switch,
and in many cases saves doing any remote delivery at all.

Since iplookup is just a rewriting router, a transport must not be specified
for it.


hosts                            Type: string                   Default: unset

    This option must be supplied. Its value is a colon-separated list of host
    names. The hosts are looked up using "gethostbyname()" (or
    "getipnodebyname()" when available) and are tried in order until one
    responds to the query. If none respond, what happens is controlled by
    "optional".

optional                        Type: boolean                  Default: false

    If "optional" is true, if no response is obtained from any host, the
    address is passed to the next router, overriding "no_more". If "optional"
    is false, delivery to the address is deferred.

port                            Type: integer                      Default: 0

    This option must be supplied. It specifies the port number for the TCP or
    UDP call.

protocol                         Type: string                     Default: udp

    This option can be set to 'udp' or 'tcp' to specify which of the two
    protocols is to be used.

query                           Type: string*
                              Default: $local_part@$domain $local_part@$domain

    This defines the content of the query that is sent to the remote hosts.
    The repetition serves as a way of checking that a response is to the
    correct query in the default case (see "response_pattern" below).

reroute                         Type: string*                  Default: unset

    If this option is not set, the rerouted address is precisely the byte
    string returned by the remote host, up to the first white space, if any.
    If set, the string is expanded to form the rerouted address. It can
    include parts matched in the response by "response_pattern" by means of
    numeric variables such as $1, $2, etc. The variable $0 refers to the
    entire input string, whether or not a pattern is in use. In all cases, the
    rerouted address must end up in the form "local_part@domain".

response_pattern                 Type: string                   Default: unset

    This option can be set to a regular expression that is applied to the
    string returned from the remote host. If the pattern does not match the
    response, the router declines. If "response_pattern" is not set, no
    checking of the response is done, unless the query was defaulted, in which
    case there is a check that the text returned after the first white space
    is the original address. This checks that the answer that has been
    received is in response to the correct question. For example, if the
    response is just a new domain, the following could be used:

      response_pattern = ^([^@]+)$
      reroute = $local_part@$1

timeout                           Type: time                       Default: 5s

    This specifies the amount of time to wait for a response from the remote
    machine. The same timeout is used for the "connect()" function for a TCP
    call. It does not apply to UDP.



                          20. THE MANUALROUTE ROUTER


The manualroute router is so-called because it provides a way of manually
routing an address according to its domain. It is mainly used when you want to
route addresses to remote hosts according to your own rules, bypassing the
normal DNS routing that looks up MX records. However, manualroute can also
route to local transports, a facility that may be useful if you want to save
messages for dial-in hosts in local files.

The manualroute router compares a list of domain patterns with the domain it
is trying to route. If there is no match, the router declines. Each pattern
has associated with it a list of hosts and some other optional data, which may
include a transport. The combination of a pattern and its data is called a
'routing rule'. For patterns that do not have an associated transport, the
generic "transport" option must specify a transport, unless the router is
being used purely for verification (see "verify_only").

In the case of verification, matching the domain pattern is sufficient for the
router to accept the address. When actually routing an address for delivery,
an address that matches a domain pattern is queued for the associated
transport. If the transport is not a local one, a host list must be associated
with the pattern; IP addresses are looked up for the hosts, and these are
passed to the transport along with the mail address. For local transports, a
host list is optional. If it is present, it is passed in $host as a single
text string.

The list of routing rules can be provided as an inline string in "route_list",
or the data can be obtained by looking up the domain in a file or database by
setting "route_data". Only one of these settings may appear in any one
instance of manualroute. The format of routing rules is described below,
following the list of private options.


20.1 Private options for manualroute

The private options for the manualroute router are as follows:


host_find_failed                 Type: string                  Default: freeze

    This option controls what happens when manualroute tries to find an IP
    address for a host, and the host does not exist. The option can be set to
    one of

      decline
      defer
      fail
      freeze
      pass

    The default assumes that this state is a serious configuration error. The
    difference between 'pass' and 'decline' is that the former forces the
    address to be passed to the next router (or the router defined by
    "pass_router"), overriding "no_more", whereas the latter passes the
    address to the next router only if "more" is true.

    This option applies only to a definite 'does not exist' state; if a host
    lookup gets a temporary error, delivery is deferred unless the generic
    "pass_on_timeout" option is set.

hosts_randomize                 Type: boolean                  Default: false

    If this option is set, the order of the items in a host list in a routing
    rule is randomized each time the list is used, unless an option in the
    routing rule overrides (see below). Randomizing the order of a host list
    can be used to do crude load sharing. However, if more than one mail
    address is routed by the same router to the same host list, the host lists
    are considered to be the same (even though they may be randomized into
    different orders) for the purpose of deciding whether to batch the
    deliveries into a single SMTP transaction.

    When "hosts_randomize" is true, a host list may be split into groups whose
    order is separately randomized. This makes it possible to set up MX-like
    behaviour. The boundaries between groups are indicated by an item that is
    just "+" in the host list. For example:

      route_list = * host1:host2:host3:+:host4:host5

    The order of the first three hosts and the order of the last two hosts is
    randomized for each use, but the first three always end up before the last
    two. If "hosts_randomize" is not set, a "+" item in the list is ignored.
    If a randomized host list is passed to an smtp transport that also has
    "hosts_randomize set", the list is not re-randomized.

route_data                      Type: string*                  Default: unset

    If this option is set, it must expand to yield the data part of a routing
    rule. Typically, the expansion string includes a lookup based on the
    domain. For example:

      route_data = ${lookup{$domain}dbm{/etc/routes}}

    If the expansion is forced to fail, or the result is an empty string, the
    router declines. Other kinds of expansion failure cause delivery to be
    deferred.

route_list          Type: string list, semicolon-separated      Default: unset

    This string is a list of routing rules, in the form defined below. Note
    that, unlike most string lists, the items are separated by semicolons.
    This is so that they may contain colon-separated host lists.

same_domain_copy_routing        Type: boolean                  Default: false

    Addresses with the same domain are normally routed by the manualroute
    router to the same list of hosts. However, this cannot be presumed,
    because the router options and preconditions may refer to the local part
    of the address. By default, therefore, Exim routes each address in a
    message independently. DNS servers run caches, so repeated DNS lookups are
    not normally expensive, and in any case, personal messages rarely have
    more than a few recipients.

    If you are running mailing lists with large numbers of subscribers at the
    same domain, and you are using a manualroute router which is independent
    of the local part, you can set "same_domain_copy_routing" to bypass
    repeated DNS lookups for identical domains in one message. In this case,
    when manualroute routes an address to a remote transport, any other
    unrouted addresses in the message that have the same domain are automati-
    cally given the same routing without processing them independently.
    However, this is only done if "headers_add" and "headers_remove" are
    unset.


20.2 Routing rules in route_list

The value of "route_list" is a string consisting of a sequence of routing
rules, separated by semicolons. If a semicolon is needed in a rule, it can be
entered as two semicolons. Empty rules are ignored. The format of each rule is

  <domain pattern>  <list of hosts>  <options>

The following example contains two rules, each with a simple domain pattern
and no options:

  route_list = \
    dict.ref.example  mail-1.ref.example:mail-2.ref.example ; \
    thes.ref.example  mail-3.ref.example:mail-4.ref.example

The three parts of a rule are separated by white space. The pattern and the
list of hosts can be enclosed in quotes if necessary, and if they are, the
usual quoting rules apply. Each rule in a "route_list" must start with a
single domain pattern, which is the only mandatory item in the rule. The
pattern is in the same format as one item in a domain list (see section 10.7), |
except that it may not be the name of an interpolated file. That is, it may be |
wildcarded, or a regular expression, or a file or database lookup (with
semicolons doubled, because of the use of semicolon as a separator in a
"route_list").

The rules in "route_list" are searched in order until one of the patterns
matches the domain that is being routed. The list of hosts and then options
are then used as described below. If there is no match, the router declines.
When "route_list" is set, "route_data" must not be set.


20.3 Routing rules in route_data

The use of "route_list" is convenient when there are only a small number of
routing rules. For larger numbers, it is easier to use a file or database to
hold the routing information, and use the "route_data" option instead. The
value of "route_data" is a list of hosts, followed by (optional) options. Most
commonly, "route_data" is set as a string that contains an expansion lookup.
For example, suppose we place two routing rules in a file like this:

  dict.ref.example:  mail-1.ref.example:mail-2.ref.example
  thes.ref.example:  mail-3.ref.example:mail-4.ref.example

This data can be accessed by setting

  route_data = ${lookup{$domain}lsearch{/the/file/name}}

Failure of the lookup results in an empty string, causing the router to
decline. However, you do not have to use a lookup in "route_data". The only
requirement is that the result of expanding the string is a list of hosts,
possibly followed by options, separated by white space. The list of hosts must
be enclosed in quotes if it contains white space.


20.4 Format of the list of hosts

A list of hosts, whether obtained via "route_data" or "route_list", is always
separately expanded before use. If the expansion fails, the router declines.
The result of the expansion must be a colon-separated list of names and/or IP
addresses. IP addresses are not enclosed in brackets.

If the list of hosts was obtained from a "route_list" item, the following
variables are set during its expansion:

 .   If the domain was matched against a regular expression, the numeric
     variables $1, $2, etc. may be set.

 .   $0 is always set to the entire domain.

 .   $1 is also set when partial matching is done in a file lookup.

 .   If the pattern that matched the domain was a lookup item, the data that
     was looked up is available in the expansion variable $value.

An unadorned name in the list of hosts is interpreted as a host name. A name   |
that is followed by "/MX" is interpreted as an indirection to a sublist of     |
hosts obtained by looking up MX records. For example:                          |
                                                                               |
  route_list = *  x.y.z:p.q.r/MX:e.f.g                                         |
                                                                               |
If the "hosts_randomize" option is set, the order of the items in the list is  |
randomized first. Then, when Exim is scanning the list to find an IP address   |
for each host, names that end with "/MX" are replaced with the list of host    |
names obtained by looking up MX records for the name. The order is determined  |
by the preference values in the MX records, according to the usual rules.      |
Because randomizing happens first, it does not affect the order that is        |
defined by MX preferences.                                                     |
                                                                               |
If the local host is present in the sublist obtained from MX records, but is   |
not the most preferred host in that list, it and any equally or less preferred |
hosts are removed before the sublist is inserted into the main list.           |
                                                                               |
If the local host is the most preferred host in the MX list, what happens      |
depends on where in the original list of hosts the "/MX" item appears. If it   |
is not the first item (that is, there are previous hosts), Exim discards this  |
name and any subsequent items in the list, just as it does for a non-MX name   |
that turns out to be the local host.                                           |
                                                                               |
If the MX item is first in the list of hosts, and the local host is the most   |
preferred host, what happens is controlled by the "self" option of the router. |
                                                                               |
DNS failures when lookup up the MX records are treated in the same way as DNS  |
failures when looking up IP addresses: "pass_on_timeout" and                   |
"host_find_failed" are used when relevant.                                     |


20.5 How the list of hosts is used

When an address is routed to an smtp transport by manualroute, each of the
hosts is tried, in the order specified, when carrying out the SMTP delivery.
However, the order can be changed by setting the "hosts_randomize" option,
either on the router (see section 20.1 above), or on the transport.


20.6 How the options are used

The options are a sequence of words; in practice no more than three are ever
present. One of the words can be the name of a transport; this overrides the
"transport" option on the router for this particular routing rule only. The
other words (if present) control randomization of the list of hosts on a per-
rule basis, and how the IP addresses of the hosts are to be found when routing
to a remote transport. These options are as follows:

 .   "randomize": randomize the order of the hosts in this list, overriding
     the setting of "hosts_randomize" for this routing rule only.

 .   "no_randomize": do not randomize the order of the hosts in this list,
     overriding the setting of "hosts_randomize" for this routing rule only.

 .   "byname": use "getipnodebyname()" ("gethostbyname()" on older systems) to
     find IP addresses. This function may ultimately cause a DNS lookup, but
     it may also look in /etc/hosts or other sources of information.

 .   "bydns": look up address records for the hosts directly in the DNS; fail
     if no address records are found. If there is a temporary DNS error (such
     as a timeout), delivery is deferred.

For example:

  route_list = domain1  host1:host2:host3  randomize bydns;\
               domain2  host4:host5

If neither "byname" nor "bydns" is given, Exim behaves as follows: First, a
DNS lookup is done. If this yields anything other than HOST_NOT_FOUND, that
result is used. Otherwise, Exim goes on to try a call to "getipnodebyname()"
or "gethostbyname()", and the result of the lookup is the result of that call.

Warning: It has been discovered that on some systems, if a DNS lookup called
via "getipnodebyname()" times out, HOST_NOT_FOUND is returned instead of
TRY_AGAIN. That is why the default action is to try a DNS lookup first. Only
if that gives a definite 'no such host' is the local function called.

If no IP address for a host can be found, what happens is controlled by the
"host_find_failed" option.

When an address is routed to a local transport, IP addresses are not looked
up. The host list is passed to the transport in the $host variable.


20.7 Manualroute examples

In some of the examples that follow, the presence of the "remote_smtp"
transport, as defined in the default configuration file, is assumed:

 .   The manualroute router can be used to forward all external mail to a
     "smart host". If you have set up, in the main part of the configuration,
     a named domain list that contains your local domains, for example,

       domainlist local_domains = my.domain.example

     you can arrange for all other domains to be routed to a smart host by
     making your first router something like this:

       smart_route:
         driver = manualroute
         domains = !+local_domains
         transport = remote_smtp
         route_list = * smarthost.ref.example

     This causes all non-local addresses to be sent to the single host
     "smarthost.ref.example". If a colon-separated list of smart hosts is
     given, they are tried in order (but you can use "hosts_randomize" to vary
     the order each time). Another way of configuring the same thing is this:

       smart_route:
         driver = manualroute
         transport = remote_smtp
         route_list = !+local_domains  smarthost.ref.example

     There is no difference in behaviour between these two routers as they
     stand. However, they behave differently if "no_more" is added to them. In
     the first example, the router is skipped if the domain does not match the
     "domains" precondition; the following router is always tried. If the
     router runs, it always matches the domain and so can never decline.
     Therefore, "no_more" would have no effect. In the second case, the router
     is never skipped; it always runs. However, if it doesn't match the
     domain, it declines. In this case "no_more" would prevent subsequent
     routers from running.

 .   A "mail hub" is a host which receives mail for a number of domains via MX
     records in the DNS and delivers it via its own private routing mechanism.
     Often the final destinations are behind a firewall, with the mail hub
     being the one machine that can connect to machines both inside and
     outside the firewall. The manualroute router is usually used on a mail
     hub to route incoming messages to the correct hosts. For a small number
     of domains, the routing can be inline, using the "route_list" option, but
     for a larger number a file or database lookup is easier to manage.

     If the domain names are in fact the names of the machines to which the
     mail is to be sent by the mail hub, the configuration can be quite
     simple. For example,

       hub_route:
         driver = manualroute
         transport = remote_smtp
         route_list = *.rhodes.tvs.example  $domain

     This configuration routes domains that match "*.rhodes.tvs.example" to
     hosts whose names are the same as the mail domains. A similar approach
     can be taken if the host name can be obtained from the domain name by a
     string manipulation that the expansion facilities can handle. Otherwise,
     a lookup based on the domain can be used to find the host:

       through_firewall:
         driver = manualroute
         transport = remote_smtp
         route_data = ${lookup {$domain} cdb {/internal/host/routes}}

     The result of the lookup must be the name or IP address of the host (or
     hosts) to which the address is to be routed. If the lookup fails, the
     route data is empty, causing the router to decline. The address then
     passes to the next router.

 .   You can use manualroute to deliver messages to pipes or files in batched
     SMTP format for onward transportation by some other means. This is one
     way of storing mail for a dial-up host when it is not connected. The
     route list entry can be as simple as a single domain name in a
     configuration like this:

       save_in_file:
         driver = manualroute
         transport = batchsmtp_appendfile
         route_list = saved.domain.example

     though often a pattern is used to pick up more than one domain. If there
     are several domains or groups of domains with different transport
     requirements, different transports can be listed in the routing
     information:

       save_in_file:
         driver = manualroute
         route_list = \
           *.saved.domain1.example  $domain  batch_appendfile; \
           *.saved.domain2.example  \
             ${lookup{$domain}dbm{/domain2/hosts}{$value}fail} \
             batch_pipe

     The first of these just passes the domain in the $host variable, which
     doesn't achieve much (since it is also in $domain), but the second does a
     file lookup to find a value to pass, causing the router to decline to
     handle the address if the lookup fails.

 .   Routing mail directly to UUCP software is a specific case of the use of
     manualroute in a gateway to another mail environment. This is an example
     of one way it can be done:

       # Transport
       uucp:
         driver = pipe
         user = nobody
         command = /usr/local/bin/uux -r - \
           ${substr_-5:$host}!rmail ${local_part}
         return_fail_output = true

       # Router
       uucphost:
         transport = uucp
         driver = manualroute
         route_data = \
           ${lookup{$domain}lsearch{/usr/local/exim/uucphosts}}

     The file /usr/local/exim/uucphosts contains entries like

       darksite.ethereal.example:           darksite.UUCP

     It can be set up more simply without adding and removing '.UUCP' but
     this way makes clear the distinction between the domain name
     "darksite.ethereal.example" and the UUCP host name "darksite".



                         21. THE QUERYPROGRAM ROUTER


The queryprogram router routes an address by running an external command and
acting on its output. This is an expensive way to route, and is intended
mainly for use in lightly-loaded systems, or for performing experiments.
However, if it is possible to use the precondition options ("domains",
"local_parts", etc) to skip this router for most addresses, it could sensibly
be used in special cases, even on a busy host. There are the following private
options:


command                         Type: string*                  Default: unset

    This option must be set. It specifies the command that is to be run. The
    command is split up into a command name and arguments, and then each is
    expanded separately (exactly as for a pipe transport, described in chapter
    29).

command_group                    Type: string                   Default: unset

    This option specifies a gid to be set when running the command. It must be
    set if "command_user" specifies a numerical uid. If it begins with a
    digit, it is interpreted as the numerical value of the gid. Otherwise it
    is looked up using "getgrnam()".

command_user                     Type: string                   Default: unset

    This option must be set. It specifies the uid which is set when running
    the command. If it begins with a digit it is interpreted as the numerical
    value of the uid. Otherwise, it is looked up using "getpwnam()" to obtain
    a value for the uid and, if "command_group" is not set, a value for the
    gid also.

current_directory                Type: string                       Default: /

    This option specifies an absolute path which is made the current directory
    before running the command.

timeout                           Type: time                       Default: 1h

    If the command does not complete within the timeout period, its process
    group is killed and the message is frozen. A value of zero time specifies
    no timeout.

The standard output of the command is connected to a pipe, which is read when
the command terminates. It should consist of a single line of output,
containing up to five fields, separated by white space. The first field is one
of the following words (case-insensitive):

 .   "Accept": routing succeeded; the remaining fields specify what to do (see
     below).

 .   "Decline": the router declines; pass the address to the next router,
     unless "no_more" is set.

 .   "Fail": routing failed; do not pass the address to any more routers. Any
     subsequent text on the line is an error message. If the router is run as
     part of address verification during an incoming SMTP message, the message
     is included in the SMTP response.

 .   "Defer": routing could not be completed at this time; try again later.
     Any subsequent text on the line is an error message which is logged. It
     is not included in any SMTP response.

 .   "Freeze": the same as "defer", except that the message is frozen.

 .   "Pass": pass the address to the next router (or the router specified by
     "pass_router"), overriding "no_more".

 .   "Redirect": the message is redirected. The remainder of the line is a
     list of new addresses, which are routed independently, starting with the
     first router, or the router specified by "redirect_router", if set.

When the first word is "accept", the remainder of the line consists of a
number of keyed data values, as follows (split into two lines here, to fit on
the page):

  ACCEPT TRANSPORT=<transport> HOSTS=<list of hosts>
         LOOKUP=byname|bydns DATA=<text>

The data items can be given in any order, and all are optional. If no
transport is included, the transport specified by the generic "transport"
option is used. The list of hosts and the lookup type are needed only if the
transport is an smtp transport that does not itself supply a list of hosts.

The format of the list of hosts is the same as for the manualroute router. As  |
well as host names and IP addresses, it may contain names followed by "/MX" to |
specify sublists of hosts that are obtained by looking up MX records.          |

If the lookup type is not specified, Exim behaves as follows when trying to
find an IP address for each host: First, a DNS lookup is done. If this yields
anything other than HOST_NOT_FOUND, that result is used. Otherwise, Exim goes
on to try a call to "getipnodebyname()" or "gethostbyname()", and the result
of the lookup is the result of that call.

If the DATA field is set, its value is placed in the $address_data variable.
For example, this return line

  accept hosts=x1.y.example:x2.y.example data="rule1"

routes the address to the default transport, with a host list containing two
hosts. When the transport runs, the string 'rule1' is in $address_data.



                           22. THE REDIRECT ROUTER


The redirect router handles several kinds of address redirection. Its most
common uses are for resolving local part aliases from a central alias file
(usually called /etc/aliases) and for handling users' personal .forward files,
but it has many other potential uses. The incoming address can be redirected
in several different ways:

 .   It can be replaced by one or more new addresses which are themselves
     routed independently.

 .   It can be routed to be delivered to a given file or directory.

 .   It can be routed to be delivered to a specified pipe command.

 .   It can cause an automatic reply to be generated.

 .   It can be forced to fail, with a custom error message.

 .   It can be temporarily deferred.

 .   It can be discarded.

The generic "transport" option must not be set for redirect routers. However,
there are some private options which define transports for delivery to files
and pipes, and for generating autoreplies. See the "file_transport",
"pipe_transport" and "reply_transport" descriptions below.


22.1 Redirection data

The router operates by interpreting a text string which it obtains either by
expanding the contents of the "data" option, or by reading the entire contents
of a file whose name is given in the "file" option. These two options are
mutually exclusive. The first is commonly used for handling system aliases, in
a configuration like this:

  system_aliases:
    driver = redirect
    data = ${lookup{$local_part}lsearch{/etc/aliases}}

If the lookup fails, causing the expanded string to be empty, the router
declines. A configuration using "file" is commonly used for handling users'
.forward files, like this:

  userforward:
    driver = redirect
    check_local_user
    file = $home/.forward
    no_verify

If the file does not exist, or causes no action to be taken (for example, it
is empty or consists only of comments), the router declines. Warning: This is
not the case when the file contains syntactically valid items that happen to
yield empty addresses, for example, items containing only RFC 2822 address
comments.


22.2 Forward files and address verification

It is usual to set "no_verify" on redirect routers which handle users'
.forward files, as in the example above. There are two reasons for this:

 .   When Exim is receiving an incoming SMTP message from a remote host, it is
     running under the Exim uid, not as root. No additional groups are set up, |
     even if the Exim uid is a member of other groups (that is, the            |
     "initgroups()" function is not run). Exim is unable to change uid to read |
     the file as the user, and it may not be able to read it as the Exim user.
     So in practice the router may not be able to operate.

 .   However, even when the router can operate, the existence of a .forward
     file is unimportant when verifying an address. What should be checked is
     whether the local part is a valid user name or not. Cutting out the
     redirection processing saves some resources.


22.3 Interpreting redirection data

The contents of the data string, whether obtained from "data" or "file", can
be interpreted in two different ways:

 .   If the "allow_filter" option is set true, and the data begins with the
     text '#Exim filter' or '#Sieve filter', it is interpreted as a list of    |
     "filtering" instructions in the form of an Exim or Sieve filter file,     |
     respectively. Details of the syntax and semantics of filter files are     |
     described in a separate document entitled "Exim's interfaces to mail
     filtering"; this document is intended for use by end users.

 .   Otherwise, the data must be a comma-separated list of redirection items,
     as described in the next section.

When a message is redirected to a file (a 'mail folder'), the file name given  |
in a non-filter redirection list must always be an absolute path. A filter may |
generate a relative path - how this is handled depends on the transport's      |
configuration. See section 26.1 for a discussion of this issue for the         |
appendfile transport.                                                          |


22.4 Items in a non-filter redirection list

When the redirection data is not an Exim or Sieve filter, for example, if it
comes from a conventional alias or forward file, it consists of a list of
addresses, file names, pipe commands, or certain special items (see section
22.6 below). The special items can be individually enabled or disabled by
means of options whose names begin with "allow_" or "forbid_", depending on
their default values. The items in the list are separated by commas or
newlines. If a comma is required in an item, the entire item must be enclosed  |
in double quotes.                                                              |

Lines starting with a # character are comments, and are ignored, and # may
also appear following a comma, in which case everything between the # and the
next newline character is ignored.

If an item is entirely enclosed in double quotes, these are removed. Otherwise
double quotes are retained because some forms of mail address require their
use (but never to enclose the entire address). In the following description,
'item' refers to what remains after any surrounding double quotes have been
removed.

Warning: If you use an Exim expansion to construct a redirection address, and
the expansion contains a reference to $local_part, you should make use of the
"quote" expansion operator, in case the local part contains special charac-
ters. For example, to redirect all mail for the domain "obsolete.example",
retaining the existing local part, you could use this setting:

  data = ${quote:$local_part}@newdomain.example


22.5 Redirecting to a local mailbox

A redirection item may safely be the same as the address currently under
consideration. This does not cause a routing loop, because a router is
automatically skipped if any ancestor of the address that is being processed
has the same local part and was processed by that router. The address is
therefore passed to the following routers, so it is handled as if there were
no redirection. When making this loop-avoidance test, the complete local part,
including any prefix or suffix, is used.

Specifying the same local part without a domain is a common usage in personal
filter files when the user wants to have messages delivered to the local
mailbox and also forwarded elsewhere. For example, the user "spqr" might have
a .forward file containing this:

  spqr, Sam.Reman@other.domain.example

For compatibility with other MTAs, such local parts may be preceeded by '\',
but this is not a requirement for loop prevention. However, it does make a
difference if more than one domain is being handled synonymously.

If an item begins with '\' and the rest of the item parses as a valid RFC 2822
address that does not include a domain, the item is qualified using the domain
of the incoming address. In the absence of a leading '\', unqualified
addresses are qualified using the value in "qualify_recipient", but you can
force the incoming domain to be used by setting "qualify_preserve_domain".

Care must be taken if there are alias names for local users. For example if
the system alias file contains

  Sam.Reman: spqr

then

  Sam.Reman, spqr@reme.elsewhere.example

in "spqr"'s forward file fails on an incoming message addressed to
"Sam.Reman". The redirect router does not process "Sam.Reman" the second time
round, because it has previously routed it, and the following routers
presumably cannot handle the alias. The forward file should really contain

  spqr, spqr@reme.elsewhere.example

but because this is such a common error, the "check_ancestor" option (see
below) exists to provide a way to get round it. This is normally set on a
redirect router that is handling users' .forward files.


22.6 Special items in redirection lists

In addition to addresses, the following types of item may appear in
redirection lists (that is, in non-filter redirection data):

 .   An item is treated as a pipe command if it begins with '|' and does not
     parse as a valid RFC 2822 address that includes a domain. A transport for
     running the command must be specified by the "pipe_transport" option.
     Either the router or the transport must specify a user and group under
     which to run the delivery. Single or double quotes can be used for
     enclosing the individual arguments of the pipe command; no interpretation
     of escapes is done for single quotes. If the command contains a comma
     character, it is necessary to put the whole item in double quotes, for
     example:

       "|/some/command ready,steady,go"

     since items in redirection lists are terminated by commas. Do not,
     however, quote just the command. An item such as

       |"/some/command ready,steady,go"

     is interpreted as a pipe with a rather strange command name, and no
     arguments.

 .   An item is interpreted as a path name if it begins with '/' and does not
     parse as a valid RFC 2822 address that includes a domain. For example,

       /home/world/minbari

     is treated as a file name, but

       /s=molari/o=babylon/@x400gate.way

     is treated as an address. For a file name, a transport must be specified
     using the "file_transport" option. However, if the generated path name
     ends with a forward slash character, it is interpreted as a directory
     name rather than a file name, and "directory_transport" is used instead.
     Either the router or the transport must specify a user and group under
     which to run the delivery. However, if a redirection item is the path
     /dev/null, delivery to it is bypassed at a high level, and the log entry
     shows '**bypassed**' instead of a transport name. This avoids the need to
     specify a user and group.

 .   If an item is of the form

       :include:<path name>

     a list of further items is taken from the given file and included at that
     point. Note: such a file can not be a filter file; it is just an out-of-
     line addition to the list. The items in the included list are separated
     by commas or newlines and are not subject to expansion. If this is the
     first item in an alias list in an lsearch file, a colon must be used to
     terminate the alias name. This example is incorrect:

       list1    :include:/opt/lists/list1

     It must be given as

       list1:   :include:/opt/lists/list1

 .   Sometimes you want to throw away mail to a particular local part. Making
     the "data" option expand to an empty string does not work, because that
     causes the router to decline. Instead, the alias item

       :blackhole:

     can be used. It does what its name implies. No delivery is done, and no
     error message is generated. This has the same effect as specifing
     /dev/null, but can be independently disabled.

     Warning: If ":blackhole:" appears anywhere in a redirection list, no      |
     delivery is done for the original local part, even if other redirection   |
     items are present. If you are generating a multi-item list (for example,  |
     by reading a database) and need the ability to provide a no-op item, you  |
     must use /dev/null.                                                       |

 .   An attempt to deliver a particular address can be deferred or forced to
     fail by redirection items of the form

       :defer:
       or
       :fail:

     respectively. When a redirection list contains such an item, it applies
     to the entire redirection; any other items in the list are ignored
     (:blackhole: is different). Any text following :fail: or :defer: is
     placed in the error text associated with the failure. For example, an
     alias file might contain:

       X.Employee:  :fail: Gone away, no forwarding address

     In the case of an address that is being verified from an ACL or as the
     subject of a VRFY command, the text is included in the SMTP error
     response by default. In an ACL, an explicitly provided message overrides
     the default, but the default message is available in the variable
     $acl_verify_message and can therefore be included in a custom message if
     this is desired. Exim sends a 451 SMTP code for a :defer:, and 550 for
     :fail:. In non-SMTP cases the text is included in the error message that
     Exim generates.

     Normally the error text is the rest of the redirection list - a comma
     does not terminate it - but a newline does act as a terminator. Newlines
     are not normally present in alias expansions. In lsearch lookups they are
     removed as part of the continuation process, but they may exist in other
     kinds of lookup and in :include: files.

     During routing for message delivery (as opposed to verification), a
     redirection containing :fail: causes an immediate failure of the incoming
     address, whereas :defer: causes the message to remain on the queue so
     that a subsequent delivery attempt can happen at a later time. If an
     address is deferred for too long, it will ultimately fail, because the
     normal retry rules still apply.

 .   Sometimes it is useful to use a single-key search type with a default
     (see chapter 9) to look up aliases. However, there may be a need for
     exceptions to the default. These can be handled by aliasing them to

       :unknown:

     This differs from :fail: in that it causes the redirect router to
     decline, whereas :fail: forces routing to fail. A lookup which results in
     an empty redirection list has the same effect.


22.7 Duplicate addresses

Exim removes duplicate addresses from the list to which it is delivering, so
as to deliver just one copy to each address. This does not apply to deliveries
routed to pipes by different immediate parent addresses, but an indirect
aliasing scheme of the type

  pipe:       |/some/command $local_part
  localpart1: pipe
  localpart2: pipe

does not work with a message that is addressed to both local parts, because
when the second is aliased to the intermediate local part 'pipe' it gets
discarded as being the same as a previously handled address. However, a scheme
such as

  localpart1: |/some/command $local_part
  localpart2: |/some/command $local_part

does result in two different pipe deliveries, because the immediate parents of
the pipes are distinct.


22.8 Repeated redirection expansion

When a message cannot be delivered to all of its recipients immediately,
leading to two or more delivery attempts, redirection expansion is carried out
afresh each time for those addresses whose children were not all previously
delivered. If redirection is being used as a mailing list, this can lead to
new members of the list receiving copies of old messages. The "one_time"
option can be used to avoid this.


22.9 Errors in redirection lists

If "skip_syntax_errors" is set, a malformed address that causes a parsing
error is skipped, and an entry is written to the main log. This may be useful
for mailing lists that are automatically managed. Otherwise, if an error is
detected while generating the list of new addresses, the original address is
deferred. See also "syntax_errors_to".


22.10 Private options for the redirect router

The private options for the redirect router are as follows:


allow_defer                     Type: boolean                  Default: false

    Setting this option allows the use of :defer: in non-filter redirection
    data, or the "defer" command in an Exim filter file.                       |

allow_fail                      Type: boolean                  Default: false

    If this option is true, the :fail: item can be used in a redirection list,
    and the "fail" command may be used in a filter file.

allow_filter                    Type: boolean                  Default: false

    Setting this option allows Exim to interpret redirection data that starts
    with '#Exim filter' or '#Sieve filter' as a set of filtering instructions. |
    There are some features of Exim filter files that some administrators may  |
    wish to lock out; see the "forbid_filter_xxx" options below. The filter is
    run using the uid and gid set by the generic "user" and "group" options.
    These take their defaults from the password data if "check_local_user" is
    set, so in the normal case of users' personal filter files, the filter is
    run as the relevant user. When "allow_filter" is set true, Exim insists
    that either "check_local_user" or "user" is set.

allow_freeze                    Type: boolean                  Default: false

    Setting this option allows the use of the "freeze" command in an Exim
    filter. This command is more normally encountered in system filters, and
    is disabled by default for redirection filters because it isn't something
    you usually want to let ordinary users do.

check_ancestor                  Type: boolean                  Default: false

    This option is concerned with handling generated addresses which are the
    same as some address in the list of redirection ancestors of the current
    address. Although it is turned off by default in the code, it is set in
    the default configuration file for handling users' .forward files. It is
    recommended for this use of the redirect router.

    When "check_ancestor" is set, if a generated address is the same as any
    ancestor of the current address, it is replaced by a copy of the current
    address. This helps in the case where local part A is aliased to B, and B
    has a .forward file pointing back to A. For example: 'Joe.Bloggs' is
    aliased to 'jb' and ~jb/.forward contains:

      \Joe.Bloggs, <other item(s)>

    Without the "check_ancestor" setting, either local part ('jb' or
    'joe.bloggs') gets processed once by each router and so ends up as it was
    originally. If 'jb' is the real mailbox name, mail to 'jb' gets delivered
    (having been turned into 'joe.bloggs' by the .forward file and back to
    'jb' by the alias), but mail to 'joe.bloggs' fails. Setting
    "check_ancestor" on the redirect router that handles the .forward file
    prevents it from turning 'jb' back into 'joe.bloggs' when that was the
    original address. See also the "repeat_use" option below.

check_group                     Type: boolean              Default: see below

    When the "file" option is used, the group owner of the file is checked
    only when this option is set. The permitted groups are those listed in
    the "owngroups" option, together with the user's default group if
    "check_local_user" is set. If the file has the wrong group, routing is
    deferred. The default setting for this option is true if
    "check_local_user" is set and the "modemask" option permits the group
    write bit, or if the "owngroups" option is set. Otherwise it is false, and
    no group check occurs.

check_owner                     Type: boolean              Default: see below

    When the "file" option is used, the owner of the file is checked only when
    this option is set. If "check_local_user" is set, the local user is
    permitted; otherwise the owner must be one of those listed in the "owners"
    option. The default value for this option is true if "check_local_user" or
    "owners" is set. Otherwise the default is false, and no owner check
    occurs.

data                            Type: string*                  Default: unset

    This option is mutually exclusive with "file". One or other of them must
    be set, but not both. The contents of "data" are expanded, and then used
    as the list of forwarding items, or as a set of filtering instructions. If
    the expansion is forced to fail, or the result is an empty string or a
    string that has no effect (consists entirely of comments), the router
    declines.

    When filtering instructions are used, the string must begin with '#Exim
    filter', and all comments in the string, including this initial one, must
    be terminated with newline characters. For example:

      data = #Exim filter\n\
             if $h_to: contains Exim then save $home/mail/exim endif

    If you are reading the data from a database where newlines cannot be
    included, you can use the ${sg} expansion item to turn the escape string
    of your choice into a newline.

directory_transport             Type: string*                  Default: unset

    A redirect router sets up a direct delivery to a directory when a path
    name ending with a slash is specified as a new 'address'. The transport
    used is specified by this option, which, after expansion, must be the
    name of a configured transport. This should normally be an appendfile
    transport.

file                            Type: string*                  Default: unset

    This option specifies the name of a file that contains the redirection
    data. It is mutually exclusive with the "data" option. The string is
    expanded before use; if the expansion is forced to fail, the router
    declines. Other expansion failures cause delivery to be deferred. The
    result of a successful expansion must be an absolute path. The entire file
    is read and used as the redirection data. If the data is an empty string
    or a string that has no effect (consists entirely of comments), the router
    declines.

    If the attempt to open the file fails with a 'does not exist' error, Exim
    runs a check on the containing directory, unless "ignore_enotdir" is true
    (see below). If the directory does not appear to exist, delivery is
    deferred. This can happen when users' .forward files are in NFS-mounted
    directories, and there is a mount problem. If the containing directory
    does exist, but the file does not, the router declines.

file_transport                  Type: string*                  Default: unset

    A redirect router sets up a direct delivery to a file when a path name not
    ending in a slash is specified as a new 'address'. The transport used is
    specified by this option, which, after expansion, must be the name of a
    configured transport. This should normally be an appendfile transport.
    When it is running, the file name is in $address_file.

forbid_blackhole                Type: boolean                  Default: false

    If this option is true, the :blackhole: item may not appear in a
    redirection list.

forbid_file                     Type: boolean                  Default: false

    If this option is true, this router may not generate a new address that
    specifies delivery to a local file or directory, either from a filter or
    from a conventional forward file. This option is forced to be true if
    "one_time" is set. It applies to Sieve filters as well as to Exim filters, |
    but if true, it locks out the Sieve's 'keep' facility.                     |

forbid_filter_existstest        Type: boolean                  Default: false

    If this option is true, string expansions in Exim filters are not allowed
    to make use of the "exists" condition.

forbid_filter_logwrite          Type: boolean                  Default: false

    If this option is true, use of the logging facility in Exim filters is not
    permitted. Logging is in any case available only if the filter is being
    run under some unprivileged uid (which is normally the case for ordinary
    users' .forward files).

forbid_filter_lookup            Type: boolean                  Default: false

    If this option is true, string expansions in Exim filter files are not
    allowed to make use of "lookup" items.

forbid_filter_perl              Type: boolean                  Default: false

    This option is available only if Exim is built with embedded Perl support.
    If it is true, string expansions in Exim filter files are not allowed to
    make use of the embedded Perl support.

forbid_filter_readfile          Type: boolean                  Default: false

    If this option is true, string expansions in Exim filter files are not
    allowed to make use of "readfile" items.

forbid_filter_readsocket        Type: boolean                  Default: false

    If this option is true, string expansions in Exim filter files are not
    allowed to make use of "readsocket" items.

forbid_filter_reply             Type: boolean                  Default: false

    If this option is true, this router may not generate an automatic reply
    message. Automatic replies can be generated only from Exim filter files,
    not from traditional forward files or Sieve filters. This option is forced
    to be true if "one_time" is set.

forbid_filter_run               Type: boolean                  Default: false

    If this option is true, string expansions in Exim filter files are not
    allowed to make use of "run" items.

forbid_include                  Type: boolean                  Default: false

    If this option is true, items of the form

      :include:<path name>

    are not permitted in non-filter redirection lists.

forbid_pipe                     Type: boolean                  Default: false

    If this option is true, this router may not generate a new address which
    specifies delivery to a pipe, either from an Exim filter or from a
    conventional forward file. This option is forced to be true if "one_time"
    is set.

hide_child_in_errmsg            Type: boolean                  Default: false

    If this option is true, it prevents Exim from quoting a child address if
    it generates a bounce or delay message for it. Instead it says 'an address
    generated from <the top level address>'. Of course, this applies only to
    bounces generated locally. If a message is forwarded to another host, its
    bounce may well quote the generated address.

ignore_eacces                   Type: boolean                  Default: false

    If this option is set and an attempt to open a redirection file yields the
    EACCES error (permission denied), the redirect router behaves as if the
    file did not exist.

ignore_enotdir                  Type: boolean                  Default: false

    If this option is set and an attempt to open a redirection file yields the
    ENOTDIR error (something on the path is not a directory), the redirect
    router behaves as if the file did not exist.

    Setting "ignore_enotdir" has another effect as well: When a redirect
    router that has the "file" option set discovers that the file does not
    exist (the ENOENT error), it tries to "stat()" the parent directory, as a
    check against unmounted NFS directories. If the parent can not be statted,
    delivery is deferred. However, it seems wrong to do this check when
    "ignore_enotdir" is set, because that option tells Exim to ignore 'some-
    thing on the path is not a directory' (the ENOTDIR error). This is a
    confusing area, because it seems that some operating systems give ENOENT
    where others give ENOTDIR.

include_directory                Type: string                   Default: unset

    If this option is set, the path names of any :include: items in a
    redirection list must start with this directory.

modemask                     Type: octal integer                 Default: 022

    This specifies mode bits which must not be set for a file specified by the
    "file" option. If any of the forbidden bits are set, delivery is deferred.

one_time                        Type: boolean                  Default: false

    Sometimes the fact that Exim re-evaluates aliases and reprocesses
    redirection files each time it tries to deliver a message causes a problem |
    when one or more of the generated addresses fails be delivered at the      |
    first attempt. The problem is not one of duplicate delivery - Exim is      |
    clever enough to handle that - but of what happens when the redirection    |
    list changes during the time that the message is on Exim's queue. This is  |
    particularly true in the case of mailing lists, where new subscribers      |
    might receive copies of messages that were posted before they subscribed.  |

    If "one_time" is set and any addresses generated by the router fail to
    deliver at the first attempt, the failing addresses are added to the
    message as 'top level' addresses, and the parent address that generated
    them is marked 'delivered'. Thus, redirection does not happen again at the
    next delivery attempt.

    Warning 1: This means that any header line addition or removal that is
    specified by this router would be lost if delivery did not succeed at the
    first attempt. For this reason, the "headers_add" and "headers_remove"
    generic options are not permitted when "one_time" is set.

    Warning 2: To ensure that the router generates only addresses (as opposed
    to pipe or file deliveries or auto-replies) "forbid_file", "forbid_pipe",
    and "forbid_filter_reply" are forced to be true when "one_time" is set.

    The original top-level address is remembered with each of the generated
    addresses, and is output in any log messages. However, any intermediate
    parent addresses are not recorded. This makes a difference to the log only
    if "all_parents" log selector is set. It is expected that "one_time" will
    typically be used for mailing lists, where there is normally just one
    level of expansion.

owners                        Type: string list                Default: unset

    This specifies a list of permitted owners for the file specified by
    "file". This list is in addition to the local user when "check_local_user"
    is set. See "check_owner" above.

owngroups                     Type: string list                Default: unset

    This specifies a list of permitted groups for the file specified by
    "file". The list is in addition to the local user's primary group when
    "check_local_user" is set. See "check_group" above.

pipe_transport                  Type: string*                  Default: unset

    A redirect router sets up a direct delivery to a pipe when a string
    starting with a vertical bar character is specified as a new 'address'.
    The transport used is specified by this option, which, after expansion,
    must be the name of a configured transport. This should normally be a
    pipe transport. When the transport is run, the pipe command is in
    $address_pipe.

qualify_preserve_domain         Type: boolean                  Default: false

    If this is set and an unqualified address (one without a domain) is
    generated, it is qualified with the domain of the incoming address instead
    of the global setting in "qualify_recipient".

repeat_use                      Type: boolean                   Default: true

    If this option is set false, the router is skipped for a child address
    that has any ancestor that was routed by this router. This test happens
    before any of the other preconditions are tested. Exim's default anti-
    looping rules skip only when the ancestor is the same as the current
    address. See also "check_ancestor" above and the generic "redirect_router"
    option.

reply_transport                 Type: string*                  Default: unset

    A redirect router sets up an automatic reply when a "mail" or "vacation"
    command is used in a filter file. The transport used is specified by this
    option, which, after expansion, must be the name of a configured trans-
    port. This should normally be an autoreply transport. Other transports are
    unlikely to do anything sensible or useful.

rewrite                         Type: boolean                   Default: true

    If this option is set false, addresses generated by the router are not
    subject to address rewriting. Otherwise, they are treated like new
    addresses and are rewritten according to the global rewriting rules.

skip_syntax_errors              Type: boolean                  Default: false

    If "skip_syntax_errors" is set, syntactically malformed addresses in non-
    filter redirection data are skipped, and each failing address is logged.
    If "syntax_errors_to" is set, a message is sent to the address it defines,
    giving details of the failures. If "syntax_errors_text" is set, its
    contents are expanded and placed at the head of the error message
    generated by "syntax_errors_to". Usually it is appropriate to set
    "syntax_errors_to" to be the same address as the generic "errors_to"
    option. The "skip_syntax_errors" option is often used when handling
    mailing lists.

    If all the addresses in a redirection list are skipped because of syntax
    errors, the router declines to handle the original address, and it is
    passed to the following routers.

    If "skip_syntax_errors" is set when an Exim filter is interpreted, any
    syntax error in the filter causes filtering to be abandoned without any
    action being taken. The incident is logged, and the router declines to
    handle the address, so it is passed to the following routers.

    Currently, any syntax errors in a Sieve filter file cause the 'keep'       |
    action to occur. The values of "skip_syntax_errors", "syntax_errors_to",   |
    and "syntax_errors_text" are not used.                                     |

    "skip_syntax_errors" can be used to specify that errors in users' forward
    lists or filter files should not prevent delivery. The "syntax_errors_to"
    option, used with an address that does not get redirected, can be used to
    notify users of these errors, by means of a router like this:

      userforward:
        driver = redirect
        allow_filter
        check_local_user
        file = $home/.forward
        file_transport = address_file
        pipe_transport = address_pipe
        reply_transport = address_reply
        no_verify
        skip_syntax_errors
        syntax_errors_to = real-$local_part@$domain
        syntax_errors_text = \
          This is an automatically generated message. An error has\n\
          been found in your .forward file. Details of the error are\n\
          reported below. While this error persists, you will receive\n\
          a copy of this message for every message that is addressed\n\
          to you. If your .forward file is a filter file, or if it is\n\
          a non-filter file containing no valid forwarding addresses,\n\
          a copy of each incoming message will be put in your normal\n\
          mailbox. If a non-filter file contains at least one valid\n\
          forwarding address, forwarding to the valid addresses will\n\
          happen, and those will be the only deliveries that occur.

    You also need a router to ensure that local addresses that are prefixed by
    "real-" are recognized, but not forwarded or filtered. For example, you
    could put this immediately before the userforward router:

      real_localuser:
        driver = accept
        check_local_user
        local_part_prefix = real-
        transport = local_delivery

syntax_errors_text              Type: string*                  Default: unset

    See "skip_syntax_errors" above.

syntax_errors_to                 Type: string                   Default: unset

    See "skip_syntax_errors" above.



                 23. ENVIRONMENT FOR RUNNING LOCAL TRANSPORTS


Local transports handle deliveries to files and pipes. (The autoreply trans-
port can be thought of as similar to a pipe.) Exim always runs transports in
subprocesses, under specified uids and gids. Typical deliveries to local
mailboxes run under the uid and gid of the local user.

Exim also sets a specific current directory while running the transport; for
some transports a home directory setting is also relevant. The pipe transport
is the only one which sets up environment variables; see section 29.3 for
details.

The values used for the uid, gid, and the directories may come from several
different places. In many cases, the router that handles the address associ-
ates settings with that address as a result of its "check_local_user",
"group", or "user" options. However, values may also be given in the
transport's own configuration, and these override anything that comes from the
router.


23.1 Uids and gids

All transports have the options "group" and "user". If "group" is set, it
overrides any group that the router set in the address, even if "user" is not
set for the transport. This makes it possible, for example, to run local mail
delivery under the uid of the recipient (set by the router), but in a special
group (set by the transport). For example:

  # Routers ...
  # User/group are set by check_local_user in this router
  local_users:
    driver = accept
    check_local_user
    transport = group_delivery

  # Transports ...
  # This transport overrides the group
  group_delivery:
    driver = appendfile
    file = /var/spool/mail/$local_part
    group = mail

If "user" is set for a transport, its value overrides what is set in the
address. If "user" is non-numeric and "group" is not set, the gid associated
with the user is used. If "user" is numeric, "group" must be set.

When the uid is taken from the transport's configuration, the "initgroups()"
function is called for the groups associated with that uid if the "initgroups"
option is set for the transport. When the uid is not specified by the
transport, but is associated with the address by a router, the option for
calling "initgroups()" is taken from the router configuration.

The pipe transport contains the special option "pipe_as_creator". If this is
set and "user" is not set, the uid of the process that called Exim to receive
the message is used, and if "group" is not set, the corresponding original gid
is also used.


23.2 Current and home directories

Routers may set current and home directories for local transports by means
of the "transport_current_directory" and "transport_home_directory" options.
However, if the transport's "current_directory" or "home_directory" options
are set, they override the router's values. In detail, the home directory for
a local transport is taken from the first of these values that is set:

 .   The "home_directory" option on the transport;

 .   The "transport_home_directory" option on the router;

 .   The password data if "check_local_user" is set on the router;

 .   The "router_home_directory" option on the router.

The current directory is taken from the first of these values that is set:

 .   The "current_directory" option on the transport;

 .   The "transport_current_directory" option on the router.

If neither the router nor the transport sets a current directory, Exim uses
the value of the home directory, if it is set. Otherwise it sets the current
directory to / before running a local transport.


23.3 Expansion variables derived from the address

Normally a local delivery is handling a single address, and in that case the
variables such as $domain and $local_part are set during local deliveries.
However, in some circumstances more than one address may be handled at once
(for example, while writing batch SMTP for onward transmission by some other
means). In this case, the variables associated with the local part are never
set, $domain is set only if all the addresses have the same domain, and
$original_domain is never set.



                      24. GENERIC OPTIONS FOR TRANSPORTS


The following generic options apply to all transports:

body_only                       Type: boolean                  Default: false

    If this option is set, the message's headers are not transported. It is
    mutually exclusive with "headers_only". If it is used with the appendfile
    or pipe transports, the settings of "message_prefix" and "message_suffix"
    should be checked, because this option does not automatically suppress
    them.

current_directory               Type: string*                  Default: unset

    This specifies the current directory that is to be set while running the
    transport, overriding any value that may have been set by the router. If   |
    the expansion fails for any reason, including forced failure, an error is  |
    logged, and delivery is deferred.                                          |

disable_logging                 Type: boolean                  Default: false

    If this option is set true, nothing is logged for any transport errors.
    You should not set this option unless you really, really know what you are
    doing.

debug_print                     Type: string*                  Default: unset

    If this option is set and debugging is enabled (see the -d command line
    option), the string is expanded and included in the debugging output when
    the transport is run. If expansion of the string fails, the error message  |
    is written to the debugging output, and Exim carries on processing. This   |
    facility is provided to help with checking out the values of variables
    and so on when debugging driver configurations. For example, if a
    "headers_add" option is not working properly, "debug_print" could be used
    to output the variables it references. A newline is added to the text if
    it does not end with one.

delivery_date_add               Type: boolean                  Default: false

    If this option is true, a Delivery-date: header is added to the message.
    This gives the actual time the delivery was made. As this is not a
    standard header, Exim has a configuration option ("delivery_date_remove")
    which requests its removal from incoming messages, so that delivered
    messages can safely be resent to other recipients.

driver                           Type: string                   Default: unset

    This specifies which of the available transport drivers is to be used.
    There is no default, and this option must be set for every transport.

envelope_to_add                 Type: boolean                  Default: false

    If this option is true, an Envelope-to: header is added to the message.
    This gives the original address(es) in the incoming envelope that caused
    this delivery to happen. More than one address may be present if the
    transport is configured to handle several addresses at once, or if more
    than one original address was redirected to the same final address. As
    this is not a standard header, Exim has a configuration option
    ("envelope_to_remove") which requests its removal from incoming messages,
    so that delivered messages can safely be resent to other recipients.

group                           Type: string*             Default: Exim group

    This option specifies a gid for running the transport process, overriding
    any value that the router supplies, and also overriding any value
    associated with "user" (see below).

headers_add                     Type: string*                  Default: unset

    This option specifies a string of text which is expanded and added to the
    header portion of a message as it is transported. If the result of the
    expansion is an empty string, or if the expansion is forced to fail, no
    action is taken. Other expansion failures are treated as errors and cause
    the delivery to be deferred. The expanded string should be in the form of
    one or more RFC 2822 header lines, separated by newlines (coded as '\n'),
    for example:

      headers_add = X-added: this is a header added at $tod_log\n\
                    X-added: this is another

    Exim does not check the syntax of these added header lines. They are added
    at the end of the existing header lines. If you include a blank line
    within the string, you can subvert this facility into adding text at the
    start of the message's body. This is not recommended. Additional header
    lines can also be specified by routers. See chapter 15 and section 44.15.

headers_only                    Type: boolean                  Default: false

    If this option is set, the message's body is not transported. It is
    mutually exclusive with "body_only". If it is used with the appendfile or
    pipe transports, the settings of "message_prefix" and "message_suffix"
    should be checked, since this option does not automatically suppress them.

headers_remove                  Type: string*                  Default: unset

    This option is expanded; the result must consist of a colon-separated list
    of header names, not including the terminating colon, for example:

      headers_remove = return-receipt-to:acknowledge-to

    Any existing headers matching those names are not included in any message
    that is transmitted by the transport. If the result of the expansion is an |
    empty string, or if the expansion is forced to fail, no action is taken.   |
    Other expansion failures are treated as errors and cause the delivery to   |
    be deferred.                                                               |

    If there are multiple instances of a header, they are all removed.
    However, added headers may have these names. Thus it is possible to
    replace a header by specifying it in "headers_remove" and supplying the
    replacement in "headers_add". Headers to be removed can also be specified
    by routers. See chapter 15 and section 44.15.

headers_rewrite                  Type: string                   Default: unset

    This option allows addresses in header lines to be rewritten at transport
    time, that is, as the message is being copied to its destination. The
    contents of the option are a colon-separated list of rewriting rules. Each
    rule is in exactly the same form as one of the general rewriting rules
    that are applied when a message is received. These are described in
    chapter 31. For example,

      headers_rewrite = a@b c@d f : \
                        x@y w@z

    changes "a@b" into "c@d" in From: header lines, and "x@y" into "w@z" in
    all address-bearing header lines. The rules are applied to the header
    lines just before they are written out at transport time, so they affect
    only those copies of the message that pass through the transport. However,
    only the message's original header lines, and any that were added by a
    system filter, are rewritten. If a router or transport adds header lines,
    they are not affected by this option. These rewriting rules are not
    applied to the envelope. You can change the return path using
    "return_path", but you cannot change envelope recipients at this time.

home_directory                  Type: string*                  Default: unset

    This option specifies a home directory setting for the transport, overrid-
    ing any value that may be set by the router. The home directory is placed
    in $home while expanding the transport's private options. It is also used
    as the current directory if no current directory is set by the "current_
    directory" option on the transport or the "transport_current_directory"
    option on the router. If the expansion fails for any reason, including     |
    forced failure, an error is logged, and delivery is deferred.              |


initgroups                      Type: boolean                  Default: false

    If this option is true and the uid for the delivery process is provided by
    the transport, the "initgroups()" function is called when running the
    transport to ensure that any additional groups associated with the uid are
    set up.

message_size_limit              Type: string*                      Default: 0

    This option controls the size of messages passed through the transport. It
    is expanded before use; the result of the expansion must be a sequence of
    digits, optionally followed by K or M. If the expansion fails for any      |
    reason, including forced failure, or if the result is not of the required  |
    form, delivery is deferred. If the value is greater than zero and the size |
    of a message exceeds this limit, the address is failed. If there is any
    chance that the resulting bounce message could be routed to the same
    transport, you should ensure that "return_size_limit" is less than the
    transport's "message_size_limit", as otherwise the bounce message will
    fail to get delivered.

rcpt_include_affixes            Type: boolean                  Default: false  |
                                                                               |
    When this option is false (the default), and an address that has had any   |
    affixes (prefixes or suffixes) removed from the local part is delivered by |
    any form of SMTP or LMTP, the affixes are not included. For example, if a  |
    router that contains                                                       |
                                                                               |
      local_part_prefix = *-                                                   |
                                                                               |
    routes the address "abc-xyz@some.domain" to an SMTP transport, the envel-  |
    ope is delivered with                                                      |
                                                                               |
      RCPT TO:<xyz@some.domain>                                                |
                                                                               |
    If "rcpt_include_affixes" is set true, the whole local part is included in |
    the RCPT command. This option applies to BSMTP deliveries by the           |
    appendfile and pipe transports as well as to the lmtp and smtp transports. |

retry_use_local_part            Type: boolean              Default: see below

    When a delivery suffers a temporary failure, a retry record is created in
    Exim's hints database. For remote deliveries, the key for the retry record
    is based on the name and/or IP address of the failing remote host. For
    local deliveries, the key is normally the entire address, including both
    the local part and the domain. This is suitable for most common cases of
    local delivery temporary failure - for example, exceeding a mailbox quota
    should delay only deliveries to that mailbox, not to the whole domain.

    However, in some special cases you may want to treat a temporary local
    delivery as a failure associated with the domain, and not with a
    particular local part. (For example, if you are storing all mail for some
    domain in files.) You can do this by setting "retry_use_local_part" false.

    For all the local transports, its default value is true. For remote
    transports, the default value is false for tidiness, but changing the
    value has no effect on a remote transport in the current implementation.

return_path                     Type: string*                  Default: unset

    If this option is set, the string is expanded at transport time and
    replaces the existing return path (envelope sender) value in the copy of
    the message that is being delivered. An empty return path is permitted.
    This feature is designed for remote deliveries, where the value of this
    option is used in the SMTP MAIL command. If you set "return_path" for a
    local transport, the only effect is to change the address that is placed
    in the Return-path: header line, if one is added to the message (see the
    next option).

    The expansion can refer to the existing value via $return_path. This is
    either the message's envelope sender, or an address set by the "errors_to"
    option on a router. If the expansion is forced to fail, no replacement
    occurs; if it fails for another reason, delivery is deferred. This option
    can be used to support VERP (Variable Envelope Return Paths) - see chapter
    43.

    Note: If a delivery error is detected locally, the bounce message is not
    sent to the value of this option, but to the previously set errors
    address.

return_path_add                 Type: boolean                  Default: false

    If this option is true, a Return-path: header is added to the message.
    Although the return path is normally available in the prefix line of BSD
    mailboxes, this is commonly not displayed by MUAs, and so the user does
    not have easy access to it.

    RFC 2821 states that the Return-path: header is added to a message 'when
    the delivery SMTP server makes the final delivery'. This implies that this
    header should not be present in incoming messages. Exim has a configur-
    ation option, "return_path_remove", which requests removal of this header
    from incoming messages, so that delivered messages can safely be resent to
    other recipients.

shadow_condition                Type: string*                  Default: unset

    See "shadow_transport" below.

shadow_transport                 Type: string                   Default: unset

    A local transport may set the "shadow_transport" option to the name of
    another local transport. Shadow remote transports are not supported.

    Whenever a delivery to the main transport succeeds, and either             |
    "shadow_condition" is unset, or its expansion does not result in the empty |
    string or one of the strings '0' or 'no' or 'false', the message is also   |
    passed to the shadow transport, with the same delivery address or          |
    addresses. If expansion fails, no action is taken except that non-forced   |
    expansion failures cause a log line to be written.                         |

    The result of the shadow transport is discarded and does not affect the
    subsequent processing of the message. Only a single level of shadowing is
    provided; the "shadow_transport" option is ignored on any transport when
    it is running as a shadow. Options concerned with output from pipes are
    also ignored.

    The log line for the successful delivery has an item added on the end, of
    the form

      ST=<shadow transport name>

    If the shadow transport did not succeed, the error message is put in
    parentheses afterwards.

    Shadow transports can be used for a number of different purposes,
    including keeping more detailed log information than Exim normally pro-
    vides, and implementing automatic acknowledgement policies based on mess-
    age headers that some sites insist on.

transport_filter                Type: string*                  Default: unset

    This option sets up a filtering (in the Unix shell sense) process for
    messages at transport time. It should not be confused with mail filtering
    as set up by individual users or via a system filter.

    When the message is about to be written out, the command specified by
    "transport_filter" is started up in a separate process, and the entire
    message, including the header lines, is passed to it on its standard input
    (this in fact is done from a third process, to avoid deadlock). The
    command must be specified as an absolute path.

    The message is passed to the filter before any SMTP-specific processing,
    such as turning '\n' into '\r\n' and escaping lines beginning with a dot,
    and also before any processing implied by the settings of "check_string"
    and "escape_string" in the appendfile or pipe transports.

    The filter's standard output is read and written to the message's
    destination. The filter can perform any transformations it likes, but of
    course should take care not to break RFC 2822 syntax. A demonstration Perl
    script is provided in util/transport-filter.pl; this makes a few arbitrary
    modifications just to show the possibilities. Exim does not check the
    result, except to test for a final newline when SMTP is in use. All
    messages transmitted over SMTP must end with a newline, so Exim supplies
    one if it is missing.

    A problem might arise if the filter increases the size of a message that
    is being sent down an SMTP connection. If the receiving SMTP server has
    indicated support for the SIZE parameter, Exim will have sent the size of
    the message at the start of the SMTP session. If what is actually sent is
    substantially more, the server might reject the message. This can be
    worked round by setting the "size_addition" option on the smtp transport,
    either to allow for additions to the message, or to disable the use of
    SIZE altogether.

    The value of the option is the command string for starting up the filter,
    which is run directly from Exim, not under a shell. The string is parsed
    by Exim in the same way as a command string for the pipe transport: Exim
    breaks it up into arguments and then expands each argument separately. The
    special argument $pipe_addresses is replaced by a number of arguments, one
    for each address that applies to this delivery. (This isn't an ideal name
    for this feature here, but as it was already implemented for the pipe
    transport, it seemed sensible not to change it.)

    The expansion variables $host and $host_address are available when the
    transport is a remote one. They contain the name and IP address of the
    host to which the message is being sent. For example:

      transport_filter = /some/directory/transport-filter.pl \
        $host $host_address $sender_address $pipe_addresses

    The filter process is run under the same uid and gid as the normal
    delivery. For remote deliveries this is the Exim uid/gid by default.

    If a transport filter is set on an autoreply transport, the original
    message is passed through the filter as it is being copied into the newly
    generated message, which happens if the "return_message" option is set.

transport_filter_timeout          Type: time                       Default: 5m |
                                                                               |
    When Exim is reading the output of a transport filter, it a applies a      |
    timeout that can be set by this option. Exceeding the timeout is treated   |
    as a temporary delivery failure.                                           |

user                            Type: string*              Default: Exim user

    This option specifies the user under whose uid the delivery process is to
    be run, overriding any uid that may have been set by the router. If the
    user is given as a name, the uid is looked up from the password data, and
    the associated group is taken as the value of the gid to be used if the
    "group" option is not set.

    For remote transports, you should leave this option unset unless you
    really are sure you know what you are doing. When a remote transport is
    running, it needs to be able to access Exim's hints databases, because
    each host may have its own retry data.



                   25. ADDRESS BATCHING IN LOCAL TRANSPORTS


The only remote transport (smtp) is normally configured to handle more than
one address at a time, so that when several addresses are routed to the same
remote host, just one copy of the message is sent. Local transports, however,
normally handle one address at a time. That is, a separate instance of the
transport is run for each address that is routed to the transport. A separate
copy of the message is delivered each time.

In special cases, it may be desirable to handle several addresses at once in a
local transport, for example:

 .   In an appendfile transport, when storing messages in files for later
     delivery by some other means, a single copy of the message with multiple
     recipients saves space.

 .   In an lmtp transport, when delivering over 'local SMTP' to some process,
     a single copy saves time, and is the normal way LMTP is expected to work.

 .   In a pipe transport, when passing the message to a scanner program or to
     some other delivery mechanism such as UUCP, multiple recipients may be
     acceptable.

The three local transports (appendfile, lmtp, and pipe) all have the same
options for controlling multiple ('batched') deliveries, namely "batch_max"
and "batch_id". To save repeating the information for each transport, these
options are described here.

The "batch_max" option specifies the maximum number of addresses that can be
delivered together in a single run of the transport. Its default value is one.
When more than one address is routed to a transport that has a "batch_max"
value greater than one, the addresses are delivered in a batch (that is, in a
single run of the transport), subject to certain conditions:

 .   If any of the transport's options contain a reference to $local_part, no
     batching is possible.

 .   If any of the transport's options contain a reference to $domain, only
     addresses with the same domain are batched.

 .   If "batch_id" is set, it is expanded for each address, and only those
     addresses with the same expanded value are batched. This allows you to
     specify customized batching conditions. Failure of the expansion for any  |
     reason, including forced failure, disables batching, but it does not stop |
     the delivery from taking place.                                           |

 .   Batched addresses must also have the same errors address (where to send
     delivery errors), the same header additions and removals, the same user
     and group for the transport, and if a host list is present, the first
     host must be the same.

If the generic "envelope_to_add" option is set for the transport, the
Envelope-to: header that is added to the message contains all the addresses
that are batched together.

The appendfile and pipe transports have an option called "use_bsmtp", which
causes them to deliver the message in 'batched SMTP' format, with the envelope
represented as SMTP commands. The "check_string" and "escape_string" options
are forced to the values

  check_string = "."
  escape_string = ".."

when batched SMTP is in use. A full description of the batch SMTP mechanism is
given in section 43.11. The lmtp transport does not have a "use_bsmtp" option,
because it always delivers using the SMTP protocol.

If you are not using BSMTP, but are using a pipe transport, you can include
$pipe_addresses as part of the command. This is not a true variable; it is a
bit of magic that causes each of the recipient addresses to be inserted into
the command as a separate argument. This provides a way of accessing all the
addresses that are being delivered in the batch.

If you are using a batching appendfile transport without "use_bsmtp", the only
way to preserve the recipient addresses is to set the "envelope_to_add"
option. This causes an Envelope-to: header line to be added to the message,
containing all the recipients.



                         26. THE APPENDFILE TRANSPORT


The appendfile transport delivers a message by appending it to an existing
file, or by creating an entirely new file in a specified directory. Single
files to which messages are appended can be in the traditional Unix mailbox
format, or optionally in the MBX format supported by the Pine MUA and
University of Washington IMAP daemon, inter alia. When each message is being
delivered as a separate file, 'maildir' format can optionally be used to give
added protection against failures that happen part-way through the delivery. A
third form of separate-file delivery known as 'mailstore' is also supported.
For all file formats, Exim attempts to create as many levels of directory as
necessary, provided that "create_directory" is set.

The code for the optional formats is not included in the Exim binary by
default. It is necessary to set SUPPORT_MBX, SUPPORT_MAILDIR and/or
SUPPORT_MAILSTORE in Local/Makefile to have the appropriate code included.

Exim recognises system quota errors, and generates an appropriate message.
Exim also supports its own quota control within the transport, for use when
the system facility is unavailable or cannot be used for some reason.

If there is an error while appending to a file (for example, quota exceeded or
partition filled), Exim attempts to reset the file's length and last modifi-
cation time back to what they were before. If there is an error while creating
an entirely new file, the new file is removed.

Before appending to a file, a number of security checks are made, and the file
is locked. A detailed description is given below, after the list of private
options.

appendfile is most commonly used for local deliveries to users' mailboxes.
However, it can also be used as a pseudo-remote transport for putting messages
into files for remote delivery by some means other than Exim. 'Batch SMTP'
format is often used in this case (see the "use_bsmtp" option).


26.1 The file and directory options

The "file" option specifies a single file, to which the message is appended;
the "directory" option specifies a directory, in which a new file containing
the message is created. Only one of these two options can be set, and for
normal deliveries to mailboxes, one of them "must" be set.

However, appendfile is also used for delivering messages to files or director-
ies whose names (or parts of names) are obtained from alias, forwarding, or
filtering operations (for example, a "save" command in a user's Exim filter).
When such a transport is running, $local_part contains the local part that was
aliased or forwarded, and $address_file contains the name (or partial name) of
the file or directory generated by the redirection operation. There are two
cases:

 .   If neither "file" nor "directory" is set, the redirection operation must
     specify an absolute path (one that begins with "/"). This is the most
     common case when users with local accounts use filtering to sort mail
     into different folders. See for example, the address_file transport in
     the default configuration. If the path ends with a slash, it is assumed
     to be the name of a directory. A delivery to a directory can also be
     forced by setting "maildir_format" or "mailstore_format".

 .   If "file" or "directory" is set for a delivery from a redirection, it is
     used to determine the file or directory name for the delivery. Normally,
     the contents of $address_file are used in some way in the string
     expansion.

As an example of the second case, consider an environment where users do not   |
have home directories. They may be permitted to use Exim filter commands of    |
the form:                                                                      |
                                                                               |
  save folder23                                                                |
                                                                               |
or Sieve filter commands of the form:                                          |
                                                                               |
  require "fileinto";                                                          |
  fileinto "folder23";                                                         |
                                                                               |
In this situation, the expansion of "file" or "directory" in the transport     |
must transform the relative path into an appropriate absolute file name. In    |
the case of Sieve filters, the name "inbox" must be handled. It is the name    |
that is used as a result of a 'keep' action in the filter. This example shows  |
one way of handling this requirement:                                          |
                                                                               |
  file = ${if eq{$address_file}{inbox} \                                       |
              {/var/mail/$local_part} \                                        |
              {${if eq{${substr_0_1:$address_file}}{/} \                       |
                    {$address_file} \                                          |
                    {$home/mail/$address_file} \                               |
              }} \                                                             |
         }                                                                     |
                                                                               |
With this setting of "file", "inbox" refers to the standard mailbox location,  |
absolute paths are used without change, and other folders are in the mail      |
directory within the home directory.                                           |
                                                                               |
Note 1: While processing an Exim filter, a relative path such as folder23 is   |
turned into an absolute path if a home directory is known to the router. In    |
particular, this is the case if "check_local_user" is set. If you want to      |
prevent this happening at routing time, you can set "router_home_directory"    |
empty. This forces the router to pass the relative path to the transport.      |

Note 2: An absolute path in $address_file is not treated specially; the "file"
or "directory" option is still used if it is set.


26.2 Private options for appendfile

allow_fifo                      Type: boolean                  Default: false

    Setting this option permits delivery to named pipes (FIFOs) as well as to
    regular files. If no process is reading the named pipe at delivery time,
    the delivery is deferred.

allow_symlink                   Type: boolean                  Default: false

    By default, appendfile will not deliver if the path name for the file is
    that of a symbolic link. Setting this option relaxes that constraint, but
    there are security issues involved in the use of symbolic links. Be sure
    you know what you are doing if you set this. Details of exactly what this
    option affects are included in the discussion which follows this list of
    options.

batch_id                        Type: string*                  Default: unset

    See the description of local delivery batching in chapter 25. However,
    batching is automatically disabled for appendfile deliveries that happen
    as a result of forwarding or aliasing or other redirection directly to a
    file.

batch_max                       Type: integer                      Default: 1

    See the description of local delivery batching in chapter 25.

check_group                     Type: boolean                  Default: false

    When this option is set, the group owner of the file defined by the "file"
    option is checked to see that it is the same as the group under which the
    delivery process is running. The default setting is false because the
    default file mode is 0600, which means that the group is irrelevant.

check_owner                     Type: boolean                   Default: true

    When this option is set, the owner of the file defined by the "file"
    option is checked to ensure that it is the same as the user under which
    the delivery process is running.

check_string                     Type: string               Default: see below

    As appendfile writes the message, the start of each line is tested for
    matching "check_string", and if it does, the initial matching characters
    are replaced by the contents of "escape_string". The value of
    "check_string" is a literal string, not a regular expression, and the case
    of any letters it contains is significant.

    If "use_bsmtp" is set the values of "check_string" and "escape_string" are
    forced to '.' and '..' respectively, and any settings in the configuration
    are ignored. Otherwise, they default to 'From ' and '>From ' when the
    "file" option is set, and unset when any of the "directory", "maildir", or
    "mailstore" options are set.

    The default settings, along with "message_prefix" and "message_suffix",
    are suitable for traditional 'BSD' mailboxes, where a line beginning with
    'From ' indicates the start of a new message. All four options need
    changing if another format is used. For example, to deliver to mailboxes
    in MMDF format:

      check_string = "\1\1\1\1\n"
      escape_string = "\1\1\1\1 \n"
      message_prefix = "\1\1\1\1\n"
      message_suffix = "\1\1\1\1\n"

However, many clients require that the certificate presented by Exim be a user
(also called "leaf" or "site") certificate, and not a self-signed certificate.
In this case, the self-signed certificate described above must be installed on
the client host as a trusted root certification authority and the certificate
used by Exim must be a user certificate signed with that self-signed
certificate.

For information on creating self-signed CA certificates and using them to sign
user certificates, see the "General implementation overview" chapter of the
Open-source PKI Book, available online at:

     http://ospkibook.sourceforge.net/


create_directory                Type: boolean                   Default: true

    When this option is true, Exim attempts to create any missing superior
    directories for the file that it is about to write. A created directory's
    mode is given by the "directory_mode" option.

create_file                      Type: string                Default: anywhere

    This option constrains the location of files and directories that are
    created by this transport. It applies to files defined by the "file"
    option and directories defined by the "directory" option. In the case of
    maildir delivery, it applies to the top level directory, not the maildir
    directories beneath.

    The option must be set to one of the words 'anywhere', 'inhome', or
    'belowhome'. In the second and third cases, a home directory must have
    been set for the transport. This option is not useful when an explicit
    file name is given for normal mailbox deliveries. It is intended for the
    case when file names are generated from users' .forward files. These are
    usually handled by an appendfile transport called "address_file". See also
    "file_must_exist".

directory                       Type: string*                  Default: unset

    This option is mutually exclusive with the "file" option, but one of
    "file" or "directory" must be set, unless the delivery is the direct
    result of a redirection (see section 26.1).

    When "directory" is set, the string is expanded, and the message is
    delivered into a new file or files in or below the given directory,
    instead of being appended to a single mailbox file. A number of different
    formats are provided (see "maildir_format" and "mailstore_format"), and
    see section 26.4 for further details of this form of delivery.

directory_file                  Type: string*
                                         Default: q${base62:$tod_epoch}-$inode

    When "directory" is set, but neither "maildir_format" nor
    "mailstore_format" is set, appendfile delivers each message into a file
    whose name is obtained by expanding this string. The default value
    generates a unique name from the current time, in base 62 form, and the
    inode of the file. The variable $inode is available only when expanding
    this option.

directory_mode               Type: octal integer                Default: 0700

    If appendfile creates any directories as a result of the
    "create_directory" option, their mode is specified by this option.

escape_string                    Type: string         Default: see description

    See "check_string" above.

file                            Type: string*                  Default: unset

    This option is mutually exclusive with the "directory" option, but one of
    "file" or "directory" must be set, unless the delivery is the direct
    result of a redirection (see section 26.1). The "file" option specifies
    a single file, to which the message is appended. One or more of
    "use_fcntl_lock", "use_flock_lock", or "use_lockfile" must be set with
    "file". If you are using more than one host to deliver over NFS into the
    same mailboxes, you should always use lock files.

    The string value is expanded for each delivery, and must yield an absolute
    path. The most common settings of this option are variations on one of
    these examples:

      file = /var/spool/mail/$local_part
      file = /home/$local_part/inbox
      file = $home/inbox

    In the first example, all deliveries are done into the same directory. If
    Exim is configured to use lock files (see "use_lockfile" below) it must be
    able to create a file in the directory, so the 'sticky' bit must be turned
    on for deliveries to be possible, or alternatively the "group" option can
    be used to run the delivery under a group id which has write access to the
    directory.

file_format                      Type: string                   Default: unset

    This option requests the transport to check the format of an existing file
    before adding to it. The check consists of matching a specific string at
    the start of the file. The value of the option consists of an even number
    of colon-separated strings. The first of each pair is the test string, and
    the second is the name of a transport. If the transport associated with a
    matched string is not the current transport, control is passed over to the
    other transport. For example, suppose the standard local_delivery trans-
    port has this added to it:

      file_format = "From       : local_delivery :\
                     \1\1\1\1\n : local_mmdf_delivery"

    Mailboxes that begin with 'From' are still handled by this transport, but
    if a mailbox begins with four binary ones followed by a newline, control
    is passed to a transport called "local_mmdf_delivery", which presumably is
    configured to do the delivery in MMDF format. If a mailbox does not exist
    or is empty, it is assumed to match the current transport. If the start of
    a mailbox doesn't match any string, or if the transport named for a given
    string is not defined, delivery is deferred.

file_must_exist                 Type: boolean                  Default: false

    If this option is true, the file specified by the "file" option must
    exist, and an error occurs if it does not. Otherwise, it is created if it
    does not exist.

lock_fcntl_timeout                Type: time                       Default: 0s

    By default, the appendfile transport uses non-blocking calls to "fcntl()"
    when locking an open mailbox file. If the call fails, the delivery process
    sleeps for "lock_interval" and tries again, up to "lock_retries" times.
    Non-blocking calls are used so that the file is not kept open during the
    wait for the lock; the reason for this is to make it as safe as possible
    for deliveries over NFS in the case when processes might be accessing an
    NFS mailbox without using a lock file. This should not be done, but
    misunderstandings and hence misconfigurations are not unknown.

    On a busy system, however, the performance of a non-blocking lock approach
    is not as good as using a blocking lock with a timeout. In this case, the
    waiting is done inside the system call, and Exim's delivery process
    acquires the lock and can proceed as soon as the previous lock holder
    releases it.

    If "lock_fcntl_timeout" is set to a non-zero time, blocking locks, with
    that timeout, are used. There may still be some retrying: the maximum
    number of retries is

      (lock_retries * lock_interval) / lock_fcntl_timeout

    rounded up to the next whole number. In other words, the total time during
    which appendfile is trying to get a lock is roughly the same, unless
    "lock_fcntl_timeout" is set very large.

    You should consider setting this option if you are getting a lot of
    delayed local deliveries because of errors of the form

      failed to lock mailbox /some/file (fcntl)

lock_flock_timeout                Type: time                       Default: 0s

    This timeout applies to file locking when using "flock()" (see
    "use_flock"); the timeout operates in a similar manner to
    "lock_fcntl_timeout".

lock_interval                     Type: time                       Default: 3s

    This specifies the time to wait between attempts to lock the file. See
    below for details of locking.

lock_retries                    Type: integer                     Default: 10

    This specifies the maximum number of attempts to lock the file. A value of
    zero is treated as 1. See below for details of locking.

lockfile_mode                Type: octal integer                Default: 0600

    This specifies the mode of the created lock file, when a lock file is
    being used (see "use_lockfile").

lockfile_timeout                  Type: time                      Default: 30m

    When a lock file is being used (see "use_lockfile"), if a lock file
    already exists and is older than this value, it is assumed to have been
    left behind by accident, and Exim attempts to remove it.

maildir_format                  Type: boolean                  Default: false

    If this option is set with the "directory" option, the delivery is into a
    new file, in the 'maildir' format that is used by other mail software.
    When the transport is activated directly from a redirect router (for
    example, the address_file transport in the default configuration), setting
    "maildir_format" causes the path received from the router to be treated as
    a directory, whether or not it ends with "/". This option is available
    only if SUPPORT_MAILDIR is present in Local/Makefile. See section 26.5
    below for further details.

maildir_retries                 Type: integer                     Default: 10

    This option specifies the number of times to retry when writing a file in
    'maildir' format. See section 26.5 below.

maildir_tag                     Type: string*                  Default: unset

    This option applies only to deliveries in maildir format, and is described
    in section 26.5 below.

maildir_use_size_file           Type: boolean                  Default: false  |
                                                                               |
    Setting this option true enables support for maildirsize files. Exim       |
    creates a maildirsize file in a maildir if one does not exist, taking the  |
    quota from the "quota" option of the transport. If "quota" is unset, the   |
    value is zero. See section 26.5 below for further details.                 |

mailstore_format                Type: boolean                  Default: false

    If this option is set with the "directory" option, the delivery is into
    two new files in 'mailstore' format. The option is available only if
    SUPPORT_MAILSTORE is present in Local/Makefile. See section 26.4 below for
    further details.

mailstore_prefix                Type: string*                  Default: unset

    This option applies only to deliveries in mailstore format, and is
    described in section 26.4 below.

mailstore_suffix                Type: string*                  Default: unset

    This option applies only to deliveries in mailstore format, and is
    described in section 26.4 below.

mbx_format                      Type: boolean                  Default: false

    This option is available only if Exim has been compiled with SUPPORT_MBX
    set in Local/Makefile. If "mbx_format" is set with the "file" option, the
    message is appended to the mailbox file in MBX format instead of
    traditional Unix format. This format is supported by Pine4 and its
    associated IMAP and POP daemons, by means of the "c-client" library that
    they all use.

    Note: The "message_prefix" and "message_suffix" options are not automati-  |
    cally changed by the use of "mbx_format". They should normally be set      |
    empty when using MBX format, so this option almost always appears in this  |
    combination:                                                               |
                                                                               |
      mbx_format = true                                                        |
      message_prefix =                                                         |
      message_suffix =                                                         |
                                                                               |
    If none of the locking options are mentioned in the configuration,
    "use_mbx_lock" is assumed and the other locking options default to false.
    It is possible to specify the other kinds of locking with "mbx_format",
    but "use_fcntl_lock" and "use_mbx_lock" are mutually exclusive. MBX lock-
    ing interworks with "c-client", providing for shared access to the
    mailbox. It should not be used if any program that does not use this form
    of locking is going to access the mailbox, nor should it be used if the
    mailbox file is NFS mounted, because it works only when the mailbox is
    accessed from a single host.

    If you set "use_fcntl_lock" with an MBX-format mailbox, you cannot use the
    standard version of "c-client", because as long as it has a mailbox open
    (this means for the whole of a Pine or IMAP session), Exim will not be
    able to append messages to it.

message_prefix                  Type: string*              Default: see below

    The string specified here is expanded and output at the start of every
    message. The default is unset unless "file" is specified and "use_bsmtp"
    is not set, in which case it is:

      message_prefix = "From ${if def:return_path{$return_path}\
        {MAILER-DAEMON}} $tod_bsdinbox\n"

message_suffix                  Type: string*              Default: see below

    The string specified here is expanded and output at the end of every
    message. The default is unset unless "file" is specified and "use_bsmtp"
    is not set, in which case it is a single newline character. The suffix can
    be suppressed by setting

      message_suffix =

mode                         Type: octal integer                Default: 0600

    If the output file is created, it is given this mode. If it already exists
    and has wider permissions, they are reduced to this mode. If it has
    narrower permissions, an error occurs unless "mode_fail_narrower" is
    false. However, if the delivery is the result of a "save" command in a
    filter file specifing a particular mode, the mode of the output file is
    always forced to take that value, and this option is ignored.

mode_fail_narrower              Type: boolean                   Default: true

    This option applies in the case when an existing mailbox file has a
    narrower mode than that specified by the "mode" option. If
    "mode_fail_narrower" is true, the delivery is deferred ('mailbox has the
    wrong mode'); otherwise Exim continues with the delivery attempt, using
    the existing mode of the file.

notify_comsat                   Type: boolean                  Default: false

    If this option is true, the "comsat" daemon is notified after every
    successful delivery to a user mailbox. This is the daemon that notifies
    logged on users about incoming mail.

quota                           Type: string*                  Default: unset

    This option imposes a limit on the size of the file to which Exim is
    appending, or to the total space used in the directory tree when the
    "directory" option is set. In the latter case, computation of the space
    used is expensive, because all the files in the directory (and any sub-
    directories) have to be individually inspected and their sizes summed (but
    see "quota_size_regex" below). Also, there is no interlock against two
    simultaneous deliveries into a multi-file mailbox. For single-file
    mailboxes, of course, an interlock is a necessity.

    A file's size is taken as its "used" value. Because of blocking effects,
    this may be a lot less than the actual amount of disk space allocated to
    the file. If the sizes of a number of files are being added up, the
    rounding effect can become quite noticeable, especially on systems that
    have large block sizes. Nevertheless, it seems best to stick to the "used"
    figure, because this is the obvious value which users understand most
    easily.

    The value of the option is expanded, and must then be a numerical value
    (decimal point allowed), optionally followed by one of the letters K or M.
    A value of zero unsets the option. The expansion happens while Exim is
    running as root, before it changes uid for the delivery. This means that
    files which are inaccessible to the end user can be used to hold quota
    values that are looked up in the expansion. When delivery fails because
    this quota is exceeded, the handling of the error is as for system quota
    failures.

    By default, Exim's quota checking mimics system quotas, and restricts the
    mailbox to the specified maximum size, though the value is not accurate to
    the last byte, owing to separator lines and additional headers that may
    get added during message delivery. When a mailbox is nearly full, large
    messages may get refused even though small ones are accepted, because the
    size of the current message is added to the quota when the check is made.
    This behaviour can be changed by setting "quota_is_inclusive" false. When
    this is done, the check for exceeding the quota does not include the
    current message. Thus, deliveries continue until the quota has been
    exceeded; thereafter, no further messages are delivered. See also
    "quota_warn_threshold".

quota_directory                 Type: string*                  Default: unset

    This option defines the directory to check for quota purposes when
    delivering into individual files. The default is the delivery directory,
    or, if a file called maildirfolder exists in a maildir directory, the
    parent of the delivery directory.

quota_filecount                 Type: string*                      Default: 0

    This option applies when the "directory" option is set. It limits the
    total number of files in the directory (compare the inode limit in system
    quotas). It can only be used if "quota" is also set. The value is
    expanded; an expansion failure causes delivery to be deferred.

quota_is_inclusive              Type: boolean                   Default: true

    See "quota" above.

quota_size_regex                 Type: string                   Default: unset

    This option applies when one of the delivery modes that writes a separate
    file for each message is being used. When Exim wants to find the size
    of one of these files in order to test the quota, it first checks
    "quota_size_regex". If this is set to a regular expression that matches
    the file name, and it captures one string, that string is interpreted as a
    representation of the file's size. The value of "quota_size_regex" is not
    expanded.

    This feature is useful only when users have no shell access to their
    mailboxes - otherwise they could defeat the quota simply by renaming the
    files. This facility can be used with maildir deliveries, by setting
    "maildir_tag" to add the file length to the file name. For example:

      maildir_tag = ,S=$message_size
      quota_size_regex = ,S=(\d+)

    The regular expression should not assume that the length is at the end of
    the file name (even though "maildir_tag" puts it there) because maildir
    MUAs sometimes add other information onto the ends of message file names.

quota_warn_message              Type: string*              Default: see below

    See below for the use of this option. If it is not set when
    "quota_warn_threshold" is set, it defaults to

      quota_warn_message = "\
        To: $local_part@$domain\n\
        Subject: Your mailbox\n\n\
        This message is automatically created \
        by mail delivery software.\n\n\
        The size of your mailbox has exceeded \
        a warning threshold that is\n\
        set by the system administrator.\n"

quota_warn_threshold            Type: string*                      Default: 0

    This option is expanded in the same way as "quota" (see above). If the
    resulting value is greater than zero, and delivery of the message causes
    the size of the file or total space in the directory tree to cross the
    given threshold, a warning message is sent. If "quota" is also set, the
    threshold may be specified as a percentage of it by following the value
    with a percent sign. For example:

      quota = 10M
      quota_warn_threshold = 75%

    If "quota" is not set, a setting of "quota_warn_threshold" that ends with
    a percent sign is ignored.

    The warning message itself is specified by the "quota_warn_message"
    option, and it must start with a To: header line containing the
    recipient(s). A Subject: line should also normally be supplied. The
    "quota" option does not have to be set in order to use this option; they
    are independent of one another except when the threshold is specified as a
    percentage.

use_bsmtp                       Type: boolean                  Default: false

    If this option is set true, appendfile writes messages in 'batch SMTP'
    format, with the envelope sender and recipient(s) included as SMTP
    commands. If you want to include a leading HELO command with such
    messages, you can do so by setting the "message_prefix" option. See
    section 43.11 for details of batch SMTP.

use_crlf                        Type: boolean                  Default: false

    This option causes lines to be terminated with the two-character CRLF
    sequence (carriage return, linefeed) instead of just a linefeed character.
    In the case of batched SMTP, the byte sequence written to the file is then
    an exact image of what would be sent down a real SMTP connection.

    The contents of the "message_prefix" and "message_suffix" options are
    written verbatim, so must contain their own carriage return characters if
    these are needed. In cases where these options have non-empty defaults,
    the values end with a single linefeed, so they must be changed to end with |
    "\r\n" if "use_crlf" is set.

use_fcntl_lock                  Type: boolean              Default: see below

    This option controls the use of the "fcntl()" function to lock a file for
    exclusive use when a message is being appended. It is set by default
    unless "use_flock_lock" is set. Otherwise, it should be turned off only
    if you know that all your MUAs use lock file locking. When both
    "use_fcntl_lock" and "use_flock_lock" are unset, "use_lockfile" must be
    set.

use_flock_lock                  Type: boolean                  Default: false

    This option is provided to support the use of "flock()" for file locking,
    for the few situations where it is needed. Most modern operating systems
    support "fcntl()" and "lockf()" locking, and these two functions interwork
    with each other. Exim uses "fcntl()" locking by default.

    This option is required only if you are using an operating system where
    "flock()" is used by programs that access mailboxes (typically MUAs), and
    where "flock()" does not correctly interwork with "fcntl()". You can use
    both "fcntl()" and "flock()" locking simultaneously if you want.

    Not all operating systems provide "flock()". Some versions of Solaris do
    not have it (and some, I think, provide a not quite right version built on
    top of "lockf()"). If the OS does not have "flock()", Exim will be built
    without the ability to use it, and any attempt to do so will cause a
    configuration error.

    Warning: "flock()" locks do not work on NFS files (unless "flock()" is
    just being mapped onto "fcntl()" by the OS).

use_lockfile                    Type: boolean              Default: see below

    If this option is turned off, Exim does not attempt to create a lock file
    when appending to a mailbox file. In this situation, the only locking is
    by "fcntl()". You should only turn "use_lockfile" off if you are absol-
    utely sure that every MUA that is ever going to look at your users'
    mailboxes uses "fcntl()" rather than a lock file, and even then only when
    you are not delivering over NFS from more than one host.

    In order to append to an NFS file safely from more than one host, it is
    necessary to take out a lock before opening the file, and the lock file
    achieves this. Otherwise, even with "fcntl()" locking, there is a risk of
    file corruption.

    The "use_lockfile" option is set by default unless "use_mbx_lock" is set.
    It is not possible to turn both "use_lockfile" and "use_fcntl_lock" off,
    except when "mbx_format" is set.

use_mbx_lock                    Type: boolean              Default: see below

    This option is available only if Exim has been compiled with SUPPORT_MBX
    set in Local/Makefile. Setting the option specifies that special MBX
    locking rules be used. It is set by default if "mbx_format" is set and
    none of the locking options are mentioned in the configuration. The
    locking rules are the same as are used by the "c-client" library that
    underlies Pine and the IMAP4 and POP daemons that come with it (see the
    discussion below). The rules allow for shared access to the mailbox.
    However, this kind of locking does not work when the mailbox is NFS
    mounted.

    You can set "use_mbx_lock" with either (or both) of "use_fcntl_lock" and
    "use_flock_lock" to control what kind of locking is used in implementing
    the MBX locking rules. The default is to use "fcntl()" if "use_mbx_lock"
    is set without "use_fcntl_lock" or "use_flock_lock".


26.3 Operational details for appending

Before appending to a file, the following preparations are made:

 .   If the name of the file is /dev/null, no action is taken, and a success
     return is given.

 .   If any directories on the file's path are missing, Exim creates them if
     the "create_directory" option is set. A created directory's mode is given
     by the "directory_mode" option.

 .   If "file_format" is set, the format of an existing file is checked. If
     this indicates that a different transport should be used, control is
     passed to that transport.

 .   If "use_lockfile" is set, a lock file is built in a way that will work
     reliably over NFS, as follows:

      .   Create a 'hitching post' file whose name is that of the lock file
          with the current time, primary host name, and process id added, by
          opening for writing as a new file. If this fails with an access
          error, delivery is deferred.

      .   Close the hitching post file, and hard link it to the lock file
          name.

      .   If the call to "link()" succeeds, creation of the lock file has
          succeeded. Unlink the hitching post name.

      .   Otherwise, use "stat()" to get information about the hitching post
          file, and then unlink hitching post name. If the number of links is
          exactly two, creation of the lock file succeeded but something (for
          example, an NFS server crash and restart) caused this fact not to be
          communicated to the "link()" call.

      .   If creation of the lock file failed, wait for "lock_interval" and
          try again, up to "lock_retries" times. However, since any program
          that writes to a mailbox should complete its task very quickly, it
          is reasonable to time out old lock files that are normally the
          result of user agent and system crashes. If an existing lock file is
          older than "lockfile_timeout" Exim attempts to unlink it before
          trying again.

 .   A call is made to "lstat()" to discover whether the main file exists, and
     if so, what its characteristics are. If "lstat()" fails for any reason
     other than non-existence, delivery is deferred.

 .   If the file does exist and is a symbolic link, delivery is deferred,
     unless the "allow_symlink" option is set, in which case the ownership of
     the link is checked, and then "stat()" is called to find out about the
     real file, which is then subjected to the checks below. The check on the
     top-level link ownership prevents one user creating a link for another's
     mailbox in a sticky directory, though allowing symbolic links in this
     case is definitely not a good idea. If there is a chain of symbolic
     links, the intermediate ones are not checked.

 .   If the file already exists but is not a regular file, or if the file's
     owner and group (if the group is being checked - see "check_group" above)
     are different from the user and group under which the delivery is
     running, delivery is deferred.

 .   If the file's permissions are more generous than specified, they are
     reduced. If they are insufficient, delivery is deferred, unless
     "mode_fail_narrower" is set false, in which case the delivery is tried
     using the existing permissions.

 .   The file's inode number is saved, and the file is then opened for
     appending. If this fails because the file has vanished, appendfile
     behaves as if it hadn't existed (see below). For any other failures,
     delivery is deferred.

 .   If the file is opened successfully, check that the inode number hasn't
     changed, that it is still a regular file, and that the owner and
     permissions have not changed. If anything is wrong, defer delivery and
     freeze the message.

 .   If the file did not exist originally, defer delivery if the
     "file_must_exist" option is set. Otherwise, check that the file is being
     created in a permitted directory if the "create_file" option is set
     (deferring on failure), and then open for writing as a new file, with the
     O_EXCL and O_CREAT options, except when dealing with a symbolic link (the
     "allow_symlink" option must be set). In this case, which can happen if
     the link points to a non-existent file, the file is opened for writing
     using O_CREAT but not O_EXCL, because that prevents link following.

 .   If opening fails because the file exists, obey the tests given above for
     existing files. However, to avoid looping in a situation where the file
     is being continuously created and destroyed, the exists/not-exists loop
     is broken after 10 repetitions, and the message is then frozen.

 .   If opening fails with any other error, defer delivery.

 .   Once the file is open, unless both "use_fcntl_lock" and "use_flock_lock"
     are false, it is locked using "fcntl()" or "flock()" or both. If
     "use_mbx_lock" is false, an exclusive lock is requested in each case.
     However, if "use_mbx_lock" is true, Exim takes out a shared lock on the
     open file, and an exclusive lock on the file whose name is

       /tmp/.<device-number>.<inode-number>

     using the device and inode numbers of the open mailbox file, in
     accordance with the MBX locking rules.

     If Exim fails to lock the file, there are two possible courses of action,
     depending on the value of the locking timeout. This is obtained from
     "lock_fcntl_timeout" or "lock_flock_timeout", as appropriate.

     If the timeout value is zero, the file is closed, Exim waits for
     "lock_interval", and then goes back and re-opens the file as above and
     tries to lock it again. This happens up to "lock_retries" times, after
     which the delivery is deferred.

     If the timeout has a value greater than zero, blocking calls to "fcntl()"
     or "flock()" are used (with the given timeout), so there has already been
     some waiting involved by the time locking fails. Nevertheless, Exim does
     not give up immediately. It retries up to

       (lock_retries * lock_interval) / <timeout>

     times (rounded up).

At the end of delivery, Exim closes the file (which releases the "fcntl()"
and/or "flock()" locks) and then deletes the lock file if one was created.


26.4 Operational details for delivery to a new file

When the "directory" option is set instead of "file", each message is
delivered into a newly-created file or set of files. When appendfile is
activated directly from a redirect router, neither "file" nor "directory" is
normally set, because the path for delivery is supplied by the router. (See
for example, the address_file transport in the default configuration.) In this
case, delivery is to a new file if either the path name ends in "/", or the
"maildir_format" or "mailstore_format" option is set.

No locking is required while writing the message to a new file, so the various
locking options of the transport are ignored. The 'From' line that by default
separates messages in a single file is not normally needed, nor is the
escaping of message lines that start with 'From', and there is no need to
ensure a newline at the end of each message. Consequently, the default values
for "check_string", "message_prefix", and "message_suffix" are all unset when
any of "directory", "maildir_format", or "mailstore_format" is set.

If Exim is required to check a "quota" setting, it adds up the sizes of all
the files in the delivery directory by default. However, you can specify a
different directory by setting "quota_directory". Also, for maildir deliveries
(see below) the maildirfolder convention is honoured.

There are three different ways in which delivery to individual files can
be done, controlled by the settings of the "maildir_format" and
"mailstore_format" options. Note that code to support maildir or mailstore
formats is not included in the binary unless SUPPORT_MAILDIR or
SUPPORT_MAILSTORE, respectively, is set in Local/Makefile.

In all three cases an attempt is made to create the directory and any
necessary sub-directories if they do not exist, provided that the
"create_directory" option is set (the default). The location of a created
directory can be constrained by setting "create_file". A created directory's
mode is given by the "directory_mode" option. If creation fails, or if the
"create_directory" option is not set when creation is required, delivery is
deferred.


26.5 Maildir delivery

If the "maildir_format" option is true, Exim delivers each message by writing
it to a file whose name is tmp/<stime>.H<mtime>P<pid>.<host> in the given
directory. If the delivery is successful, the file is renamed into the new
subdirectory.

In the file name, <stime> is the current time of day in seconds, and <mtime>
is the microsecond fraction of the time. After a maildir delivery, Exim checks
that the time-of-day clock has moved on by at least one microsecond before
terminating the delivery process. This guarantees uniqueness for the file
name. However, as a precaution, Exim calls "stat()" for the file before
opening it. If any response other than ENOENT (does not exist) is given, Exim
waits 2 seconds and tries again, up to "maildir_retries" times.

If Exim is required to check a "quota" setting before a maildir delivery, and
"quota_directory" is not set, it looks for a file called maildirfolder in the
maildir directory (alongside new, cur, tmp). If this exists, Exim assumes the
directory is a maildir++ folder directory, which is one level down from the
user's top level mailbox directory. This causes it to start at the parent
directory instead of the current directory when calculating the amount of
space used.

If "maildir_use_sizefile" is true, Exim implements the maildir++ rules for     |
storing quota and message size information in a file called maildirsize within |
the maildir directory. If this file does not exist, Exim creates it, setting   |
the quota from the "quota" option of the transport. If this is unset, the      |
quota will be zero. The maildirsize file is used to hold information about the |
sizes of messages in the maildir, thus speeding up quota calculations.         |

If "maildir_tag" is set, the string is expanded for each delivery. When the
maildir file is renamed into the new sub-directory, the tag is added to its
name. However, if adding the tag takes the length of the name to the point
where the test "stat()" call fails with ENAMETOOLONG, the tag is dropped and
the maildir file is created with no tag.

Tags can be used to encode the size of files in their names; see
"quota_size_regex" above for an example. The expansion of "maildir_tag"
happens after the message has been written. The value of the $message_size
variable is set to the number of bytes actually written. If the expansion is
forced to fail, the tag is ignored, but a non-forced failure causes delivery
to be deferred. The expanded tag may contain any printing characters except
'/'. Non-printing characters in the string are ignored; if the resulting
string is empty, it is ignored. If it starts with an alphanumeric character, a
leading colon is inserted.


26.6 Mailstore delivery

If the "mailstore_format" option is true, each message is written as two files
in the given directory. A unique base name is constructed from the message id
and the current delivery process, and the files that are written use this base
name plus the suffixes .env and .msg. The .env file contains the message's
envelope, and the .msg file contains the message itself.

During delivery, the envelope is first written to a file with the suffix .tmp.
The .msg file is then written, and when it is complete, the .tmp file is
renamed as the .env file. Programs that access messages in mailstore format
should wait for the presence of both a .msg and a .env file before accessing
either of them. An alternative approach is to wait for the absence of a .tmp
file.

The envelope file starts with any text defined by the "mailstore_prefix"
option, expanded and terminated by a newline if there isn't one. Then follows
the sender address on one line, then all the recipient addresses, one per
line. There can be more than one recipient only if the "batch_max" option is
set greater than one. Finally, "mailstore_suffix" is expanded and the result
appended to the file, followed by a newline if it does not end with one.

If expansion of "mailstore_prefix" or "mailstore_suffix" ends with a forced
failure, it is ignored. Other expansion errors are treated as serious
configuration errors, and delivery is deferred.


26.7 Non-special new file delivery

If neither "maildir_format" nor "mailstore_format" is set, a single new file
is created directly in the named directory. For example, when delivering
messages into files in batched SMTP format for later delivery to some host
(see section 43.11), a setting such as

  directory = /var/bsmtp/$host

might be used. A message is written to a file with a temporary name, which is
then renamed when the delivery is complete. The final name is obtained by
expanding the contents of the "directory_file" option.



                         27. THE AUTOREPLY TRANSPORT


The autoreply transport is not a true transport in that it does not cause the
message to be transmitted. Instead, it generates another mail message. It is
usually run as the result of mail filtering, a 'vacation' message being the
standard example. However, it can also be run directly from a router like any
other transport. To reduce the possibility of message cascades, messages
created by the autoreply transport always have empty envelope sender
addresses, like bounce messages.

The parameters of the message to be sent can be specified in the configuration
by options described below. However, these are used only when the address
passed to the transport does not contain its own reply information. When the
transport is run as a consequence of a "mail" or "vacation" command in a
filter file, the parameters of the message are supplied by the filter, and
passed with the address. The transport's options that define the message are
then ignored (so they are not usually set in this case). The message is
specified entirely by the filter or by the transport; it is never built from a
mixture of options. However, the "file_optional", "mode", and "return_message"
options apply in all cases.

Autoreply is implemented as a local transport. When used as a result of a
command in a user's filter file, autoreply normally runs under the uid and gid
of the user, and with appropriate current and home directories (see chapter
23).

There is a subtle difference between routing a message to a pipe transport
that generates some text to be returned to the sender, and routing it to an
autoreply transport. This difference is noticeable only if more than one
address from the same message is so handled. In the case of a pipe, the
separate outputs from the different addresses are gathered up and returned to
the sender in a single message, whereas if autoreply is used, a separate
message is generated for each address that is passed to it.

Non-printing characters are not permitted in the header lines generated for
the message that autoreply creates, with the exception of newlines that are
immediately followed by whitespace. If any non-printing characters are found,
the transport defers. Whether characters with the top bit set count as
printing characters or not is controlled by the "print_topbitchars" global
option.

If any of the generic options for manipulating headers (for example,
"headers_add") are set on an autoreply transport, they apply to the copy
of the original message that is included in the generated message when
"return_message" is set. They do not apply to the generated message itself.

If the autoreply transport receives return code 2 from Exim when it submits
the message, indicating that there were no recipients, it does not treat this
as an error. This means that autoreplies sent to $sender_address when this is
empty (because the incoming message is a bounce message) do not cause
problems. They are just discarded.


27.1 Private options for autoreply

bcc                             Type: string*                  Default: unset

    This specifies the addresses that are to receive 'blind carbon copies' of
    the message when the message is specified by the transport.

cc                              Type: string*                  Default: unset

    This specifies recipients of the message and the contents of the Cc:
    header when the message is specified by the transport.

file                            Type: string*                  Default: unset

    The contents of the file are sent as the body of the message when the
    message is specified by the transport. If both "file" and "text" are set,
    the text string comes first.

file_expand                     Type: boolean                  Default: false

    If this is set, the contents of the file named by the "file" option are
    subjected to string expansion as they are added to the message.

file_optional                   Type: boolean                  Default: false

    If this option is true, no error is generated if the file named by the
    "file" option or passed with the address does not exist or cannot be read.

from                            Type: string*                  Default: unset

    This specifies the contents of the From: header when the message is
    specified by the transport.

headers                         Type: string*                  Default: unset

    This specifies additional RFC 2822 headers that are to be added to the
    message when the message is specified by the transport. Several can be
    given by using '\n' to separate them. There is no check on the format.

log                             Type: string*                  Default: unset

    This option names a file in which a record of every message sent is logged
    when the message is specified by the transport.

mode                         Type: octal integer                Default: 0600

    If either the log file or the 'once' file has to be created, this mode is
    used.

once                            Type: string*                  Default: unset

    This option names a file or DBM database in which a record of each To:
    recipient is kept when the message is specified by the transport. Note:
    This does not apply to Cc: or Bcc: recipients. If "once_file_size" is not
    set, a DBM database is used, and it is allowed to grow as large as
    necessary. If a potential recipient is already in the database, no message
    is sent by default. However, if "once_repeat" specifies a time greater
    than zero, the message is sent if that much time has elapsed since a
    message was last sent to this recipient. If "once" is unset, the message
    is always sent.

    If "once_file_size" is set greater than zero, it changes the way Exim
    implements the "once" option. Instead of using a DBM file to record every
    recipient it sends to, it uses a regular file, whose size will never get
    larger than the given value. In the file, it keeps a linear list of
    recipient addresses and times at which they were sent messages. If the
    file is full when a new address needs to be added, the oldest address is
    dropped. If "once_repeat" is not set, this means that a given recipient
    may receive multiple messages, but at unpredictable intervals that depend
    on the rate of turnover of addresses in the file. If "once_repeat" is set,
    it specifies a maximum time between repeats.

once_file_size                  Type: integer                      Default: 0

    See "once" above.

once_repeat                      Type: time*                      Default: 0s

    See "once" above. After expansion, the value of this option must be a
    valid time value.

reply_to                        Type: string*                  Default: unset

    This specifies the contents of the Reply-To: header when the message is
    specified by the transport.

return_message                  Type: boolean                  Default: false

    If this is set, a copy of the original message is returned with the new
    message, subject to the maximum size set in the "return_size_limit" global
    configuration option.

subject                         Type: string*                  Default: unset

    This specifies the contents of the Subject: header when the message is
    specified by the transport.

text                            Type: string*                  Default: unset

    This specifies a single string to be used as the body of the message when
    the message is specified by the transport. If both "text" and "file" are
    set, the text comes first.

to                              Type: string*                  Default: unset

    This specifies recipients of the message and the contents of the To:
    header when the message is specified by the transport.



                            28. THE LMTP TRANSPORT


The lmtp transport runs the LMTP protocol (RFC 2033) over a pipe to a
specified command or by interacting with a Unix domain socket. This transport
is something of a cross between the pipe and smtp transports. Exim also has
support for using LMTP over TCP/IP; this is implemented as an option for the
smtp transport. Because LMTP is expected to be of minority interest, the
default build-time configure in src/EDITME has it commented out. You need to
ensure that

  TRANSPORT_LMTP=yes

is present in your Local/Makefile in order to have the lmtp transport included
in the Exim binary.

The private options of the lmtp transport are as follows:

batch_id                        Type: string*                  Default: unset

    See the description of local delivery batching in chapter 25.

batch_max                       Type: integer                      Default: 1

    This limits the number of addresses that can be handled in a single
    delivery. Most LMTP servers can handle several addresses at once, so it is
    normally a good idea to increase this value. See the description of local
    delivery batching in chapter 25.

command                         Type: string*                  Default: unset

    This option must be set if "socket" is not set. The string is a command
    which is run in a separate process. It is split up into a command name and
    list of arguments, each of which is separately expanded (so expansion
    cannot change the number of arguments). The command is run directly, not
    via a shell. The message is passed to the new process using the standard
    input and output to operate the LMTP protocol.

socket                          Type: string*                  Default: unset

    This option must be set if "command" is not set. The result of expansion
    must be the name of a Unix domain socket. The transport connects to the
    socket and delivers the message to it using the LMTP protocol.

timeout                           Type: time                       Default: 5m

    The transport is aborted if the created process or Unix domain socket does
    not respond to LMTP commands or message input within this timeout.

Here is an example of a typical LMTP transport:

  lmtp:
    driver = lmtp
    command = /some/local/lmtp/delivery/program
    batch_max = 20
    user = exim

This delivers up to 20 addresses at a time, in a mixture of domains if
necessary, running as the user "exim".



                            29. THE PIPE TRANSPORT


The pipe transport is used to deliver messages via a pipe to a command running
in another process. This can happen in one of two ways:

 .   A router routes an address to a transport in the normal way, and the
     transport is configured as a pipe transport. In this case, $local_part
     contains the address (as usual), and the command which is run is
     specified by the "command" option on the transport. An example of this is
     the use of pipe as a pseudo-remote transport for passing messages to some
     other delivery mechanism (such as UUCP).

 .   A router redirects an address directly to a pipe command (for example,
     from an alias or forward file). In this case, $local_part contains the
     local part that was redirected, and $address_pipe contains the text of
     the pipe command itself. The "command" option on the transport is
     ignored.

The pipe transport is a non-interactive delivery method. Exim can also deliver
messages over pipes using the LMTP interactive protocol. This is implemented
by the lmtp transport.

In the case when pipe is run as a consequence of an entry in a local user's
.forward file, the command runs under the uid and gid of that user. In other
cases, the uid and gid have to be specified explicitly, either on the
transport or on the router that handles the address. Current and 'home'
directories are also controllable. See chapter 23 for details of the local
delivery environment.


29.1 Returned status and data

If the command exits with a non-zero return code, the delivery is deemed to
have failed, unless either the "ignore_status" option is set (in which case
the return code is treated as zero), or the return code is one of those listed
in the "temp_errors" option, which are interpreted as meaning 'try again
later'. In this case, delivery is deferred. Details of a permanent failure are
logged, but are not included in the bounce message, which merely contains
'local delivery failed'.

If the return code is greater than 128 and the command being run is a shell
script, it normally means that the script was terminated by a signal whose
value is the return code minus 128.

If Exim is unable to run the command (that is, if "execve()" fails), the       |
return code is set to 127. This is the value that a shell returns if it is     |
asked to run a non-existent command. The wording for the log line suggests     |
that a non-existent command may be the problem.                                |

The "return_output" option can affect the result of a pipe delivery. If it is
set and the command produces any output on its standard output or standard
error streams, the command is considered to have failed, even if it gave a
zero return code or if "ignore_status" is set. The output from the command is
included as part of the bounce message. The "return_fail_output" option is
similar, except that output is returned only when the command exits with a
failure return code, that is, a value other than zero or a code that matches
"temp_errors".


29.2 How the command is run

The command line is (by default) broken down into a command name and arguments
by the pipe transport itself. The "allow_commands" and "restrict_to_path"
options can be used to restrict the commands that may be run. Unquoted
arguments are delimited by white space. If an argument appears in double
quotes, backslash is interpreted as an escape character in the usual way. If
an argument appears in single quotes, no escaping is done.

String expansion is applied to the command line except when it comes from a
traditional .forward file (commands from a filter file are expanded). The
expansion is applied to each argument in turn rather than to the whole line.
For this reason, any string expansion item that contains white space must be
quoted so as to be contained within a single argument. A setting such as

  command = /some/path ${if eq{$local_part}{postmaster}{xxx}{yyy}}

will not work, because the expansion item gets split between several argu-
ments. You have to write

  command = /some/path "${if eq{$local_part}{postmaster}{xxx}{yyy}}"

to ensure that it is all in one argument. The expansion is done in this way,
argument by argument, so that the number of arguments cannot be changed as a
result of expansion, and quotes or backslashes in inserted variables do not
interact with external quoting.

Special handling takes place when an argument consists of precisely the text
'$pipe_addresses'. This is not a general expansion variable; the only place
this string is recognized is when it appears as an argument for a pipe or
transport filter command. It causes each address that is being handled to be
inserted in the argument list at that point as a separate argument. This
avoids any problems with spaces or shell metacharacters, and is of use when a
pipe transport is handling groups of addresses in a batch.

After splitting up into arguments and expansion, the resulting command is run
in a subprocess directly from the transport, not under a shell. The message
that is being delivered is supplied on the standard input, and the standard
output and standard error are both connected to a single pipe that is read by
Exim. The "max_output" option controls how much output the command may
produce, and the "return_output" and "return_fail_output" options control what
is done with it.

Not running the command under a shell (by default) lessens the security risks
in cases when a command from a user's filter file is built out of data that
was taken from an incoming message. If a shell is required, it can of course
be explicitly specified as the command to be run. However, there are
circumstances where existing commands (for example, in .forward files) expect
to be run under a shell and cannot easily be modified. To allow for these
cases, there is an option called "use_shell", which changes the way the pipe
transport works. Instead of breaking up the command line as just described, it
expands it as a single string and passes the result to /bin/sh. The
"restrict_to_path" option and the $pipe_addresses facility cannot be used with
"use_shell", and the whole mechanism is inherently less secure.


29.3 Environment variables

The environment variables listed below are set up when the command is invoked.
This list is a compromise for maximum compatibility with other MTAs. Note that
the "environment" option can be used to add additional variables to this
environment.

  DOMAIN                 the domain of the address
  HOME                   the home directory, if set
  HOST                   the host name when called from a router (see below)
  LOCAL_PART             see below
  LOCAL_PART_PREFIX      see below
  LOCAL_PART_SUFFIX      see below
  LOGNAME                see below
  MESSAGE_ID             the message's id
  PATH                   as specified by the "path" option below
  QUALIFY_DOMAIN         the sender qualification domain
  RECIPIENT              the complete recipient address
  SENDER                 the sender of the message (empty if a bounce)
  SHELL                  '/bin/sh'
  TZ                     the value of the "timezone" option, if set
  USER                   see below

When a pipe transport is called directly from (for example) an accept router,
LOCAL_PART is set to the local part of the address. When it is called as a
result of a forward or alias expansion, LOCAL_PART is set to the local part of
the address that was expanded. In both cases, any affixes are removed from the
local part, and made available in LOCAL_PART_PREFIX and LOCAL_PART_SUFFIX,
respectively. LOGNAME and USER are set to the same value as LOCAL_PART for
compatibility with other MTAs.

HOST is set only when a pipe transport is called from a router that associates
hosts with an address, typically when using pipe as a pseudo-remote transport.
HOST is set to the first host name specified by the router.

If the transport's generic "home_directory" option is set, its value is used
for the HOME environment variable. Otherwise, a home directory may be set by
the router's "transport_home_directory" option, which defaults to the user's
home directory if "check_local_user" is set.


29.4 Private options for pipe

allow_commands                Type: string list*                Default: unset

    The string is expanded, and is then interpreted as a colon-separated list
    of permitted commands. If "restrict_to_path" is not set, the only commands
    permitted are those in the "allow_commands" list. They need not be
    absolute paths; the "path" option is still used for relative paths. If
    "restrict_to_path" is set with "allow_commands", the command must either
    be in the "allow_commands" list, or a name without any slashes that is
    found on the path. In other words, if neither "allow_commands" nor
    "restrict_to_path" is set, there is no restriction on the command, but
    otherwise only commands that are permitted by one or the other are
    allowed. For example, if

      allow_commands = /usr/bin/vacation

    and "restrict_to_path" is not set, the only permitted command is
    /usr/bin/vacation. The "allow_commands" option may not be set if
    "use_shell" is set.

batch_id                        Type: string*                  Default: unset

    See the description of local delivery batching in chapter 25.

batch_max                       Type: integer                      Default: 1

    This limits the number of addresses that can be handled in a single
    delivery. See the description of local delivery batching in chapter 25.

check_string                     Type: string                   Default: unset

    As pipe writes the message, the start of each line is tested for matching
    "check_string", and if it does, the initial matching characters are
    replaced by the contents of "escape_string", provided both are set. The
    value of "check_string" is a literal string, not a regular expression, and
    the case of any letters it contains is significant. When "use_bsmtp" is
    set, the contents of "check_string" and "escape_string" are forced to
    values that implement the SMTP escaping protocol. Any settings made in the
    configuration file are ignored.

command                         Type: string*                  Default: unset

    This option need not be set when pipe is being used to deliver to pipes
    obtained directly from address redirections. In other cases, the option
    must be set, to provide a command to be run. It need not yield an absolute
    path (see the "path" option below). The command is split up into separate
    arguments by Exim, and each argument is separately expanded, as described
    in section 29.2 above.

environment                     Type: string*                  Default: unset

    This option is used to add additional variables to the environment in
    which the command runs (see section 29.3 for the default list). Its value
    is a string which is expanded, and then interpreted as a colon-separated
    list of environment settings of the form '<name>=<value>'.

escape_string                    Type: string                   Default: unset

    See "check_string" above.

freeze_exec_fail                Type: boolean                  Default: false

    Failure to exec the command in a pipe transport is by default treated
    like any other failure while running the command. However, if
    "freeze_exec_fail" is set, failure to exec is treated specially, and
    causes the message to be frozen, whatever the setting of "ignore_status".

ignore_status                   Type: boolean                  Default: false

    If this option is true, the status returned by the subprocess that is set
    up to run the command is ignored, and Exim behaves as if zero had been
    returned. Otherwise, a non-zero status or termination by signal causes an  |
    error return from the transport unless the status value is one of those
    listed in "temp_errors"; these cause the delivery to be deferred and tried
    again later.

log_defer_output                Type: boolean                  Default: false

    If this option is set, and the status returned by the command is one of
    the codes listed in "temp_errors" (that is, delivery was deferred), and
    any output was produced, the first line of it is written to the main log.

log_fail_output                 Type: boolean                  Default: false

    If this option is set, and the command returns any output, and also ends
    with a return code that is neither zero nor one of the return codes listed
    in "temp_errors" (that is, the delivery failed), the first line of output
    is written to the main log.

log_output                      Type: boolean                  Default: false

    If this option is set and the command returns any output, the first line
    of output is written to the main log, whatever the return code.

max_output                      Type: integer                    Default: 20K

    This specifies the maximum amount of output that the command may produce
    on its standard output and standard error file combined. If the limit is
    exceeded, the process running the command is killed. This is intended as a
    safety measure to catch runaway processes. The limit is applied indepen-
    dently of the settings of the options that control what is done with such
    output (for example, "return_output"). Because of buffering effects, the
    amount of output may exceed the limit by a small amount before Exim
    notices.

message_prefix                  Type: string*              Default: see below

    The string specified here is expanded and output at the start of every
    message. The default is unset if "use_bsmtp" is set. Otherwise it is

      message_prefix = \
        From ${if def:return_path{$return_path}{MAILER-DAEMON}}\
        ${tod_bsdinbox}\n

    This is required by the commonly used /usr/bin/vacation program. However,
    it must not be present if delivery is to the Cyrus IMAP server, or to the
    "tmail" local delivery agent. The prefix can be suppressed by setting

      message_prefix =

message_suffix                  Type: string*              Default: see below

    The string specified here is expanded and output at the end of every
    message. The default is unset if "use_bsmtp" is set. Otherwise it is a
    single newline. The suffix can be suppressed by setting

      message_suffix =

path                             Type: string                Default: /usr/bin

    This option specifies the string that is set up in the PATH environment
    variable of the subprocess. If the "command" option does not yield an
    absolute path name, the command is sought in the PATH directories, in the
    usual way. Warning: This does not apply to a command specified as a
    transport filter.

pipe_as_creator                 Type: boolean                  Default: false

    If the generic "user" option is not set and this option is true, the
    delivery process is run under the uid that was in force when Exim was
    originally called to accept the message. If the group id is not otherwise
    set (via the generic "group" option), the gid that was in force when Exim
    was originally called to accept the message is used.

restrict_to_path                Type: boolean                  Default: false

    When this option is set, any command name not listed in "allow_commands"
    must contain no slashes. The command is searched for only in the
    directories listed in the "path" option. This option is intended for use
    in the case when a pipe command has been generated from a user's .forward
    file. This is usually handled by a pipe transport called "address_pipe".

return_fail_output              Type: boolean                  Default: false

    If this option is true, and the command produced any output and ended with
    a return code other than zero or one of the codes listed in "temp_errors"
    (that is, the delivery failed), the output is returned in the bounce
    message. However, if the message has a null sender (that is, it is itself
    a bounce message), output from the command is discarded.

return_output                   Type: boolean                  Default: false

    If this option is true, and the command produced any output, the delivery
    is deemed to have failed whatever the return code from the command, and
    the output is returned in the bounce message. Otherwise, the output is
    just discarded. However, if the message has a null sender (that is, it is
    a bounce message), output from the command is always discarded, whatever
    the setting of this option.

temp_errors                   Type: string list            Default: see below

    This option contains either a colon-separated list of numbers, or a single
    asterisk. If "ignore_status" is false and the command exits with a non-
    zero return code, the failure is treated as temporary and the delivery is
    deferred if the return code matches one of the numbers, or if the setting
    is a single asterisk. Otherwise, non-zero return codes are treated as
    permanent errors. The default setting contains the codes defined by
    EX_TEMPFAIL and EX_CANTCREAT in sysexits.h. If Exim is compiled on a
    system that does not define these macros, it assumes values of 75 and 73,
    respectively.

timeout                           Type: time                       Default: 1h

    If the command fails to complete within this time, it is killed. This
    normally causes the delivery to fail. A zero time interval specifies no
    timeout. In order to ensure that any subprocesses created by the command
    are also killed, Exim makes the initial process a process group leader,
    and kills the whole process group on a timeout. However, this can be
    defeated if one of the processes starts a new process group.

umask                        Type: octal integer                 Default: 022

    This specifies the umask setting for the subprocess that runs the command.

use_bsmtp                       Type: boolean                  Default: false

    If this option is set true, the pipe transport writes messages in 'batch
    SMTP' format, with the envelope sender and recipient(s) included as SMTP
    commands. If you want to include a leading HELO command with such
    messages, you can do so by setting the "message_prefix" option. See
    section 43.11 for details of batch SMTP.

use_crlf                        Type: boolean                  Default: false

    This option causes lines to be terminated with the two-character CRLF
    sequence (carriage return, linefeed) instead of just a linefeed character.
    In the case of batched SMTP, the byte sequence written to the pipe is then
    an exact image of what would be sent down a real SMTP connection.

    The contents of the "message_prefix" and "message_suffix" options are
    written verbatim, so must contain their own carriage return characters if
    these are needed. Since the default values for both "message_prefix" and
    "message_suffix" end with a single linefeed, their values must be changed  |
    to end with "\r\n" if "use_crlf" is set.

use_shell                       Type: boolean                  Default: false

    If this option is set, it causes the command to be passed to /bin/sh
    instead of being run directly from the transport, as described in section
    29.2. This is less secure, but is needed in some situations where the
    command is expected to be run under a shell and cannot easily be modified.
    The "allow_commands" and "restrict_to_path" options, and the
    '$pipe_addresses' facility are incompatible with "use_shell". The command
    is expanded as a single string, and handed to /bin/sh as data for its -c
    option.


29.5 Using an external local delivery agent

The pipe transport can be used to pass all messages that require local
delivery to a separate local delivery agent such as "procmail". When doing
this, care must be taken to ensure that the pipe is run under an appropriate
uid and gid. In some configurations one wants this to be a uid that is trusted
by the delivery agent to supply the correct sender of the message. It may be
necessary to recompile or reconfigure the delivery agent so that it trusts an
appropriate user. The following is an example transport and router configur-
ation for "procmail":

  # transport
  procmail_pipe:
    driver = pipe
    command = /usr/local/bin/procmail -d $local_part
    return_path_add
    delivery_date_add
    envelope_to_add
    check_string = "From "
    escape_string = ">From "
    user = $local_part
    group = mail

  # router
  procmail:
    driver = accept
    check_local_user
    transport = procmail_pipe

In this example, the pipe is run as the local user, but with the group set to
"mail". An alternative is to run the pipe as a specific user such as "mail" or
"exim", but in this case you must arrange for "procmail" to trust that user to
supply a correct sender address. If you do not specify either a "group" or a
"user" option, the pipe command is run as the local user. The home directory
is the user's home directory by default.

Note that the command that the pipe transport runs does not begin with

  IFS=" "

as shown in the "procmail" documentation, because Exim does not by default use
a shell to run pipe commands.

The next example shows a transport and a router for a system where local
deliveries are handled by the Cyrus IMAP server.

  # transport
  local_delivery_cyrus:
    driver = pipe
    command = /usr/cyrus/bin/deliver \
              -m ${substr_1:$local_part_suffix} -- $local_part
    user = cyrus
    group = mail
    return_output
    log_output
    message_prefix =
    message_suffix =

  # router
  local_user_cyrus:
    driver = accept
    check_local_user
    local_part_suffix = .*
    transport = local_delivery_cyrus

Note the unsetting of "message_prefix" and "message_suffix", and the use of
"return_output" to cause any text written by Cyrus to be returned to the
sender.



                            30. THE SMTP TRANSPORT


The smtp transport delivers messages over TCP/IP connections using the SMTP or
LMTP protocol. The list of hosts to try can either be taken from the address
that is being processed (having been set up by the router), or specified
explicitly for the transport. Timeout and retry processing (see chapter 32) is
applied to each IP address independently.


30.1 Multiple messages on a single connection

The sending of multiple messages over a single TCP/IP connection can arise in
two ways:

 .   If a message contains more than "max_rcpt" (see below) addresses that are
     routed to the same host, more than one copy of the message has to be sent
     to that host. In this situation, multiple copies may be sent in a single
     run of the smtp transport over a single TCP/IP connection. (What Exim
     actually does when it has too many addresses to send in one message also
     depends on the value of the global "remote_max_parallel" option. Details
     are given in section 43.1.)

 .   When a message has been successfully delivered over a TCP/IP connection,
     Exim looks in its hints database to see if there are any other messages
     awaiting a connection to the same host. If there are, a new delivery
     process is started for one of them, and the current TCP/IP connection is
     passed on to it. The new process may in turn send multiple copies and
     possibly create yet another process.

For each copy sent over the same TCP/IP connection, a sequence counter is
incremented, and if it ever gets to the value of "connection_max_messages", no
further messages are sent over that connection.


30.2 Use of the $host variable

At the start of a run of the smtp transport, the values of $host and
$host_address are the name and IP address of the first host on the host list
passed by the router. However, when the transport is about to connect to a
specific host, and while it is connected to that host, $host and $host_address
are set to the values for that host. These are the values that are in force
when the "helo_data", "hosts_try_auth", "interface", "serialize_hosts", and
the various TLS options are expanded.


30.3 Private options for smtp

The private options of the smtp transport are as follows:

allow_localhost                 Type: boolean                  Default: false

    When a host specified in "hosts" or "fallback_hosts" (see below) turns out
    to be the local host, or is listed in "hosts_treat_as_local", delivery is
    deferred by default. However, if "allow_localhost" is set, Exim goes on to
    do the delivery anyway. This should be used only in special cases when the
    configuration ensures that no looping will result (for example, a differ-
    ently configured Exim is listening on the port to which the message is
    sent).

authenticated_sender            Type: string*                  Default: unset

    When Exim has authenticated as a client, this option sets a value for the  |
    AUTH= item on outgoing MAIL commands, overriding any existing              |
    authenticated sender value. If the string expansion is forced to fail, the |
    option is ignored. Other expansion failures cause delivery to be deferred. |
    If the result of expansion is an empty string, that is also ignored.       |
                                                                               |
    If the SMTP session is not authenticated, the expansion of                 |
    "authenticated_sender" still happens (and can cause the delivery to be     |
    deferred if it fails), but no AUTH= item is added to MAIL commands.        |

    This option allows you to use the smtp transport in LMTP mode to deliver
    mail to Cyrus IMAP and provide the proper local part as the 'authenticated
    sender', via a setting such as:

      authenticated_sender = $local_part

    This removes the need for IMAP subfolders to be assigned special ACLs to
    allow direct delivery to those subfolders.

    Because of expected uses such as that just described for Cyrus (when no
    domain is involved), there is no checking on the syntax of the provided
    value.

command_timeout                   Type: time                       Default: 5m

    This sets a timeout for receiving a response to an SMTP command that has
    been sent out. It is also used when waiting for the initial banner line
    from the remote host. Its value must not be zero.

connect_timeout                   Type: time                       Default: 5m

    This sets a timeout for the "connect()" function, which sets up a TCP/IP
    call to a remote host. A setting of zero allows the system timeout
    (typically several minutes) to act. To have any effect, the value of this
    option must be less than the system timeout. However, it has been observed
    that on some systems there is no system timeout, which is why the default
    value for this option is 5 minutes, a value recommended by RFC 1123.


connection_max_messages         Type: integer                    Default: 500

    This controls the maximum number of separate message deliveries that are
    sent over a single TCP/IP connection. If the value is zero, there is no
    limit. For testing purposes, this value can be overridden by the -oB
    command line option.

data_timeout                      Type: time                       Default: 5m

    This sets a timeout for the transmission of each block in the data portion
    of the message. As a result, the overall timeout for a message depends
    on the size of the message. Its value must not be zero. See also
    "final_timeout".

delay_after_cutoff              Type: boolean                   Default: true

    This option controls what happens when all remote IP addresses for a given
    domain have been inaccessible for so long that they have passed their
    retry cutoff times.

    In the default state, if the next retry time has not been reached for any
    of them, the address is bounced without trying any deliveries. In other
    words, Exim delays retrying an IP address after the final cutoff time
    until a new retry time is reached, and can therefore bounce an address
    without ever trying a delivery, when machines have been down for a long
    time. Some people are unhappy at this prospect, so...

    If "delay_after_cutoff" is set false, Exim behaves differently. If all IP
    addresses are past their final cutoff time, Exim tries to deliver to those
    IP addresses that have not been tried since the message arrived. If there
    are none, of if they all fail, the address is bounced. In other words, it
    does not delay when a new message arrives, but immediately tries those
    expired IP addresses that haven't been tried since the message arrived. If
    there is a continuous stream of messages for the dead hosts, unsetting
    "delay_after_cutoff" means that there will be many more attempts to
    deliver to them.

dns_qualify_single              Type: boolean                   Default: true

    If the "hosts" or "fallback_hosts" option is being used, and the
    "gethostbyname" option is false, the RES_DEFNAMES resolver option is set.
    See the "qualify_single" option in chapter 17 for more details.

dns_search_parents              Type: boolean                  Default: false

    If the "hosts" or "fallback_hosts" option is being used, and the
    "gethostbyname" option is false, the RES_DNSRCH resolver option is set.
    See the "search_parents" option in chapter 17 for more details.

fallback_hosts                Type: string list                Default: unset

    String expansion is not applied to this option. The argument must be a
    colon-separated list of host names or IP addresses. Fallback hosts can
    also be specified on routers, which associate them with the addresses
    they process. As for the "hosts" option without "hosts_override",
    "fallback_hosts" specified on the transport is used only if the address
    does not have its own associated fallback host list. Unlike "hosts", a
    setting of "fallback_hosts" on an address is not overridden by
    "hosts_override". However, "hosts_randomize" does apply to fallback host
    lists.

    If Exim is unable to deliver to any of the hosts for a particular address,
    and the errors are not permanent rejections, the address is put on a
    separate transport queue with its host list replaced by the fallback
    hosts, unless the address was routed via MX records and the current host
    was in the original MX list. In that situation, the fallback host list is
    not used.

    Once normal deliveries are complete, the fallback queue is delivered by
    re-running the same transports with the new host lists. If several failing
    addresses have the same fallback hosts (and "max_rcpt" permits it), a
    single copy of the message is sent.

    The resolution of the host names on the fallback list is controlled by the
    "gethostbyname" option, as for the "hosts" option. Fallback hosts apply
    both to cases when the host list comes with the address and when it is
    taken from "hosts". This option provides a 'use a smart host only if
    delivery fails' facility.

final_timeout                     Type: time                      Default: 10m

    This is the timeout that applies while waiting for the response to the
    final line containing just '.' that terminates a message. Its value must
    not be zero.

gethostbyname                   Type: boolean                  Default: false

    If this option is true when the "hosts" and/or "fallback_hosts" options
    are being used, names are looked up using "gethostbyname()" (or
    "getipnodebyname()" when available) instead of using the DNS. Of course,
    that function may in fact use the DNS, but it may also consult other
    sources of information such as /etc/hosts.


helo_data                       Type: string*      Default: $primary_hostname

    The value of this option is expanded, and used as the argument for the
    EHLO or HELO command that starts the outgoing SMTP session.

hosts                         Type: string list*                Default: unset

    Hosts are associated with an address by a router such as dnslookup, which
    finds the hosts by looking up the address domain in the DNS. However,
    addresses can be passed to the smtp transport by any router, and not all
    of them can provide an associated host list. The "hosts" option specifies
    a list of hosts which are used if the address being processed does not
    have any hosts associated with it. The hosts specified by "hosts" are also
    used, whether or not the address has its own hosts, if "hosts_override" is
    set.

    The string is first expanded, before being interpreted as a colon-
    separated list of host names or IP addresses. If the expansion fails,
    delivery is deferred. Unless the failure was caused by the inability to
    complete a lookup, the error is logged to the panic log as well as the
    main log. Host names are looked up either by searching directly for
    address records in the DNS or by calling "gethostbyname()" (or
    "getipnodebyname()" when available), depending on the setting of the
    "gethostbyname" option. When Exim is compiled with IPv6 support, if a host
    that is looked up in the DNS has both IPv4 and IPv6 addresses, both types
    of address are used.

    During delivery, the hosts are tried in order, subject to their retry
    status, unless "hosts_randomize" is set.

hosts_avoid_esmtp              Type: host list*                 Default: unset |
                                                                               |
    This option is for use with broken hosts that announce ESMTP facilities    |
    (for example, PIPELINING) and then fail to implement them properly. When a |
    host matches "hosts_avoid_esmtp", Exim sends HELO rather than EHLO at the  |
    start of the SMTP session. This means that it cannot use any of the ESMTP  |
    facilities such as AUTH, PIPELINING, SIZE, and STARTTLS.                   |

hosts_avoid_tls                Type: host list*                 Default: unset

    Exim will not try to start a TLS session when delivering to any host that
    matches this list. See chapter 37 for details of TLS.

hosts_max_try                   Type: integer                      Default: 5

    This option limits the number of IP addresses that are tried for any one
    delivery, in cases where attempts to connect suffer host errors. The limit
    does not apply, for example, to cases where temporary address errors
    (4"xx" responses to RCPT commands) are received. Section 30.4 describes in
    detail how the value of this option is used.

hosts_nopass_tls               Type: host list*                 Default: unset

    For any host that matches this list, a connection on which a TLS session
    has been started will not be passed to a new delivery process for sending
    another message on the same connection. See section 37.5 for an expla-
    nation of when this might be needed.

hosts_override                  Type: boolean                  Default: false

    If this option is set and the "hosts" option is also set, any hosts that
    are attached to the address are ignored, and instead the hosts specified
    by the "hosts" option are always used. This option does not apply to
    "fallback_hosts".

hosts_randomize                 Type: boolean                  Default: false

    If this option is set, and either the list of hosts is taken from the
    "hosts" or the "fallback_hosts" option, or the hosts supplied by the
    router were not obtained from MX records (this includes fallback hosts
    from the router), and were not randomizied by the router, the order of
    trying the hosts is randomized each time the transport runs. Randomizing
    the order of a host list can be used to do crude load sharing.

    When "hosts_randomize" is true, a host list may be split into groups whose
    order is separately randomized. This makes it possible to set up MX-like
    behaviour. The boundaries between groups are indicated by an item that is
    just "+" in the host list. For example:

      hosts = host1:host2:host3:+:host4:host5

    The order of the first three hosts and the order of the last two hosts is
    randomized for each use, but the first three always end up before the last
    two. If "hosts_randomize" is not set, a "+" item in the list is ignored.


hosts_require_auth             Type: host list*                 Default: unset

    This option provides a list of servers for which authentication must
    succeed before Exim will try to transfer a message. If authentication
    fails for servers which are not in this list, Exim tries to send
    unauthenticated. If authentication fails for one of these servers,
    delivery is deferred. This temporary error is detectable in the retry
    rules, so it can be turned into a hard failure if required. See also
    "hosts_try_auth", and chapter 33 for details of authentication.

hosts_require_tls              Type: host list*                 Default: unset

    Exim will insist on using a TLS session when delivering to any host that
    matches this list. See chapter 37 for details of TLS. Note: This option    |
    affects outgoing mail only. To insist on TLS for incoming messages, use an |
    appropriate ACL.                                                           |


hosts_try_auth                 Type: host list*                 Default: unset

    This option provides a list of servers to which, provided they announce
    authentication support, Exim will attempt to authenticate as a client when
    it connects. If authentication fails, Exim will try to transfer the
    message unauthenticated. See also "hosts_require_auth", and chapter 33 for
    details of authentication.


interface                     Type: string list*                Default: unset

    This option specifies which interface to bind to when making an outgoing
    SMTP call. The variables $host and $host_address refer to the host to
    which a connection is about to be made during the expansion of the string.
    Forced expansion failure, or an empty string result causes the option to
    be ignored. Otherwise, after expansion, the string must be a colon-
    separated list of IP addresses, for example:

      interface = <; 192.168.123.123 ; 3ffe:ffff:836f::fe86:a061

    The first interface of the correct type (IPv4 or IPv6) is used for the
    outgoing connection. If none of them are the correct type, the option is
    ignored. If "interface" is not set, or is ignored, the system's IP
    functions choose which interface to use if the host has more than one.

keepalive                       Type: boolean                   Default: true

    This option controls the setting of SO_KEEPALIVE on outgoing TCP/IP socket
    connections. When set, it causes the kernel to probe idle connections
    periodically, by sending packets with 'old' sequence numbers. The other
    end of the connection should send a acknowledgement if the connection is
    still okay or a reset if the connection has been aborted. The reason for
    doing this is that it has the beneficial effect of freeing up certain
    types of connection that can get stuck when the remote host is discon-
    nected without tidying up the TCP/IP call properly. The keepalive mechan-
    ism takes several hours to detect unreachable hosts.

max_rcpt                        Type: integer                    Default: 100

    This option limits the number of RCPT commands that are sent in a single
    SMTP message transaction. Each set of addresses is treated independently,
    and so can cause parallel connections to the same host if
    "remote_max_parallel" permits this.

multi_domain                    Type: boolean                   Default: true

    When this option is set, the smtp transport can handle a number of
    addresses containing a mixture of different domains provided they all
    resolve to the same list of hosts. Turning the option off restricts the
    transport to handling only one domain at a time. This is useful if you
    want to use $domain in an expansion for the transport, because it is set
    only when there is a single domain involved in a remote delivery.

port                            Type: string*              Default: see below  |
                                                                               |
    This option specifies the TCP/IP port on the server to which Exim          |
    connects. If it begins with a digit it is taken as a port number;          |
    otherwise it is looked up using "getservbyname()". The default value is    |
    normally 'smtp', but if "protocol" is set to 'lmtp', the default is        |
    'lmtp'. If the expansion fails, or if a port number cannot be found,       |
    delivery is deferred.                                                      |

protocol                         Type: string                    Default: smtp

    If this option is set to 'lmtp' instead of 'smtp', the default value for
    the "port" option changes to 'lmtp', and the transport operates the LMTP
    protocol (RFC 2033) instead of SMTP. This protocol is sometimes used for
    local deliveries into closed message stores. Exim also has support for
    running LMTP over a pipe to a local process - see chapter 28.

retry_include_ip_address        Type: boolean                   Default: true

    Exim normally includes both the host name and the IP address in the key it
    constructs for indexing retry data after a temporary delivery failure.
    This means that when one of several IP addresses for a host is failing, it
    gets tried periodically (controlled by the retry rules), but use of the
    other IP addresses is not affected.

    However, in some dialup environments hosts are assigned a different IP
    address each time they connect. In this situation the use of the IP
    address as part of the retry key leads to undesirable behaviour. Setting
    this option false causes Exim to use only the host name. This should
    normally be done on a separate instance of the smtp transport, set up
    specially to handle the dialup hosts.

serialize_hosts                Type: host list*                 Default: unset

    Because Exim operates in a distributed manner, if several messages for the
    same host arrive at around the same time, more than one simultaneous
    connection to the remote host can occur. This is not usually a problem
    except when there is a slow link between the hosts. In that situation it
    may be helpful to restrict Exim to one connection at a time. This can be
    done by setting "serialize_hosts" to match the relevant hosts.

    Exim implements serialization by means of a hints database in which a
    record is written whenever a process connects to one of the restricted
    hosts. The record is deleted when the connection is completed. Obviously
    there is scope for records to get left lying around if there is a system
    or program crash. To guard against this, Exim ignores any records that are
    more than six hours old.

    If you set up this kind of serialization, you should also arrange to
    delete the relevant hints database whenever your system reboots. The names
    of the files start with misc and they are kept in the spool/db directory.
    There may be one or two files, depending on the type of DBM in use. The
    same files are used for ETRN serialization.

size_addition                   Type: integer                   Default: 1024

    If a remote SMTP server indicates that it supports the SIZE option of the
    MAIL command, Exim uses this to pass over the message size at the start of
    an SMTP transaction. It adds the value of "size_addition" to the value it
    sends, to allow for headers and other text that may be added during
    delivery by configuration options or in a transport filter. It may be
    necessary to increase this if a lot of text is added to messages.

    Alternatively, if the value of "size_addition" is set negative, it
    disables the use of the SIZE option altogether.

tls_certificate                 Type: string*                  Default: unset

    The value of this option must be the absolute path to a file which
    contains the client's certificate, for use when sending a message over an
    encrypted connection. The values of $host and $host_address are set to the
    name and address of the server during the expansion. See chapter 37 for
    details of TLS.

    Note: This option must be set if you want Exim to use TLS when sending     |
    messages as a client. The global option of the same name specifies the     |
    certificate for Exim as a server; it is not automatically assumed that the |
    same certificate should be used when Exim is operating as a client.        |

tls_privatekey                  Type: string*                  Default: unset

    The value of this option must be the absolute path to a file which
    contains the client's private key, for use when sending a message over an
    encrypted connection. The values of $host and $host_address are set to the
    name and address of the server during the expansion. If this option is
    unset, the private key is assumed to be in the same file as the
    certificate. See chapter 37 for details of TLS.

tls_require_ciphers             Type: string*                  Default: unset

    The value of this option must be a list of permitted cipher suites, for
    use when setting up an encrypted connection. The values of $host and
    $host_address are set to the name and address of the server during the
    expansion. See chapter 37 for details of TLS; note that this option is
    used in different ways by OpenSSL and GnuTLS (see section 37.1).

tls_tempfail_tryclear           Type: boolean                   Default: true

    When the server host is not in "hosts_require_tls", and there is a problem
    in setting up a TLS session, this option determines whether or not Exim
    should try to deliver the message unencrypted. If it is set false,
    delivery to the current host is deferred; if there are other hosts, they
    are tried. If this option is set true, Exim attempts to deliver
    unencrypted after a 4"xx" response to STARTTLS. Also, if STARTTLS is
    accepted, but the subsequent TLS negotiation fails, Exim closes the
    current connection (because it is in an unknown state), opens a new one to
    the same host, and then tries the delivery in clear.

tls_verify_certificates         Type: string*                  Default: unset

    The value of this option must be the absolute path to a file containing    |
    permitted server certificates, for use when setting up an encrypted        |
    connection. Alternatively, if you are using OpenSSL, you can set           |
    "tls_verify_certificates" to the name of a directory containing certifi-   |
    cate files. This does not work with GnuTLS; the option must be set to the  |
    name of a single file if you are using GnuTLS. The values of $host and     |
    $host_address are set to the name and address of the server during the     |
    expansion of this option. See chapter 37 for details of TLS.               |


30.4 How the value of hosts_max_try is used

The "hosts_max_try" option limits the number of hosts that are tried for a
single delivery. However, despite the term 'host' in its name, the option
actually applies to each IP address independently. In other words, a
multihomed host is treated as several independent hosts, just as it is for
retrying.

Many of the larger ISPs have multiple MX records which often point to
multihomed hosts. As a result, a list of a dozen or more IP addresses may be
created as a result of routing one of these domains.

Trying every single IP address on such a long list does not seem sensible; if
several at the top of the list fail, it is reasonable to assume there is some
problem that is likely to affect all of them. Roughly speaking, the value of
"hosts_max_try" is the maximum number that are tried before deferring the
delivery. However, the logic cannot be quite that simple.

Firstly, IP addresses that are skipped because their retry times have not
arrived do not count, and in addition, addresses that are past their retry
limits are also not counted, even when they are tried. This means that when
some IP addresses are past their retry limits, more than the value of
"hosts_max_retry" may be tried. The reason for this behaviour is to ensure
that all IP addresses are considered before timing out an email address.

Secondly, when the "hosts_max_try" limit is reached, Exim looks down the host
list to see if there is a subsequent host with a different (higher valued) MX.
If there is, that host is used next, and the current IP address is used but
not counted. This behaviour helps in the case of a domain with a retry rule
that hardly ever delays any hosts, as is now explained:

Consider the case of a long list of hosts with one MX value, and a few with a
higher MX value. If "hosts_max_try" is small (the default is 5) only a few
hosts at the top of the list are tried at first. With the default retry rule,
which specifies increasing retry times, the higher MX hosts are eventually
tried when those at the top of the list are skipped because they have not
reached their retry times.

However, it is common practice to put a fixed short retry time on domains for
large ISPs, on the grounds that their servers are rarely down for very long.
Unfortunately, these are exactly the domains that tend to resolve to long
lists of hosts. The short retry time means that the lowest MX hosts are tried
every time. The attempts may be in a different order because of random
sorting, but without the special MX check mentioned about, the higher MX hosts
would never be tried at all because the lower MX hosts are never all past
their retry times.

With the special check, Exim tries least one address from each MX value, even
if the "hosts_max_try" limit has already been reached.



                            31. ADDRESS REWRITING


There are some circumstances in which Exim automatically rewrites domains in
addresses. The two most common are when an address is given without a domain
(referred to as an 'unqualified address') or when an address contains an
abbreviated domain that is expanded by DNS lookup.

Unqualified envelope addresses are accepted only for locally submitted mess-
ages, or messages from hosts that match "sender_unqualified_hosts" or
"recipient_unqualified_hosts", respectively. Unqualified addresses in header
lines are qualified if they are in locally submitted messages, or messages
from hosts that are permitted to send unqualified envelope addresses.
Otherwise, unqualified addresses in header lines are neither qualified nor
rewritten.

One situation in which Exim does not rewrite a domain is when it is the name
of a CNAME record in the DNS. The older RFCs suggest that such a domain should
be rewritten using the 'canonical' name, and some MTAs do this. The new RFCs
do not contain this suggestion.

This chapter is about address rewriting that is explicitly specified in the
configuration. Some people believe that configured rewriting is a Mortal Sin.
Others believe that life is not possible without it. Exim provides the
facility; you do not have to use it.

In general, rewriting addresses from your own system or domain has some
legitimacy. Rewriting other addresses should be done only with great care and
in special circumstances. The author of Exim believes that rewriting should be
used sparingly, and mainly for 'regularizing' addresses in your own domains.
Although it can sometimes be used as a routing tool, this is very strongly
discouraged.

There are two commonly encountered circumstances where rewriting is used, as
illustrated by these examples:

 .   The company whose domain is "hitch.fict.example" has a number of hosts
     that exchange mail with each other behind a firewall, but there is only
     a single gateway to the outer world. The gateway rewrites
     "*.hitch.fict.example" as "hitch.fict.example" when sending mail off-
     site.

 .   A host rewrites the local parts of its own users so that, for example,
     "fp42@hitch.fict.example" becomes "Ford.Prefect@hitch.fict.example".

Configured address rewriting can take place at several different stages of a
message's processing. The main rewriting happens when a message is received,
but it can also happen when a new address is generated during routing (for
example, by aliasing), and when a message is transported.

The rewriting rules that appear in the 'rewrite' section of the configuration
file apply to addresses in incoming messages, and to addresses that are
generated from the envelope recipients by redirection, unless "no_rewrite" is
set on the relevant routers. Roughly speaking, they apply to each address the
first time Exim sees it. These rules operate both on envelope addresses and on
addresses in header lines. Each rule specifies to which types of address it
applies.

At transport time, additional rewriting of addresses in header lines can be
specified by setting the generic "headers_rewrite" option on a transport. This
option contains rules that are identical in form to those in the rewrite
section of the configuration file. In addition, the outgoing envelope sender
can be rewritten by means of the "return_path" transport option. However, it
is not possible to rewrite envelope recipients at transport time.

Rewriting of addresses in header lines applies only to those headers that were
received with the message, and, in the case of transport rewriting, those that
were added by a system filter. That is, it applies only to those headers that
are common to all copies of the message. Header lines that are added by
individual routers or transports (and which are therefore specific to individ-
ual recipient addresses) are not rewritten.

The remainder of this chapter describes the rewriting rules that are used in
the main rewrite section of the configuration file, and also in the generic
"headers_rewrite" option that can be set on any transport.


31.1 Testing the rewriting rules that apply on input

Exim's input rewriting configuration appears in a part of the run time
configuration file headed by 'begin rewrite'. It can be tested by the -brw
command line option. This takes an address (which can be a full RFC 2822
address) as its argument. The output is a list of how the address would be
transformed by the rewriting rules for each of the different places it might
appear in an incoming message, that is, for each different header and for the
envelope sender and recipient fields. For example,

  exim -brw ph10@exim.workshop.example

might produce the output

    sender: Philip.Hazel@exim.workshop.example
      from: Philip.Hazel@exim.workshop.example
        to: ph10@exim.workshop.example
        cc: ph10@exim.workshop.example
       bcc: ph10@exim.workshop.example
  reply-to: Philip.Hazel@exim.workshop.example
  env-from: Philip.Hazel@exim.workshop.example
    env-to: ph10@exim.workshop.example

which shows that rewriting has been set up for that address when used in any
of the source fields, but not when it appears as a recipient address. At the
present time, there is no equivalent way of testing rewriting rules that are
set for a particular transport.


31.2 Rewriting rules

The rewrite section of the configuration file consists of lines of rewriting
rules in the form

  <source pattern>  <replacement>  <flags>

Rewriting rules that are specified for the "headers_rewrite" generic transport
option are given as a colon-separated list. Each item in the list takes the
same form as a line in the main rewriting configuration (except that any
colons must be doubled, of course).

The formats of source patterns and replacement strings are described below.
Each is terminated by white space, unless enclosed in double quotes, in which
case normal quoting conventions apply inside the quotes. The flags are single
characters which may appear in any order. Spaces and tabs between them are
ignored.

For each address that could potentially be rewritten, the rules are scanned in
order, and replacements for the address from earlier rules can themselves be
replaced by later rules (but see the 'q' and 'R' flags).

The order in which addresses are rewritten is undefined, may change between
releases, and must not be relied on, with one exception: when a message is
received, the envelope sender is always rewritten first, before any header
lines are rewritten. For example, the replacement string for a rewrite of an
address in To: must not assume that the message's address in From: has (or has
not) already been rewritten. However, a rewrite of From: may assume that the
envelope sender has already been rewritten.

The variables $local_part and $domain can be used in the replacement string to
refer to the address that is being rewritten. Note that lookup-driven
rewriting can be done by a rule of the form

  *@*   ${lookup ...

where the lookup key uses $1 and $2 or $local_part and $domain to refer to the
address that is being rewritten.


31.3 Rewriting patterns

The source pattern in a rewriting rule is any item which may appear in an
address list (see section 10.16). It is in fact processed as a single-item
address list, which means that it is expanded before being tested against the
address.

Domains in patterns should be given in lower case. Local parts in patterns are
case-sensitive. If you want to do case-insensitive matching of local parts,
you can use a regular expression that starts with "^(?i)".

After matching, the numerical variables $1, $2, etc. may be set, depending on
the type of match which occurred. These can be used in the replacement string
to insert portions of the incoming address. $0 always refers to the complete
incoming address. When a regular expression is used, the numerical variables
are set from its capturing subexpressions. For other types of pattern they are
set as follows:

 .   If a local part or domain starts with an asterisk, the numerical
     variables refer to the character strings matched by asterisks, with $1
     associated with the first asterisk, and $2 with the second, if present.
     For example, if the pattern

       *queen@*.fict.example

     is matched against the address "hearts-queen@wonderland.fict.example"
     then

       $0 = hearts-queen@wonderland.fict.example
       $1 = hearts-
       $2 = wonderland

     Note that if the local part does not start with an asterisk, but the
     domain does, it is $1 that contains the wild part of the domain.

 .   If the domain part of the pattern is a partial lookup, the wild and fixed
     parts of the domain are placed in the next available numerical variables.
     Suppose, for example, that the address "foo@bar.baz.example" is processed
     by a rewriting rule of the form

       *@partial-dbm;/some/dbm/file    <replacement string>

     and the key in the file that matches the domain is "*.baz.example". Then

       $1 = foo
       $2 = bar
       $3 = baz.example

     If the address "foo@baz.example" is looked up, this matches the same
     wildcard file entry, and in this case $2 is set to the empty string, but
     $3 is still set to "baz.example". If a non-wild key is matched in a
     partial lookup, $2 is again set to the empty string and $3 is set to the
     whole domain. For non-partial domain lookups, no numerical variables are
     set.


31.4 Rewriting replacements

If the replacement string for a rule is a single asterisk, addresses that
match the pattern and the flags are not rewritten, and no subsequent rewriting
rules are scanned. For example,

  hatta@lookingglass.fict.example  *  f

specifies that "hatta@lookingglass.fict.example" is never to be rewritten in
From: headers.

If the replacement string is not a single asterisk, it is expanded, and
must yield a fully qualified address. Within the expansion, the variables
$local_part and $domain refer to the address that is being rewritten. Any
letters they contain retain their original case - they are not lower cased.
The numerical variables are set up according to the type of pattern that
matched the address, as described above. If the expansion is forced to fail by
the presence of 'fail' in a conditional or lookup item, rewriting by the
current rule is abandoned, but subsequent rules may take effect. Any other
expansion failure causes the entire rewriting operation to be abandoned, and
an entry written to the panic log.


31.5 Rewriting flags

There are three different kinds of flag that may appear on rewriting rules:

 .   Flags that specify which headers and envelope addresses to rewrite: E, F,
     T, b, c, f, h, r, s, t.

 .   A flag that specifies rewriting at SMTP time: S.

 .   Flags that control the rewriting process: Q, q, R, w.

For rules that are part of the "headers_rewrite" generic transport option, E,
F, T, and S are not permitted.


31.6 Flags specifying which headers and envelope addresses to rewrite

If none of the following flag letters, nor the 'S' flag (see section 31.7) are
present, a main rewriting rule applies to all headers and to both the sender
and recipient fields of the envelope, whereas a transport-time rewriting rule
just applies to all headers. Otherwise, the rewriting rule is skipped unless
the relevant addresses are being processed.

  E       rewrite all envelope fields
  F       rewrite the envelope From field
  T       rewrite the envelope To field
  b       rewrite the Bcc: header
  c       rewrite the Cc: header
  f       rewrite the From: header
  h       rewrite all headers
  r       rewrite the Reply-To: header
  s       rewrite the Sender: header
  t       rewrite the To: header

You should be particularly careful about rewriting Sender: headers, and
restrict this to special known cases in your own domains.


31.7 The SMTP-time rewriting flag

The rewrite flag 'S' specifies a rewrite of incoming envelope addresses at
SMTP time, as soon as an address is received in a MAIL or RCPT command, and
before any other processing; even before syntax checking. The pattern is
required to be a regular expression, and it is matched against the whole of
the data for the command, including any surrounding angle brackets.

This form of rewrite rule allows for the handling of addresses that are not
compliant with RFCs 2821 and 2822 (for example, 'bang paths' in batched SMTP
input). Because the input is not required to be a syntactically valid address,
the variables $local_part and $domain are not available during the expansion
of the replacement string. The result of rewriting replaces the original
address in the MAIL or RCPT command.


31.8 Flags controlling the rewriting process

There are four flags which control the way the rewriting process works. These
take effect only when a rule is invoked, that is, when the address is of the
correct type (matches the flags) and matches the pattern:

 .   If the 'Q' flag is set on a rule, the rewritten address is permitted to
     be an unqualified local part. It is qualified with "qualify_recipient".
     In the absence of 'Q' the rewritten address must always include a domain.

 .   If the 'q' flag is set on a rule, no further rewriting rules are
     considered, even if no rewriting actually takes place because of a 'fail'
     in the expansion. The 'q' flag is not effective if the address is of the
     wrong type (does not match the flags) or does not match the pattern.

 .   The 'R' flag causes a successful rewriting rule to be re-applied to the
     new address, up to ten times. It can be combined with the 'q' flag, to
     stop rewriting once it fails to match (after at least one successful
     rewrite).

 .   When an address in a header is rewritten, the rewriting normally applies
     only to the working part of the address, with any comments and RFC 2822
     'phrase' left unchanged. For example, rewriting might change

       From: Ford Prefect <fp42@restaurant.hitch.fict.example>

     into

       From: Ford Prefect <prefectf@hitch.fict.example>

     Sometimes there is a need to replace the whole address item, and this can
     be done by adding the flag letter 'w' to a rule. If this is set on a rule
     that causes an address in a header line to be rewritten, the entire
     address is replaced, not just the working part. The replacement must be a
     complete RFC 2822 address, including the angle brackets if necessary. If
     text outside angle brackets contains a character whose value is greater
     than 126 or less than 32 (except for tab), the text is encoded according
     to RFC 2047. The character set is taken from "headers_charset", which     |
     defaults to ISO-8859-1.                                                   |

     When the 'w' flag is set on a rule that causes an envelope address to be
     rewritten, all but the working part of the replacement address is
     discarded.


31.9 Rewriting examples

Here is an example of the two common rewriting paradigms:

  *@*.hitch.fict.example  $1@hitch.fict.example
  *@hitch.fict.example    ${lookup{$1}dbm{/etc/realnames}\
                       {$value}fail}@hitch.fict.example bctfrF

Note the use of 'fail' in the lookup expansion in the second rule, forcing the
string expansion to fail if the lookup does not succeed. In this context it
has the effect of leaving the original address unchanged, but Exim goes on to
consider subsequent rewriting rules, if any, because the 'q' flag is not
present in that rule. An alternative to 'fail' would be to supply $1
explicitly, which would cause the rewritten address to be the same as before,
at the cost of a small bit of processing. Not supplying either of these is an
error, since the rewritten address would then contain no local part.

The first example above replaces the domain with a superior, more general
domain. This may not be desirable for certain local parts. If the rule

  root@*.hitch.fict.example  *

were inserted before the first rule, rewriting would be suppressed for the
local part "root" at any domain ending in "hitch.fict.example".

Rewriting can be made conditional on a number of tests, by making use of ${if
in the expansion item. For example, to apply a rewriting rule only to messages
that originate outside the local host:

  *@*.hitch.fict.example  "${if !eq {$sender_host_address}{}\
                           {$1@hitch.fict.example}fail}"

The replacement string is quoted in this example because it contains white
space.

Exim does not handle addresses in the form of 'bang paths'. If it sees such an
address it treats it as an unqualified local part which it qualifies with the
local qualification domain (if the source of the message is local or if the
remote host is permitted to send unqualified addresses). Rewriting can
sometimes be used to handle simple bang paths with a fixed number of
components. For example, the rule

  \N^([^!]+)!(.*)@your.domain.example$\N   $2@$1

rewrites a two-component bang path "host.name!user" as the domain address
"user@host.name". However, there is a security implication in using this as a
global rewriting rule for envelope addresses. It can provide a backdoor method
for using your system as a relay, because the incoming addresses appear to be
local. If the bang path addresses are received via SMTP, it is safer to use
the 'S' flag to rewrite them as they are received, so that relay checking can
be done on the rewritten addresses.



                           32. RETRY CONFIGURATION


The 'retry' section of the run time configuration file contains a list of
retry rules which control how often Exim tries to deliver messages that cannot
be delivered at the first attempt. If there are no retry rules, temporary
errors are treated as permanent. The -brt command line option can be used to
test which retry rule will be used for a given address or domain.

The most common cause of retries is temporary failure to deliver to a remote
host because the host is down, or inaccessible because of a network problem.
Exim's retry processing in this case is applied on a per-host (strictly, per
IP address) basis, not on a per-message basis. Thus, if one message has
recently been delayed, delivery of a new message to the same host is not
immediately tried, but waits for the host's retry time to arrive. If the
"retry_defer" log selector is set, the message 'retry time not reached' is
written to the main log whenever a delivery is skipped for this reason.
Section 43.2 contains more details of the handling of errors during remote
deliveries.

Retry processing applies to routing as well as to delivering, except as
covered in the next paragraph. The retry rules do not distinguish between
these actions. It is not possible, for example, to specify different behaviour
for failures to route the domain "snark.fict.example" and failures to deliver
to the host "snark.fict.example". I didn't think anyone would ever need this
added complication, so did not implement it. However, although they share the
same retry rule, the actual retry times for routing and transporting a given
domain are maintained independently.

When a delivery is not part of a queue run (typically an immediate delivery on
receipt of a message), the routers are always run, and local deliveries are
always attempted, even if retry times are set for them. This makes for better
behaviour if one particular message is causing problems (for example, causing
quota overflow, or provoking an error in a filter file). If such a delivery
suffers a temporary failure, the retry data is updated as normal, and
subsequent delivery attempts from queue runs occur only when the retry time
for the local address is reached.


32.1 Retry rules

Each retry rule occupies one line and consists of three parts, separated by
white space: a pattern, an error name, and a list of retry parameters. The
pattern must be enclosed in double quotes if it contains white space. The
rules are searched in order until one is found whose pattern matches the
failing host or address.

The pattern is any single item that may appear in an address list (see section
10.16). It is in fact processed as a one-item address list, which means that
it is expanded before being tested against the address that has been delayed.
Address list processing treats a plain domain name as if it were preceded by
'*@', which makes it possible for many retry rules to start with just a
domain. For example,

  lookingglass.fict.example        *  F,24h,30m;

provides a rule for any address in the "lookingglass.fict.example" domain,
whereas

  alice@lookingglass.fict.example  *  F,24h,30m;

applies only to temporary failures involving the local part "alice". In        |
practice, almost all rules start with a domain name pattern without a local    |
part.                                                                          |

Warning: If you use a regular expression in a routing rule, it must match a
complete address, not just a domain, because that is how regular expressions
work in address lists.

  ^\Nxyz\d+\.abc\.example$\N        *  G,1h,10m,2     "Wrong"
  ^\N[^@]+@xyz\d+\.abc\.example$\N  *  G,1h,10m,2     "Right"

                                                                               |
32.2 Choosing which retry rule to use                                          |
                                                                               |
When Exim is looking for a retry rule after a routing attempt has failed (for  |
example, after a DNS timeout), each line in the retry configuration is tested  |
against the complete address only if "retry_use_local_part" is set for the     |
router. Otherwise, only the domain is used, except when matching against a     |
regular expression, when the local part of the address is replaced with '*'. A |
domain on its own can match a domain pattern, or a pattern that starts with    |
'*@'. By default, "retry_use_local_part" is true for routers where             |
"check_local_user" is true, and false for other routers.                       |
                                                                               |
Similarly, when Exim is looking for a retry rule after a local delivery has    |
failed (for example, after a mailbox full error), each line in the retry       |
configuration is tested against the complete address only if                   |
"retry_use_local_part" is set for the transport (it defaults true for all      |
local transports).                                                             |
                                                                               |
When looking for a retry rule after a remote delivery attempt has failed (for  |
example, a connection timeout), each line in the retry configuration is        |
checked twice. First, the name of the remote host is used as a domain name     |
(preceded by '*@' when matching a regular expression). If this does not match  |
the line, the domain from the email address is tried in a simlar fashion. For  |
example, suppose the MX records for "a.b.c.example" are

  a.b.c.example  MX  5  x.y.z.example
                 MX  6  p.q.r.example
                 MX  7  m.n.o.example

and the retry rules are

  p.q.r.example    *      F,24h,30m;
  a.b.c.example    *      F,4d,45m;

and a delivery to the host "x.y.z.example" fails. Exim scans the retry rules.
The first rule matches neither the host nor the domain, so Exim looks at the
second rule. This does not match the host, but it does match the domain, so it
is used to calculate the retry time for the host "x.y.z.example". Meanwhile,
Exim tries to deliver to "p.q.r.example". If this fails, the first retry rule
is used, because it matches the host.

In other words, failures to deliver to host "p.q.r.example" use the first rule
to determine retry times, but for all the other hosts for the domain
"a.b.c.example", the second rule is used. The second rule is also used if
routing to "a.b.c.example" suffers a temporary failure.


32.3 Retry rules for specific errors

The second field in a retry rule is the name of a particular error, or an
asterisk, which matches any error. The errors that can be tested for are:

     "auth_failed": authentication failed when trying to send to a host in the
     "hosts_require_auth" list in an smtp transport

     "refused_MX": connection refused from a host obtained from an MX record

     "refused_A": connection refused from a host not obtained from an MX
     record

     "refused": any connection refusal

     "timeout_connect_MX": connection timeout from a host obtained from an MX  |
     record                                                                    |
                                                                               |
     "timeout_connect_A": connection timeout from a host not obtained from an  |
     MX record                                                                 |
                                                                               |
     "timeout_connect": any connection timeout                                 |
                                                                               |
     "timeout_MX": any timeout from a host obtained from an MX record          |
                                                                               |
     "timeout_A": any timeout from a host not obtained from an MX record       |

     "timeout": any timeout

     "quota": quota exceeded in local delivery by appendfile

     "quota_"<time>: quota exceeded in local delivery, and the mailbox has not
     been read for <time>. For example, "quota_4d" applies to a quota error
     when the mailbox has not been read for four days. Warning: this applies
     only when the mailbox consists of a single file whose access time can be
     checked. If you are using multi-file mailboxes, a retry rule that uses
     this type of error field is never matched.

The quota errors apply both to system-enforced quotas and to Exim's own quota
mechanism in the appendfile transport. The "quota" error also applies when a
local delivery is deferred because a partition is full (the ENOSPC error).


32.4 Retry rule parameters

The third field in a retry rule is a sequence of retry parameter sets,
separated by semicolons. Each set consists of

  <letter>,<cutoff time>,<arguments>

The letter identifies the algorithm for computing a new retry time; the cutoff
time is the time beyond which this algorithm no longer applies, and the
arguments vary the algorithm's action. The cutoff time is measured from the
time that the first failure for the domain (combined with the local part if
relevant) was detected, not from the time the message was received. The
available algorithms are:

 .   "F": retry at fixed intervals. There is a single time parameter specify-
     ing the interval.

 .   "G": retry at geometrically increasing intervals. The first argument
     specifies a starting value for the interval, and the second a multiplier,
     which is used to increase the size of the interval at each retry.

When computing the next retry time, the algorithm definitions are scanned in
order until one whose cutoff time has not yet passed is reached. This is then
used to compute a new retry time that is later than the current time. In the
case of fixed interval retries, this simply means adding the interval to the
current time. For geometrically increasing intervals, retry intervals are
computed from the rule's parameters until one that is greater than the
previous interval is found. The main configuration variable
"retry_interval_max" limits the maximum interval between retries.

A single remote domain may have a number of hosts associated with it, and each
host may have more than one IP address. Retry algorithms are selected on the
basis of the domain name, but are applied to each IP address independently.
If, for example, a host has two IP addresses and one is unusable, Exim will
generate retry times for it and will not try to use it until its next retry
time comes. Thus the good IP address is likely to be tried first most of the
time.

Retry times are hints rather than promises. Exim does not make any attempt to
run deliveries exactly at the computed times. Instead, a queue runner process
starts delivery processes for delayed messages periodically, and these attempt
new deliveries only for those addresses that have passed their next retry
time. If a new message arrives for a deferred address, an immediate delivery
attempt occurs only if the address has passed its retry time. In the absence
of new messages, the minimum time between retries is the interval between
queue runner processes. There is not much point in setting retry times of five
minutes if your queue runners happen only once an hour, unless there are a
significant number of incoming messages (which might be the case on a system
that is sending everything to a smart host, for example).

The data in the retry hints database can be inspected by using the
"exim_dumpdb" or "exim_fixdb" utility programs (see chapter 46). The latter
utility can also be used to change the data. The "exinext" utility script can
be used to find out what the next retry times are for the hosts associated
with a particular mail domain, and also for local deliveries that have been
deferred.


32.5 Retry rule examples

Here are some example retry rules:

  alice@wonderland.fict.example quota_5d  F,7d,3h
  wonderland.fict.example       quota_5d
  wonderland.fict.example       *         F,1h,15m; G,2d,1h,2;
  lookingglass.fict.example     *         F,24h,30m;
  *                          refused_A F,2h,20m;
  *                          *         F,2h,15m; G,16h,1h,1.5; F,5d,8h

The first rule sets up special handling for mail to
"alice@wonderland.fict.example" when there is an over-quota error and the
mailbox has not been read for at least 5 days. Retries continue every three
hours for 7 days. The second rule handles over-quota errors for all other
local parts at "wonderland.fict.example"; the absence of a local part has the
same effect as supplying '*@'. As no retry algorithms are supplied, messages
that fail are bounced immediately if the mailbox has not been read for at
least 5 days.

The third rule handles all other errors at "wonderland.fict.example"; retries
happen every 15 minutes for an hour, then with geometrically increasing
intervals until two days have passed since a delivery first failed. After the
first hour there is a delay of one hour, then two hours, then four hours, and
so on (this is a rather extreme example).

The fourth rule controls retries for the domain "lookingglass.fict.example".
They happen every 30 minutes for 24 hours only. The remaining two rules handle
all other domains, with special action for connection refusal from hosts that
were not obtained from an MX record.

The final rule in a retry configuration should always have asterisks in the
first two fields so as to provide a general catch-all for any addresses that
do not have their own special handling. This example tries every 15 minutes
for 2 hours, then with intervals starting at one hour and increasing by a
factor of 1.5 up to 16 hours, then every 8 hours up to 5 days.


32.6 Timeout of retry data

Exim timestamps the data that it writes to its retry hints database. When it
consults the data during a delivery it ignores any that is older than the
value set in "retry_data_expire" (default 7 days). If, for example, a host
hasn't been tried for 7 days, Exim will try to deliver to it immediately a
message arrives, and if that fails, it will calculate a retry time as if it
were failing for the first time.

This improves the behaviour for messages routed to rarely-used hosts such as
MX backups. If such a host was down at one time, and happens to be down again
when Exim tries a month later, using the old retry data would imply that it
had been down all the time, which is not a justified assumption.

If a host really is permanently dead, this behaviour causes a burst of retries
every now and again, but only if messages routed to it are rare. It there is a
message at least once every 7 days the retry data never expires.


32.7 Long-term failures

Special processing happens when an email address has been failing for so long
that the cutoff time for the last algorithm is reached. For example, using the
default retry rule:

  * * F,2h,15m; G,16h,1h,1.5; F,4d,6h

the cutoff time is four days. Reaching the retry cutoff is independent of how
long any specific message has been failing; it is the length of continuous
failure for the recipient address that counts.

When the cutoff time is reached for a local delivery, or for all the IP
addresses associated with a remote delivery, a subsequent delivery failure
causes Exim to give up on the address, and a bounce message is generated. In
order to cater for new messages that use the failing address, a next retry
time is still computed from the final algorithm, and is used as follows:

For local deliveries, one delivery attempt is always made for any subsequent
messages. If this delivery fails, the address fails immediately. The post-
cutoff retry time is not used.

If the delivery is remote, there are two possibilities, controlled by the
"delay_after_cutoff" option of the smtp transport. The option is true by
default and in that case:

     Until the post-cutoff retry time for one of the IP addresses is reached,
     the failing email address is bounced immediately, without a delivery
     attempt taking place. After that time, one new delivery attempt is made
     to those IP addresses that are past their retry times, and if that still
     fails, the address is bounced and new retry times are computed.

In other words, when all the hosts for a given email address have been failing
for a long time, Exim bounces rather then defers until one of the hosts' retry
times is reached. Then it tries once, and bounces if that attempt fails. This
behaviour ensures that few resources are wasted in repeatedly trying to
deliver to a broken destination, but if the host does recover, Exim will
eventually notice.

If "delay_after_cutoff" is set false, Exim behaves differently. If all IP
addresses are past their final cutoff time, Exim tries to deliver to those IP
addresses that have not been tried since the message arrived. If there are no
suitable IP addresses, or if they all fail, the address is bounced. In other
words, it does not delay when a new message arrives, but tries the expired
addresses immediately, unless they have been tried since the message arrived.
If there is a continuous stream of messages for the failing domains, setting
"delay_after_cutoff" false means that there will be many more attempts to
deliver to permanently failing IP addresses than when "delay_after_cutoff" is
true.


32.8 Ultimate address timeout

An additional rule is needed to cope with cases where a host is intermittently
available, or when a message has some attribute that prevents its delivery
when others to the same address get through. In this situation, because some
messages are successfully delivered, the 'retry clock' for the address keeps
getting restarted, and so a message could remain on the queue for ever. To
prevent this, if a message has been on the queue for longer than the cutoff
time of any applicable retry rule for a given address, a delivery is attempted
for that address, even if it is not yet time, and if this delivery fails, the
address is timed out. A new retry time is not computed in this case, so that
other messages for the same address are considered immediately.



                           33. SMTP AUTHENTICATION


The 'authenticators' section of Exim's run time configuration is concerned
with SMTP authentication. This facility is an extension to the SMTP protocol,
described in RFC 2554, which allows a client SMTP host to authenticate itself
to a server. This is a common way for a server to recognize clients that are
permitted to use it as a relay. SMTP authentication is not of relevance to the
transfer of mail between servers that have no managerial connection with each
other.

Very briefly, the way SMTP authentication works is as follows:

 .   The server advertises a number of authentication "mechanisms" in response
     to the client's EHLO command.

 .   The client issues an AUTH command, naming a specific mechanism. The
     command may, optionally, contain some authentication data.

 .   The server may issue one or more "challenges", to which the client must
     send appropriate responses. In simple authentication mechanisms, the
     challenges are just prompts for user names and passwords. The server does
     not have to issue any challenges - in some mechanisms the relevant data
     may all be transmitted with the AUTH command.

 .   The server either accepts or denies authentication.

 .   If authentication succeeds, the client may optionally make use of the
     AUTH option on the MAIL command to pass an authenticated sender in
     subsequent mail transactions. Authentication lasts for the remainder of
     the SMTP connection.

 .   If authentication fails, the client may give up, or it may try a
     different authentication mechanism, or it may try transferring mail over
     the unauthenticated connection.

If you are setting up a client, and want to know which authentication
mechanisms the server supports, you can use Telnet to connect to port 25 (the
SMTP port) on the server, and issue an EHLO command. The response to this
includes the list of supported mechanisms. For example:

  $ telnet server.example 25
  Trying 192.168.34.25...
  Connected to server.example.
  Escape character is '^]'.
  220 server.example ESMTP Exim 4.20 ...
  ehlo client.example
  250-server.example Hello client.example [10.8.4.5]
  250-SIZE 52428800
  250-PIPELINING
  250-AUTH PLAIN
  250 HELP

The second-last line of this example output shows that the server supports
authentication using the PLAIN mechanism. In Exim, the different
authentication mechanisms are configured by specifying "authenticator"
drivers. Like the routers and transports, which authenticators are included in
the binary is controlled by build-time definitions. The following are cur-
rently available, included by setting

  AUTH_CRAM_MD5=yes
  AUTH_PLAINTEXT=yes
  AUTH_SPA=yes

in Local/Makefile, respectively. The first of these supports the CRAM-MD5
authentication mechanism (RFC 2195), and the second can be configured to
support the PLAIN authentication mechanism (RFC 2595) or the LOGIN mechanism,
which is not formally documented, but used by several MUAs. The third
authenticator supports Microsoft's "Secure Password Authentication" mechanism.

The authenticators are configured using the same syntax as other drivers (see
section 6.16). If no authenticators are required, no authentication section
need be present in the configuration file. Each authenticator can in principle
have both server and client functions. When Exim is receiving SMTP mail, it is
acting as a server; when it is sending out messages over SMTP, it is acting as
a client. Authenticator configuration options are provided for use in both
these circumstances.

To make it clear which options apply to which situation, the prefixes
"server_" and "client_" are used on option names that are specific to either
the server or the client function, respectively. Server and client functions
are disabled if none of their options are set. If an authenticator is to be
used for both server and client functions, a single definition, using both
sets of options, is required. For example:

  cram:
    driver = cram_md5
    public_name = CRAM-MD5
    server_secret = ${if eq{$1}{ph10}{secret1}fail}
    client_name = ph10
    client_secret = secret2

The "server_" option is used when Exim is acting as a server, and the
"client_" options when it is acting as a client.

Descriptions of the individual authenticators are given in subsequent chap-
ters. The remainder of this chapter covers the generic options for the
authenticators, followed by general discussion of the way authentication works
in Exim.


33.1 Generic options for authenticators

driver                           Type: string                   Default: unset

    This option must always be set. It specifies which of the available
    authenticators is to be used.

public_name                      Type: string                   Default: unset

    This option specifies the name of the authentication mechanism that the
    driver implements, and by which it is known to the outside world. These
    names should contain only upper case letters, digits, underscores, and
    hyphens (RFC 2222), but Exim in fact matches them caselessly. If
    "public_name" is not set, it defaults to the driver's instance name.

server_advertise_condition      Type: string*                  Default: unset

    When a server is about to advertise an authentication mechanism, the
    condition is expanded. If it yields the empty string, '0', 'no', or
    'false', the mechanism is not advertised. If the expansion fails, the      |
    mechanism is not advertised. If the failure was not forced, and was not    |
    caused by a lookup defer, the incident is logged. See section 33.3 below   |
    for further discussion.

server_debug_print              Type: string*                  Default: unset

    If this option is set and authentication debugging is enabled (see the -d
    command line option), the string is expanded and included in the debugging
    output when the authenticator is run as a server. This can help with
    checking out the values of variables. If expansion of the string fails,    |
    the error message is written to the debugging output, and Exim carries on  |
    processing.                                                                |

server_set_id                   Type: string*                  Default: unset

    When an Exim server successfully authenticates a client, this string is
    expanded using data from the authentication, and preserved for any
    incoming messages in the variable $authenticated_id. It is also included
    in the log lines for incoming messages. For example, a user/password
    authenticator configuration might preserve the user name that was used to
    authenticate, and refer to it subsequently during delivery of the message. |
    If expansion fails, the option is ignored.                                 |

server_mail_auth_condition      Type: string*                  Default: unset

    This option allows a server to discard authenticated sender addresses
    supplied as part of MAIL commands in SMTP connections that are
    authenticated by the driver on which "server_mail_auth_condition" is set.
    The option is not used as part of the authentication process; instead its
    (unexpanded) value is remembered for later use. How it is used is          |
    described in the following section.                                        |

                                                                               |
33.2 The AUTH parameter on MAIL commands                                       |
                                                                               |
When a client supplied an AUTH= item on a MAIL command, Exim applies the       |
following checks before accepting it as the authenticated sender of the        |
message:                                                                       |
                                                                               |
 .   If the connection is not using extended SMTP (that is, HELO was used      |
     rather than EHLO), the use of AUTH= is a syntax error.                    |
                                                                               |
 .   If the value of the AUTH= parameter is '<>', it is ignored.               |
                                                                               |
 .   If "acl_smtp_mailauth" is defined, the ACL it specifies is run. While it  |
     is running, the value of $authenticated_sender is set to the value        |
     obtained from the AUTH= parameter. If the ACL does not yield 'accept',    |
     the value of $authenticated_sender is deleted. The "acl_smtp_mailauth"    |
     ACL may not return 'drop' or 'discard'. If it defers, a temporary error   |
     code (451) is given for the MAIL command.                                 |
                                                                               |
 .   If "acl_smtp_mailauth" is not defined, the value of the AUTH= parameter   |
     is accepted and placed in $authenticated_sender only if the client has    |
     authenticated.                                                            |
                                                                               |
 .   If the AUTH= value was accepted by either of the two previous rules, and  |
     the client has authenticated, and the authenticator has a setting for the |
     "server_mail_auth_condition", the condition is checked at this point. The |
     valued that was saved from the authenticator is expanded. If the          |
     expansion fails, or yields an empty string, '0', 'no', or 'false', the    |
     value of $authenticated_sender is deleted. If the expansion yields any    |
     other value, the value of $authenticated_sender is retained and passed on |
     with the message.                                                         |
                                                                               |
When $authenticated_sender is set for a message, it is passed on to other      |
hosts to which Exim authenticates as a client. Do not confuse this value with  |
$authenticated_id, which is a string obtained from the authentication process, |
and which is not usually a complete email address.                             |
                                                                               |
Whenever an AUTH= value is ignored, the incident is logged. The ACL for MAIL,  |
if defined, is run after AUTH= is accepted or ignored. It can therefore        |
make use of $authenticated_sender. The converse is not true: the value of      |
$sender_address is not yet set up when the "acl_smtp_mailauth" ACL is run.     |


33.3 Authentication on an Exim server

When Exim receives an EHLO command, it advertises the public names of those
authenticators that are configured as servers, subject to the following
conditions:

 .   The client host must match "auth_advertise_hosts" (default *).

 .   It the "server_advertise_condition" option is set, its expansion must not
     yield the empty string, '0', 'no', or 'false'.

The order in which the authenticators are defined controls the order in which
the mechanisms are advertised.

Some mail clients (for example, some versions of Netscape) require the user to
provide a name and password for authentication whenever AUTH is advertised,
even though authentication may not in fact be needed (for example, Exim may be
set up to allow unconditional relaying from the client by an IP address
check). You can make such clients more friendly by not advertising AUTH to
them. For example, if clients on the 10.9.8.0/24 network are permitted (by the
ACL that runs for RCPT) to relay without authentication, you should set

  auth_advertise_hosts = ! 10.9.8.0/24

so that no authentication mechanisms are advertised to them.

The "server_advertise_condition" controls the advertisement of individual
authentication mechanisms. For example, it can be used to restrict the
advertisement of a patricular mechanism to encrypted connections, by a setting
such as:

  server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}}

If the session is encrypted, $tls_cipher is not empty, and so the expansion
yields 'yes', which allows the advertisement to happen.

When an Exim server receives an AUTH command from a client, it rejects it
immediately if AUTH was not advertised in response to an earlier EHLO command.
This is the case if

 .   The client host does not match "auth_advertise_hosts"; or

 .   No authenticators are configured with server options; or

 .   Expansion of "server_advertise_condition" blocked the advertising of all
     the server authenticators.

Otherwise, Exim runs the ACL specified by "acl_smtp_auth" in order to decide
whether to accept the command. If "acl_smtp_auth" is not set, AUTH is accepted
from any client host.

If AUTH is not rejected by the ACL, Exim searches its configuration for a
server authentication mechanism that was advertised in response to EHLO and
that matches the one named in the AUTH command. If it finds one, it runs the
appropriate authentication protocol, and authentication either succeeds or
fails. If there is no matching advertised mechanism, the AUTH command is
rejected with a 504 error.

When a message is received from an authenticated host, the value of
$received_protocol is set to 'asmtp' instead of 'esmtp', and
$sender_host_authenticated contains the name (not the public name) of the
authenticator driver that successfully authenticated the client from which the
message was received. This variable is empty if there was no successful
authentication.


33.4 Testing server authentication

Exim's -bh option can be useful for testing server authentication configur-
ations. The data for the AUTH command has to be sent using base64 encoding. A
quick way to produce such data for testing is the following Perl script:

  use MIME::Base64;
  printf ("%s", encode_base64(eval "\"$ARGV[0]\""));

This interprets its argument as a Perl string, and then encodes it. The
interpretation as a Perl string allows binary zeros, which are required for
some kinds of authentication, to be included in the data. For example, a
command line to run this script on such data might be

  encode '\0user\0password'

Note the use of single quotes to prevent the shell interpreting the
backslashes, so that they can be interpreted by Perl to specify characters
whose code value is zero.

Warning 1: If either of the user or password strings starts with an octal
digit, you must use three zeros instead of one after the leading backslash. If
you do not, the octal digit that starts your string will be incorrectly
interpreted as part of the code for the first character.

Warning 2: If there are characters in the strings that Perl interprets
specially, you must use a Perl escape to prevent them being misinterpreted.
For example, a command such as

  encode '\0user@domain.com\0pas$$word'

gives an incorrect answer because of the unescaped '@' and '$' characters.

If you have the "mimencode" command installed, another way to do produce
base64-encoded strings is to run the command

  echo -e -n `\0user\0password' | mimencode

The -e option of "echo" enables the interpretation of backslash escapes in the
argument, and the -n option specifies no newline at the end of its output.
However, not all versions of "echo" recognize these options, so you should
check your version before relying on this suggestion.


33.5 Authentication by an Exim client

The smtp transport has two options called "hosts_require_auth" and
"hosts_try_auth". When the smtp transport connects to a server that announces
support for authentication, and the host matches an entry in either of these
options, Exim (as a client) tries to authenticate as follows:

 .   For each authenticator that is configured as a client, it searches the
     authentication mechanisms announced by the server for one whose name
     matches the public name of the authenticator.

 .   When it finds one that matches, it runs the authenticator's client code.
     The variables $host and $host_address are available for any string
     expansions that the client might do. They are set to the server's name
     and IP address. If any expansion is forced to fail, the authentication
     attempt is abandoned, and Exim moves on to the next authenticator.
     Otherwise an expansion failure causes delivery to be deferred.

 .   If the result of the authentication attempt is a temporary error or a
     timeout, Exim abandons trying to send the message to the host for the
     moment. It will try again later. If there are any backup hosts available,
     they are tried in the usual way.

 .   If the response to authentication is a permanent error (5xx code), Exim
     carries on searching the list of authenticators and tries another one if
     possible. If all authentication attempts give permanent errors, or if
     there are no attempts because no mechanisms match (or option expansions
     force failure), what happens depends on whether the host matches
     "hosts_require_auth" or "hosts_try_auth". In the first case, a temporary
     error is generated, and delivery is deferred. The error can be detected
     in the retry rules, and thereby turned into a permanent error if you
     wish. In the second case, Exim tries to deliver the message
     unauthenticated.

When Exim has authenticated itself to a remote server, it adds the AUTH
parameter to the MAIL commands it sends, if it has an authenticated sender for
the message. If the message came from a remote host, the authenticated sender
is the one that was receiving on an incoming MAIL command, provided that the
incoming connection was authenticated and the "server_mail_auth" condition
allowed the authenticated sender to be retained. If a local process calls Exim
to send a message, the sender address that is built from the login name
and "qualify_domain" is treated as authenticated. However, if the
"authenticated_sender" option is set on the smtp transport, it overrides the
authenticated sender that was received with the message.



                       34. THE PLAINTEXT AUTHENTICATOR


The plaintext authenticator can be configured to support the PLAIN and LOGIN
authentication mechanisms, both of which transfer authentication data as plain
(unencrypted) text (though base64 encoded). The use of plain text is a
security risk. If you use one of these mechanisms without also making use of
SMTP encryption (see chapter 37) you should not use the same passwords for
SMTP connections as you do for login accounts.


34.1 Using plaintext in a server

When running as a server, plaintext performs the authentication test by
expanding a string. It has the following options:

server_prompts                  Type: string*                  Default: unset

    The contents of this option, after expansion, must be a colon-separated
    list of prompt strings. If expansion fails, a temporary authentication     |
    rejection is given.                                                        |

server_condition                Type: string*                  Default: unset

    This option must be set in order to configure the driver as a server. Its
    use is described below.

The data sent by the client with the AUTH command, or in response to
subsequent prompts, is base64 encoded, and so may contain any byte values when
decoded. If any data is supplied with the command, it is treated as a list of
NUL-separated strings which are placed in the expansion variables $1, $2, etc.
If there are more strings in "server_prompts" than the number of strings
supplied with the AUTH command, the remaining prompts are used to obtain more
data. Each response from the client may be a list of NUL-separated strings.

Once a sufficient number of data strings have been received,
"server_condition" is expanded. If the expansion is forced to fail,
authentication fails. Any other expansion failure causes a temporary error
code to be returned. If the result of a successful expansion is an empty
string, '0', 'no', or 'false', authentication fails. If the result of the
expansion is '1', 'yes', or 'true', authentication succeeds and the generic
"server_set_id" option is expanded and saved in $authenticated_id. For any
other result, a temporary error code is returned, with the expanded string as
the error text.

Warning: If you use a lookup in the expansion to find the user's password, be
sure to make the authentication fail if the user is unknown. There are good
and bad examples at the end of the next section.


34.2 The PLAIN authentication mechanism

The PLAIN authentication mechanism (RFC 2595) specifies that three strings be
sent as one item of data (that is, one combined string containing two NUL
separators). The data is sent either as part of the AUTH command, or
subsequently in response to an empty prompt from the server.

The second and third strings are a user name and a corresponding password.
Using a single fixed user name and password as an example, this could be
configured as follows:

  fixed_plain:
    driver = plaintext
    public_name = PLAIN
    server_prompts = :
    server_condition = \
      ${if and {{eq{$2}{ph10}}{eq{$3}{secret}}}{yes}{no}}
    server_set_id = $2

The "server_prompts" setting specifies a single, empty prompt (empty items at
the end of a string list are ignored). If all the data comes as part of the
AUTH command, as is commonly the case, the prompt is not used. This
authenticator is advertised in the response to EHLO as

  250-AUTH PLAIN

and a client host can authenticate itself by sending the command

  AUTH PLAIN AHBoMTAAc2VjcmV0

As this contains three strings (more than the number of prompts), no further
data is required from the client. Alternatively, the client may just send

  AUTH PLAIN

to initiate authentication, in which case the server replies with an empty
prompt. The client must respond with the combined data string.

The data string is base64 encoded, as required by the RFC. This example, when
decoded, is '<NUL>ph10<NUL>secret', where <NUL> represents a zero byte. This
is split up into three strings, the first of which is empty. The condition
checks that the second two are 'ph10' and 'secret' respectively.

A more sophisticated instance of this authenticator could make use of the user
name in $2 to look up a password in a file or database, and maybe do an
encrypted comparison (see "crypteq" in chapter 11). Here is a example of this
approach, where the passwords are looked up in a DBM file. Warning: This is an
incorrect example:

  server_condition = \
    ${if eq{$3}{${lookup{$2}dbm{/etc/authpwd}}}{yes}{no}}

The expansion uses the user name ($2) as the key to look up a password, which
it then compares to the supplied password ($3). Why is this example incorrect?
It works fine for existing users, but consider what happens if a non-existent
user name is given. The lookup fails, but as no success/failure strings are
given for the lookup, it yields an empty string. Thus, to defeat the
authentication, all a client has to do is to supply a non-existent user name
and an empty password. The correct way of writing this test is:

  server_condition = ${lookup{$2}dbm{/etc/authpwd}\
    {${if eq{$value}{$3}{yes}{no}}}{no}}

In this case, if the lookup succeeds, the result is checked; if the lookup
fails, authentication fails. If "crypteq" is being used instead of "eq", the
first example is in fact safe, because "crypteq" always fails if its second
argument is empty. However, the second way of writing the test makes the logic
clearer.


34.3 The LOGIN authentication mechanism

The LOGIN authentication mechanism is not documented in any RFC, but is in use
in a number of programs. No data is sent with the AUTH command. Instead, a
user name and password are supplied separately, in response to prompts. The
plaintext authenticator can be configured to support this as in this example:

  fixed_login:
    driver = plaintext
    public_name = LOGIN
    server_prompts = User Name : Password
    server_condition = \
      ${if and {{eq{$1}{ph10}}{eq{$2}{secret}}}{yes}{no}}
    server_set_id = $1

Because of the way plaintext operates, this authenticator accepts data
supplied with the AUTH command (in contravention of the specification of
LOGIN), but if the client does not supply it (as is the case for LOGIN
clients), the prompt strings are used to obtain two data items.

Some clients are very particular about the precise text of the prompts. For
example, Outlook Express is reported to recognize only 'Username:' and
'Password:'. Here is an example of a LOGIN authenticator which uses those
strings, and which uses the "ldapauth" expansion condition to check the user
name and password by binding to an LDAP server:

  login:
    driver = plaintext
    public_name = LOGIN
    server_prompts = Username:: : Password::
    server_condition = ${if ldapauth \
      {user="cn=${quote_ldap_dn:$1},ou=people,o=example.org" \
      pass=${quote:$2} \
      ldap://ldap.example.org/}{yes}{no}}
    server_set_id = uid=$1,ou=people,o=example.org

Note the use of the "quote_ldap_dn" operator to correctly quote the DN for
authentication. However, the basic "quote" operator, rather than any of the
LDAP quoting operators, is the correct one to use for the password, because
quoting is needed only to make the password conform to the Exim syntax. At the
LDAP level, the password is an uninterpreted string.


34.4 Support for different kinds of authentication

A number of string expansion features are provided for the purpose of
interfacing to different ways of user authentication. These include checking
traditionally encrypted passwords from /etc/passwd (or equivalent), PAM,
Radius, "ldapauth", and "pwcheck". For details see section 11.6.


34.5 Using plaintext in a client

The plaintext authenticator has just one client option:

client_send                     Type: string*                  Default: unset

    The string is a colon-separated list of authentication data strings. Each
    string is independently expanded before being sent to the server. The
    first string is sent with the AUTH command; any more strings are sent in
    response to prompts from the server.

    Note: you cannot use expansion to create multiple strings, because         |
    splitting takes priority and happens first.                                |

    Because the PLAIN authentication mechanism requires NUL (zero) bytes in
    the data, further processing is applied to each string before it is sent.
    If there are any single circumflex characters in the string, they are
    converted to NULs. Should an actual circumflex be required as data, it
    must be doubled in the string.

This is an example of a client configuration that implements the PLAIN
authentication mechanism with a fixed user name and password:

  fixed_plain:
    driver = plaintext
    public_name = PLAIN
    client_send = ^ph10^secret

The lack of colons means that the entire text is sent with the AUTH command,
with the circumflex characters converted to NULs. A similar example that uses
the LOGIN mechanism is:

  fixed_login:
    driver = plaintext
    public_name = LOGIN
    client_send = : ph10 : secret

The initial colon means that the first string is empty, so no data is sent
with the AUTH command itself. The remaining strings are sent in response to
prompts.



                        35. THE CRAM_MD5 AUTHENTICATOR


The CRAM-MD5 authentication mechanism is described in RFC 2195. The server
sends a challenge string to the client, and the response consists of a user
name and the CRAM-MD5 digest of the challenge string combined with a secret
string (password) which is known to both server and client. Thus, the secret
is not sent over the network as plain text, which makes this authenticator
more secure than plaintext. However, the downside is that the secret has to be
available in plain text at either end.


35.1 Using cram_md5 as a server

This authenticator has one server option, which must be set to configure the
authenticator as a server:

server_secret                   Type: string*                  Default: unset

    When the server receives the client's response, the user name is placed in
    the expansion variable $1, and "server_secret" is expanded to obtain the
    password for that user. The server then computes the CRAM-MD5 digest that
    the client should have sent, and checks that it received the correct
    string. If the expansion of "server_secret" is forced to fail,
    authentication fails. If the expansion fails for some other reason, a
    temporary error code is returned to the client.

For example, the following authenticator checks that the user name given by
the client is 'ph10', and if so, uses 'secret' as the password. For any other
user name, authentication fails.

  fixed_cram:
    driver = cram_md5
    public_name = CRAM-MD5
    server_secret = ${if eq{$1}{ph10}{secret}fail}
    server_set_id = $1

If authentication succeeds, the setting of "server_set_id" preserves the user
name in $authenticated_id. A more tyical configuration might look up the
secret string in a file, using the user name as the key. For example:

  lookup_cram:
    driver = cram_md5
    public_name = CRAM-MD5
    server_secret = ${lookup{$1}lsearch{/etc/authpwd}{$value}fail}
    server_set_id = $1

Note that this expansion explicitly forces failure if the lookup fails because
$1 contains an unknown user name.


35.2 Using cram_md5 as a client

When used as a client, the cram_md5 authenticator has two options:

client_name                     Type: string*  Default: the primary host name

    This string is expanded, and the result used as the user name data when
    computing the response to the server's challenge.

client_secret                   Type: string*                  Default: unset

    This option must be set for the authenticator to work as a client. Its
    value is expanded and the result used as the secret string when computing
    the response.

Different user names and secrets can be used for different servers by
referring to $host or $host_address in the options.

Forced failure of either expansion string is treated as an indication that
this authenticator is not prepared to handle this case. Exim moves on to the
next configured client authenticator. Any other expansion failure causes Exim
to give up trying to send the message to the current server.

A simple example configuration of a cram_md5 authenticator, using fixed
strings, is:

  fixed_cram:
    driver = cram_md5
    public_name = CRAM-MD5
    client_name = ph10
    client_secret = secret



                          36. THE SPA AUTHENTICATOR


The spa authenticator provides client support for Microsoft's "Secure Password
Authentication" mechanism, which is also sometimes known as NTLM (NT LanMan).
The code for client side of this authenticator was contributed by Marc
Prud'hommeaux, and much of it is taken from the Samba project
(http://www.samba.org). The code for the server side was subsequently contrib-
uted by Tom Kistner.

The mechanism works as follows:

 .   After the AUTH command has been accepted, the client sends an SPA
     authentication request based on the user name and optional domain.

 .   The server sends back a challenge.

 .   The client builds a challenge response which makes use of the user's
     password and sends it to the server, which then accepts or rejects it.

Encryption is used to protect the password in transit.


36.1 Using spa as a server

The spa authenticator has just one server option:

server_password                 Type: string*                  Default: unset

    This option is expanded, and the result must be the cleartext password for
    the authenticating user, whose name is at this point in $1. For example:

      spa:
        driver = spa
        public_name = NTLM
        server_password = ${lookup{$1}lsearch{/etc/exim/spa_clearpass}}

    If the expansion is forced to fail, authentication fails. Any other
    expansion failure causes a temporary error code to be returned.


36.2 Using spa as a client

The spa authenticator has the following client options:

client_domain                   Type: string*                  Default: unset

    This option specifies an optional domain for the authentication.

client_password                 Type: string*                  Default: unset

    This option specifies the user's password, and must be set.

client_username                 Type: string*                  Default: unset

    This option specifies the user name, and must be set.

Here is an example of a configuration of this authenticator for use with the
mail servers at "msn.com":

  msn:
    driver = spa
    public_name = MSN
    client_username = msn/msn_username
    client_password = msn_plaintext_password
    client_domain = DOMAIN_OR_UNSET



                 37. ENCRYPTED SMTP CONNECTIONS USING TLS/SSL


Support for TLS (Transport Layer Security), otherwise known as SSL (Secure
Sockets Layer), is implemented by making use of the OpenSSL library or the
GnuTLS library. There is no cryptographic code in the Exim distribution itself
for implementing TLS. In order to use this feature you must install OpenSSL or
GnuTLS, and then build a version of Exim that includes TLS support (see
section 4.6). You also need to understand the basic concepts of encryption at
a managerial level, and in particular, the way that public keys, private keys,
and certificates are used.

RFC 2487 defines how SMTP connections can make use of encryption. Once a
connection is established, the client issues a STARTTLS command. If the server
accepts this, the client and the server negotiate an encryption mechanism. If
the negotiation succeeds, the data that subsequently passes between them is
encrypted.

Exim also has support for legacy clients that do not use the STARTTLS
mechanism. Instead, they connect to a different port on the server (usually
called the 'ssmtp' port), and expect to negotiate a TLS session as soon as the
connection to the server is established. The -tls-on-connect command line
option can be used to run an Exim server in this way from "inetd", and it can
also be used to run a special daemon that operates in this manner (use -oX to
specify the port).

Exim's ACLs can detect whether the current SMTP session is encrypted or not,
and if so, what cipher suite is in use, whether the client supplied a
certificate, and whether or not that certificate was verified. This makes it
possible for an Exim server to deny or accept certain commands based on the
encryption state.

Warning: certain types of firewall and certain anti-virus products can disrupt |
TLS connections. You need to turn off SMTP scanning for these products in      |
order to get TLS to work.                                                      |


37.1 OpenSSL vs GnuTLS

The first TLS support in Exim was implemented using OpenSSL. Support for
GnuTLS followed later, when the first versions of GnuTLS were released. To
build Exim to use GnuTLS, you need to set

  USE_GNUTLS=yes

in Local/Makefile, in addition to

  SUPPORT_TLS=yes

You must also set TLS_LIBS and TLS_INCLUDE appropriately, so that the include
files and libraries for GnuTLS can be found.

There are some differences in usage when using GnuTLS instead of OpenSSL:

 .   The "tls_dhparam" option is ignored, because early versions of GnuTLS had
     no facility for varying its Diffie-Hellman parameters. I understand that
     this has changed, but Exim has not been updated to provide this facility.

 .   GnuTLS uses RSA and D-H parameters that take a substantial amount of time
     to compute. It is unreasonable to re-compute them for every TLS session.
     Therefore, Exim keeps this data in a file in its spool directory, called
     gnutls-params. The file is owned by the Exim user and is readable only by
     its owner. Every Exim process that start up GnuTLS reads the RSA and D-H
     parameters from this file. If the file does not exist, the first Exim
     process that needs it computes the data and writes it to a temporary file
     which is renamed once it is complete. It does not matter if several Exim
     processes do this simultaneously (apart from wasting a few resources).
     Once a file is in place, new Exim processes immediately start using it.

     For maximum security, the parameters that are stored in this file should
     be recalculated periodically, the frequency depending on your paranoia
     level. Arranging this is easy; just delete the file when you want new
     values to be computed.

 .   OpenSSL identifies cipher suites using hyphens as separators, for
     example: DES-CBC3-SHA. GnuTLS uses underscores, for example:
     RSA_ARCFOUR_SHA. What is more, OpenSSL complains if underscores are
     present in a cipher list. To make life simpler, Exim changes underscores
     to hyhens for OpenSSL and hyphens to underscores for GnuTLS when
     processing the list of cipher suites in the "require_ciphers" option of
     the smtp transport.

 .   The "require_ciphers" option of the smtp transport operates differently.
     When using OpenSSL, Exim can pass the list to the library before starting
     the session, and the library takes care of the checking. In GnuTLS, all
     that can be done, as far as I know, is to check after the session has
     been set up that the cipher is acceptable. This makes "require_ciphers"
     rather less useful.


37.2 Configuring an Exim server to use TLS

When Exim has been built with TLS support, it advertises the availability of
the STARTTLS command to client hosts that match "tls_advertise_hosts", but not
to any others. The default value of this option is unset, which means that
STARTTLS is not advertised at all. This default is chosen because you need to
set some other options in order to make TLS avaliable, and also it is sensible
for systems that want to use TLS only as a client.

If a client issues a STARTTLS command and there is some configuration problem
in the server, the command is rejected with a 454 error. If the client
persists in trying to issue SMTP commands, all except QUIT are rejected with
the error

  554 Security failure

If a STARTTLS command is issued within an existing TLS session, it is rejected
with a 554 error code.

To enable TLS operations on a server, you must set "tls_advertise_hosts" to
match some hosts. You can, of course, set it to * to match all hosts. However,
this is not all you need to do. TLS sessions to a server won't work without
some further configuration at the server end.

It is rumoured that all existing clients that support TLS/SSL use RSA
encryption. To make this work you need to set, in the server,

  tls_certificate = /some/file/name
  tls_privatekey = /some/file/name

The first file contains the server's X509 certificate, and the second contains
the private key that goes with it. These files need to be readable by the Exim
user, and must always be given as full path names. They can be the same
file if both the certificate and the key are contained within it. If
"tls_privatekey" is not set, this is assumed to be the case. The certificate
file may also contain intermediate certificates that need to be sent to the
client to enable it to authenticate the server's certificate.

If you do not understand about certificates and keys, please try to find a
source of this background information, which is not Exim-specific. (There are
a few comments below in section 37.6.)

Note: These options do not apply when Exim is operating as a client - they     |
apply only in the case of a server. For a client, you must set the options of  |
the same name in an smtp transport.                                            |

With just these options, Exim will work as a server with clients such as
Netscape. It does not require the client to have a certificate (but see below
for how to insist on this). There is one other option that may be needed in
other situations. If

  tls_dhparam = /some/file/name

is set, the SSL library is initialized for the use of Diffie-Hellman ciphers
with the parameters contained in the file. This increases the set of cipher
suites that the server supports. See the command

  openssl dhparam

for a way of generating this data. At present, "tls_dhparam" is used only when
Exim is linked with OpenSSL. It is ignored if GnuTLS is being used.

The strings supplied for these three options are expanded every time a client
host connects. It is therefore possible to use different certificates and keys
for different hosts, if you so wish, by making use of the client's IP address
in $sender_host_address to control the expansion. If a string expansion is
forced to fail, Exim behaves as if the option is not set.

The variable $tls_cipher is set to the cipher suite that was negotiated for an
incoming TLS connection. It is included in the Received: header of an incoming
message (by default - you can, of course, change this), and it is also
included in the log line that records a message's arrival, keyed by 'X=',
unless the "tls_cipher" log selector is turned off. The "encrypted" condition
can be used to test for specific cipher suites in ACLs.

The ACLs that run for subsequent SMTP commands can check the name of the
cipher suite and vary their actions accordingly. The cipher suite names are
those used by OpenSSL. These may differ from the names used elsewhere. For
example, OpenSSL uses the name DES-CBC3-SHA for the cipher suite which in
other contexts is known as TLS_RSA_WITH_3DES_EDE_CBC_SHA. Check the OpenSSL
documentation for more details.


37.3 Requesting and verifying client certificates

If you want an Exim server to request a certificate when negotiating a TLS
session with a client, you must set either "tls_verify_hosts" or
"tls_try_verify_hosts". You can, of course, set either of them to * to apply
to all TLS connections. For any host that matches one of these options, Exim
requests a certificate as part of the setup of the TLS session. The contents
of the certificate are verified by comparing it with a list of expected
certificates. These must be available in a file or directory that is
identified by "tls_verify_certificates".

A file can contain multiple certificates, concatenated end to end. If a
directory is used, each certificate must be in a separate file, with a name
(or a symbolic link) of the form <hash>.0, where <hash> is a hash value
constructed from the certificate. You can compute the relevant hash by running
the command

  openssl x509 -hash -noout -in /cert/file

where /cert/file contains a single certificate.

The difference between "tls_verify_hosts" and "tls_try_verify_hosts" is what
happens if the client does not supply a certificate, or if the certificate
does not match any of the certificates in the collection named by
"tls_verify_certificates". If the client matches "tls_verify_hosts", the
attempt to set up a TLS session is aborted, and the incoming connection is
dropped. If the client matches "tls_try_verify_hosts", the (encrypted) SMTP
session continues. ACLs that run for subsequent SMTP commands can detect the
fact that no certificate was verified, and vary their actions accordingly. For
example, you can insist on a certificate before accepting a message for
relaying, but not when the message is destined for local delivery.

When a client supplies a certificate (whether it verifies or not), the value
of the Distinguished Name of the certificate is made available in the variable
$tls_peerdn during subsequent processing of the message. Because it is often a
long text string, it is not included in the log line or Received: header by
default. You can arrange for it to be logged, keyed by 'DN=', by setting the
"tls_peerdn" log selector, and you can use "received_header_text" to change
the Received: header. When no certificate is supplied, $tls_peerdn is empty.


37.4 Configuring an Exim client to use TLS

The "tls_cipher" and "tls_peerdn" log selectors apply to outgoing SMTP
deliveries as well as to incoming, the latter one causing logging of the
server certificate's DN. The remaining client configuration for TLS is all
within the smtp transport.

It is not necessary to set any options to have TLS work in the smtp transport.
If Exim is built with TLS support, and TLS is advertised by a server, the smtp
transport always tries to start a TLS session. However, this can be prevented
by setting "hosts_avoid_tls" (an option of the transport) to a list of server
hosts for which TLS should not be used.

If you do not want Exim to attempt to send messages unencrypted when an
attempt to set up an encrypted connection fails in any way, you can set
"hosts_require_tls" to a list of hosts for which encryption is mandatory. For
those hosts, delivery is always deferred if an encrypted connection cannot be
set up. If there are any other hosts for the address, they are tried in the
usual way.

When the server host is not in "hosts_require_tls", Exim may try to deliver
the message unencrypted. It always does this if the response to STARTTLS is a
5"xx" code. For a temporary error code, or for a failure to negotiate a TLS
session after a success response code, what happens is controlled by the
"tls_tempfail_tryclear" option of the smtp transport. If it is false, delivery
to this host is deferred, and other hosts (if available) are tried. If it is
true, Exim attempts to deliver unencrypted after a 4"xx" response to STARTTLS,
and if STARTTLS is accepted, but the subsequent TLS negotiation fails, Exim
closes the current connection (because it is in an unknown state), opens a new
one to the same host, and then tries the delivery unencrypted.

The "tls_certificate" and "tls_privatekey" options of the smtp transport
provide the client with a certificate, which is passed to the server if it
requests it. If the server is Exim, it will request a certificate only if
"tls_verify_hosts" or "tls_try_verify_hosts" matches the client. Note: these   |
options must be set in the smtp transport for Exim to use TLS when it is       |
operating as a client. Exim does not assume that a server certificate (set by  |
the global options of the same name) should also be used when operating as a   |
client.                                                                        |

If "tls_verify_certificates" is set, it must name a file or directory that
contains a collection of expected certificates. The client verifies the
server's certificate against this collection. If "tls_require_ciphers" is set
on the smtp transport, it must contain a list of permitted cipher suites. If
either of these checks fails, delivery to the current host is abandoned, and
the smtp transport tries to deliver to alternative hosts, if any.

All the TLS options in the smtp transport are expanded before use, with $host
and $host_address containing the name and address of the server to which the
client is connected. Forced failure of an expansion causes Exim to behave as
if the relevant option were unset.


37.5 Multiple messages on the same encrypted TCP/IP connection

Exim sends multiple messages down the same TCP/IP connection by starting up an
entirely new delivery process for each message, passing the socket from one
process to the next. This implementation does not fit well with the use of
TLS, because there is quite a lot of state information associated with a TLS
connection, not just a socket identification. Passing all the state infor-
mation to a new process is not feasible. Consequently, Exim shuts down an
existing TLS session before passing the socket to a new process. The new
process may then try to start a new TLS session, and if successful, may try to
re-authenticate if AUTH is in use, before sending the next message.

The RFC is not clear as to whether or not an SMTP session continues in clear
after TLS has been shut down, or whether TLS may be restarted again later,
as just described. However, if the server is Exim, this shutdown and
reinitialization works. It is not known which (if any) other servers operate
successfully if the client closes a TLS session and continues with unencrypted
SMTP, but there are certainly some that do not work. For such servers, Exim
should not pass the socket to another process, because the failure of the
subsequent attempt to use it would cause Exim to record a temporary host
error, and delay other deliveries to that host.

To test for this case, Exim sends an EHLO command to the server after closing
down the TLS session. If this fails in any way, the connection is closed
instead of being passed to a new delivery process, but no retry information is
recorded.

There is also a manual override; you can set "hosts_nopass_tls" on the smtp
transport to match those hosts for which Exim should not pass connections to
new processes if TLS has been used.


37.6 Certificates and all that

In order to understand fully how TLS works, you need to know about certifi-
cates, certificate signing, and certificate authorities. This is not the place
to give a tutorial, especially as I do not know very much about it myself.
Some helpful introduction can be found in the FAQ for the SSL addition to
Apache, currently at

  http://www.modssl.org/docs/2.7/ssl_faq.html#ToC24

Other parts of the "modssl" documentation are also helpful, and have links to
further files. Eric Rescorla's book, "SSL and TLS", published by Addison-
Wesley (ISBN 0-201-61598-3), contains both introductory and more in-depth
descriptions. Some sample programs taken from the book are available from

  http://www.rtfm.com/openssl-examples/


37.7 Certificate chains

The file named by "tls_certificate" may contain more than one certificate.
This is useful in the case where the certificate that is being sent is
validated by an intermediate certificate which the other end does not have.
Multiple certificates must be in the correct order in the file. First the
host's certificate itself, then the first intermediate certificate to validate
the issuer of the host certificate, then the next intermediate certificate to
validate the issuer of the first intermediate certificate, and so on, until
finally (optionally) the root certificate. The root certificate must already
be trusted by the recipient for validation to succeed, of course, but if it's
not preinstalled, sending the root certificate along with the rest makes it
available for the user to install if the receiving end is a client MUA that
can interact with a user.


37.8 Self-signed certificates

You can create a self-signed certificate using the "req" command provided with
OpenSSL, like this:

  openssl req -x509 -newkey rsa:1024 -keyout file1 -out file2 \
              -days 9999 -nodes

file1 and file2 can be the same file; the key and the certificate are
delimited and so can be identified independently. The -days option specifies a
period for which the certificate is valid. The -nodes option is important: if
you do not set it, the key is encrypted with a passphrase that you are
prompted for, and any use that is made of the key causes more prompting for
the passphrase. This is not helpful if you are going to use this certificate
and key in an MTA, where prompting is not possible.

A self-signed certificate made in this way is sufficient for testing, and may
be adequate for all your requirements if you are mainly interested in
encrypting transfers, and not in secure identification.

However, many clients require that the certificate presented by the server be
a user (also called 'leaf' or 'site') certificate, and not a self-signed
certificate. In this situation, the self-signed certificate described above
must be installed on the client host as a trusted root "certification
authority" (CA), and the certificate used by Exim must be a user certificate
signed with that self-signed certificate.

For information on creating self-signed CA certificates and using them to sign
user certificates, see the "General implementation overview" chapter of the
Open-source PKI book, available online at http://ospkibook.sourceforge.net/.



                           38. ACCESS CONTROL LISTS


Access Control Lists (ACLs) are defined in a separate section of the run time
configuration file, headed by 'begin acl'. Each ACL definition starts with a
name, terminated by a colon. Here is a complete ACL section which contains
just one very small ACL:

  begin acl

  small_acl:
    accept   hosts = one.host.only

You can have as many lists as you like in the ACL section, and the order in
which they appear does not matter. The lists are self-terminating.

The majority of ACLs are used to control Exim's behaviour when it receives
certain SMTP commands. This applies both to incoming TCP/IP connections, and
when a local process submits a message over a pipe (using the -bs option). The
most common use is for controlling which recipients are accepted in incoming
messages. In addition, you can also define an ACL that is used to check local
non-SMTP messages. The default configuration file contains an example of a
realistic ACL for checking RCPT commands. This is discussed in chapter 7.


38.1 Testing ACLs

The -bh command line option provides a way of testing your ACL configuration
locally by running a fake SMTP session with which you interact. The host
"relay-test.mail-abuse.org" provides a service for checking your relaying
configuration (see section 38.27 for more details).


38.2 Specifying when ACLs are used

In order to cause an ACL to be used, you have to name it in one of the
relevant options in the main part of the configuration. These options are:

  "acl_not_smtp"              ACL for non-SMTP messages
  "acl_smtp_auth"             ACL for AUTH
  "acl_smtp_connect"          ACL for start of SMTP connection
  "acl_smtp_data"             ACL after DATA
  "acl_smtp_etrn"             ACL for ETRN
  "acl_smtp_expn"             ACL for EXPN
  "acl_smtp_helo"             ACL for HELO or EHLO
  "acl_smtp_mail"             ACL for MAIL
  "acl_smtp_mailauth"         ACL for the AUTH parameter of MAIL               |
  "acl_smtp_rcpt"             ACL for RCPT
  "acl_smtp_starttls"         ACL for STARTTLS
  "acl_smtp_vrfy"             ACL for VRFY

For example, if you set

  acl_smtp_rcpt = small_acl

the little ACL defined above is used whenever Exim receives a RCPT command in
an SMTP dialogue. The majority of policy tests on incoming messages can be
done when RCPT commands arrive. A rejection of RCPT should cause the sending
MTA to give up on the recipient address contained in the RCPT command, whereas
rejection at other times may cause the client MTA to keep on trying to deliver
the message. It is therefore recommended that you do as much testing as
possible at RCPT time.

However, you cannot test the contents of the message, for example, to verify
addresses in the headers, at RCPT time. Such tests have to appear in the ACL
that is run after the message has been received, before the final response to
the DATA command is sent. This is the ACL specified by "acl_smtp_data". At
this time, it is no longer possible to reject individual recipients. An error
response should reject the entire message. Unfortunately, it is known that
some MTAs do not treat hard (5xx) errors correctly at this point - they keep
the message on their queues and try again later, but that is their problem,
though it does waste some of your resources.

The ACL test specified by "acl_smtp_connect" happens after the test specified
by "host_reject_connection" (which is now an anomaly) and any TCP Wrappers
testing (if configured).

The non-SMTP ACL applies to all non-interactive incoming messages, that is, it
applies to batch SMTP as well as to non-SMTP messages. (Batch SMTP is not
really SMTP.) This ACL is run just before the "local_scan()" function. Any
kind of rejection is treated as permanent, because there is no way of sending
a temporary error for these kinds of message. Many of the ACL conditions (for
example, host tests, and tests on the state of the SMTP connection such as
encryption and authentication) are not relevant and are forbidden in this ACL.


38.3 ACL return codes

The result of running an ACL is either 'accept' or 'deny', or, if some test
cannot be completed (for example, if a database is down), 'defer'. These
results cause 2xx, 5xx, and 4xx return codes, respectively, to be used in the
SMTP dialogue. A fourth return, 'error', occurs when there is an error such as
invalid syntax in the ACL. This also causes a 4xx return code.

The ACLs that are relevant to message reception may also return 'discard'.
This has the effect of 'accept', but causes either the entire message or an
individual recipient address to be discarded. In other words, it is a
blackholing facility. Use it with great care.

If the ACL for MAIL returns 'discard', all recipients are discarded, and no
ACL is run for subsequent RCPT commands. The effect of 'discard' in a RCPT ACL
is to discard just the one address. If there are no recipients left when the
message's data is received, the DATA ACL is not run. A 'discard' return from
the DATA or the non-SMTP ACL discards all the remaining recipients.

The "local_scan()" function is always run, even if there are no remaining
recipients; it may create new recipients.


38.4 Unset ACL options

The default actions when any of the "acl_smtp_xxx" options are unset are not
all the same.

For "acl_not_smtp", "acl_smtp_auth", "acl_smtp_connect", "acl_smtp_data",
"acl_smtp_helo", "acl_smtp_mail", "acl_smtp_mailauth", and                     |
"acl_smtp_starttls", the default action is 'accept'.

For the others ("acl_smtp_etrn", "acl_smtp_expn", "acl_smtp_rcpt", and
"acl_smtp_vrfy"), the default action is 'deny'. This means that
"acl_smtp_rcpt" must be defined in order to receive any messages over an SMTP
connection. For an example, see the ACL in the default configuration file.


38.5 Data for message ACLs

When an ACL for MAIL, RCPT, or DATA is being run, the variables that contain
information about the host and the message's sender (for example,
$sender_host_address and $sender_address) are set, and can be used in ACL
statements. In the case of RCPT (but not MAIL or DATA), $domain and
$local_part are set from the argument address.

When an ACL for the AUTH parameter of MAIL is being run, the variables that    |
contain information about the host are set, but $sender_address is not yet     |
set.                                                                           |

The $message_size variable is set to the value of the SIZE parameter on the
MAIL command at MAIL and RCPT time, or -1 if that parameter was not given. Its
value is updated to the true message size by the time the ACL after DATA is
run.

The $rcpt_count variable increases by one for each RCPT command received. The
$recipients_count variable increases by one each time a RCPT command is
accepted, so while an ACL for RCPT is being processed, it contains the number
of previously accepted recipients. At DATA time, $rcpt_count contains the
total number of RCPT commands, and $recipients_count contains the total number
of accepted recipients.


38.6 Data for non-message ACLs

When an ACL for AUTH, ETRN, EXPN, STARTTLS, or VRFY is being run, the
remainder of the SMTP command line is placed in $smtp_command_argument. This
can be tested using a "condition" condition. For example, here is an ACL for
use with AUTH, which insists that either the session is encrypted, or the
CRAM-MD5 authentication method is used. In other words, it does not permit
authentication methods that use cleartext passwords on unencrypted
connections.

  acl_check_auth:
    accept encrypted = *
    accept condition = ${if eq{${uc:$smtp_command_argument}}\
                        {CRAM-MD5}{yes}{no}}
    deny   message   = TLS encryption or CRAM-MD5 required

(Another way of applying this restriction is to arrange for the authenticators
that use cleartext passwords not to be advertised when the connection is not
encrypted. You can use the generic "server_advertise_condition" authenticator
option to do this.)


38.7 Use of the ACL selection options

The value of an "acl_smtp_xxx" option is expanded before use, so you can use
different ACLs in different circumstances, and in fact the resulting string
does not have to be the name of a configured list. Having expanded the string,
Exim searches for an ACL as follows:

 .   If the string begins with a slash, Exim attempts to open the file and
     read its contents as an ACL. The lines are processed in the same way as   |
     lines in the Exim configuration file. In particular, continuation lines   |
     are supported, blank lines are ignored, as are lines whose first non-     |
     whitespace character is '#'. If the file does not exist or cannot be      |
     read, an error occurs (typically causing a temporary failure of whatever
     caused the ACL to be run). For example:

       acl_smtp_data = /etc/acls/\
         ${lookup{$sender_host_address}lsearch\
         {/etc/acllist}{$value}{default}}

     This looks up an ACL file to use on the basis of the host's IP address,
     falling back to a default if the lookup fails. If an ACL is successfully
     read from a file, it is retained in memory for the duration of the Exim
     process, so that it can be re-used without having to re-read the file.

 .   If the string does not start with a slash, and does not contain any
     spaces, Exim searches the ACL section of the configuration for a list
     whose name matches the string.

 .   If no named ACL is found, or if the string contains spaces, Exim parses
     the string as an inline ACL. This can save typing in cases where you just
     want to have something like

       acl_smtp_vrfy = accept

     in order to allow free use of the VRFY command. Such a string may contain |
     newlines; it is processed in the same way as an ACL that is read from a   |
     file.                                                                     |


38.8 Format of an ACL

An individual ACL consists of a number of statements. Each statement starts
with a verb, optionally followed by a number of conditions and other
modifiers. If all the conditions are met, the verb is obeyed. The same         |
condition may be used (with different arguments) more than once in the same    |
statement. This provides a means of specifying an 'and' conjunction between    |
conditions. For example:                                                       |
                                                                               |
  deny  dnslists = list1.example                                               |
        dnslists = list2.example                                               |
                                                                               |
If there are no conditions, the verb is always obeyed. What happens if any of
the conditions are not met depends on the verb (and in one case, on a special
modifier). Not all the conditions make sense at every testing point. For
example, you cannot test a sender address in the ACL that is run for a VRFY
command.

The verbs are as follows:

 .   "accept": If all the conditions are met, the ACL returns 'accept'. If any
     of the conditions are not met, what happens depends on whether "endpass"
     appears among the conditions (for syntax see below). If the failing
     condition precedes "endpass", control is passed to the next ACL state-
     ment; if it follows "endpass", the ACL returns 'deny'. Consider this
     statement, used to check a RCPT command:

       accept domains = +local_domains
              endpass
              verify = recipient

     If the recipient domain does not match the "domains" condition, control
     passes to the next statement. If it does match, the recipient is
     verified, and the command is accepted if verification succeeds. However,
     if verification fails, the ACL yields 'deny', because the failing
     condition is after "endpass".

 .   "defer": If all the conditions are met, the ACL returns 'defer' which, in
     an SMTP session, causes a 4"xx" response to be given. For a non-SMTP ACL,
     "defer" is the same as "deny", because there is no way of sending a
     temporary error. For a RCPT command, "defer" is much the same as using a
     redirect router and ":defer:" while verifying, but the "defer" verb can
     be used in any ACL, and even for a recipient it might be a simpler
     approach.

 .   "deny": If all the conditions are met, the ACL returns 'deny'. If any of
     the conditions are not met, control is passed to the next ACL statement.
     For example,

       deny dnslists = blackholes.mail-abuse.org

     rejects commands from hosts that are on a DNS black list.

 .   "discard": This verb behaves like "accept", except that it returns
     'discard' from the ACL instead of 'accept'. It is permitted only on ACLs
     that are concerned with receiving messages, and it causes recipients to
     be discarded. If the "log_message" modifier is set when "discard"         |
     operates, its contents are added to the line that is automatically        |
     written to the log.                                                       |

     If "discard" is used in an ACL for RCPT, just the one recipient is
     discarded; if used for MAIL, DATA or in the non-SMTP ACL, all the
     message's recipients are discarded. Recipients that are discarded before
     DATA do not appear in the log line when the "log_recipients" log selector
     is set.

 .   "drop": This verb behaves like "deny", except that an SMTP connection is
     forcibly closed after the 5"xx" error message has been sent. For example:

       drop   message   = I don't take more than 20 RCPTs
              condition = ${if > {$rcpt_count}{20}{yes}{no}}

     There is no difference between "deny" and "drop" for the connect-time
     ACL. The connection is always dropped after sending a 550 response.

 .   "require": If all the conditions are met, control is passed to the next
     ACL statement. If any of the conditions are not met, the ACL returns
     'deny'. For example, when checking a RCPT command,

       require verify = sender

     passes control to subsequent statements only if the message's sender can
     be verified. Otherwise, it rejects the command.

 .   "warn": If all the conditions are met, some warning action may be taken.  |
     In all cases, control passes to the next ACL statement. Warning actions   |
     are specified by modifiers; if none are set, a "warn" verb does nothing,  |
     other than check its conditions.                                          |
                                                                               |
     In the case of testing an incoming message, one warning action is to add  |
     header lines. These are specified by the "message" modifier, as in this   |
     example:                                                                  |
                                                                               |
       warn message = X-blacklisted-at: $dnslist_domain                        |
            dnslists = blackholes.mail-abuse.org : \                           |
                       dialup.mail-abuse.org                                   |
                                                                               |
     If an identical header line is requested several times (provoked, for     |
     example, by multiple RCPT commands), only one copy is actually added to   |
     the message.                                                              |
                                                                               |
     Header lines that are added by an ACL at MAIL or RCPT time are not        |
     visible in string expansions in the ACL for subsequent RCPT commands.     |
     However they are are visible in string expansions in the ACL that is run  |
     after DATA. If you want to preserve data between MAIL and RCPT ACLs, you  |
     can use ACL variables, as described in the next section. If a message is  |
     rejected after DATA, all added header lines are included in the entry     |
     that is written to the reject log.                                        |
                                                                               |
     If a "message" modifier is present on a "warn" verb in an ACL that is not |
     testing an incoming message, it is ignored, and the incident is logged.   |
                                                                               |
     Any "warn" statement may use the "log_message" modifier to cause a line   |
     to be written to the main log when the statement's conditions are true.   |
     If "log_message" is not used, nothing is logged. When one of the          |
     conditions is an address verification that fails, the text of the         |
     verification failure message is in $acl_verify_message. If you want this  |
     logged, you must set it up explicitly. For example:                       |
                                                                               |
       warn   !verify = sender                                                 |
               log_message = sender verify failed: $acl_verify_message         |
                                                                               |
     If any condition on a "warn" statement cannot be completed (that is,
     there is some sort of defer), the warning action does not take place. The
     incident is logged, but the ACL continues to be processed.

At the end of each ACL there is an implicit unconditional "deny".

As you can see from the examples above, the conditions and modifiers are
written one to a line, with the first one on the same line as the verb, and
subsequent ones on following lines. If you have a very long condition, you can
continue it onto several physical lines by the usual \ continuation mechanism.
It is conventional to align the conditions vertically.


38.9 ACL variables

There are some special variables that can be set during ACL processing. They   |
can be used to pass information between different ACLs, different invocations  |
of the same ACL in the same SMTP connection, and between ACLs and the routers, |
transports, and filters that are used to deliver a message. There are two sets |
of these variables:                                                            |
                                                                               |
 .   The values of $acl_c0 to $acl_c9 persist throughout an SMTP connection.   |
     They are never reset. Thus, a value that is set while receiving one       |
     message is still available when receiving the next message on the same    |
     SMTP connection.                                                          |
                                                                               |
 .   The values of $acl_m0 to $acl_m9 persist only while a message is being    |
     received. They are reset afterwards. They are also reset by MAIL, RSET,   |
     EHLO, HELO, and after starting up a TLS session.                          |
                                                                               |
When a message is accepted, the current values of all the ACL variables are    |
preserved with the message and are subsequently made available at delivery     |
time.                                                                          |
                                                                               |
The ACL variables are set by modifier called "set". For example:               |
                                                                               |
  accept hosts = whatever                                                      |
         set acl_m4 = some value                                               |
                                                                               |
Note that the leading dollar sign is not used when naming a variable that is   |
to be set. If you want to set a variable without taking any action, you can    |
use a "warn" verb without any other modifiers.                                 |


38.10 Condition and modifier processing

An exclamation mark preceding a condition negates its result. For example,

  deny   domains = *.dom.example
        !verify = recipient

causes the ACL to return 'deny' if the recipient domain ends in "dom.example",
but the recipient address cannot be verified.

The arguments of conditions and modifiers are expanded. A forced failure of an
expansion causes a condition to be ignored, that is, it behaves as if the
condition is true. Consider these two statements:

  accept  senders = ${lookup{$host_name}lsearch\
                    {/some/file}{$value}fail}
  accept  senders = ${lookup{$host_name}lsearch\
                    {/some/file}{$value}{}}

Each attempts to look up a list of acceptable senders. If the lookup succeeds,
the returned list is searched, but if the lookup fails the behaviour is
different in the two cases. The "fail" in the first statement causes the
condition to be ignored, leaving no further conditions. The "accept" verb
therefore succeeds. The second statement, however, generates an empty list
when the lookup fails. No sender can match an empty list, so the condition
fails, and therefore the "accept" also fails.

ACL modifiers appear mixed in with conditions in ACL statements. Some of them  |
specify actions that are taken as the conditions for a statement are checked;  |
others specify text for messages that are used when access is denied or a      |
warning is generated.                                                          |

The positioning of the modifiers in an ACL statement important, because the
processing of a verb ceases as soon as its outcome is known. Only those
modifiers that have already been encountered will take effect. For the
"accept" and "require" statements, this means that processing stops as soon as
a false condition is met. For example, consider this use of the "message"
modifier:

  require message = Can't verify sender
          verify = sender
          message = Can't verify recipient
          verify = recipient
          message = This message cannot be used

If sender verification fails, Exim knows that the result of the statement is
'deny', so it goes no further. The first "message" modifier has been seen, so
its text is used as the error message. If sender verification succeeds, but
recipient verification fails, the second message is used. If recipient
verification succeeds, the third message becomes 'current', but is never used
because there are no more conditions to cause failure.

For the "deny" verb, on the other hand, it is always the last "message"
modifier that is used, because all the conditions must be true for rejection
to happen. Specifying more than one "message" modifier does not make sense,
and the message can even be specified after all the conditions. For example:

  deny   hosts = ...
        !senders = *@my.domain.example
         message = Invalid sender from client host

The 'deny' result does not happen until the end of the statement is reached,
by which time Exim has set up the message.


38.11 ACL modifiers

The ACL modifiers are as follows:

control = <text>

   This modifier may appear only in ACLs for commands relating to incoming
   messages. It affects the subsequent processing of the message, provided
   that the message is eventually accepted. The text must be one of the words
   'freeze' or 'queue_only'. These cause the message to be frozen or just
   queued (without immediate delivery), respectively. Once one of these
   controls is set, it remains set for the message. For example, if "control"
   is used in a RCPT ACL, it applies to the whole message, not just the
   individual recipient. The "control" modifier can be used in several
   different ways. For example:

    .   It can be at the end of an "accept" statement:

          accept  ...some conditions...
                  control = queue_only

        In this case, the control is applied when this statement yields
        'accept'.

    .   It can be in the middle of an "accept" statement:

          accept  ...some conditions...
                  control = queue_only
                  ...some more conditions...

        If the first set of conditions are true, the control is applied, even
        if the statement does not accept because one of the second set of
        conditions is false. In this case, some subsequent statement must
        yield 'accept' for the control to be relevant.

    .   It can be used with "warn" to apply the control, leaving the decision
        about accepting or denying to a subsequent verb. For example:

          warn    ...some conditions...
                  control = freeze
          accept  ...

        Note: If neither "message" nor "log_message" is set for a "warn"
        statement, it does not add anything to the message and does not write
        a log entry.

delay = <time>

   This modifier causes Exim to wait for the time interval before proceeding.
   The time is given in the usual Exim notation. This modifier may appear in
   any ACL. The delay happens as soon as the modifier is processed. Like
   "control", "delay" can be used with "accept" or "deny", for example:

     deny    ...some conditions...
             delay = 30s

   The delay happens if all the conditions are true, before the statement
   returns 'deny'. Compare this with:

     deny    delay = 30s
             ...some conditions...

   which waits for 30s before processing the conditions. The "delay" modifier
   can also be used with "warn" and together with "control":

     warn    ...some conditions...
             delay = 2m
             control = freeze
     accept  ...

endpass

   This modifier, which has no argument, is recognized only in "accept"
   statements. It marks the boundary between the conditions whose failure
   causes control to pass to the next statement, and the conditions whose
   failure causes the ACL to return 'deny'. See the description of "accept"
   above.

log_message = <text>

   This modifier sets up a message that is used as part of the log message if
   the ACL denies access. For example:

     require log_message = wrong cipher suite $tls_cipher
             encrypted   = DES-CBC3-SHA

   "log_message" adds to any underlying error message that may exist because
   of the condition failure. For example, while verifying a recipient address,
   a :fail: redirection might have already set up a message. Although the
   message is usually defined before the conditions to which it applies, the
   expansion does not happen until Exim decides that access is to be denied.
   This means that any variables that are set by the condition are available
   for inclusion in the message. For example, the $dnslist_<xxx> variables
   are set after a DNS black list lookup succeeds. If the expansion of
   "log_message" fails, or if the result is an empty string, the modifier is
   ignored.

   If "log_message" is used with a "warn" statement, 'Warning:' is added to
   the start of the logged message. In the absence of "log_message", nothing   |
   is logged for a "warn" statement. If you want to use a "warn" statement to  |
   log the result of an address verification, you can use $acl_verify_message  |
   to include the verification error message.                                  |

   If "log_message" is not present and there is no underlying error message
   (for example, from the failure of address verification), but "message" is
   present, the "message" text is used for logging rejections. However, if any |
   text for logging contains newlines, only the first line is logged. In the   |
   absence of both "log_message" and "message", a default built-in message is
   used for logging rejections.

logwrite = <text>                                                              |
                                                                               |
   This modifier writes a message to a log file as soon as it is encountered   |
   when processing an ACL. (Compare "log_message", which is used only if the   |
   ACL statement denies access.) The "logwrite" modifier can be used to log    |
   special incidents in ACLs. For example:                                     |
                                                                               |
     accept <some special conditions>                                          |
            control  = freeze                                                  |
            logwrite = froze message because ...                               |
                                                                               |
   By default, the message is written to the main log. However, it may begin   |
   with a colon, followed by a comma-separated list of log names, and then     |
   another colon, to specify exactly which logs are to be written. For         |
   example:                                                                    |
                                                                               |
     logwrite = :main,reject: text for main and reject logs                    |
     logwrite = :panic: text for panic log only                                |
                                                                               |
message = <text>

   This modifier sets up a text string that is expanded and used as an error
   message if the current statement causes the ACL to deny access. The
   expansion happens at the time Exim decides that access is to be denied, not
   at the time it processes "message". If the expansion fails, or generates an
   empty string, the modifier is ignored. For ACLs that are triggered by SMTP
   commands, the message is returned as part of the SMTP error response.

   The "message" modifier is also used with the "warn" verb to specify one or
   more header lines to be added to an incoming message when all the
   conditions are true. If "message" is used with "warn" in an ACL that is not |
   concerned with receiving a message, it has no effect.                       |

   The text is literal; any quotes are taken as literals, but because the
   string is expanded, backslash escapes are processed anyway. If the message
   contains newlines, this gives rise to a multi-line SMTP response. Like
   "log_message", the contents of "message" are not expanded until after a
   condition has failed.

   If "message" is used on a statement that verifies an address, the message
   specified overrides any message that is generated by the verification
   process. However, the original message is available in the variable
   $acl_verify_message, so you can incorporate it into your message if you
   wish. In particular, if you want the text from ":fail:" items in redirect
   routers to be passed back as part of the SMTP response, you should either
   not use a "message" modifier, or make use of $acl_verify_message.

set <acl_name> = <value>

   This modifier puts a value into one of the ACL variables (see section
   38.9).


38.12 ACL conditions

Not all conditions are relevant in all circumstances. For example, testing
senders and recipients does not make sense in an ACL that is being run as the
result of the arrival of an ETRN command, and checks on message headers can be
done only in the ACLs specified by "acl_smtp_data" and "acl_not_smtp". You can |
use the same condition (obviously with different parameters) more than once in |
the same ACL statement. This provides a way of specifying an 'and' conjunc-    |
tion. The conditions are as follows:                                           |

acl = <name of acl or ACL string or file name >

   The possible values of the argument are the same as for the "acl_smtp_xxx"
   options. The named or inline ACL is run. If it returns 'accept' the
   condition is true; if it returns 'deny' the condition is false; if it
   returns 'defer', the current ACL returns 'defer'. If it returns 'drop' and  |
   the outer condition denies access, the connection is dropped. If it returns |
   'discard', the verb must be "accept" or "discard", and the action is taken  |
   immediately - no further conditions are tested.                             |

   ACLs may be nested up to 20 deep; the limit exists purely to catch runaway
   loops. This condition allows you to use different ACLs in different
   circumstances. For example, different ACLs can be used to handle RCPT
   commands for different local users or different local domains.

authenticated = <string list>

   If the SMTP connection is not authenticated, the condition is false.
   Otherwise, the name of the authenticator is tested against the list. To
   test for authentication by any authenticator, you can set

     authenticated = *

condition = <string>

   This feature allows you to make up custom conditions. If the result of
   expanding the string is an empty string, the number zero, or one of the
   strings 'no' or 'false', the condition is false. If the result is any non-
   zero number, or one of the strings 'yes' or 'true', the condition is true.
   For any other values, some error is assumed to have occured, and the ACL
   returns 'defer'.

dnslists = <list of domain names and other data>

   This condition checks for entries in DNS black lists. These are also known
   as 'RBL lists', after the original Realtime Blackhole List, but note that
   the use of the lists at "mail-abuse.org" now carries a charge. There are    |
   too many different variants of this condition to describe briefly here. See |
   sections 38.13-38.19 for details.                                           |

domains = <domain list>

   This condition is relevant only after a RCPT command. It checks that the
   domain of the recipient address is in the domain list. If percent-hack
   processing is enabled, it is done before this test is done. If the check
   succeeds with a lookup, the result of the lookup is placed in $domain_data
   until the next "domains" test.

encrypted = <string list>

   If the SMTP connection is not encrypted, the condition is false. Otherwise,
   the name of the cipher suite in use is tested against the list. To test for
   encryption without testing for any specific cipher suite(s), set

     encrypted = *

hosts = < host list>

   This condition tests that the calling host matches the host list. If you
   have name lookups or wildcarded host names and IP addresses in the same
   host list, you should normally put the IP addresses first. For example, you
   could have:

     accept hosts = 10.9.8.7 : dbm;/etc/friendly/hosts

   The reason for this lies in the left-to-right way that Exim processes
   lists. It can test IP addresses without doing any DNS lookups, but when it
   reaches an item that requires a host name, it fails if it cannot find a
   host name to compare with the pattern. If the above list is given in the
   opposite order, the "accept" statement fails for a host whose name cannot
   be found, even if its IP address is 10.9.8.7.

   If you really do want to do the name check first, and still recognize the
   IP address even if the name lookup fails, you can rewrite the ACL like
   this:

     accept hosts = dbm;/etc/friendly/hosts
     accept hosts = 10.9.8.7

   The default action on failing to find the host name is to assume that the
   host is not in the list, so the first "accept" statement fails. The second
   statement can then check the IP address.

   If a "hosts" condition is satisfied by means of a lookup, the result of the
   lookup is made available in the $host_data variable. This allows you, for
   example, to set up a statement like this:

     deny  hosts = net-lsearch;/some/file
           message = $host_data

   which gives a custom error message for each denied host.

local_parts = <local part list>

   This condition is relevant only after a RCPT command. It checks that the
   local part of the recipient address is in the list. If percent-hack
   processing is enabled, it is done before this test. If the check succeeds
   with a lookup, the result of the lookup is placed in $local_part_data until
   the next "local_parts" test.

recipients = <address list>

   This condition is relevant only after a RCPT command. It checks the entire
   recipient address against a list of recipients.

sender_domains = <domain list>

   This condition tests the domain of the sender of the message against the
   given domain list. Note: the domain of the sender address is in             |
   $sender_address_domain. It is "not" put in $domain during the testing of    |
   this condition. This is an exception to the general rule for testing domain |
   lists. It is done this way so that, if this condition is used in an ACL for |
   a RCPT command, the recipient's domain (which is in $domain) can be used to |
   influence the sender checking.                                              |

senders = <address list>

   This condition tests the sender of the message against the given list. To
   test for a bounce message, which has an empty sender, set

     senders = :

verify = certificate

   This condition is true in an SMTP session if the session is encrypted, and
   a certificate was received from the client, and the certificate was
   verified. The server requests a certificate only if the client matches
   "tls_verify_hosts" or "tls_try_verify_hosts" (see chapter 37).

verify = header_sender/<options>

   This condition is relevant only in an ACL that is run after a message has
   been received, that is, in an ACL specified by "acl_smtp_data". It checks
   that there is a verifiable sender address in at least one of the Sender:,
   Reply-To:, or From: header lines. Details of address verification and the
   options are given in the next section. You can combine this condition with
   the "senders" condition to restrict it to bounce messages only:

     deny    senders = :
             message = A valid sender header is required for bounces
            !verify  = header_sender

verify = header_syntax

   This condition is relevant only in an ACL that is run after a message
   has been received, that is, in an ACL specified by "acl_smtp_data" or
   "acl_not_smtp". It checks the syntax of all header lines that can contain
   lists of addresses (Sender:, From:, Reply-To:, To:, Cc:, and Bcc:).
   Unqualified addresses (local parts without domains) are permitted only in
   locally generated messages and from hosts that match
   "sender_unqualified_hosts" or "recipient_unqualified_hosts", as
   appropriate.

   Note that this condition is a syntax check only. However, a common spamming
   ploy is to send syntactically invalid headers such as

     To: @

   and this condition can be used to reject such messages.

verify = helo

   This condition is true if a HELO or EHLO command has been received from the
   client host, and its contents have been verified. Verification of these
   commands does not happen by default. See the description of the
   "helo_verify_hosts" and "helo_try_verify_hosts" options for details of how
   to request it.

verify = recipient/<options>

   This condition is relevant only after a RCPT command. It verifies the
   current recipient. Details of address verification are given in the next
   section. After a recipient has been verified, the value of $address_data is
   the last value that was set while routing the address. This applies even if
   the verification fails. When an address that is being verified is redirec-
   ted to a single address, verification continues with the new address, and
   in that case, the subsequent value of $address_data is the value for the
   child address.

verify = reverse_host_lookup

   This condition ensures that a verified host name has been looked up from
   the IP address of the client host. (This may have happened already if the
   host name was needed for checking a host list, or if the host matched
   "host_lookup".) Verification ensures that the host name obtained from a
   reverse DNS lookup, or one of its aliases, does, when it is itself looked
   up in the DNS, yield the original IP address.

   If this condition is used for a locally generated message (that is, when
   there is no client host involved), it always succeeds.

verify = sender/<options>

   This condition is relevant only after a MAIL or RCPT command, or after a
   message has been received (the "acl_smtp_data" or "acl_not_smtp" ACLs). If
   the message's sender is empty (that is, this is a bounce message), the
   condition is true. Otherwise, the sender address is verified. Details of
   verification are given in the next section. Exim caches the result of
   sender verification, to avoid doing it more than once per message.

verify = sender=address/<options>

   This is a variation of the previous option, in which a modified address is
   verified as a sender.

                                                                               |
38.13 Using DNS lists                                                          |
                                                                               |
In its simplest form, the "dnslists" condition tests whether the calling host  |
is on a DNS list by looking up the inverted IP address in one or more DNS      |
domains. For example, if the calling host's IP address is 192.168.62.43, and   |
the ACL statement is                                                           |
                                                                               |
  deny dnslists = blackholes.mail-abuse.org : \                                |
                  dialups.mail-abuse.org                                       |
                                                                               |
the following domains are looked up:                                           |
                                                                               |
  43.62.168.192.blackholes.mail-abuse.org                                      |
  43.62.168.192.dialups.mail-abuse.org                                         |
                                                                               |
If a DNS lookup times out or otherwise fails to give a decisive answer, Exim   |
behaves as if the host is not on the relevant list. This is usually the        |
required action when "dnslists" is used with "deny" (which is the most common  |
usage), because it prevents a DNS failure from blocking mail. However, you can |
change this behaviour by putting one of the following special items in the     |
list:                                                                          |
                                                                               |
  +include_unknown    behave as if the item is on the list                     |
  +exclude_unknown    behave as if the item is not on the list (default)       |
  +defer_unknown      give a temporary error                                   |
                                                                               |
Each of these applies to any subsequent items on the list. For example:        |
                                                                               |
  deny dnslists = +defer_unknown : foo.bar.example                             |
                                                                               |
Testing the list of domains stops as soon as a match is found. If you want to  |
warn for one list and block for another, you can use two different statements: |
                                                                               |
  deny  dnslists = blackholes.mail-abuse.org                                   |
  warn  message  = X-Warn: sending host is on dialups list                     |
        dnslists = dialups.mail-abuse.org                                      |
                                                                               |
DNS list lookups are cached by Exim for the duration of the SMTP session, so a |
lookup based on the IP address is done at most once for any incoming           |
connection. Exim does not share information between multiple incoming connec-  |
tions (but your local name server cache should be active).                     |
                                                                               |
                                                                               |
38.14 DNS lists keyed on domain names                                          |
                                                                               |
There are some lists that are keyed on domain names rather than inverted IP    |
addresses (see for example the "domain based zones" link at http://www.rfc-    |
ignorant.org/). You can change the name that is looked up by adding additional |
data to a "dnslists" item, introduced by a slash. For example,                 |
                                                                               |
  deny  message  = Sender's domain is listed at $dnslist_domain                |
        dnslists = dsn.rfc-ignorant.org/$sender_address_domain                 |
                                                                               |
This particular example is useful only in ACLs that are obeyed after the RCPT  |
or DATA commands, when a sender address is available. If (for example) the     |
message's sender is "user@tld.example" the name that is looked up is           |
                                                                               |
  tld.example.dsn.rfc-ignorant.org                                             |
                                                                               |
You can mix entries with and without additional data in the same "dnslists"    |
condition.                                                                     |
                                                                               |
                                                                               |
38.15 Data returned by DNS lists                                               |
                                                                               |
DNS lists are constructed using address records in the DNS. The original RBL   |
just used the address 127.0.0.1 on the right hand side of each record, but the |
RBL+ list and some other lists use a number of values with different meanings. |
The values used on the RBL+ list are:                                          |
                                                                               |
  127.1.0.1    RBL                                                             |
  127.1.0.2    DUL                                                             |
  127.1.0.3    DUL and RBL                                                     |
  127.1.0.4    RSS                                                             |
  127.1.0.5    RSS and RBL                                                     |
  127.1.0.6    RSS and DUL                                                     |
  127.1.0.7    RSS and DUL and RBL                                             |
                                                                               |
Some DNS lists may return more than one address record.                        |
                                                                               |
                                                                               |
38.16 Variables set from DNS lists                                             |
                                                                               |
When an entry is found in a DNS list, the variable $dnslist_domain contains    |
the name of the domain that matched, $dnslist_value contains the data from the |
entry, and $dnslist_text contains the contents of any associated TXT record.   |
If more than one address record is returned by the DNS lookup, all the IP      |
addresses are included in $dnslist_value, separated by commas and spaces.      |
                                                                               |
You can use these variables in "message" or "log_message" modifiers - although |
these appear before the condition in the ACL, they are not expanded until      |
after it has failed. For example:                                              |
                                                                               |
  deny    hosts = !+local_networks                                             |
          message = $sender_host_address is listed \                           |
                    at $dnslist_domain                                         |
          dnslists = rbl-plus.mail-abuse.example                               |
                                                                               |
                                                                               |
38.17 Additional matching conditions for DNS lists                             |
                                                                               |
If you add an equals sign and an IP address after a "dnslists" domain name,    |
you can restrict its action to DNS records with a matching right hand side.    |
For example,                                                                   |
                                                                               |
  deny dnslists = rblplus.mail-abuse.org=127.0.0.2                             |
                                                                               |
rejects only those hosts that yield 127.0.0.2. Without this additional data,   |
any address record is considered to be a match. If more than one address       |
record is found on the list, they are all checked for a matching right-hand    |
side.                                                                          |
                                                                               |
If you want to specify a constraining address and also change the name that is |
looked up, the address list must be specified first. For example:              |
                                                                               |
  deny dnslists = dsn.rfc-ignorant.org\                                        |
                  =127.0.0.2/$sender_address_domain                            |
                                                                               |
More than one IP address may be given for checking, using a comma as a         |
separator. These are alternatives - if any one of them matches, the "dnslists" |
condition is true. For example:                                                |
                                                                               |
  deny  dnslists = a.b.c=127.0.0.2,127.0.0.3                                   |
                                                                               |
If the character '&' is used instead of '=', the comparison for each listed IP |
address is done by a bitwise 'and' instead of by an equality test. In other    |
words, the listed addresses are used as bit masks. The comparison is true if   |
all the bits in the mask are present in the address that is being tested. For  |
example:                                                                       |
                                                                               |
  dnslists = a.b.c&0.0.0.3                                                     |
                                                                               |
matches if the address is "x.x.x."3, "x.x.x."7, "x.x.x."11, etc. If you want   |
to test whether one bit or another bit is present (as opposed to both being    |
present), you must use multiple values. For example:                           |
                                                                               |
  dnslists = a.b.c&0.0.0.1,0.0.0.2                                             |
                                                                               |
matches if the final component of the address is an odd number or two times an |
odd number.                                                                    |
                                                                               |
                                                                               |
38.18 Negated DNS matching conditions                                          |
                                                                               |
You can supply a negative list of IP addresses as part of a "dnslists"         |
condition. Whereas                                                             |
                                                                               |
  deny  dnslists = a.b.c=127.0.0.2,127.0.0.3                                   |
                                                                               |
means 'deny if the host is in the black list at the domain "a.b.c" and the IP  |
address yielded by the list is either 127.0.0.2 or 127.0.0.3',                 |
                                                                               |
  deny  dnslists = a.b.c!=127.0.0.2,127.0.0.3                                  |
                                                                               |
means 'deny if the host is in the black list at the domain "a.b.c" and the IP  |
address yielded by the list is not 127.0.0.2 and not 127.0.0.3'. In other      |
words, the result of the test is inverted if an exclamation mark appears       |
before the '=' (or the '&') sign.                                              |
                                                                               |
Note: this kind of negation is not the same as negation in a domain, host, or  |
address list (which is why the syntax is different).                           |
                                                                               |
If you are using just one list, the negation syntax does not gain you much.    |
The previous example is precisely equivalent to                                |
                                                                               |
  deny  dnslists = a.b.c                                                       |
       !dnslists = a.b.c=127.0.0.2,127.0.0.3                                   |
                                                                               |
However, if you are using multiple lists, the negation syntax is clearer.      |
Consider this example:                                                         |
                                                                               |
  deny  dnslists = sbl.spamhaus.org : \                                        |
                   list.dsbl.org : \                                           |
                   dnsbl.njabl.org!=127.0.0.3 : \                              |
                   relays.ordb.org                                             |
                                                                               |
Using only positive lists, this would have to be:                              |
                                                                               |
  deny  dnslists = sbl.spamhaus.org : \                                        |
                   list.dsbl.org                                               |
  deny  dnslists = dnsbl.njabl.org                                             |
       !dnslists = dnsbl.njabl.org=127.0.0.3                                   |
  deny  dnslists = relays.ordb.org                                             |
                                                                               |
which is less clear, and harder to maintain.                                   |
                                                                               |
                                                                               |
38.19 DNS lists and IPv6                                                       |
                                                                               |
If Exim is asked to do a dnslist lookup for an IPv6 address, it inverts it     |
nibble by nibble. For example, if the calling host's IP address is             |
3ffe:ffff:836f:0a00:000a:0800:200a:c031, Exim might look up                    |
                                                                               |
  1.3.0.c.a.0.0.2.0.0.8.0.a.0.0.0.0.0.a.0.f.6.3.8.                             |
    f.f.f.f.e.f.f.3.blackholes.mail-abuse.org                                  |
                                                                               |
(split over two lines here to fit on the page). Unfortunately, some of the DNS |
lists contain wildcard records, intended for IPv4, that interact badly with    |
IPv6. For example, the DNS entry                                               |
                                                                               |
  *.3.some.list.example.    A    127.0.0.1                                     |
                                                                               |
is probably intended to put the entire 3.0.0.0/8 IPv4 network on the list.     |
Unfortunately, it also matches the entire 3::/124 IPv6 network.                |
                                                                               |
You can exclude IPv6 addresses from DNS lookups by making use of a suitable    |
"condition" condition, as in this example:                                     |
                                                                               |
  deny   condition = ${if isip4{$sender_host_address}{yes}{no}}                |
         dnslists  = some.list.example                                         |
                                                                               |

38.20 Address verification

Several of the "verify" conditions described in the previous section cause
addresses to be verified. These conditions can be followed by options that
modify the verification process. The options are separated from the keyword
and from each other by slashes, and some of them contain parameters. For
example:

  verify = sender/callout
  verify = recipient/defer_ok/callout=10s,defer_ok

The first stage of verification is to run the address through the routers, in
'verify mode'. Routers can detect the difference between verification and
routing for delivery, and their actions can be varied by a number of generic
options such as "verify" and "verify_only" (see chapter 15).

If there is a defer error while doing this verification routing, the ACL
normally returns 'defer'. However, if you include "defer_ok" in the options,
the condition is forced to be true instead.


38.21 Callout verification

For non-local addresses, routing verifies the domain, but is unable to do any
checking of the local part. There are situations where some means of verifying
the local part is desirable. One way this can be done is to make an SMTP
"callback" to the sending host (for a sender address) or a "callforward" to a
subsequent host (for a recipient address), to see if the host accepts the
address. We use the term "callout" to cover both cases. This facility should
be used with care, because it can add a lot of resource usage to the cost of
verifying an address. However, Exim does cache the results of callouts, which
helps to reduce the cost. Details are in the next section.

A successful callout does not guarantee that a real delivery to the address
would succeed; on the other hand, a failing callout does guarantee that it
would fail.

If the "callout" option is present on a condition that verifies an address, a
second stage of verification occurs if the address is successfully routed to
one or more remote hosts. The usual case is routing by a dnslookup or a
manualroute router, where the router specifies the hosts. However, if a router
that does not set up hosts routes to an smtp transport with a "hosts" setting,
the transport's hosts are used. If an smtp transport transport has
"hosts_override" set, its hosts are always used, whether or not the router
supplies a host list.

The port that is used is taken from the transport, if it is specified and is a |
remote transport. (For routers that do verification only, no transport need be |
specified.) Otherwise, the default SMTP port is used. If a remote transport    |
specifies an outgoing interface, this is used; otherwise the interface is not  |
specified.                                                                     |
                                                                               |
For a sender callout check, Exim makes SMTP connections to the remote hosts,   |
to test whether a bounce message could be delivered to the sender address. The |
following SMTP commands are sent:                                              |
                                                                               |
  HELO <primary host name>                                                     |
  MAIL FROM:<>                                                                 |
  RCPT TO:<the address to be tested>                                           |
  QUIT                                                                         |
                                                                               |
LHLO is used instead of HELO if the transport's "protocol" option is set to    |
'lmtp'. A recipient callout check is similar, except that the MAIL command     |
contains the sender address of the incoming message.                           |

If the response to the RCPT command is a 2xx code, the verification succeeds.
If it is 5xx, the verification fails. For any other condition, Exim tries the
next host, if any. If there is a problem with all the remote hosts, the ACL
yields 'defer', unless the "defer_ok" parameter of the "callout" option is
given, in which case the condition is forced to succeed.


38.22 Additional parameters for callouts

The "callout" option can be followed by an equals sign and a number of
optional parameters, separated by commas. For example:

  verify = recipient/callout=10s,defer_ok

The old syntax, which had "callout_defer_ok" and "check_postmaster" as separ-
ate verify options, is retained for backwards compatibility, but is now
deprecated. The additional parameters for "callout" are as follows:

 .   <a time>: This specifies the timeout that applies for the callout attempt
     to each host. For example:

       verify = sender/callout=5s

     The default is 30 seconds. The timeout is used for each response from the
     remote host.

 .   "defer_ok": Failure to contact any host, or any other kind of temporary
     error is treated as success by the ACL. However, the cache is not updated
     in this circumstance.

 .   "no_cache": The callout cache is neither read nor updated.

 .   "postmaster": A successful callout check is followed by a similar check
     for the local part "postmaster" at the same domain. If this address is
     rejected, the callout fails. The result of the postmaster check is
     recorded in a cache record; if it is a failure, this is used to fail
     subsequent callouts for the domain without a connection being made, until
     the cache record expires.

 .   "random": Before doing the normal callout check, Exim does a check for a
     'random' local part at the same domain. The local part is not really
     random - it is defined by the expansion of the option
     "callout_random_local_part", which defaults to

       $primary_host_name-$tod_epoch-testing

     The idea here is to try to determine whether the remote host accepts all
     local parts without checking. If it does, there is no point in doing
     callouts for specific local parts. If the 'random' check succeeds, the
     result is saved in a cache record, and used to force the current and
     subsequent callout checks to succeed without a connection being made,
     until the cache record expires.


38.23 Callout caching

Exim caches the results of callouts in order to reduce the amount of resources
used, unless you specify the "no_cache" parameter with the "callout" option. A
hints database called 'callout' is used for the cache. Two different record
types are used: one records the result of a callout check for a specific
address, and the other records information that applies to the entire domain
(for example, that it accepts the local part "postmaster").

When an original callout fails, a detailed SMTP error message is given about
the failure. However, for subsequent failures use the cache data, this message
is not available.

The expiry times for negative and positive address cache records are indepen-
dent, and can be set by the global options "callout_negative_expire" (default
2h) and "callout_positive_expire" (default 24h), respectively.

If a host gives a negative response to an SMTP connection, or rejects any
commands up to and including

  MAIL FROM:<>

any callout attempt is bound to fail. Exim remembers such failures in a domain
cache record, which it uses to fail callouts for the domain without making new
connections, until the domain record times out. There are two separate expiry
times for domain cache records: "callout_domain_negative_expire" (default 3h)
and "callout_domain_positive_expire" (default 7d).

Domain records expire when the negative expiry time is reached if callouts
cannot be made for the domain, or if the postmaster check failed. Otherwise,
they expire when the positive expiry time is reached. This ensures that, for
example, a host that stops accepting 'random' local parts will eventually be
noticed.

The callout caching mechanism is based entirely on the domain of the address
that is being tested. If the domain routes to several hosts, it is assumed
that their behaviour will be the same.


38.24 Sender address verification reporting

When sender verification fails in an ACL, the details of the failure are given
as additional output lines before the 550 response to the relevant SMTP
command (RCPT or DATA). For example, if sender callout is in use, you might
see:

  MAIL FROM:<xyz@abc.example>
  250 OK
  RCPT TO:<pqr@def.example>
  550-Verification failed for <xyz@abc.example>
  550-Called:   192.168.34.43
  550-Sent:     RCPT TO:<xyz@abc.example>
  550-Response: 550 Unknown local part xyz in <xyz@abc.example>
  550 Sender verification failed

If more than one RCPT command fails in the same way, the details are given
only for the first of them. However, some administrators do not want to
send out this much information. You can suppress the details by adding
'/no_details' to the ACL statement that requests sender verification. For
example:

  verify = sender/no_details


38.25 Redirection while verifying

A dilemma arises when a local address is redirected by aliasing or forwarding
during verification: should the generated addresses themselves be verified, or
should the successful expansion of the original address be enough to verify
it? Exim takes the following pragmatic approach:

 .   When an incoming address is redirected to just one child address,
     verification continues with the child address, and if that fails to
     verify, the original verification also fails.

 .   When an incoming address is redirected to more than one child address,
     verification does not continue. A success result is returned.

This seems the most reasonable behaviour for the common use of aliasing as a
way of redirecting different local parts to the same mailbox. It means, for
example, that a pair of alias entries of the form

  A.Wol:   aw123
  aw123:   :fail: Gone away, no forwarding address

work as expected, with both local parts causing verification failure. When a
redirection generates more than one address, the behaviour is more like a
mailing list, where the existence of the alias itself is sufficient for
verification to succeed.


38.26 Using an ACL to control relaying

An MTA is said to "relay" a message if it receives it from some host and
delivers it directly to another host as a result of a remote address contained
within it. Redirecting a local address via an alias or forward file and then
passing the message on to another host is not relaying, but a redirection as a
result of the 'percent hack' is.

Two kinds of relaying exist, which are termed 'incoming' and 'outgoing'. A
host which is acting as a gateway or an MX backup is concerned with incoming
relaying from arbitrary hosts to a specific set of domains. On the other hand,
a host which is acting as a smart host for a number of clients is concerned
with outgoing relaying from those clients to the Internet at large. Often the
same host is fulfilling both functions, as illustrated in the diagram below,
but in principle these two kinds of relaying are entirely independent. What is
not wanted is the transmission of mail from arbitrary remote hosts through
your system to arbitrary domains.

     --------------    -----------
     | Arbitrary  |    |Arbitrary|
     |remote hosts|    | domains |
     --------------    -----------
   I       v                ^       O
   n       v                ^       u
   c    ---v----------------^---    t
   o    |  v     Local      ^  |    g
   m    |  v      host      ^  |    o
   i    ---v----------------^---    i
   n       v                ^       n
   g       v                ^       g
        Specific         Specific
        domains           hosts

You can implement relay control by means of suitable statements in the ACL
that runs for each RCPT command. For convenience, it is often easiest to use
Exim's named list facility to define the domains and hosts involved. For
example, suppose you want to do the following:

 .   Deliver a number of domains to mailboxes on the local host (or process
     them locally in some other way). Let's say these are "my.dom1.example"
     and "my.dom2.example".

 .   Relay mail for a number of other domains for which you are the secondary
     MX. These might be "friend1.example" and "friend2.example".

 .   Relay mail from the hosts on your local LAN, to whatever domains are
     involved. Suppose your LAN is 192.168.45.0/24.

In the main part of the configuration, you put the following definitions:

  domainlist local_domains = my.dom1.example : my.dom2.example
  domainlist relay_domains = friend1.example : friend2.example
  hostlist   relay_hosts   = 192.168.45.0/24

Now you can use these definitions in the ACL that is run for every RCPT
command:

  acl_check_rcpt:
    accept domains = +local_domains : +relay_domains
    accept hosts   = +relay_hosts

The first statement accepts any RCPT command that contains an address in the
local or relay domains. For any other domain, control passes to the second
statement, which accepts the command only if it comes from one of the relay
hosts. In practice, you will probably want to make your ACL more sophisticated
than this, for example, by including sender and recipient verification. The
default configuration includes a more comprehensive example, which is
described in chapter 7.


38.27 Checking a relay configuration

You can check the relay characteristics of your configuration in the same way
that you can test any ACL behaviour for an incoming SMTP connection, by using
the -bh option to run a fake SMTP session with which you interact.

For specifically testing for unwanted relaying, the host "relay-test.mail-
abuse.org" provides a useful service. If you telnet to this host from the host
on which Exim is running, using the normal telnet port, you will see a normal
telnet connection message and then quite a long delay. Be patient. The remote
host is making an SMTP connection back to your host, and trying a number of
common probes to test for open relay vulnerability. The results of the tests
will eventually appear on your terminal.



                   39. ADDING A LOCAL SCAN FUNCTION TO EXIM


In these days of email worms, viruses, and ever-increasing spam, some sites
want to apply a lot of checking to messages before accepting them. You can do
a certain amount through string expansions and the "condition" condition in
the ACL that runs after the SMTP DATA command or the ACL for non-SMTP messages
(see chapter 38), but this has its limitations.

An increasingly popular way of doing additional checking is to make use of the |
Exiscan patch for Exim, which adds ACL conditions that perform body scans of   |
various kinds. This is available from http://duncanthrax.net/exiscan-acl/.     |

To allow for even more general checking that can be customized to a site's own
requirements, there is the possibility of linking Exim with a private message
scanning function, written in C. If you want to run code that is written in
something other than C, you can of course use a little C stub to call it.

The local scan function is run once for every incoming message, at the point   |
when Exim is just about to accept the message. It can therefore be used to     |
control non-SMTP messages from local processes as well as messages arriving
via SMTP.

Exim applies a timeout to calls of the local scan function, and there is an
option called "local_scan_timeout" for setting it. The default is 5 minutes.
Zero means 'no timeout'. Exim also sets up signal handlers for SIGSEGV,
SIGILL, SIGFPE, and SIGBUS before calling the local scan function, so that the
most common types of crash are caught. If the timeout is exceeded or one of
those signals is caught, the incoming message is rejected with a temporary
error if it is an SMTP message. For a non-SMTP message, the message is dropped
and Exim ends with a non-zero code. The incident is logged on the main and
reject logs.


39.1 Building Exim to use a local scan function

To make use of the local scan function feature, you must tell Exim where your
function is before building Exim, by setting LOCAL_SCAN_SOURCE in your
Local/Makefile. A recommended place to put it is in the Local directory, so
you might set

  LOCAL_SCAN_SOURCE=Local/local_scan.c

for example. The function must be called "local_scan()". It is called by Exim
after it has received a message, when the success return code is about to be
sent. This is after all the ACLs have been run. The return code from your
function controls whether the message is actually accepted or not. There is a
commented template function (that just accepts the message) in the file
src/local_scan.c.

If you want to make use of Exim's run time configuration file to set options
for your "local_scan()" function, you must also set

  LOCAL_SCAN_HAS_OPTIONS=yes

in Local/Makefile (see section 39.3 below).


39.2 API for local_scan()

You must include this line near the start of your code:

  #include "local_scan.h"

This header file defines a number of variables and other values, and the
prototype for the function itself. Exim is coded to use unsigned char values
almost exclusively, and one of the things this header defines is a shorthand
for "unsigned char" called "uschar". It also contains the following macro
definitions, to simplify casting character strings and pointers to character
strings:

  #define CS   (char *)
  #define CCS  (const char *)
  #define CSS  (char **)
  #define US   (unsigned char *)
  #define CUS  (const unsigned char *)
  #define USS  (unsigned char **)

The function prototype for "local_scan()" is:

  extern int local_scan(int fd, uschar **return_text);

The arguments are as follows:

 .   "fd" is a file descriptor for the file that contains the body of the
     message (the -D file). The file is open for reading and writing, but
     updating it is not recommended. Warning: You must "not" close this file
     descriptor.

     The descriptor is positioned at character 19 of the file, which is the
     first character of the body itself, because the first 19 characters are
     the message id followed by "-D" and a newline. If you rewind the file,
     you should use the macro SPOOL_DATA_START_OFFSET to reset to the start of
     the data, just in case this changes in some future version.

 .   "return_text" is an address which you can use to return a pointer to a
     text string at the end of the function. The value it points to on entry
     is NULL.

The function must return an "int" value which is one of the following macros:

 .   "LOCAL_SCAN_ACCEPT"

     The message is accepted. If you pass back a string of text, it is saved
     with the message, and made available in the variable $local_scan_data. No
     newlines are permitted (if there are any, they are turned into spaces)
     and the maximum length of text is 1000 characters.

 .   "LOCAL_SCAN_ACCEPT_FREEZE"

     This behaves as LOCAL_SCAN_ACCEPT, except that the accepted message is
     queued without immediate delivery, and is frozen.

 .   "LOCAL_SCAN_ACCEPT_QUEUE"

     This behaves as LOCAL_SCAN_ACCEPT, except that the accepted message is
     queued without immediate delivery.

 .   "LOCAL_SCAN_REJECT"

     The message is rejected; the returned text is used as an error message
     which is passed back to the sender and which is also logged. Newlines are
     permitted - they cause a multiline response for SMTP rejections, but are
     converted to "\n" in log lines. If no message is given, 'Administrative
     prohibition' is used.

 .   "LOCAL_SCAN_TEMPREJECT"

     The message is temporarily rejected; the returned text is used as an
     error message as for LOCAL_SCAN_REJECT. If no message is given,
     'Temporary local problem' is used.

 .   "LOCAL_SCAN_REJECT_NOLOGHDR"

     This behaves as LOCAL_SCAN_REJECT, except that the header of the rejected
     message is not written to the reject log. It has the effect of unsetting
     the "rejected_header" log selector for just this rejection. If
     "rejected_header" is already unset (see the discussion of the
     "log_selection" option in section 45.15), this code is the same as
     LOCAL_SCAN_REJECT.

 .   "LOCAL_SCAN_TEMPREJECT_NOLOGHDR"

     This code is a variation of LOCAL_SCAN_TEMPREJECT in the same way that
     LOCAL_SCAN_REJECT_NOLOGHDR is a variation of LOCAL_SCAN_REJECT.

If the message is not being received by interactive SMTP, rejections are
reported by writing to "stderr" or by sending an email, as configured by the
-oe command line options.


39.3 Configuration options for local_scan()

It is possible to have option settings in the main configuration file that set
values in static variables in the "local_scan()" module. If you want to do
this, you must have the line

  LOCAL_SCAN_HAS_OPTIONS=yes

in your Local/Makefile when you build Exim. (This line is in OS/Makefile-
Default, commented out). Then, in the "local_scan()" source file, you must
define static variables to hold the option values, and a table to define them.

The table must be a vector called "local_scan_options", of type "optionlist".
Each entry is a triplet, consisting of a name, an option type, and a pointer
to the variable that holds the value. The entries must appear in alphabetical
order. Following "local_scan_options" you must also define a variable called
"local_scan_options_count" that contains the number of entries in the table.
Here is a short example, showing two kinds of option:

  static int my_integer_option = 42;
  static uschar *my_string_option = US"a default string";

  optionlist local_scan_options[] = {
    { "my_integer", opt_int,       &my_integer_option },
    { "my_string",  opt_stringptr, &my_string_option }
  };
  int local_scan_options_count =
    sizeof(local_scan_options)/sizeof(optionlist);

The values of the variables can now be changed from Exim's runtime configur-
ation file by including a local scan section as in this example:

  begin local_scan
  my_integer = 99
  my_string = some string of text...

The available types of option data are as follows:

opt_bool

   This specifies a boolean (true/false) option. The address should point to a
   variable of type "BOOL", which will be set to TRUE or FALSE, which are
   macros that are defined as '1' and '0', respectively. If you want to detect
   whether such a variable has been set at all, you can initialize it to
   TRUE_UNSET. (BOOL variables are integers underneath, so can hold more than
   two values.)

opt_fixed

   This specifies a fixed point number, such as is used for load averages. The
   address should point to a variable of type "int". The value is stored
   multiplied by 1000, so, for example, 1.4142 is truncated and stored as
   1414.

opt_int

   This specifies an integer; the address should point to a variable of type
   "int". The value may be specified in any of the integer formats accepted by
   Exim.

opt_mkint

   This is the same as "opt_int", except that when such a value is output in a
   -bP listing, if it is an exact number of kilobytes or megabytes, it is
   printed with the suffix K or M.

opt_octint

   This also specifies an integer, but the value is always interpeted as an
   octal integer, whether or not it starts with the digit zero, and it is
   always output in octal.

opt_stringptr

   This specifies a string value; the address must be a pointer to a variable
   that points to a string (for example, of type "uschar *").

opt_time

   This specifies a time interval value. The address must point to a variable
   of type "int". The value that is placed there is a number of seconds.

If the -bP command line option is followed by "local_scan", Exim prints out
the values of all the "local_scan()" options.


39.4 Available Exim variables

The header local_scan.h gives you access to a number of C variables. These are
the only ones that are guaranteed to be maintained from release to release.
Note, however, that you can obtain the value of any Exim variable by calling
"expand_string()". The exported variables are as follows:

unsigned int debug_selector

   This variable is set to zero when no debugging is taking place. Otherwise,
   it is a bitmap of debugging selectors. Two bits are identified for use in
   "local_scan()"; they are defined as macros:

    .   The "D_v" bit is set when -v was present on the command line. This is
        a testing option that is not privileged - any caller may set it. All
        the other selector bits can be set only by admin users.

    .   The "D_local_scan" bit is provided for use by "local_scan()"; it is
        set by the "+local_scan" debug selector. It is not included in the
        default set of debugging bits.

   Thus, to write to the debugging output only when "+local_scan" has been
   selected, you should use code like this:

     if ((debug_selector & D_local_scan) != 0)
       debug_printf("xxx", ...);

uschar *expand_string_message

   After a failing call to "expand_string()" (returned value NULL), the
   variable "expand_string_message" contains the error message, zero-
   terminated.

header_line *header_list

   A pointer to a chain of header lines. The "header_line" structure is
   discussed below.

header_line *header_last

   A pointer to the last of the header lines.

uschar *headers_charset                                                        |
                                                                               |
   The value of the "headers_charset" configuration option.                    |

BOOL host_checking

   This variable is TRUE during a host checking session that is initiated by
   the -bh command line option.

uschar *interface_address

   The IP address of the interface that received the message, as a string.
   This is NULL for locally submitted messages.

int interface_port

   The port on which this message was received.

uschar *message_id

   This variable contains the message id for the incoming message as a zero-
   terminated string.

uschar *received_protocol

   The name of the protocol by which the message was received.

int recipients_count

   The number of accepted recipients.

recipient_item *recipients_list

   The list of accepted recipients, held in a vector of length
   "recipients_count". The "recipient_item" structure is discussed below. You
   can add additional recipients by calling "receive_add_recipient()" (see
   below). You can delete recipients by removing them from the vector and
   adusting the value in "recipients_count". In particular, by setting
   "recipients_count" to zero you remove all recipients. If you then return
   the value "LOCAL_SCAN_ACCEPT", the message is accepted, but immediately
   blackholed. To replace the recipients, set "recipients_count" to zero and
   then call "receive_add_recipient()" as often as needed.

uschar *sender_address

   The envelope sender address. For bounce messages this is the empty string.

uschar *sender_host_address

   The IP address of the sending host, as a string. This is NULL for locally-
   submitted messages.

uschar *sender_host_authenticated

   The name of the authentication mechanism that was used, or NULL if the
   message was not received over an authenticated SMTP connection.

uschar *sender_host_name

   The name of the sending host, if known.

int sender_host_port

   The port on the sending host.

BOOL smtp_input                                                                |
                                                                               |
   This variable is TRUE for all SMTP input, including BSMTP.                  |
                                                                               |
BOOL smtp_batched_input                                                        |
                                                                               |
   This variable is TRUE for BSMTP input.                                      |

int store_pool

   The contents of this variable control which pool of memory is used for new
   requests. See section 39.8 for details.


39.5 Structure of header lines

The "header_line" structure contains the members listed below. You can add
additional header lines by calling the "header_add()" function (see below).
You can cause header lines to be ignored (deleted) by setting their type to *.

struct header_line *next

   A pointer to the next header line, or NULL for the last line.

int type

   A code identifying certain headers that Exim recognizes. The codes are
   printing characters, and are documented in chapter 49 of this manual.
   Notice in particular that any header line whose type is * is not
   transmitted with the message. This flagging is used for header lines that
   have been rewritten, or are to be removed (for example, Envelope-sender:
   header lines.) Effectively, * means 'deleted'.

int slen

   The number of characters in the header line, including the terminating and
   any internal newlines.

uschar *text

   A pointer to the text of the header. It always ends with a newline,
   followed by a zero byte. Internal newlines are preserved.


39.6 Structure of recipient items

The "recipient_item" structure contains these members:

uschar *address

   This is a pointer to the recipient address as it was received.

int pno

   This is used in later Exim processing when top level addresses are created
   by the "one_time" option. It is not relevant at the time "local_scan()" is
   run and must always contain -1 at this stage.

uschar *errors_to

   If this value is not NULL, bounce messages caused by failing to deliver to
   the recipient are sent to the address it contains. In other words, it
   overrides the envelope sender for this one recipient. (Compare the
   "errors_to" generic router option.) If a "local_scan()" function sets an
   "errors_to" field to an unqualified address, Exim qualifies it using the
   domain from "qualify_recipient". When "local_scan()" is called, the
   "errors_to" field is NULL for all recipients.


39.7 Available Exim functions

The header local_scan.h gives you access to a number of Exim functions. These
are the only ones that are guaranteed to be maintained from release to
release:

pid_t child_open(uschar **argv, uschar **envp, int newumask, int *infdptr, int
   *outfdptr, BOOL make_leader)

   This function creates a child process that runs the command specified by
   "argv". The environment for the process is specified by "envp", which can
   be NULL if no environment variables are to be passed. A new umask is
   supplied for the process in "newumask".

   Pipes to the standard input and output of the new process are set up and
   returned to the caller via the "infdptr" and "outfdptr" arguments. The
   standard error is cloned to the standard output. If there are any file
   descriptors 'in the way' in the new process, they are closed. If the final
   argument is TRUE, the new process is made into a process group leader.

   The function returns the pid of the new process, or -1 if things go wrong.

int child_close(pid_t pid, int timeout)

   This function waits for a child process to terminate, or for a timeout (in
   seconds) to expire. A timeout value of zero means wait as long as it takes.
   The return value is as follows:

    .   >= 0

        The process terminated by a normal exit and the value is the process
        ending status.

    .   < 0 and > -256

        The process was terminated by a signal and the value is the negation
        of the signal number.

    .   -256

        The process timed out.

    .   -257

        The was some other error in wait(); "errno" is still set.

pid_t child_open_exim(int *fd)

   This function provide you with a means of submitting a new message to Exim.
   (Of course, you can also call /usr/sbin/sendmail yourself if you want, but
   this packages it all up for you.) The function creates a pipe, forks a
   subprocess that is running

     exim -t -oem -oi -f <>

   and returns to you (via the "int *" argument) a file descriptor for the
   pipe that is connected to the standard input. The yield of the function is
   the PID of the subprocess. You can then write a message to the file
   descriptor, with recipients in To:, Cc:, and/or Bcc: header lines.

   When you have finished, call "child_close()" to wait for the process to
   finish and to collect its ending status. A timeout value of zero is usually
   fine in this circumstance. Unless you have made a mistake with the
   recipient addresses, you should get a return code of zero.

void debug_printf(char *, ...)

   This is Exim's debugging function, with arguments as for "(printf()". The
   output is written to the standard error stream. If no debugging is
   selected, calls to "debug_printf()" have no effect. Normally, you should
   make calls conditional on the "local_scan" debug selector by coding like
   this:

     if ((debug_selector & D_local_scan) != 0)
       debug_printf("xxx", ...);

uschar *expand_string(uschar *string)

   This is an interface to Exim's string expansion code. The return value is
   the expanded string, or NULL if there was an expansion failure. The C
   variable "expand_string_message" contains an error message after an expan-
   sion failure. If expansion does not change the string, the return value is
   the pointer to the input string. Otherwise, the return value points to a
   new block of memory that was obtained by a call to "store_get()". See
   section 39.8 below for a discussion of memory handling.

void header_add(int type, char *format, ...)

   This function allows you to add additional header lines. The first argument
   is the type, and should normally be a space character. The second argument
   is a format string and any number of substitution arguments as for
   "sprintf()". You may include internal newlines if you want, and you must
   ensure that the string ends with a newline.

uschar *lss_b64encode(uschar *cleartext, int length)

   This function base64-encodes a string, which is passed by address and
   length. The text may contain bytes of any value, including zero. The result
   is passed back in dynamic memory that is obtained by calling "store_get()".
   It is zero-terminated.

int lss_b64decode(uschar *codetext, uschar **cleartext)

   This function decodes a base64-encoded string. Its arguments are a zero-
   terminated base64-encoded string and the address of a variable that is set
   to point to the result, which is in dynamic memory. The length of the
   decoded string is the yield of the function. If the input is invalid base64
   data, the yield is -1. A zero byte is added to the end of the output string
   to make it easy to interpret as a C string (assuming it contains no zeros
   of its own). The added zero byte is not included in the returned count.

int lss_match_domain(uschar *domain, uschar *list)

   This function checks for a match in a domain list. Domains are always
   matched caselessly. The return value is one of the following:

     OK     match succeeded
     FAIL   match failed
     DEFER  match deferred

   DEFER is usually caused by some kind of lookup defer, such as the inability
   to contact a database.

int lss_match_local_part(uschar *localpart, uschar *list, BOOL caseless)

   This function checks for a match in a local part list. The third argument
   controls case-sensitivity. The return values are as for
   "lss_match_domain()".

int lss_match_address(uschar *address, uschar *list, BOOL caseless)

   This function checks for a match in an address list. The third argument
   controls the case-sensitivity of the local part match. The domain is always
   matched caselessly. The return values are as for "lss_match_domain()".

int lss_match_host(uschar *host_name, uschar *host_address, uschar *list)

   This function checks for a match in a host list. The most common usage is
   expected to be

     lss_match_host(sender_host_name, sender_host_address, ...)

   An empty address field matches an empty item in the host list. If the host
   name is NULL, the name corresponding to $sender_host_address is automati-
   cally looked up if a host name is required to match an item in the list.
   The return values are as for "lss_match_domain()", but in addition,
   "lss_match_host()" returns ERROR in the case when it had to look up a host
   name, but the lookup failed.

void log_write(unsigned int selector, int which, char *format, ...)

   This function writes to Exim's log files. The first argument should be
   zero (it is concerned with "log_selector"). The second argument can be
   "LOG_MAIN" or "LOG_REJECT" or "LOG_PANIC" or the inclusive 'or' of any
   combination of them. It specifies to which log or logs the message is
   written. The remaining arguments are a format and relevant insertion
   arguments. The string should not contain any newlines, not even at the end.

void receive_add_recipient(uschar *address, int pno)

   This function adds an additional recipient to the message. The first
   argument is the recipient address. If if is unqualified (has no domain), it
   is qualified with the "qualify_recipient" domain. The second argument must
   always be -1.

   This function does not allow you to specify a private "errors_to" address
   (as described with the structure of "recipient_item" above), because it
   pre-dates the addition of that field to the structure. However, it is easy
   to add such a value afterwards. For example:

     receive_add_recipient(US"monitor@mydom.example", -1);
     recipients_list[recipients_count-1].errors_to =
       US"postmaster@mydom.example";

uschar *rfc2047_decode(uschar *string, BOOL lencheck, uschar *target, int      |
   zeroval, int *lenptr, uschar **error)                                       |
                                                                               |
   This function decodes strings that are encoded according to RFC 2047.       |
   Typically these are the contents of header lines. First, each encoded       |
   'word' is decoded from the Q or B encoding into a byte-string. Then, if     |
   provided with the name of a charset encoding, and if the "iconv()" function |
   is available, an attempt is made to translate the result to the named       |
   character set. If this fails, the binary string is returned with an error   |
   message.                                                                    |
                                                                               |
   The first argument is the string to be decoded. If "lencheck" is TRUE, the  |
   maximum MIME word length is enforced. The third argument is the target      |
   encoding, or NULL if no translation is wanted.                              |
                                                                               |
   If a binary zero is encountered in the decoded string, it is replaced by    |
   the contents of the "zeroval" argument. For use with Exim headers, the      |
   value must not be 0 because header lines are handled as zero-terminated     |
   strings.                                                                    |
                                                                               |
   The function returns the result of processing the string, zero-terminated;  |
   if "lenptr" is not NULL, the length of the result is set in the variable to |
   which it points. When "zeroval" is 0, "lenptr" should not be NULL.          |
                                                                               |
   If an error is encountered, the function returns NULL and uses the "error"  |
   argument to return an error message. The variable pointed to by "error" is  |
   set to NULL if there is no error; it may be set non-NULL even when the      |
   function returns a non-NULL value if decoding was successful, but there was |
   a problem with translation.                                                 |
                                                                               |
int smtp_fflush(void)                                                          |
                                                                               |
   This function is used in conjunction with "smtp_printf()", as described     |
   below.                                                                      |
                                                                               |
void smtp_printf(char *, ...)                                                  |
                                                                               |
   The arguments of this function are like "printf()"; it writes to the SMTP   |
   output stream. You should use this function only when there is an SMTP      |
   output stream, that is, when the incoming message is being received via     |
   interactive SMTP. This is the case when "smtp_input" is TRUE and            |
   "smtp_batched_input" is FALSE. If you want to test for an incoming message  |
   from another host (as opposed to a local process that used the -bs command  |
   line option), you can test the value of "sender_host_address", which is     |
   non-NULL when a remote host is involved.                                    |
                                                                               |
   If an SMTP TLS connection is established, "smtp_printf()" uses the TLS      |
   output function, so it can be used for all forms of SMTP connection.        |
                                                                               |
   Strings that are written by "smtp_printf()" from within "local_scan()" must |
   start with an appropriate response code: 550 if you are going to return     |
   LOCAL_SCAN_REJECT, 451 if you are going to return LOCAL_SCAN_TEMPREJECT,    |
   and 250 otherwise. Because you are writing the initial lines of a multi-    |
   line response, the code must be followed by a hyphen to indicate that the   |
   line is not the final response line. You must also ensure that the lines    |
   you write terminate with CRLF. For example:                                 |
                                                                               |
     smtp_printf("550-this is some extra info\r\n");                           |
     return LOCAL_SCAN_REJECT;                                                 |
                                                                               |
   Note that you can also create multi-line responses by including newlines in |
   the data returned via the "return_text" argument. The added value of using  |
   "smtp_printf()" is that, for instance, you could introduce delays between   |
   multiple output lines.                                                      |
                                                                               |
   The "smtp_printf()" function does not return any error indication, because  |
   it does not automatically flush pending output, and therefore does not test |
   the state of the stream. (In the main code of Exim, flushing and error      |
   detection is done when Exim is ready for the next SMTP input command.) If   |
   you want to flush the output and check for an error (for example, the       |
   dropping of a TCP/IP connection), you can call "smtp_fflush()", which has   |
   no arguments. It flushes the output stream, and returns a non-zero value if |
   there is an error.                                                          |

void *store_get(int)

   This function accesses Exim's internal store (memory) manager. It gets a
   new chunk of memory whose size is given by the argument. Exim bombs out if
   it ever runs out of memory. See the next section for a discussion of memory
   handling.

void *store_get_perm(int)

   This function is like "store_get()", but it always gets memory from the
   permanent pool. See the next section for a discussion of memory handling.

uschar *string_copy(uschar *string)

uschar *string_copyn(uschar *string, int length)

uschar *string_sprintf(char *format, ...)

   These three functions create strings using Exim's dynamic memory facili-
   ties. The first makes a copy of an entire string. The second copies up to a
   maximum number of characters, indicated by the second argument. The third
   uses a format and insertion arguments to create a new string. In each case,
   the result is a pointer to a new string in the current memory pool. See the
   next section for more discussion.


39.8 More about Exim's memory handling

No function is provided for freeing memory, because that is never needed. The
dynamic memory that Exim uses when receiving a message is automatically
recycled if another message is received by the same process (this applies only
to incoming SMTP connections - other input methods can supply only one message
at a time). After receiving the last message, a reception process terminates.

Because it is recycled, the normal dynamic memory cannot be used for holding
data that must be preserved over a number of incoming messages on the same
SMTP connection. However, Exim in fact uses two pools of dynamic memory; the
second one is not recycled, and can be used for this purpose.

If you want to allocate memory that remains available for subsequent messages
in the same SMTP connection, you should set

  store_pool = POOL_PERM

before calling the function that does the allocation. There is no need to
restore the value if you do not need to; however, if you do want to revert to
the normal pool, you can either restore the previous value of "store_pool" or
set it explicitly to POOL_MAIN.

The pool setting applies to all functions that get dynamic memory, including
"expand_string()", "store_get()", and the "string_xxx()" functions. There is
also a convenience function called "store_get_perm()" that gets a block of
memory from the permanent pool while preserving the value of "store_pool".



                      40. SYSTEM-WIDE MESSAGE FILTERING


The previous chapters (on ACLs and the local scan function) describe checks
that can be applied to messages before they are accepted by a host. There is
also a mechanism for checking messages once they have been received, but
before they are delivered. This is called the system filter.

The system filter operates in a similar manner to users' filter files, but it
is run just once per message (however many recipients the message has). It
should not normally be used as a substitute for routing, because "deliver"
commands in a system router provide new envelope recipient addresses. The      |
system filter must be an Exim filter. It cannot be a Sieve filter.             |

The system filter is run at the start of a delivery attempt, before any
routing is done. If a message fails to be completely delivered at the first
attempt, the system filter is run again at the start of every retry. If you
want your filter to do something only once per message, you can make use of
the "first_delivery" condition in an "if" command in the filter to prevent it
happening on retries.

Warning: Because the system filter runs just once, variables that are specific
to individual recipient addresses, such as $local_part and $domain, are not
set, and the 'personal' condition is not meaningful. If you want to run a
centrally-specified filter for each recipient address independently, you can
do so by setting up a suitable redirect router, as described in section 40.8
below.


40.1 Specifying a system filter

The name of the file that contains the system filter must be specified by
setting "system_filter". If you want the filter to run under a uid and gid
other than root, you must also set "system_filter_user" and
"system_filter_group" as appropriate. For example:

  system_filter = /etc/mail/exim.filter
  system_filter_user = exim

If a system filter generates any deliveries directly to files or pipes (via
the "save" or "pipe" commands), transports to handle these deliveries must
be specified by setting "system_filter_file_transport" and
"system_filter_pipe_transport", respectively. Similarly,
"system_filter_reply_transport" must be set to handle any messages generated
by the "reply" command.


40.2 Testing a system filter

You can run simple tests of a system filter in the same way as for a user
filter, but you should use -bF rather than -bf, so that features that are
permitted only in system filters are recognized.


40.3 Contents of a system filter

The language used to specify system filters is the same as for users' filter
files. It is described in the separate end-user document "Exim's interface to
mail filtering". However, there are some additional features that are avail-
able only in system filters; these are described in subsequent sections. If
they are encountered in a user's filter file or when testing with -bf, they
cause errors.

There are two special conditions which, though available in users' filter
files, are designed for use in system filters. The condition "first_delivery"
is true only for the first attempt at delivering a message, and
"manually_thawed" is true only if the message has been frozen, and subse-
quently thawed by an admin user. An explicit forced delivery counts as a
manual thaw, but thawing as a result of the "auto_thaw" setting does not.

Warning: If a system filter uses the "first_delivery" condition to specify an
'unseen' (non-significant) delivery, and that delivery does not succeed, it
will not be tried again. If you want Exim to retry an unseen delivery until it
succeeds, you should arrange to set it up every time the filter runs.

When a system filter finishes running, the values of the variables $n0 - $n9
are copied into $sn0 - $sn9 and are thereby made available to users' filter
files. Thus a system filter can, for example, set up 'scores' to which users'
filter files can refer.


40.4 Additional variable for system filters

The expansion variable $recipients, containing a list of all the recipients of
the message (separated by commas and white space), is available in system
filters. It is not available in users' filters for privacy reasons.


40.5 Defer, freeze, and fail commands for system filters

There are three extra commands ("defer", "freeze" and "fail") which are always
available in system filters, but are not normally enabled in users' filters.
(See the "allow_defer", "allow_freeze" and "allow_fail" options for the
redirect router.) These commands can optionally be followed by the word "text"
and a string containing an error message, for example:

  fail text "this message looks like spam to me"

The keyword "text" is optional if the next character is a double quote.

The "defer" command defers delivery of the original recipients of the message.
The "fail" command causes all the original recipients to be failed, and a
bounce message to be created. The "freeze" command suspends all delivery
attempts for the original recipients. In all cases, any new deliveries that
are specified by the filter are attempted as normal after the filter has run.

The "freeze" command is ignored if the message has been manually unfrozen and
not manually frozen since. This means that automatic freezing by a system
filter can be used as a way of checking out suspicious messages. If a message
is found to be all right, manually unfreezing it allows it to be delivered.

The text given with a fail command is used as part of the bounce message as
well as being written to the log. If the message is quite long, this can fill
up a lot of log space when such failures are common. To reduce the size of the
log message, Exim interprets the text in a special way if it starts with the
two characters "<<" and contains ">>" later. The text between these two
strings is written to the log, and the rest of the text is used in the bounce
message. For example:

  fail "<<filter test 1>>Your message is rejected \
       because it contains attachments that we are \
       not prepared to receive."

Take great care with the "fail" command when basing the decision to fail on
the contents of the message, because the bounce message will of course include
the contents of the original message and will therefore trigger the "fail"
command again (causing a mail loop) unless steps are taken to prevent this.
Testing the "error_message" condition is one way to prevent this. You could
use, for example

  if $message_body contains "this is spam" and not error_message
    then fail text "spam is not wanted here" endif

though of course that might let through unwanted bounce messages. The
alternative is clever checking of the body and/or headers to detect bounces
generated by the filter.

The interpretation of a system filter file ceases after a "defer", "freeze",
or "fail" command is obeyed. However, any deliveries that were set up earlier
in the filter file are honoured, so you can use a sequence such as

  mail ...
  freeze

to send a specified message when the system filter is freezing (or deferring
or failing) a message. The normal deliveries for the message do not, of
course, take place.


40.6 Adding and removing headers in a system filter

Two filter commands that are available only in system filters are:

  headers add <<string>>
  headers remove <<string>>

The argument for the "headers add" is a string which is expanded and then
added to the end of the message's headers. It is the responsibility of the
filter maintainer to make sure it conforms to RFC 2822 syntax. Leading white
space is ignored, and if the string is otherwise empty, or if the expansion is
forced to fail, the command has no effect.

If the message is not delivered at the first attempt, header lines that were
added by the system filter are stored with the message, and so are still
present at the next delivery attempt. For that reason, it is usual to make the
"headers add" command conditional on "first_delivery".

You can use '\n' within the string, followed by white space, to specify
continued header lines. More than one header may be added in one command by
including '\n' within the string without any following white space (though
this is not recommended, for a reason given below). For example:

  headers add "X-header-1: ....\n  \
               continuation of X-header-1 ...\n\
               X-header-2: ...."

Note that the header line continuation white space after the first newline
must be placed before the backslash that continues the input string, because
white space after input continuations is ignored.

Header lines that are added by a system filter are visible to users' filter
files and to all routers and transports. However, if you add multiple header
lines in a single "headers add" command, the entire text is returned if a
subsequent string expansion refers to the first of them; the second and
subsequent header lines are not accessible. If you want the individual added
header lines to be accessible in string expansions, you must use a separate
"headers add" command for each one.

The argument for "headers remove" is a colon-separated list of header names.
This command applies only to those headers that are stored with the message;
those that are added at delivery time (such as Envelope-To: and Return-Path:)
cannot be removed by this means. If there is more than one header with the
same name, they are all removed.


40.7 Setting an errors address in a system filter

In a system filter, if a "deliver" command is followed by

  errors_to <some address>

in order to change the envelope sender (and hence the error reporting) for
that delivery, any address may be specified. (In a user filter, only the
current user's address can be set.) For example, if some mail is being
monitored, you might use

  unseen deliver monitor@spying.example errors_to root@local.example

to take a copy which would not be sent back to the normal error reporting
address if its delivery failed.


40.8 Per-address filtering

In contrast to the system filter, which is run just once per message for each
delivery attempt, it is also possible to set up a system-wide filtering
operation that runs once for each recipient address. In this case, variables
such as $local_part and $domain can be used, and indeed, the choice of filter
file could be made dependent on them. This is an example of a router which
implements such a filter:

  central_filter:
    driver = redirect
    domains = +local_domains
    file = /central/filters/$local_part
    no_verify
    allow_filter
    allow_freeze

Care should be taken to ensure that none of the commands in the filter file
specify a significant delivery if the message is to go on to be delivered to
its intended recipient. The router will not then claim to have dealt with the
address, so it will be passed on to subsequent routers to be delivered in the
normal way.



                 41. CUSTOMIZING BOUNCE AND WARNING MESSAGES


When a message fails to be delivered, or remains on the queue for more than a
configured amount of time, Exim sends a message to the original sender, or to
an alternative configured address. The text of these messages is built into
the code of Exim, but it is possible to change it, either by adding a single
string, or by replacing each of the paragraphs by text supplied in a file.

The From: and To: header lines are automatically generated; you can cause a    |
Reply-To: line to be added by setting the "errors_reply_to" option. Exim also  |
adds the line                                                                  |
                                                                               |
  Auto-Submitted: auto-generated                                               |
                                                                               |
to all warning and bounce messages,                                            |


41.1 Customizing bounce messages

If "bounce_message_text" is set, its contents are included in the default
message immediately after 'This message was created automatically by mail
delivery software.' The string is not expanded. It is not used if
"bounce_message_file" is set.

When "bounce_message_file" is set, it must point to a template file for
constructing error messages. The file consists of a series of text items,
separated by lines consisting of exactly four asterisks. If the file cannot be
opened, default text is used and a message is written to the main and panic
logs. If any text item in the file is empty, default text is used for that
item.

Each item of text that is read from the file is expanded, and there are two
expansion variables which can be of use here: $bounce_recipient is set to
the recipient of an error message while it is being created, and
$return_size_limit contains the value of the "return_size_limit" option,
rounded to a whole number.

The items must appear in the file in the following order:

 .   The first item is included in the headers, and should include at least a
     Subject: header. Exim does not check the syntax of these headers.

 .   The second item forms the start of the error message. After it, Exim
     lists the failing addresses with their error messages.

 .   The third item is used to introduce any text from pipe transports that is
     to be returned to the sender. It is omitted if there is no such text.

 .   The fourth item is used to introduce the copy of the message that is
     returned as part of the error report.

 .   The fifth item is added after the fourth one if the returned message is
     truncated because it is bigger than "return_size_limit".

 .   The sixth item is added after the copy of the original message.

The default state ("bounce_message_file" unset) is equivalent to the following
file, in which the sixth item is empty. The Subject: line has been split into
two here in order to fit it on the page:
______________________________________________________________________________

Subject: Mail delivery failed
  ${if eq{$sender_address}{$bounce_recipient}{: returning message to sender}}
****
This message was created automatically by mail delivery software.

A message ${if eq{$sender_address}{$bounce_recipient}{that you sent }{sent by

  <$sender_address>

}}could not be delivered to all of its recipients.
The following address(es) failed:
****
The following text was generated during the delivery attempt(s):
****
------ This is a copy of the message, including all the headers. ------
****
------ The body of the message is $message_size characters long; only the first
------ $return_size_limit or so are included here.
****

______________________________________________________________________________


41.2 Customizing warning messages

The option "warn_message_file" can be pointed at a template file for use when
warnings about message delays are created. In this case there are only three
text sections:

 .   The first item is included in the headers, and should include at least a
     Subject: header. Exim does not check the syntax of these headers.

 .   The second item forms the start of the warning message. After it, Exim
     lists the delayed addresses.

 .   The third item then ends the message.

The default state is equivalent to the following file, except that the line
starting 'A message' has been split here, in order to fit it on the page:
______________________________________________________________________________

Subject: Warning: message $message_id delayed $warn_message_delay
****
This message was created automatically by mail delivery software.

A message ${if eq{$sender_address}{$warn_message_recipients}
  {that you sent }{sent by

  <$sender_address>

}}has not been delivered to all of its recipients after
more than $warn_message_delay on the queue on $primary_hostname.

The message identifier is:     $message_id
The subject of the message is: $h_subject
The date of the message is:    $h_date

The following address(es) have not yet been delivered:
****
No action is required on your part. Delivery attempts will continue for
some time, and this warning may be repeated at intervals if the message
remains undelivered. Eventually the mail delivery software will give up,
and when that happens, the message will be returned to you.

______________________________________________________________________________

except that in the default state the subject and date lines are omitted
if no appropriate headers exist. During the expansion of this file,
$warn_message_delay is set to the delay time in one of the forms '<n> minutes'
or '<n> hours', and $warn_message_recipients contains a list of recipients for
the warning message. There may be more than one if there are multiple
addresses with different "errors_to" settings on the routers that handled
them.



                  42. SOME COMMON CONFIGURATION REQUIREMENTS


This chapter discusses some configuration requirements that seem to be fairly
common. More examples and discussion can be found in the Exim book.


42.1 Sending mail to a smart host

If you want to send all mail for non-local domains to a 'smart host', you
should replace the default dnslookup router with a router which does the
routing explicitly:

  send_to_smart_host:
    driver = manualroute
    route_list = !+local_domains smart.host.name
    transport = remote_smtp

You can use the smart host's IP address instead of the name if you wish.


42.2 Using Exim to handle mailing lists

Exim can be used to run simple mailing lists, but for large and/or complicated
requirements, the use of additional specialized mailing list software such as
Majordomo or Mailman is recommended.

The redirect router can be used to handle mailing lists where each list is
maintained in a separate file, which can therefore be managed by an indepen-
dent manager. The "domains" router option can be used to run these lists in a
separate domain from normal mail. For example:

  lists:
    driver = redirect
    domains = lists.example
    file = /usr/lists/$local_part
    forbid_pipe
    forbid_file
    errors_to = $local_part-request@lists.example
    no_more

This router is skipped for domains other than "lists.example". For addresses
in that domain, it looks for a file that matches the local part. If there is
no such file, the router declines, but because "no_more" is set, no subsequent
routers are tried, and so the whole delivery fails.

The "forbid_pipe" and "forbid_file" options prevent a local part from being
expanded into a file name or a pipe delivery, which is usually inappropriate
in a mailing list.

The "errors_to" option specifies that any delivery errors caused by addresses
taken from a mailing list are to be sent to the given address rather than the
original sender of the message. However, before acting on this, Exim verifies
the error address, and ignores it if verification fails.

For example, using the configuration above, mail sent to "dicts@lists.example"
is passed on to those addresses contained in /usr/lists/dicts, with error
reports directed to "dicts-request@lists.example", provided that this address
can be verified. There could be a file called /usr/lists/dicts-request
containing the address(es) of this particular list's manager(s), but other
approaches, such as setting up an earlier router (possibly using the
"local_part_prefix" or "local_part_suffix" options) to handle addresses of the
form "owner-xxx" or "xxx-request", are also possible.


42.3 Syntax errors in mailing lists

If an entry in redirection data contains a syntax error, Exim normally defers
delivery of the original address. That means that a syntax error in a mailing
list holds up all deliveries to the list. This may not be appropriate when a
list is being maintained automatically from data supplied by users, and the
addresses are not rigorously checked.

If the "skip_syntax_errors" option is set, the redirect router just skips
entries that fail to parse, noting the incident in the log. If in addition
"syntax_errors_to" is set to a verifiable address, a message is sent to it
whenever a broken address is skipped. It is usually appropriate to set
"syntax_errors_to" to the same address as "errors_to".


42.4 Re-expansion of mailing lists

Exim remembers every individual address to which a message has been delivered,
in order to avoid duplication, but it normally stores only the original
recipient addresses with a message. If all the deliveries to a mailing list
cannot be done at the first attempt, the mailing list is re-expanded when the
delivery is next tried. This means that alterations to the list are taken into
account at each delivery attempt, so addresses that have been added to the
list since the message arrived will therefore receive a copy of the message,
even though it pre-dates their subscription.

If this behaviour is felt to be undesirable, the "one_time" option can be set
on the redirect router. If this is done, any addresses generated by the router
that fail to deliver at the first attempt are added to the message as 'top
level' addresses, and the parent address that generated them is marked
'delivered'. Thus, expansion of the mailing list does not happen again at the
subsequent delivery attempts. The disadvantage of this is that if any of the
failing addresses are incorrect, correcting them in the file has no effect on
pre-existing messages.

The original top-level address is remembered with each of the generated
addresses, and is output in any log messages. However, any intermediate parent
addresses are not recorded. This makes a difference to the log only if the
"all_parents" selector is set, but for mailing lists there is normally only
one level of expansion anyway.


42.5 Closed mailing lists

The examples so far have assumed open mailing lists, to which anybody may send
mail. It is also possible to set up closed lists, where mail is accepted from
specified senders only. This is done by making use of the generic "senders"
option to restrict the router that handles the list.

The following example uses the same file as a list of recipients and as a list
of permitted senders. It requires three routers:

  lists_request:
    driver = redirect
    domains = lists.example
    local_part_suffix = -request
    file = /usr/lists/$local_part$local_part_suffix
    no_more

  lists_post:
    driver = redirect
    domains = lists.example
    senders = ${if exists {/usr/lists/$local_part}\
               {lsearch;/usr/lists/$local_part}{*}}
    file = /usr/lists/$local_part
    forbid_pipe
    forbid_file
    errors_to = $local_part-request@lists.example
    no_more

  lists_closed:
    driver = redirect
    domains = lists.example
    allow_fail
    data = :fail: $local_part@lists.example is a closed mailing list

All three routers have the same "domains" setting, so for any other domains,
they are all skipped. The first router runs only if the local part ends in
"-request". It handles messages to the list manager(s) by means of an open
mailing list.

The second router runs only if the "senders" precondition is satisfied. It
checks for the existence of a list that corresponds to the local part, and
then checks that the sender is on the list by means of a linear search. It is
necessary to check for the existence of the file before trying to search it,
because otherwise Exim thinks there is a configuration error. If the file does
not exist, the expansion of "senders" is *, which matches all senders. This
means that the router runs, but because there is no list, declines, and
"no_more" ensures that no further routers are run. The address fails with an
'unrouteable address' error.

The third router runs only if the second router is skipped, which happens when
a mailing list exists, but the sender is not on it. This router forcibly fails
the address, giving a suitable error message.


42.6 Virtual domains

The phrase "virtual domain" is unfortunately used with two rather different
meanings:

 .   A domain for which there are no real mailboxes; all valid local parts are
     aliases for other email addresses. Common examples are organizational
     top-level domains and 'vanity' domains.

 .   One of a number of independent domains that are all handled by the same
     host, with mailboxes on that host, but where the mailbox owners do not
     necessarily have login accounts on that host.

The first usage is probably more common, and does seem more 'virtual' than the
second. This kind of domain can be handled in Exim with a straightforward
aliasing router. One approach is to create a separate alias file for each
virtual domain. Exim can test for the existence of the alias file to determine
whether the domain exists. The dsearch lookup type is useful here, leading to
a router of this form:

  virtual:
    driver = redirect
    domains = dsearch;/etc/mail/virtual
    data = ${lookup{$local_part}lsearch{/etc/mail/virtual/$domain}}
    no_more

The "domains" option specifies that the router is to be skipped, unless there
is a file in the /etc/mail/virtual directory whose name is the same as the
domain that is being processed. When the router runs, it looks up the local
part in the file to find a new address (or list of addresses). The "no_more"
setting ensures that if the lookup fails (leading to "data" being an empty
string), Exim gives up on the address without trying any subsequent routers.

This one router can handle all the virtual domains because the alias file
names follow a fixed pattern. Permissions can be arranged so that appropriate
people can edit the different alias files. A successful aliasing operation
results in a new envelope recipient address, which is then routed from
scratch.

The other kind of 'virtual' domain can also be handled in a straightforward
way. One approach is to create a file for each domain containing a list of
valid local parts, and use it in a router like this:

  my_domains:
    driver = accept
    domains = dsearch;/etc/mail/domains
    local_parts = lsearch;/etc/mail/domains/$domain
    transport = my_mailboxes

The address is accepted if there is a file for the domain, and the local part
can be found in the file. The "domains" option is used to check for the file's
existence because "domains" is tested before the "local_parts" option (see
section 3.11). You can't use "require_files", because that option is tested
after "local_parts". The transport is as follows:

  my_mailboxes:
    driver = appendfile
    file = /var/mail/$domain/$local_part
    user = mail

This uses a directory of mailboxes for each domain. The "user" setting is
required, to specify which uid is to be used for writing to the mailboxes.

The configuration shown here is just one example of how you might support this
requirement. There are many other ways this kind of configuration can be set
up, for example, by using a database instead of separate files to hold all the
information about the domains.


42.7 Multiple user mailboxes

Heavy email users often want to operate with multiple mailboxes, into which
incoming mail is automatically sorted. A popular way of handling this is to
allow users to use multiple sender addresses, so that replies can easily be
identified. Users are permitted to add prefixes or suffixes to their local
parts for this purpose. The wildcard facility of the generic router options
"local_part_prefix" and "local_part_suffix" can be used for this. For example,
consider this router:

  userforward:
    driver = redirect
    check_local_user
    file = $home/.forward
    local_part_suffix = -*
    local_part_suffix_optional
    allow_filter

It runs a user's .forward file for all local parts of the form "username-*".
Within the filter file the user can distinguish different cases by testing the
variable $local_part_suffix. For example:

  if $local_part_suffix contains -special then
    save /home/$local_part/Mail/special
  endif

If the filter file does not exist, or does not deal with such addresses, they
fall through to subsequent routers, and, assuming no subsequent use of the
"local_part_suffix" option is made, they presumably fail. Thus, users have
control over which suffixes are valid.

Alternatively, a suffix can be used to trigger the use of a different .forward
file - which is the way a similar facility is implemented in another MTA:

  userforward:
    driver = redirect
    check_local_user
    file = $home/.forward$local_part_suffix
    local_part_suffix = -*
    local_part_suffix_optional
    allow_filter

If there is no suffix, .forward is used; if the suffix is "-special", for
example, .forward-special is used. Once again, if the appropriate file does
not exist, or does not deal with the address, it is passed on to subsequent
routers, which could, if required, look for an unqualified .forward file to
use as a default.


42.8 Simplified vacation processing

The traditional way of running the "vacation" program is for a user to set up
a pipe command in a .forward file (see section 22.6 for syntax details). This  |
is prone to error by inexperienced users. There are two features of Exim that
can be used to make this process simpler for users:

 .   A local part prefix such as 'vacation-' can be specified on a router
     which can cause the message to be delivered directly to the "vacation"
     program, or alternatively can use Exim's autoreply transport. The con-
     tents of a user's .forward file are then much simpler. For example:

       spqr, vacation-spqr

 .   The "require_files" generic router option can be used to trigger a
     vacation delivery by checking for the existence of a certain file in the
     user's home directory. The "unseen" generic option should also be used,
     to ensure that the original delivery also proceeds. In this case, all the
     user has to do is to create a file called, say, .vacation, containing a
     vacation message.

Another advantage of both these methods is that they both work even when the
use of arbitrary pipes by users is locked out.


42.9 Taking copies of mail

Some installations have policies that require archive copies of all messages
to be made. A single copy of each message can easily be taken by an
appropriate command in a system filter, which could, for example, use a
different file for each day's messages.

There is also a shadow transport mechanism that can be used to take copies of
messages that are successfully delivered by local transports, one copy per
delivery. This could be used, inter alia, to implement automatic notification
of delivery by sites that insist on doing such things.


42.10 Intermittently connected hosts

It has become quite common (because it is cheaper) for hosts to connect to the
Internet periodically rather than remain connected all the time. The normal
arrangement is that mail for such hosts accumulates on a system that is
permanently connected.

Exim was designed for use on permanently connected hosts, and so it is not
particularly well-suited to use in an intermittently connected environment.
Nevertheless there are some features that can be used.


42.11 Exim on the upstream server host

It is tempting to arrange for incoming mail for the intermittently connected
host to remain on Exim's queue until the client connects. However, this
approach does not scale very well. Two different kinds of waiting message are
being mixed up in the same queue - those that cannot be delivered because of
some temporary problem, and those that are waiting for their destination host
to connect. This makes it hard to manage the queue, as well as wasting
resources, because each queue runner scans the entire queue.

A better approach is to separate off those messages that are waiting for an
intermittently connected host. This can be done by delivering these messages
into local files in batch SMTP, 'mailstore', or other envelope-preserving
format, from where they are transmitted by other software when their desti-
nation connects. This makes it easy to collect all the mail for one host in a
single directory, and to apply local timeout rules on a per-message basis if
required.

On a very small scale, leaving the mail on Exim's queue can be made to work.
If you are doing this, you should configure Exim with a long retry period for
the intermittent host. For example:

  cheshire.wonderland.fict.example    *   F,5d,24h

This stops a lot of failed delivery attempts from occurring, but Exim
remembers which messages it has queued up for that host. Once the intermittent
host comes online, forcing delivery of one message (either by using the -M or
-R options, or by using the ETRN SMTP command (see section 43.9) causes all
the queued up messages to be delivered, often down a single SMTP connection.
While the host remains connected, any new messages get delivered immediately.

If the connecting hosts do not have fixed IP addresses, that is, if a host is
issued with a different IP address each time it connects, Exim's retry
mechanisms on the holding host get confused, because the IP address is
normally used as part of the key string for holding retry information. This
can be avoided by unsetting "retry_include_ip_address" on the smtp transport.
Since this has disadvantages for permanently connected hosts, it is best to
arrange a separate transport for the intermittently connected ones.


42.12 Exim on the intermittently connected client host

The value of "smtp_accept_queue_per_connection" should probably be increased,
or even set to zero (that is, disabled) on the intermittently connected host,
so that all incoming messages down a single connection get delivered
immediately.

Mail waiting to be sent from an intermittently connected host will probably
not have been routed, because without a connection DNS lookups are not
possible. This means that if a normal queue run is done at connection time,
each message is likely to be sent in a separate SMTP session. This can be
avoided by starting the queue run with a command line option beginning with
-qq instead of -q. In this case, the queue is scanned twice. In the first
pass, routing is done but no deliveries take place. The second pass is a
normal queue run; since all the messages have been previously routed, those
destined for the same host are likely to get sent as multiple deliveries in a
single SMTP connection.



                             43. SMTP PROCESSING


Exim supports a number of different ways of using the SMTP protocol, and its
LMTP variant, which is an interactive protocol for transferring messages into
a closed mail store application. This chapter contains details of how SMTP is
processed. For incoming mail, the following are available:

 .   SMTP over TCP/IP (Exim daemon or "inetd");

 .   SMTP over the standard input and output (the -bs option);

 .   Batched SMTP on the standard input (the -bS option).

For mail delivery, the following are available:

 .   SMTP over TCP/IP (the smtp transport);

 .   LMTP over TCP/IP (the smtp transport with the "protocol" option set to
     'lmtp');

 .   LMTP over a pipe to a process running in the local host (the lmtp
     transport);

 .   Batched SMTP to a file or pipe (the appendfile and pipe transports with
     the "use_bsmtp" option set).

"Batched SMTP" is the name for a process in which batches of messages are
stored in or read from files (or pipes), in a format in which SMTP commands
are used to contain the envelope information.


43.1 Outgoing SMTP and LMTP over TCP/IP

Outgoing SMTP and LMTP over TCP/IP is implemented by the smtp transport. The
"protocol" option selects which protocol is to be used, but the actual
processing is the same in both cases.

If, in response to its EHLO command, Exim is told that the SIZE parameter is
supported, it adds SIZE=<n> to each subsequent MAIL command. The value of <n>
is the message size plus the value of the "size_addition" option (default
1024) to allow for additions to the message such as per-transport header
lines, or changes made in a transport filter. If "size_addition" is set
negative, the use of SIZE is suppressed.

If the remote server advertises support for PIPELINING, Exim uses the
pipelining extension to SMTP (RFC 2197) to reduce the number of TCP/IP packets
required for the transaction.

If the remote server advertises support for the STARTTLS command, and Exim was
built to support TLS encryption, it tries to start a TLS session unless the
server matches "hosts_avoid_tls". See chapter 37 for more details.

If the remote server advertises support for the AUTH command, Exim scans the
authenticators configuration for any suitable client settings, as described in
chapter 33.

Responses from the remote host are supposed to be terminated by CR followed by
LF. However, there are known to be hosts that do not send CR characters, so in
order to be able to interwork with such hosts, Exim treats LF on its own as a
line terminator.

If a message contains a number of different addresses, all those with the same
characteristics (for example, the same envelope sender) that resolve to the
same set of hosts, in the same order, are sent in a single SMTP transaction,
even if they are for different domains, unless there are more than the setting
of the "max_rcpts" option in the smtp transport allows, in which case they are
split into groups containing no more than "max_rcpts" addresses each. If
"remote_max_parallel" is greater than one, such groups may be sent in parallel
sessions. The order of hosts with identical MX values is not significant when
checking whether addresses can be batched in this way.

When the smtp transport suffers a temporary failure that is not message-
related, Exim updates its transport-specific database, which contains records
indexed by host name that remember which messages are waiting for each
particular host. It also updates the retry database with new retry times.
Exim's retry hints are based on host name plus IP address, so if one address
of a multi-homed host is broken, it will soon be skipped most of the time. See
the next section for more detail about error handling.

When a message is successfully delivered over a TCP/IP SMTP connection, Exim
looks in the hints database for the transport to see if there are any queued
messages waiting for the host to which it is connected. If it finds one, it
creates a new Exim process using the -MC option (which can only be used by a
process running as root or the Exim user) and passes the TCP/IP socket to it
so that it can deliver another message using the same socket. The new process
does only those deliveries that are routed to the connected host, and may in
turn pass the socket on to a third process, and so on.

The "connection_max_messages" option of the smtp transport can be used to
limit the number of messages sent down a single TCP/IP connection. The second
and subsequent messages delivered down an existing connection are identified
in the main log by the addition of an asterisk after the closing square
bracket of the IP address.


43.2 Errors in outgoing SMTP

Three different kinds of error are recognized for outgoing SMTP: host errors,
message errors, and recipient errors.

(1)  A host error is not associated with a particular message or with a
     particular recipient of a message. The host errors are:

      .   Connection refused or timed out,

      .   Any error response code on connection,

      .   Any error response code to EHLO or HELO,

      .   Loss of connection at any time, except after '.',

      .   I/O errors at any time,

      .   Timeouts during the session, other than in response to MAIL, RCPT or
          the '.' at the end of the data.

     For a host error, a permanent error response on connection, or in
     response to EHLO, causes all addresses routed to the host to be failed.
     Any other host error causes all addresses to be deferred, and retry data
     to be created for the host. It is not tried again, for any message, until
     its retry time arrives. If the current set of addresses are not all
     delivered in this run (to some alternative host), the message is added to
     the list of those waiting for this host, so if it is still undelivered
     when a subsequent successful delivery is made to the host, it will be
     sent down the same SMTP connection.

(2)  A message error is associated with a particular message when sent to a
     particular host, but not with a particular recipient of the message. The
     message errors are:

      .   Any error response code to MAIL, DATA, or the '.' that terminates
          the data,

      .   Timeout after MAIL,

      .   Timeout or loss of connection after the '.' that terminates the
          data. A timeout after the DATA command itself is treated as a host
          error, as is loss of connection at any other time.

     For a message error, a permanent error response (5xx) causes all
     addresses to be failed, and a delivery error report to be returned to the
     sender. A temporary error response (4xx), or one of the timeouts, causes
     all addresses to be deferred. Retry data is not created for the host, but
     instead, a retry record for the combination of host plus message id is
     created. The message is not added to the list of those waiting for this
     host. This ensures that the failing message will not be sent to this host
     again until the retry time arrives. However, other messages that are
     routed to the host are not affected, so if it is some property of the
     message that is causing the error, it will not stop the delivery of other
     mail.

     If the remote host specified support for the SIZE parameter in its
     response to EHLO, Exim adds SIZE=nnn to the MAIL command, so an over-
     large message will cause a message error because the error arrives as a
     response to MAIL.

(3)  A recipient error is associated with a particular recipient of a message.
     The recipient errors are:

      .   Any error response to RCPT,

      .   Timeout after RCPT.

     For a recipient error, a permanent error response (5xx) causes the
     recipient address to be failed, and a bounce message to be returned to
     the sender. A temporary error response (4xx) or a timeout causes the
     failing address to be deferred, and routing retry data to be created for
     it. This is used to delay processing of the address in subsequent queue
     runs, until its routing retry time arrives. This applies to all messages,
     but because it operates only in queue runs, one attempt will be made to
     deliver a new message to the failing address before the delay starts to
     operate. This ensures that, if the failure is really related to the
     message rather than the recipient ('message too big for this recipient'
     is a possible example), other messages have a chance of getting
     delivered. If a delivery to the address does succeed, the retry infor-
     mation gets cleared, so all stuck messages get tried again, and the retry
     clock is reset.

     The message is not added to the list of those waiting for this host. Use
     of the host for other messages is unaffected, and except in the case of a
     timeout, other recipients are processed independently, and may be suc-
     cessfully delivered in the current SMTP session. After a timeout it is of
     course impossible to proceed with the session, so all addresses get
     deferred. However, those other than the one that failed do not suffer any
     subsequent retry delays. Therefore, if one recipient is causing trouble,
     the others have a chance of getting through when a subsequent delivery
     attempt occurs before the failing recipient's retry time.

In all cases, if there are other hosts (or IP addresses) available for the
current set of addresses (for example, from multiple MX records), they are
tried in this run for any undelivered addresses, subject of course to their
own retry data. In other words, recipient error retry data does not take
effect until the next delivery attempt.

Some hosts have been observed to give temporary error responses to every MAIL
command at certain times ('insufficient space' has been seen). It would be
nice if such circumstances could be recognized, and defer data for the host
itself created, but this is not possible within the current Exim design. What
actually happens is that retry data for every (host, message) combination is
created.

The reason that timeouts after MAIL and RCPT are treated specially is that
these can sometimes arise as a result of the remote host's verification
procedures. Exim makes this assumption, and treats them as if a temporary
error response had been received. A timeout after '.' is treated specially
because it is known that some broken implementations fail to recognize the end
of the message if the last character of the last line is a binary zero. Thus,
it is helpful to treat this case as a message error.

Timeouts at other times are treated as host errors, assuming a problem with
the host, or the connection to it. If a timeout after MAIL, RCPT, or '.' is
really a connection problem, the assumption is that at the next try the
timeout is likely to occur at some other point in the dialogue, causing it
then to be treated as a host error.

There is experimental evidence that some MTAs drop the connection after the
terminating '.' if they do not like the contents of the message for some
reason, in contravention of the RFC, which indicates that a 5xx response
should be given. That is why Exim treats this case as a message rather than a
host error, in order not to delay other messages to the same host.


43.3 Variable Envelope Return Paths (VERP)

Variable Envelope Return Paths - see
ftp://koobera.math.uic.edu/www/proto/verp.txt - can be supported in Exim by
using the "return_path" generic transport option to rewrite the return path at
transport time. For example, the following could be used on an smtp transport:

  return_path = \
    ${if match {$return_path}{^(.+?)-request@your.dom.example\$}\
    {$1-request=$local_part%$domain@your.dom.example}fail}

This has the effect of rewriting the return path (envelope sender) on all
outgoing SMTP messages, if the local part of the original return path ends in
'-request', and the domain is "your.dom.example". The rewriting inserts the
local part and domain of the recipient into the return path. Suppose, for
example, that a message whose return path has been set to "somelist-
request@your.dom.example" is sent to "subscriber@other.dom.example". In the
transport, the return path is rewritten as

  somelist-request=subscriber%other.dom.example@your.dom.example

For this to work, you must arrange for outgoing messages that have '-request'
in their return paths to have just a single recipient. This can be done by
setting

  max_rcpt = 1

in the smtp transport. Otherwise a single copy of a message might be addressed
to several different recipients in the same domain, in which case $local_part
is not available (because it is not unique). Of course, if you do start
sending out messages with this kind of return path, you must also configure
Exim to accept the bounce messages that come back to those paths. Typically
this would be done by setting an "local_part_suffix" option for a suitable
router.

The overhead incurred in using VERP depends very much on the size of the
message, the number of recipient addresses that resolve to the same remote
host, and the speed of the connection over which the message is being sent. If
a lot of addresses resolve to the same host and the connection is slow,
sending a separate copy of the message for each address may take substantially
longer than sending a single copy with many recipients (for which VERP cannot
be used).


43.4 Incoming SMTP messages over TCP/IP

Incoming SMTP messages can be accepted in one of two ways: by running a
listening daemon, or by using "inetd". In the latter case, the entry in
/etc/inetd.conf should be like this:

  smtp  stream  tcp  nowait  exim  /opt/exim/bin/exim  in.exim  -bs

Exim distinguishes between this case and the case of a locally running user
agent using the -bs option by checking whether or not the standard input is a
socket. When it is, either the port must be privileged (less than 1024), or
the caller must be root or the Exim user. If any other user passes a socket
with an unprivileged port number, Exim prints a message on the standard error
stream and exits with an error code.

By default, Exim does not make a log entry when a remote host connects or
disconnects (either via the daemon or "inetd"), unless the disconnection is
unexpected. It can be made to write such log entries by setting the
"smtp_connection" log selector.

Commands from the remote host are supposed to be terminated by CR followed by
LF. However, there are known to be hosts that do not send CR characters. In
order to be able to interwork with such hosts, Exim treats LF on its own as a
line terminator. Furthermore, because common code is used for receiving        |
messages from all sources, a CR on its own is also interpreted as a line       |
terminator. However, the sequence 'CR, dot, CR' does not terminate incoming    |
SMTP data.                                                                     |

One area that sometimes gives rise to problems concerns the EHLO or HELO
commands. Some clients send syntactically invalid versions of these commands,
which Exim rejects by default. (This is nothing to do with verifying the data
that is sent, so "helo_verify_hosts" is not relevant.) You can tell Exim not
to apply a syntax check by setting "helo_accept_junk_hosts" to match the
broken hosts that send invalid commands.

The amount of disk space available is checked whenever SIZE is received on
a MAIL command, independently of whether "message_size_limit" or
"check_spool_space" is configured, unless "smtp_check_spool_space" is set
false. A temporary error is given if there is not enough space. If
"check_spool_space" is set, the check is for that amount of space plus the
value given with SIZE, that is, it checks that the addition of the incoming
message will not reduce the space below the threshold.

When a message is successfully received, Exim includes the local message id in
its response to the final '.' that terminates the data. If the remote host
logs this text it can help with tracing what has happened to a message.

The Exim daemon can limit the number of simultaneous incoming connections it
is prepared to handle (see the "smtp_accept_max" option). It can also limit
the number of simultaneous incoming connections from a single remote host (see
the "smtp_accept_max_per_host" option). Additional connection attempts are
rejected using the SMTP temporary error code 421.

The Exim daemon does not rely on the SIGCHLD signal to detect when a
subprocess has finished, as this can get lost at busy times. Instead, it looks
for completed subprocesses every time it wakes up. Provided there are other
things happening (new incoming calls, starts of queue runs), completed
processes will be noticed and tidied away. On very quiet systems you may
sometimes see a 'defunct' Exim process hanging about. This is not a problem;
it will be noticed when the daemon next wakes up.

When running as a daemon, Exim can reserve some SMTP slots for specific hosts,
and can also be set up to reject SMTP calls from non-reserved hosts at times
of high system load - for details see the "smtp_accept_reserve",
"smtp_load_reserve", and "smtp_reserve_hosts" options. The load check applies
in both the daemon and "inetd" cases.

Exim normally starts a delivery process for each message received, though this
can be varied by means of the -odq command line option and the "queue_only",
"queue_only_file", and "queue_only_load" options. The number of simultaneously
running delivery processes started in this way from SMTP input can be limited
by the "smtp_accept_queue" and "smtp_accept_queue_per_connection" options.
When either limit is reached, subsequently received messages are just put on
the input queue without starting a delivery process.

The controls that involve counts of incoming SMTP calls ("smtp_accept_max",
"smtp_accept_queue", "smtp_accept_reserve") are not available when Exim is
started up from the "inetd" daemon, because in that case each connection is
handled by an entirely independent Exim process. Control by load average is,
however, available with "inetd".

Exim can be configured to verify addresses in incoming SMTP commands as they
are received. See chapter 38 for details. It can also be configured to rewrite
addresses at this time - before any syntax checking is done. See section 31.7.

Exim can also be configured to limit the rate at which a client host submits
MAIL and RCPT commands in a single SMTP session. See the
"smtp_ratelimit_hosts" option.


43.5 Unrecognized SMTP commands

If Exim receives more than "smtp_max_unknown_commands" unrecognized SMTP
commands during a single SMTP connection, it drops the connection after
sending the error response to the last command. The default value for
"smtp_max_unknown_commands" is 3. This is a defence against some kinds of
abuse that subvert web servers into making connections to SMTP ports; in these
circumstances, a number of non-SMTP lines are sent first.

                                                                               |
43.6 Syntax and protocol errors in SMTP commands                               |
                                                                               |
A syntax error is detected if an SMTP command is recognized, but there is      |
something syntactically wrong with its data, for example, a malformed email    |
address in a RCPT command. Protocol errors include invalid command sequencing  |
such as RCPT before MAIL. If Exim receives more than "smtp_max_synprot_errors" |
such commands during a single SMTP connection, it drops the connection after   |
sending the error response to the last command. The default value for "smtp_   |
max_synprot_errors" is 3. This is a defence against broken clients that loop   |
sending bad commands (yes, it has been seen).                                  |


43.7 Use of non-mail SMTP commands

The 'non-mail' SMTP commands are those other than MAIL, RCPT, and DATA. Exim
counts such commands, and drops the connection if there are too many of them
in a single SMTP session. This action catches some denial-of-service attempts
and things like repeated failing AUTHs, or a mad client looping sending EHLO.
The global option "smtp_accept_max_nonmail" defines what 'too many' means. Its
default value is 10.

When a new message is expected, one occurrence of RSET is not counted. This
allows a client to send one RSET between messages (this is not necessary, but
some clients do it). Exim also allows one uncounted occurence of HELO or EHLO,
and one occurrence of STARTTLS between messages. After starting up a TLS
session, another EHLO is expected, and so it too is not counted.

The first occurrence of AUTH in a connection, or immediately following
STARTTLS is also not counted. Otherwise, all commands other than MAIL, RCPT,
DATA, and QUIT are counted.

You can control which hosts are subject to the limit set by
"smtp_accept_max_nonmail" by setting "smtp_accept_max_nonmail_hosts". The
default value is "*", which makes the limit apply to all hosts. This option
means that you can exclude any specific badly-behaved hosts that you have to
live with.


43.8 The VRFY and EXPN commands

When Exim receives a VRFY or EXPN command on a TCP/IP connection, it runs the
ACL specified by "acl_smtp_vrfy" or "acl_smtp_expn" (as appropriate) in order
to decide whether the command should be accepted or not. If no ACL is defined,
the command is rejected.

When VRFY is accepted, it runs exactly the same code as when Exim is called
with the -bv option. When EXPN is accepted, a single-level expansion of the
address is done. EXPN is treated as an 'address test' (similar to the -bt
option) rather than a verification (the -bv option). If an unqualified local
part is given as the argument to EXPN, it is qualified with "qualify_domain".
Rejections of VRFY and EXPN commands are logged on the main and reject logs,
and VRFY verification failures are logged on the main log for consistency with
RCPT failures.


43.9 The ETRN command

RFC 1985 describes an SMTP command called ETRN that is designed to overcome
the security problems of the TURN command (which has fallen into disuse). When
Exim receives an ETRN command on a TCP/IP connection, it runs the ACL
specified by "acl_smtp_etrn" in order to decide whether the command should be
accepted or not. If no ACL is defined, the command is rejected.

The ETRN command is concerned with 'releasing' messages that are awaiting
delivery to certain hosts. As Exim does not organize its message queue by
host, the only form of ETRN that is supported by default is the one where the
text starts with the '#' prefix, in which case the remainder of the text is
specific to the SMTP server. A valid ETRN command causes a run of Exim with
the -R option to happen, with the remainder of the ETRN text as its argument.
For example,

  ETRN #brigadoon

runs the command

  exim -R brigadoon

which causes a delivery attempt on all messages with undelivered addresses
containing the text 'brigadoon'. When "smtp_etrn_serialize" is set (the
default), Exim prevents the simultaneous execution of more than one queue run
for the same argument string as a result of an ETRN command. This stops a
misbehaving client from starting more than one queue runner at once.

Exim implements the serialization by means of a hints database in which a
record is written whenever a process is started by ETRN, and deleted when the
process completes. However, Exim does not keep the SMTP session waiting for
the ETRN process to complete. Once ETRN is accepted, the client is sent a
'success' return code. Obviously there is scope for hints records to get left
lying around if there is a system or program crash. To guard against this,
Exim ignores any records that are more than six hours old.

For more control over what ETRN does, the "smtp_etrn_command" option can used.
This specifies a command that is run whenever ETRN is received, whatever the
form of its argument. For example:

  smtp_etrn_command = /etc/etrn_command $domain $sender_host_address

The string is split up into arguments which are independently expanded. The
expansion variable $domain is set to the argument of the ETRN command, and no
syntax checking is done on the contents of this argument. Exim does not wait
for the command to complete, so its status code is not checked. Exim runs
under its own uid and gid when receiving incoming SMTP, so it is not possible
for it to change them before running the command.


43.10 Incoming local SMTP

Some user agents use SMTP to pass messages to their local MTA using the
standard input and output, as opposed to passing the envelope on the command
line and writing the message to the standard input. This is supported by the
-bs option. This form of SMTP is handled in the same way as incoming messages
over TCP/IP (including the use of ACLs), except that the envelope sender given
in a MAIL command is ignored unless the caller is trusted. In an ACL you can
detect this form of SMTP input by testing for an empty host identification. It
is common to have this as the first line in the ACL that runs for RCPT
commands:

  accept hosts = :

This accepts SMTP messages from local processes without doing any other tests.


43.11 Outgoing batched SMTP

Both the appendfile and pipe transports can be used for handling batched SMTP.
Each has an option called "use_bsmtp" which causes messages to be output in
BSMTP format. No SMTP responses are possible for this form of delivery. All it
is doing is using SMTP commands as a way of transmitting the envelope along
with the message.

The message is written to the file or pipe preceded by the SMTP commands MAIL
and RCPT, and followed by a line containing a single dot. Lines in the message
that start with a dot have an extra dot added. The SMTP command HELO is not
normally used. If it is required, the "message_prefix" option can be used to
specify it.

Because appendfile and pipe are both local transports, they accept only one
recipient address at a time by default. However, you can arrange for them to
handle several addresses at once by setting the "batch_max" option. When this
is done for BSMTP, messages may contain multiple RCPT commands. See chapter 25
for more details.

When one or more addresses are routed to a BSMTP transport by a router that
sets up a host list, the name of the first host on the list is available to
the transport in the variable $host. Here is an example of such a transport
and router:

  begin routers
  route_append:
    driver = manualroute
    transport = smtp_appendfile
    route_list = domain.example  batch.host.example

  begin transports
  smtp_appendfile:
    driver = appendfile
    directory = /var/bsmtp/$host
    batch_max = 1000
    use_bsmtp
    user = exim

This causes messages addressed to "domain.example" to be written in BSMTP
format to /var/bsmtp/batch.host.example, with only a single copy of each
message (unless there are more than 1000 recipients).


43.12 Incoming batched SMTP

The -bS command line option causes Exim to accept one or more messages by
reading SMTP on the standard input, but to generate no responses. If the
caller is trusted, the senders in the MAIL commands are believed; otherwise
the sender is always the caller of Exim. Unqualified senders and receivers are
not rejected (there seems little point) but instead just get qualified. HELO
and EHLO act as RSET; VRFY, EXPN, ETRN and HELP, act as NOOP; QUIT quits.

No policy checking is done for BSMTP input. That is, no ACL is run at anytime.
In this respect it is like non-SMTP local input.

If an error is detected while reading a message, including a missing '.' at
the end, Exim gives up immediately. It writes details of the error to the
standard output in a stylized way that the calling program should be able to
make some use of automatically, for example:

  554 Unexpected end of file
  Transaction started in line 10
  Error detected in line 14

It writes a more verbose version, for human consumption, to the standard error
file, for example:

  An error was detected while processing a file of BSMTP input.
  The error message was:

    501 '>' missing at end of address

  The SMTP transaction started in line 10.
  The error was detected in line 12.
  The SMTP command at fault was:

     rcpt to:<malformed@in.com.plete

  1 previous message was successfully processed.
  The rest of the batch was abandoned.

The return code from Exim is zero only if there were no errors. It is 1 if
some messages were accepted before an error was detected, and 2 if no messages
were accepted.



                            44. MESSAGE PROCESSING


Exim performs various transformations on the sender and recipient addresses of
all messages that it handles, and also on the messages' header lines. Some of
these are optional and configurable, while others always take place. All of
this processing, except rewriting as a result of routing, and the addition or
removal of header lines while delivering, happens when a message is received,
before it is placed on Exim's queue.

Some of the automatic processing takes place only for 'locally-originated'     |
messages. This adjective is used to describe messages that are not received    |
over TCP/IP, but instead are passed to an Exim process on its standard input.  |
This includes the interactive 'local SMTP' case that is set up by the -bs      |
command line option. Note: messages received over TCP/IP on the loopback       |
interface (127.0.0.1 or ::1) are not considered to be locally-originated. Exim |
does not treat the loopback interface specially in any way.                    |
                                                                               |
                                                                               |
44.1 Line endings                                                              |
                                                                               |
RFC 2821 specifies that CRLF (two characters: carriage-return, followed by     |
linefeed) is the line ending for messages transmitted over the Internet using  |
SMTP over TCP/IP. However, within individual operating systems, different      |
conventions are used. For example, Unix-like systems use just LF, but others   |
use CRLF or just CR.                                                           |
                                                                               |
Exim was designed for Unix-like systems, and internally, it stores messages    |
using the system's convention of a single LF as a line terminator. When        |
receiving a message, all line endings are translated to this standard format.  |
Originally, it was thought that programs that passed messages directly to an   |
MTA within an operating system would use that system's convention. Experience  |
has shown that this is not the case; for example, there are Unix applications  |
that use CRLF in this circumstance. For this reason, and for compatibility     |
with other MTAs, the way Exim handles line endings for all messages is now as  |
follows:                                                                       |
                                                                               |
 .   LF not preceded by CR is treated as a line ending.                        |
                                                                               |
 .   CR is treated as a line ending; if it is immediately followed by LF, the  |
     LF is ignored.                                                            |
                                                                               |
 .   The sequence 'CR, dot, CR' does not terminate an incoming SMTP message,   |
     nor a local message in the state where a line containing only a dot is a  |
     terminator.                                                               |
                                                                               |
 .   If a bare CR is encountered within a header line, an extra space is added |
     after the line terminator so as not to end the header line. The reasoning |
     behind this is that bare CRs in header lines are most likely either to be |
     mistakes, or people trying to play silly games.                           |
                                                                               |

44.2 Unqualified addresses

By default, Exim expects every address it receives from an external host to be
fully qualified. Unqualified addresses cause negative responses to SMTP
commands. However, because SMTP is used as a means of transporting messages
from MUAs running on personal workstations, there is sometimes a requirement
to accept unqualified addresses from specific hosts or IP networks.

Exim has two options that separately control which hosts may send unqualified
sender or receipient addresses in SMTP commands, namely "sender_unqualified_
hosts" and "recipient_unqualified_hosts". In both cases, if an unqualified
address is accepted, it is qualified by adding the value of "qualify_domain"
or "qualify_recipient", as appropriate.


44.3 The UUCP From line

Messages that have come from UUCP (and some other applications) often begin
with a line containing the envelope sender and a timestamp, following the word
'From'. Examples of two common formats are:

  From a.oakley@berlin.mus Fri Jan  5 12:35 GMT 1996
  From f.butler@berlin.mus Fri, 7 Jan 97 14:00:00 GMT

This line precedes the RFC 2822 header lines. For compatibility with Sendmail,
Exim recognizes such lines at the start of messages that are submitted to it
via the command line (that is, on the standard input). It does not recognize
such lines in incoming SMTP messages, unless the sending host matches
"ignore_fromline_hosts" or the -bs option was used for a local message and
"ignore_fromline_local" is set. The recognition is controlled by a regular
expression that is defined by the "uucp_from_pattern" option, whose default
value matches the two common cases shown above and puts the address that
follows 'From' into $1.

When the caller of Exim for a non-SMTP message that contains a 'From' line is
a trusted user, the message's sender address is constructed by expanding the
contents of "uucp_sender_address", whose default value is '$1'. This is then
parsed as an RFC 2822 address. If there is no domain, the local part is
qualified with "qualify_domain" unless it is the empty string. However, if the
command line -f option is used, it overrides the 'From' line.

If the caller of Exim is not trusted, the 'From' line is recognized, but the
sender address is not changed. This is also the case for incoming SMTP
messages that are permitted to contain 'From' lines.

Only one 'From' line is recognized. If there is more than one, the second is
treated as a data line that starts the body of the message, as it is not valid
as a header line. This also happens if a 'From' line is present in an incoming
SMTP message from a source that is not permitted to send them.


44.4 Resent- header lines

RFC 2822 makes provision for sets of header lines starting with the string
"Resent-" to be added to a message when it is resent by the original recipient
to somebody else. These headers are Resent-Date:, Resent-From:, Resent-
Sender:, Resent-To:, Resent-Cc:, Resent-Bcc: and Resent-Message-ID:. The RFC
says:

"Resent fields are strictly informational. They MUST NOT be used in the normal
processing of replies or other such automatic actions on messages."

This leaves things a bit vague as far as other processing actions such as
address rewriting are concerned. Exim treats "Resent-" header lines as
follows:

 .   A Resent-From: line that just contains the login id of the submitting
     user is automatically rewritten in the same way as From: (see below).

 .   If there's a rewriting rule for a particular header line, it is also
     applied to "Resent-" header lines of the same type. For example, a rule
     that rewrites From: also rewrites Resent-From:.

 .   For local messages, if Sender: is removed on input, Resent-Sender: is
     also removed.

 .   For a locally-submitted message, if there are any "Resent-" header lines  |
     but no Resent-Date:, Resent-From:, or Resent-Message-Id:, they are added
     as necessary. It is the contents of Resent-Message-Id: (rather than
     Message-Id:) which are included in log lines in this case.

 .   The logic for adding Sender: is duplicated for Resent-Sender: when any
     "Resent-" header lines are present.

                                                                               |
44.5 The Auto-Submitted: header line                                           |
                                                                               |
Whenever Exim generates a bounce or a delay warning message, it includes the   |
header line                                                                    |
                                                                               |
  Auto-Submitted: auto-generated                                               |
                                                                               |

44.6 The Bcc: header line

If Exim is called with the -t option, to take recipient addresses from a       |
message's header, it removes any Bcc: header line that may exist (after        |
extracting its addresses). If -t is not present on the command line, any       |
existing Bcc: is not removed.                                                  |


44.7 The Date: header line

If a locally-generated message has no Date: header line, Exim adds one, using  |
the current date and time.


44.8 The Delivery-date: header line

Delivery-date: header lines are not part of the standard RFC 2822 header set.
Exim can be configured to add them to the final delivery of messages. (See the
generic "delivery_date_add" transport option.) They should not be present in
messages in transit. If the "delivery_date_remove" configuration option is
set (the default), Exim removes Delivery-date: header lines from incoming
messages.


44.9 The Envelope-to: header line

Envelope-to: header lines are not part of the standard RFC 2822 header set.
Exim can be configured to add them to the final delivery of messages. (See the
generic "envelope_to_add" transport option.) They should not be present in
messages in transit. If the "envelope_to_remove" configuration option is set
(the default), Exim removes Envelope-to: header lines from incoming messages.


44.10 The From: header line

If a locally-generated incoming message does not contain a From: header line,  |
Exim adds one containing the sender's address. The calling user's login name   |
and full name are used to construct the address, as described in section       |
44.16. They are obtained from the password data by calling "getpwuid()" (but   |
see the "unknown_login" configuration option). The address is qualified with
"qualify_domain".

For compatibility with Sendmail, if an incoming, non-SMTP message has a From:
header line containing just the unqualified login name of the calling user,
this is replaced by an address containing the user's login name and full name
as described in section 44.16.


44.11 The Message-ID: header line

If a locally-generated incoming message does not contain a Message-ID: or      |
Resent-Message-ID: header line, Exim adds one to the message. If there are any
Resent-: headers in the message, it creates Resent-Message-ID:. The id is
constructed from Exim's internal message id, preceded by the letter E to
ensure it starts with a letter, and followed by @ and the primary host name.
Additional information can be included in this header line by setting the
"message_id_header_text" and/or "message_id_header_domain" options.


44.12 The Received: header line

A Received: header line is added at the start of every message. The contents
are defined by the "received_header_text" configuration option, and Exim
automatically adds a semicolon and a timestamp to the configured string.


44.13 The Return-path: header line

Return-path: header lines are defined as something an MTA may insert when it
does the final delivery of messages. (See the generic "return_path_add"
transport option.) Therefore, they should not be present in messages in
transit. If the "return_path_remove" configuration option is set (the
default), Exim removes Return-path: header lines from incoming messages.


44.14 The Sender: header line

For a locally-originated message from an untrusted user, Exim may remove an
existing Sender: header line, and it may add a new one. You can modify these
actions by setting "local_sender_retain" true or "local_from_check" false. No
processing of Sender: header lines is done for messages received by TCP/IP or
for messages submitted by trusted users.

When a local message is received from an untrusted user and "local_from_check"
is true (the default), a check is made to see if the address given in the
From: header line is the correct (local) sender of the message. The address
that is expected has the login name as the local part and the value of
"qualify_domain" as the domain. Prefixes and suffixes for the local part
can be permitted by setting "local_from_prefix" and "local_from_suffix"
appropriately. If From: does not contain the correct sender, a Sender: line is
added to the message.

If you set "local_from_check" false, this checking does not occur. However,
the removal of an existing Sender: line still happens, unless you also set
"local_sender_retain" to be true. It is not possible to set both of these
options true at the same time.


44.15 Adding and removing header lines

When a message is delivered, the addition and removal of header lines can be
specified on any of the routers and transports, and also in the system filter.
Changes specified in the system filter affect all deliveries of a message.

Header changes specified on a router affect all addresses handled by that
router, and also any new addresses it generates. If an address passes through
several routers, the changes are cumulative. When a message is processed by a
transport, the message's original set of header lines is output, except for
those named in any "headers_remove" options that the address has encountered
as it was processed, and any in the transport's own "headers_remove" option.
Then the new header lines from "headers_add" options are output.


44.16 Constructed addresses

When Exim constructs a sender address for a locally-generated message, it uses
the form

  <user name> <<login>@<qualify_domain>>

For example:

  Zaphod Beeblebrox <zaphod@end.univ.example>

The user name is obtained from the -F command line option if set, or otherwise
by looking up the calling user by "getpwuid()" and extracting the 'gecos'
field from the password entry. If the 'gecos' field contains an ampersand
character, this is replaced by the login name with the first letter upper
cased, as is conventional in a number of operating systems. See the
"gecos_name" option for a way to tailor the handling of the 'gecos' field. The
"unknown_username" option can be used to specify user names in cases when
there is no password file entry.

In all cases, the user name is made to conform to RFC 2822 by quoting all or
parts of it if necessary. In addition, if it contains any non-printing
characters, it is encoded as described in RFC 2047, which defines a way of
including non-ASCII characters in header lines. The value of the               |
"headers_charset" option specifies the name of the encoding that is used       |
(the characters are assumed to be in this encoding). The setting of            |
"print_topbitchars" controls whether characters with the top bit set (that is,
with codes greater than 127) count as printing characters or not.


44.17 Case of local parts

RFC 2822 states that the case of letters in the local parts of addresses
cannot be assumed to be non-significant. Exim preserves the case of local
parts of addresses, but by default it uses a lower-cased form when it is
routing, because on most Unix systems, usernames are in lower case and case-
insensitive routing is required. However, any particular router can be made to
use the original case for local parts by setting the "caseful_local_part"
generic router option.

If you must have mixed-case user names on your system, the best way to
proceed, assuming you want case-independent handling of incoming email, is to
set up your first router to convert incoming local parts in your domains to
the correct case by means of a file lookup. For example:

  correct_case:
    driver = redirect
    domains = +local_domains
    data = ${lookup{$local_part}cdb\
                {/etc/usercased.cdb}{$value}fail}\
                @$domain

For this router, the local part is forced to lower case by the default action
("caseful_local_part" is not set). The lower-cased local part is used to look
up a new local part in the correct case. If you then set "caseful_local_part"
on any subsequent routers which process your domains, they will operate on
local parts with the correct case in a case-sensitive manner.


44.18 Dots in local parts

RFC 2822 forbids empty components in local parts. That is, an unquoted local
part may not begin or end with a dot, nor have two consecutive dots in the
middle. However, it seems that many MTAs do not enforce this, so Exim permits
empty components for compatibility.


44.19 Rewriting addresses

Rewriting of sender and recipient addresses, and addresses in headers, can
happen automatically, or as the result of configuration options, as described
in chapter 31. The headers that may be affected by this are Bcc:, Cc:, From:,
Reply-To:, Sender:, and To:.

Automatic rewriting includes qualification, as mentioned above. The other case
in which it can happen is when an incomplete non-local domain is given. The
routing process may cause this to be expanded into the full domain name. For
example, a header such as

  To: hare@teaparty

might get rewritten as

  To: hare@teaparty.wonderland.fict.example

Rewriting as a result of routing is the one kind of message processing that
does not happen at input time, as it cannot be done until the address has been
routed.

Strictly, one should not do any deliveries of a message until all its
addresses have been routed, in case any of the headers get changed as a result
of routing. However, doing this in practice would hold up many deliveries for
unreasonable amounts of time, just because one address could not immediately
be routed. Exim therefore does not delay other deliveries when routing of one
or more addresses is deferred.



                                45. LOG FILES


Exim writes three different logs, referred to as the main log, the reject log,
and the panic log:

 .   The main log records the arrival of each message and each delivery in a
     single line in each case. The format is as compact as possible, in an
     attempt to keep down the size of log files. Two-character flag sequences
     make it easy to pick out these lines. A number of other events are
     recorded in the main log. Some of them are optional, in which case the
     "log_selector" option controls whether they are included or not. A Perl
     script called "eximstats" that does simple analysis of main log files is
     provided in the Exim distribution (see section 46.6).

 .   The reject log records information from messages that are rejected as a
     result of a configuration option (that is, for policy reasons). If the
     message's header has been read at the time the log is written, its
     contents are written to this log, following a copy of the one-line
     message that is written to the main log. You can use the reject log to
     check that your policy controls are working correctly.

 .   When certain serious errors occur, Exim writes entries to its panic log.
     If the error is sufficiently disastrous, Exim bombs out afterwards. Panic
     log entries are usually written to the main log as well, but can get lost
     amid the mass of other entries. The panic log should be empty under
     normal circumstances. It is therefore a good idea to check it (or to have
     a "cron" script check it) regularly, in order to become aware of any
     problems. When Exim cannot open its panic log, it tries as a last resort
     to write to the system log (syslog). This is opened with LOG_PID+LOG_CONS
     and the facility code of LOG_MAIL. The message itself is written at
     priority LOG_CRIT.

Every log line starts with a timestamp, in the format shown in this example:

  2001-09-16 16:09:47 SMTP connection from [127.0.0.1] closed by QUIT

By default, the timestamps are in the local timezone. There are two ways of
changing this:

 .   You can set the "timezone" option to a different time zone; in particu-
     lar, if you set

       timezone = UTC

     the timestamps will be in UTC (aka GMT).

 .   If you set "log_timezone" true, the time zone is added to the timestamp,
     for example:

       2003-04-25 11:17:07 +0100 Start queue run: pid=12762


45.1 Where the logs are written

The logs may be written to local files, or to syslog, or both. However, it
should be noted that many syslog implementations use UDP as a transport, and
are therefore unreliable in the sense that messages are not guaranteed to
arrive at the loghost, nor is the ordering of messages necessarily maintained.
It has also been reported that on large log files (tens of megabytes) you may
need to tweak syslog to prevent it syncing the file with each write - on Linux
this has been seen to make syslog take 90% plus of CPU time.

The destination for Exim's logs is configured by setting LOG_FILE_PATH in
Local/Makefile or by setting "log_file_path" in the run time configuration.
This latter string is expanded, so it can contain, for example, references to
the host name:

  log_file_path = /var/log/$primary_hostname/exim_%slog

It is generally advisable, however, to set the string in Local/Makefile rather
than at run time, because then the setting is available right from the start
of Exim's execution. Otherwise, if there's something it wants to log before it
has read the configuration file (for example, an error in the configuration
file) it will not use the path you want, and may not be able to log at all.

The value of LOG_FILE_PATH or "log_file_path" is a colon-separated list,
currently limited to at most two items. This is one option where the facility
for changing a list separator may not be used. The list must always be colon-
separated. If an item in the list is 'syslog' then syslog is used; otherwise
the item must either be an absolute path, containing "%s" at the point where
'main', 'reject', or 'panic' is to be inserted, or be empty, implying the use
of a default path.

When Exim encounters an empty item in the list, it searches the list defined
by LOG_FILE_PATH, and uses the first item it finds that is neither empty nor
'syslog'. This means that an empty item in "log_file_path" can be used to mean
'use the path specified at build time'. It no such item exists, log files are
written in the log subdirectory of the spool directory. This is equivalent to
the setting:

  log_file_path = $spool_directory/log/%slog

If you do not specify anything at build time or run time, that is where the
logs are written.

A log file path may also contain "%D" if datestamped log file names are in use
- see section 45.3 below.

Here are some examples of possible settings:

  LOG_FILE_PATH=syslog                               syslog only
  LOG_FILE_PATH=:syslog                              syslog and default path
  LOG_FILE_PATH=syslog : /usr/log/exim_%s            syslog and specified path
  LOG_FILE_PATH=/usr/log/exim_%s                     specified path only

If there are more than two paths in the list, the first is used and a panic
error is logged.


45.2 Logging to local files that are periodically 'cycled'

Some operating systems provide centralized and standardised methods for
cycling log files. For those that do not, a utility script called "exicyclog"
is provided (see section 46.5). This renames and compresses the main and
reject logs each time it is called. The maximum number of old logs to keep can
be set. It is suggested this script is run as a daily "cron" job.

An Exim delivery process opens the main log when it first needs to write to
it, and it keeps the file open in case subsequent entries are required - for
example, if a number of different deliveries are being done for the same
message. However, remote SMTP deliveries can take a long time, and this means
that the file may be kept open long after it is renamed if "exicyclog" or
something similar is being used to rename log files on a regular basis. To
ensure that a switch of log files is noticed as soon as possible, Exim calls
"stat()" on the main log's name before reusing an open file, and if the file
does not exist, or its inode has changed, the old file is closed and Exim
tries to open the main log from scratch. Thus, an old log file may remain open
for quite some time, but no Exim processes should write to it once it has been
renamed.


45.3 Datestamped log files

Instead of cycling the main and reject log files by renaming them period-
ically, some sites like to use files whose names contain a datestamp, for
example, mainlog-20031225. The datestamp is in the form yyyymmdd. Exim has
support for this way of working. It is enabled by setting the "log_file_path"
option to a path that includes "%D" at the point where the datestamp is
required. For example:

  log_file_path = /var/spool/exim/log/%slog-%D
  log_file_path = /var/log/exim-%s-%D.log
  log_file_path = /var/spool/exim/log/%D-%slog

As before, "%s" is replaced by 'main' or 'reject'; the following are examples
of names generated by the above examples:

  /var/spool/exim/log/mainlog-20021225
  /var/log/exim-reject-20021225.log
  /var/spool/exim/log/20021225-mainlog

When this form of log file is specified, Exim automatically switches to new
files at midnight. It does not make any attempt to compress old logs; you will
need to write your own script if you require this. You should not run
"exicyclog" with this form of logging.

The location of the panic log is also determined by "log_file_path", but it is
not datestamped, because rotation of the panic log does not make sense. When
generating the name of the panic log, "%D" is removed from the string. In
addition, if it immediately follows a slash, a following non-alphanumeric
character is removed; otherwise a preceding non-alphanumeric character is
removed. Thus, the three examples above would give these panic log names:

  /var/spool/exim/log/paniclog
  /var/log/exim-panic.log
  /var/spool/exim/log/paniclog


45.4 Logging to syslog

The use of syslog does not change what Exim logs or the format of its
messages, except in one respect. If "syslog_timestamp" is set false, the
timestamps on Exim's log lines are omitted when these lines are sent to
syslog. Apart from that, the same strings are written to syslog as to log
files. The syslog 'facility' is set to LOG_MAIL, and the program name to
'exim' by default, but you can change these by setting the "syslog_facility"
and "syslog_processname" options, respectively. If Exim was compiled with
SYSLOG_LOG_PID set in Local/Makefile (this is the default in src/EDITME),
then, on systems that permit it (all except ULTRIX), the LOG_PID flag is set
so that the "syslog()" call adds the pid as well as the time and host name to
each line. The three log streams are mapped onto syslog priorities as follows:

     "mainlog" is mapped to LOG_INFO

     "rejectlog" is mapped to LOG_NOTICE

     "paniclog" is mapped to LOG_ALERT

Many log lines are written to both "mainlog" and "rejectlog", and some are     |
written to both "mainlog" and "paniclog", so there will be duplicates if these |
are routed by syslog to the same place. You can suppress this duplication by   |
setting "syslog_duplication" false.                                            |

Exim's log lines can sometimes be very long, and some of its "rejectlog"
entries contain multiple lines when headers are included. To cope with both
these cases, entries written to syslog are split into separate "syslog()"
calls at each internal newline, and also after a maximum of 870 data
characters. (This allows for a total syslog line length of 1024, when
additions such as timestamps are added.) If you are running a syslog
replacement that can handle lines longer than the 1024 characters allowed by
RFC 3164, you should set

  SYSLOG_LONG_LINES=yes

in Local/Makefile before building Exim. That stops Exim from splitting long
lines, but it still splits at internal newlines in "reject" log entries.

To make it easy to re-assemble split lines later, each component of a split
entry starts with a string of the form '[<n>/<m>]' or '[<n>\<m>]' where <n> is
the component number and <m> is the total number of components in the entry.
The / delimiter is used when the line was split because it was too long; if it
was split because of an internal newline, the \ delimiter is used. For
example, supposing the length limit to be 70 instead of 1000, the following
would be the result of a typical rejection message to "mainlog" (LOG_INFO),
each line in addition being preceded by the time, host name, and pid as added
by syslog:

[1/3] 2002-09-16 16:09:43 16RdAL-0006pc-00 rejected from [127.0.0.1] (ph10):
[2/3]  syntax error in 'From' header when scanning for sender: missing or ma
[3/3] lformed local part in "<>" (envelope sender is <ph10@cam.example>)

The same error might cause the following lines to be written to 'rejectlog'
(LOG_NOTICE):

[1/14] 2002-09-16 16:09:43 16RdAL-0006pc-00 rejected from [127.0.0.1] (ph10):
[2/14]  syntax error in 'From' header when scanning for sender: missing or ma
[3\14] lformed local part in "<>" (envelope sender is <ph10@cam.example>)
[4\14] Recipients: ph10@some.domain.cam.example
[5\14] P Received: from [127.0.0.1] (ident=ph10)
[6\14]        by xxxxx.cam.example with smtp (Exim 4.00)
[7\14]        id 16RdAL-0006pc-00
[8\14]        for ph10@cam.example; Mon, 16 Sep 2002 16:09:43 +0100
[9\14] F From: <>
[10\14]   Subject: this is a test header
[11\14]   X-something: this is another header
[12\14] I Message-Id: <E16RdAL-0006pc-00@xxxxx.cam.example>
[13\14] B Bcc:
[14/14]   Date: Mon, 16 Sep 2002 16:09:43 +0100

Log lines that are neither too long nor contain newlines are written to syslog
without modification.

If only syslog is being used, the Exim monitor is unable to provide a log tail
display, unless syslog is routing "mainlog" to a file on the local host and
the environment variable EXIMON_LOG_FILE_PATH is set to tell the monitor where
it is.


45.5 Log line flags

One line is written to the main log for each message received, and for each
successful, unsuccessful, and delayed delivery. These lines can readily be
picked out by the distinctive two-character flags that immediately follow the
timestamp. The flags are:

  <=    message arrival
  =>    normal message delivery
  ->    additional address in same delivery
  *>    delivery suppressed by -N
  **    delivery failed; address bounced
  ==    delivery deferred; temporary problem


45.6 Logging message reception

The format of the single-line entry in the main log that is written for every
message received is shown in the basic example below, which is split over
several lines in order to fit it on the page:

2002-10-31 08:57:53 16ZCW1-0005MB-00 <= kryten@dwarf.fict.example
  H=mailer.fict.example [192.168.123.123] U=exim
  P=smtp S=5678 id=<incoming message id>

The address immediately following '<=' is the envelope sender address. A
bounce message is shown with the sender address '<>', and if it is locally
generated, this is followed by an item of the form

  R=<message id>

which is a reference to the message that caused the bounce to be sent.

For messages from other hosts, the H and U fields identify the remote host and
record the RFC 1413 identity of the user that sent the message, if one was
received. The number given in square brackets is the IP address of the sending
host. If there is a single, unparenthesized host name in the H field, as
above, it has been verified to correspond to the IP address (see the
"host_lookup" option). If the name is in parentheses, it was the name quoted
by the remote host in the SMTP HELO or EHLO command, and has not been
verified. If verification yields a different name to that given for HELO or
EHLO, the verified name appears first, followed by the HELO or EHLO name in
parentheses.

Misconfigured hosts (and mail forgers) sometimes put an IP address, with or
without brackets, in the HELO or EHLO command, leading to entries in the log
containing text like these examples:

  H=(10.21.32.43) [192.168.8.34]
  H=([10.21.32.43]) [192.168.8.34]

This can be confusing. Only the final address in square brackets can be relied
on.

For locally generated messages (that is, messages not received over TCP/IP),
the H field is omitted, and the U field contains the login name of the caller
of Exim.

For all messages, the P field specifies the protocol used to receive the
message. This is set to 'asmtp' for messages received from hosts which have
authenticated themselves using the SMTP AUTH command. In this case there is an
additional item A= followed by the name of the authenticator that was used.
If an authenticated identification was set up by the authenticator's
"server_set_id" option, this is logged too, separated by a colon from the
authenticator name.

The id field records the existing message id, if present. The size of the
received message is given by the S field. When the message is delivered,
headers may get removed or added, so that the size of delivered copies of the
message may not correspond with this value (and indeed may be different to
each other).

The "log_selector" option can be used to request the logging of additional
data when a message is received. See section 45.15 below.


45.7 Logging deliveries

The format of the single-line entry in the main log that is written for every
delivery is shown in one of the examples below, for local and remote
deliveries, respectively. Each example has been split into two lines in order
to fit it on the page:

2002-10-31 08:59:13 16ZCW1-0005MB-00 => marv <marv@hitch.fict.example>
  R=localuser T=local_delivery
2002-10-31 09:00:10 16ZCW1-0005MB-00 => monk@holistic.fict.example
  R=dnslookup T=remote_smtp H=holistic.fict.example [192.168.234.234]

For ordinary local deliveries, the original address is given in angle brackets
after the final delivery address, which might be a pipe or a file. If
intermediate address(es) exist between the original and the final address, the
last of these is given in parentheses after the final address. The R and T
fields record the router and transport that were used to process the address.

If a shadow transport was run after a successful local delivery, the log line
for the successful delivery has an item added on the end, of the form

  ST=<shadow transport name>

If the shadow transport did not succeed, the error message is put in
parentheses afterwards.

When more than one address is included in a single delivery (for example, two
SMTP RCPT commands in one transaction) the second and subsequent addresses are
flagged with '->' instead of '=>'. When two or more messages are delivered
down a single SMTP connection, an asterisk follows the IP address in the log
lines for the second and subsequent messages.

The generation of a reply message by a filter file gets logged as a 'delivery'
to the addressee, preceded by '>'.

The "log_selector" option can be used to request the logging of additional
data when a message is delivered. See section 45.15 below.


45.8 Discarded deliveries

When a message is discarded as a result of the command 'seen finish' being
obeyed in a filter file which generates no deliveries, a log entry of the form

  2002-12-10 00:50:49 16auJc-0001UB-00 => discarded
    <low.club@bridge.example> R=userforward

is written, to record why no deliveries are logged. When a message is
discarded because it is aliased to ':blackhole:' the log line is like this:

  1999-03-02 09:44:33 10HmaX-0005vi-00 => :blackhole:
    <hole@nowhere.example> R=blackhole_router


45.9 Deferred deliveries

When a delivery is deferred, a line of the following form is logged:

2002-12-19 16:20:23 16aiQz-0002Q5-00 == marvin@endrest.example
  R=dnslookup T=smtp defer (146): Connection refused

In the case of remote deliveries, the error is the one that was given for the
last IP address that was tried. Details of individual SMTP failures are also
written to the log, so the above line would be preceded by something like

2002-12-19 16:20:23 16aiQz-0002Q5-00 Failed to connect to
  mail1.endrest.example [192.168.239.239]: Connection refused

When a deferred address is skipped because its retry time has not been
reached, a message is written to the log, but this can be suppressed by
setting an appropriate value in "log_selector".


45.10 Delivery failures

If a delivery fails because an address cannot be routed, a line of the
following form is logged:

1995-12-19 16:20:23 0tRiQz-0002Q5-00 ** jim@trek99.example
  <jim@trek99.example>: unknown mail domain

If a delivery fails at transport time, the router and transport are shown, and
the response from the remote host is included, as in this example:

2002-07-11 07:14:17 17SXDU-000189-00 ** ace400@pb.example R=dnslookup
  T=remote_smtp: SMTP error from remote mailer after
  RCPT TO:<ace400@pb.example>: host pbmail3.py.example
  [192.168.63.111]: 553 5.3.0 <ace400@pb.example>...
  Addressee unknown

The log lines for all forms of delivery failure are flagged with "**".


45.11 Fake deliveries

If a delivery does not actually take place because the -N option has been used
to suppress it, a normal delivery line is written to the log, except that '=>'
is replaced by '*>'.


45.12 Completion

A line of the form

  2002-10-31 09:00:11 16ZCW1-0005MB-00 Completed

is written to the main log when a message is about to be removed from the
spool at the end of its processing.


45.13 Summary of Fields in Log Lines

A summary of the field identifiers that are used in log lines is shown in the
following table:

  A       authenticator name (and optional id)
  C       SMTP confirmation on delivery
  DN      DN from peer certificate
  F       sender address (on delivery lines)
  H       host name and IP address
  id      message id for incoming message
  P       protocol for incoming message
  R       on "<=" lines: reference for local bounce
          on "=>" lines: router name
  S       size of message
  ST      shadow transport name
  T       on "<=" lines: message subject (topic)
          on "=>" lines: transport name
  U       local user or RFC 1413 identity
  X       TLS cipher suite


45.14 Other log entries

Various other types of log entry are written from time to time. Most should be
self-explanatory. Among the more common are:

 .   "retry time not reached"  An address previously suffered a temporary
     error during routing or local delivery, and the time to retry has not yet
     arrived. This message is not written to an individual message log file
     unless it happens during the first delivery attempt.

 .   "retry time not reached for any host"  An address previously suffered
     temporary errors during remote delivery, and the retry time has not yet
     arrived for any of the hosts to which it is routed.

 .   "spool file locked"  An attempt to deliver a message cannot proceed
     because some other Exim process is already working on the message. This
     can be quite common if queue running processes are started at frequent
     intervals. The "exiwhat" utility script can be used to find out what Exim
     processes are doing.


45.15 Reducing or increasing what is logged

By setting the "log_selector" global option, you can disable some of Exim's
default logging, or you can request additional logging. The value of
"log_selector" is made up of names preceded by plus or minus characters. For
example:

  log_selector = +arguments -retry_defer

The list of optional log items is in the following table, with the default
selection marked by asterisks:

   address_rewrite                      address rewriting
   all_parents                          all parents in => lines
   arguments                            command line arguments
  *connection_reject                    connection rejections
  *delay_delivery                       immediate delivery delayed (message queued)
   delivery_size                        add S=nnn to => lines
  *dnslist_defer                        defers of DNS list (aka RBL) lookups
  *etrn                                 ETRN commands
  *host_lookup_failed                   as it says
   incoming_interface                   incoming interface on <= lines
   incoming_port                        incoming port on <= lines
  *lost_incoming_connection             as it says (includes timeouts)
  *queue_run                            start and end queue runs
   received_recipients                  recipients on <= lines
   received_sender                      sender on <= lines
  *rejected_header                      header contents on reject log
  *retry_defer                          'retry time not reached'
   sender_on_delivery                   add sender to => lines
  *size_reject                          rejection because too big
  *skip_delivery                        'message is frozen', 'spool file is locked'
   smtp_confirmation                    SMTP confirmation on <= lines
   smtp_connection                      SMTP connections
   smtp_incomplete_transaction          incomplete SMTP transactions           |
   smtp_protocol_error                  SMTP protocol errors
   smtp_syntax_error                    SMTP syntax errors
   subject                              contents of Subject: on <= lines
  *tls_cipher                           TLS cipher suite on <= and => lines
   tls_peerdn                           TLS peer DN on <= and => lines

   all                                  all of the above

More details on each of these items follows:

 .   "address_rewrite": This applies both to global rewrites and per-transport
     rewrites.

 .   "all_parents": Normally only the original and final addresses are logged
     on delivery lines; with this selector, intermediate parents are given in
     parentheses between them.

 .   "arguments": This causes Exim to write the arguments with which it was
     called to the main log, preceded by the current working directory. This
     is a debugging feature, added to make it easier to find out how certain
     MUAs call /usr/sbin/sendmail. The logging does not happen if Exim has
     given up root privilege because it was called with the -C or -D options.
     Arguments that are empty or that contain whitespace are quoted. Non-
     printing characters are shown as escape sequences. This facility cannot
     log unrecognized arguments, because the arguments are checked before the
     configuration file is read. The only way to log such cases is to
     interpose a script such as util/logargs.sh between the caller and Exim.

 .   "connection_reject": A log entry is written whenever an incoming SMTP
     connection is rejected, for whatever reason.

 .   "delay_delivery": A log entry is written whenever a delivery process is
     not started for an incoming message because the load is too high or too
     many messages were received on one connection. Logging does not occur if
     no delivery process is started because "queue_only" is set or -odq was
     used.

 .   "delivery_size": For each delivery, the size of message delivered is
     added to the '=>' line, tagged with S=.

 .   "dnslist_defer": A log entry is written if an attempt to look up a host
     in a DNS black list suffers a temporary error.

 .   "etrn": Every legal ETRN command that is received is logged, before the
     ACL is run to determine whether or not it is actually accepted. An
     invalid ETRN command, or one received within a message transaction is
     not logged by this selector (see "smtp_syntax_error" and
     "smtp_protocol_error").

 .   "host_lookup_failed": When a lookup of a host's IP addresses fails to
     find any addresses, or when a lookup of an IP address fails to find a
     host name, a log line is written. This logging does not apply to direct
     DNS lookups when routing email addresses, but it does apply to 'byname'
     lookups.

 .   "incoming_interface": The interface on which a message was received is
     added to the '<=' line as an IP address in square brackets, tagged by I=
     and followed by a colon and the port number.

 .   "incoming_port": The remote port number from which a message was received
     is added to log entries and Received: header lines, following the IP
     address in square brackets, and separated from it by a colon. This is
     implemented by changing the value that is put in the $sender_fullhost and
     $sender_rcvhost variables. Recording the remote port number has become
     more important with the widening use of NAT (see RFC 2505).

 .   "lost_incoming_connection": A log line is written when an incoming SMTP
     connection is unexpectedly dropped.

 .   "queue_run": The start and end of every queue run are logged.

 .   "received_recipients": The recipients of a message are listed in the main
     log as soon as the message is received. The list appears at the end of
     the log line that is written when a message is received, preceded by the
     word 'for'. The addresses are listed after they have been qualified, but
     before any rewriting has taken place. Recipients that were discarded by
     an ACL for MAIL or RCPT do not appear in the list.

 .   "received_sender": The unrewritten original sender of a message is added
     to the end of the log line that records the message's arrival, after the
     word 'from' (before the recipients if "received_recipients" is also set).

 .   "rejected_header": If a message's header has been received at the time a
     rejection is written to the reject log, the complete header is added to
     the log. Header logging can be turned off individually for messages that
     are rejected by the "local_scan()" function (see section 39.2).

 .   "retry_defer": A log line is written if a delivery is deferred because a
     retry time has not yet been reached. However, this 'retry time not
     reached' message is always omitted from individual message logs after the
     first delivery attempt.

 .   "sender_on_delivery": The message's sender address is added to every
     delivery and bounce line, tagged by F= (for 'from').

 .   "size_reject": A log line is written whenever a message is rejected
     because it is too big.

 .   "skip_delivery": A log line is written whenever a message is skipped
     during a queue run because it is frozen or because another process is
     already delivering it.

 .   "smtp_confirmation": The response to the final '.' in the SMTP dialogue
     for outgoing messages is added to delivery log lines in the form
     'C="<text>"'. A number of MTAs (including Exim) return an identifying
     string in this response.

 .   "smtp_connection": A log line is written whenever an SMTP connection is
     established or closed. (By contrast, "lost_incoming_connection" applies
     only when the closure is unexpected.) This applies to connections from
     local processes that use -bs as well as to TCP/IP connections. If a
     connection is dropped in the middle of a message, a log line is always
     written, whether this selector is set or not, but otherwise nothing is
     written at the start and end of connections unless this selector is
     enabled.

     For TCP/IP connections to an Exim daemon, the current number of connec-
     tions is included in the log message for each new connection, but note
     that the count is reset if the daemon is restarted. Also, because         |
     connections are closed (and the closure is logged) in subprocesses, the   |
     count may not include connections that have been closed but whose         |
     termination the daemon has not yet noticed. Thus, while it is possible to |
     match up the opening and closing of connections in the log, the value of  |
     the logged counts may not be entirely accurate.                           |

 .   "smtp_incomplete_transaction": When a mail transaction is aborted by      |
     RSET, QUIT, loss of connection, or otherwise, the incident is logged, and |
     the message sender plus any accepted recipients are included in the log   |
     line. This can provide evidence of dictionary attacks.                    |

 .   "smtp_protocol_error": A log line is written for every SMTP protocol
     error encountered.

 .   "smtp_syntax_error": A log line is written for every SMTP syntax error
     encountered. An unrecognized command is treated as a syntax error. For an
     external connection, the host identity is given; for an internal connec-
     tion using -bs the sender identification (normally the calling user) is
     given.

 .   "subject": The subject of the message is added to the arrival log line,
     preceded by 'T=' (T for 'topic', since S is already used for 'size'). Any |
     MIME 'words' in the subject are decoded. The "print_topbitchars" option   |
     specifies whether characters with values greater than 127 should be       |
     logged unchanged, or whether they should be rendered as escape sequences. |

 .   "tls_cipher": When a message is sent or received over an encrypted
     connection, the cipher suite used is added to the log line, preceded by
     X=.

 .   "tls_peerdn": When a message is sent or received over an encrypted
     connection, and a certificate is supplied by the remote host, the peer DN
     is added to the log line, preceded by DN=.


45.16 Message log

In addition to the general log files, Exim writes a log file for each message
that it handles. The names of these per-message logs are the message ids, and
they are kept in the msglog sub-directory of the spool directory. Each message
log contains copies of the log lines that apply to the message. This makes it
easier to inspect the status of an individual message without having to search
the main log. A message log is deleted when processing of the message is
complete, unless "preserve_message_logs" is set, but this should be used only
with great care because they can fill up your disk very quickly.

On a heavily loaded system, it may be desirable to disable the use of per-
message logs, in order to reduce disk I/O. This can be done by setting the
"message_logs" option false.



                              46. EXIM UTILITIES


A number of utility scripts and programs are supplied with Exim and are
described in this chapter. There is also the Exim Monitor, which is covered in
the next chapter. The utilities described here are:

  46.1  "exiwhat"           list what Exim processes are doing
  46.2  "exiqgrep"          grep the queue
  46.3  "exiqsumm"          summarize the queue
  46.4  "exigrep"           search the main log
  46.5  "exicyclog"         cycle (rotate) log files
  46.6  "eximstats"         extract statistics from the log
  46.7  "exim_checkaccess"  check address acceptance from given IP
  46.8  "exim_dbmbuild"     build a DBM file
  46.9  "exinext"           extract retry information
  46.10  "exim_dumpdb"      dump a hints database
  46.10  "exim_tidydb"      clean up a hints database
  46.10  "exim_fixdb"       patch a hints database
  46.11  "exim_lock"        lock a mailbox file


46.1 Finding out what Exim processes are doing (exiwhat)

On operating systems that can restart a system call after receiving a signal
(most modern OS), an Exim process responds to the SIGUSR1 signal by writing a
line describing what it is doing to the file exim-process.info in the Exim
spool directory. The "exiwhat" script sends the signal to all Exim processes
it can find, having first emptied the file. It then waits for one second to
allow the Exim processes to react before displaying the results. In order to
run "exiwhat" successfully you have to have sufficient privilege to send the
signal to the Exim processes, so it is normally run as root.

Unfortunately, the "ps" command which "exiwhat" uses to find Exim processes
varies in different operating systems. Not only are different options used,
but the format of the output is different. For this reason, there are some
system configuration options that configure exactly how "exiwhat" works. If it
doesn't seem to be working for you, check the following compile-time options:

  EXIWHAT_PS_CMD     the command for running "ps"
  EXIWHAT_PS_ARG     the argument for "ps"
  EXIWHAT_EGREP_ARG  the argument for "egrep" to select from "ps" output
  EXIWHAT_KILL_ARG   the argument for the "kill" command

An example of typical output from "exiwhat" is

  164 daemon: -q1h, listening on port 25
10483 running queue: waiting for 0tAycK-0002ij-00 (10492)
10492 delivering 0tAycK-0002ij-00 to mail.ref.example [10.19.42.42]
  (editor@ref.example)
10592 handling incoming call from [192.168.243.242]
10628 accepting a local non-SMTP message

The first number in the output line is the process number. The third line has
been split here, in order to fit it on the page.


46.2 Selective queue listing (exiqgrep)

This utility is a Perl script contributed by Matt Hubbard. It runs

  exim -bpu

to obtain a queue listing with undelivered recipients only, and then greps the
output to select messages that match given criteria. The following selection
options are available:

-f<regex>
       Match the sender address. The field that is tested is enclosed in angle
       brackets, so you can test for bounce messages with

         exiqgrep -f '^<>$'

-r<regex>
       Match a recipient address. The field that is tested is not enclosed in
       angle brackets.

-s<regex>
       Match against the size field.

-y<seconds>
       Match messages that are younger than the given time.

-o<seconds>
       Match messages that are older than the given time.

-z     Match only frozen messages.

-x     Match only non-frozen messages.

The following options control the format of the output:

-c     Display only the count of matching messages.

-l     Long format - display the full message information as output by Exim.
       This is the default.

-i     Display message ids only.

-b     Brief format - one line per message.

-R     Display messages in reverse order.

There is one more option, -h, which outputs a list of options.


46.3 Summarising the queue (exiqsumm)

The "exiqsumm" utility is a Perl script which reads the output of "exim -bp"
and produces a summary of the messages on the queue. Thus, you use it by
running a command such as

  exim -bp | exiqsumm

The output consists of one line for each domain that has messages waiting for
it, as in the following example:

    3   2322   74m   66m  msn.com.example

Each line lists the number of pending deliveries for a domain, their total     |
volume, and the length of time that the oldest and the newest messages have    |
been waiting. Note that the number of pending deliveries is greater than the   |
number of messages when messages have more than one recipient.                 |

A summary line is output at the end. By default the output is sorted on the
domain name, but "exiqsumm" has the options -a and -c, which cause the output
to be sorted by oldest message and by count of messages, respectively.

The output of "exim -bp" contains the original addresses in the message, so
this also applies to the output from "exiqsumm". No domains from addresses
generated by aliasing or forwarding are included (unless the "one_time" option
of the redirect router has been used to convert them into 'top level'
addresses).


46.4 Extracting specific information from the log (exigrep)

The "exigrep" utility is a Perl script that searches one or more main log
files for entries that match a given pattern. When it finds a match, it
extracts all the log entries for the relevant message, not just those that
match the pattern. Thus, "exigrep" can extract complete log entries for a
given message, or all mail for a given user, or for a given host, for example.
The usage is:

  exigrep [-l] [-t<n>] <pattern> [<log file>] ...                              |

The -t argument specifies a number of seconds. It adds an additional condition |
for message selection. Messages that are complete are shown only if they spent |
more than <n> seconds on the queue.                                            |

The -l flag means 'literal', that is, treat all characters in the pattern as
standing for themselves. Otherwise the pattern must be a Perl regular
expression. The pattern match is case-insensitive. If no file names are given
on the command line, the standard input is read.

If the location of a "zcat" command is known from the definition of
ZCAT_COMMAND in Local/Makefile, "exigrep" automatically passes any file whose
name ends in COMPRESS_SUFFIX through "zcat" as it searches it.


46.5 Cycling log files (exicyclog)

The "exicyclog" script can be used to cycle (rotate) "mainlog" and "rejectlog"
files. This is not necessary if only syslog is being used, or if you are using
log files with datestamps in their names (see section 45.3). Some operating
systems have their own standard mechanisms for log cycling, and these can be
used instead of "exicyclog" if preferred.

Each time "exicyclog" is run the file names get 'shuffled down' by one. If the
main log file name is mainlog (the default) then when "exicyclog" is run
mainlog becomes mainlog.01, the previous mainlog.01 becomes mainlog.02 and so
on, up to a limit which is set in the script, and which defaults to 10. Reject
logs are handled similarly.

If no mainlog file exists, the script does nothing. Files that 'drop off' the
end are deleted. All files with numbers greater than 01 are compressed, using
a compression command which is configured by the COMPRESS_COMMAND setting in
Local/Makefile. It is usual to run "exicyclog" daily from a root "crontab"
entry of the form

  1 0 * * * su exim -c /usr/exim/bin/exicyclog

assuming you have used the name 'exim' for the Exim user. You can run
"exicyclog" as root if you wish, but there is no need.


46.6 Mail statistics (eximstats)

A Perl script called "eximstats" is provided for extracting statistical
information from log files. The output is either plain text, or HTML. Exim log
files are also suported by the "Lire" system produced by the LogReport
Foundation (http://www.logreport.org).

The "eximstats" script has been hacked about quite a bit over time. The latest
version is the result of some extensive revision by Steve Campbell. A lot of
information is given by default, but there are options for suppressing various
parts of it. Following any options, the arguments to the script are a list of
files, which should be main log files. For example:

  eximstats -nr /var/spool/exim/log/mainlog.01

By default, "eximstats" extracts information about the number and volume of
messages received from or delivered to various hosts. The information is
sorted both by message count and by volume, and the top fifty hosts in each
category are listed on the standard output. Similar information, based on
email addresses or domains instead of hosts can be requested by means of
various options. For messages delivered and received locally, similar stat-
istics are also produced per user.

The output also includes total counts and statistics about delivery errors,
and histograms showing the number of messages received and deliveries made in
each hour of the day. A delivery with more than one address in its envelope
(for example, an SMTP transaction with more than one RCPT command) is counted
as a single delivery by "eximstats".

Though normally more deliveries than receipts are reported (as messages may
have multiple recipients), it is possible for "eximstats" to report more
messages received than delivered, even though the queue is empty at the start
and end of the period in question. If an incoming message contains no valid
recipients, no deliveries are recorded for it. A bounce message is handled as
an entirely separate message.

"eximstats" always outputs a grand total summary giving the volume and number
of messages received and deliveries made, and the number of hosts involved in
each case. It also outputs the number of messages that were delayed (that is,
not completely delivered at the first attempt), and the number that had at
least one address that failed.

The remainder of the output is in sections that can be independently disabled
or modified by various options. It consists of a summary of deliveries by
transport, histograms of messages received and delivered per time interval
(default per hour), information about the time messages spent on the queue, a
list of relayed messages, lists of the top fifty sending hosts, local senders,
destination hosts, and destination local users by count and by volume, and a
list of delivery errors that occurred.

The relay information lists messages that were actually relayed, that is, they
came from a remote host and were directly delivered to some other remote host,
without being processed (for example, for aliasing or forwarding) locally.

The options for "eximstats" are as follows:


-bydomain
       The 'league tables' are computed on the basis of the superior domains
       of the sending hosts instead of the sending and receiving hosts. This
       option may be combined with -byhost and/or -byemail.

-byedomain
       This is a synonym for -byemaildomain.

-byemail
       The 'league tables' are computed on the basis of complete email
       addresses, instead of sending and receiving hosts. This option may be
       combined with -byhost and/or -bydomain.

-byemaildomain
       The 'league tables' are computed on the basis of the sender's email
       domain instead of the sending and receiving hosts. This option may be
       combined with -byhost, -bydomain, or -byemail.

-byhost
       The 'league tables' are computed on the basis of sending and receiving
       hosts. This is the default option. It may be combined with -bydomain
       and/or -byemail.

-cache Cache results of "timegm()" lookups. This results in a significant
       speedup when processing hundreds of thousands of messages, at a cost of
       increasing the memory utilisation.

-chartdir<dir>
       When -charts is specified, create the charts in the directory <dir>.

-chartrel<dir>
       When -charts is specified, this option specifies the relative directory
       for the "img src=" tags from where to include the charts.

-charts
       Create graphical charts to be displayed in HTML output. This requires
       the "GD", "GDTextUtil", and "GDGraph" Perl modules, which can be
       obtained from http://www.cpan.org/modules/01modules.index.html.

       To install these, download and unpack them, then use the normal Perl
       installation procedure:

         perl Makefile.PL
         make
         make test
         make install

-d     This is a debug flag. It causes "eximstats" to output the "eval()"'d
       parser to the standard output, which makes it easier to trap errors in
       the eval section. Remember to add one to the line numbers to allow for
       the title.

-help  Show help information about "eximstats"' options.

-h<n>  This option controls the histograms of messages received and deliveries
       per time interval. By default the time interval is one hour. If -h0 is
       given, the histograms are suppressed; otherwise the value of <n> gives
       the number of divisions per hour. Valid values are 0, 1, 2, 3, 5, 10,
       15, 20, 30 or 60, so -h2 sets an interval of 30 minutes, and the
       default is equivalent to -h1.

-html  Output the results in HTML instead of plain text.

-merge This option causes "eximstats" to merge old reports into a combined
       report. When this option is used, the input files must be outputs from
       previous calls to "eximstats", not raw log files. For example, you
       could produce a set of daily reports and a weekly report by commands
       such as

         eximstats mainlog.sun > report.sun.txt
         eximstats mainlog.mon > report.mon.txt
         eximstats mainlog.tue > report.tue.txt
         eximstats mainlog.wed > report.wed.txt
         eximstats mainlog.thu > report.thu.txt
         eximstats mainlog.fri > report.fri.txt
         eximstats mainlog.sat > report.sat.txt
         eximstats -merge -html report.*.txt > weekly_report.html

       You can merge text or html reports and output the results as text or
       html. You can use all the normal "eximstats" output options, but only
       data included in the original reports can be shown. When merging
       reports, some loss of accuracy may occur in the 'league tables',
       towards the ends of the lists. The order of items in the 'league
       tables' may vary when the data volumes round to the same value.

-ne    Suppress the display of information about failed deliveries (errors).

-nr    Suppress information about messages relayed through this host.

-nr/pattern/
       Suppress information about relayed messages that match the pattern,
       which is matched against a string of the following form (split over two
       lines here in order to fit it on the page):

         H=<host> [<ip address>] A=<sender address> =>
           H=<host> A=<recipient address>

       for example

         H=in.host [1.2.3.4] A=from@some.where.example =>
           H=out.host A=to@else.where.example

       The sending host name appears in parentheses if it has not been
       verified as matching the IP address. The mail addresses are taken from
       the envelope, not the headers. This option allows you to screen out
       hosts whom you are happy to have using your host as a relay.

-nt    Suppress the statistics about delivery by transport.

-nt/<pattern>/                                                                 |
       Suppress the statistics about delivery by any transport whose name      |
       matches the pattern. If you are using one transport to send all         |
       messages to a scanning mechanism before doing the real delivery, this   |
       feature can be used to omit that transport from your normal statistics  |
       (on the grounds that it is of no interest).                             |
                                                                               |
-pattern "<description>" /<pattern>/                                           |
       Count lines matching specified patterns and show them in the results.   |
       For example:                                                            |
                                                                               |
         -pattern 'Refused connections' '/refused connection/'                 |
                                                                               |
       This option can be specified multiple times.                            |

-q0    Suppress information about times messages spend on the queue.

-q<n1>...
       This option sets an alternative list of time intervals for the queueing
       information. The values are separated by commas and are in seconds, but
       can involve arithmetic multipliers, so for example you can set 3*60 to
       specify 3 minutes. A setting such as

         -q60,5*60,10*60

       causes "eximstats" to give counts of messages that stayed on the queue
       for less than one minute, less than five minutes, less than ten
       minutes, and over ten minutes.

-t<n>  Sets the 'top' count to <n>. This controls the listings of the 'top
       <n>' hosts and users by count and volume. The default is 50, and
       setting 0 suppresses the output altogether.

-tnl   Omit local information from the 'top' listings.

-t_remote_users
       Include remote users in the 'top' listings.


46.7 Checking access policy (exim_checkaccess)

The -bh command line argument allows you to run a fake SMTP session with
debugging output, in order to check what Exim is doing when it is applying
policy controls to incoming SMTP mail. However, not everybody is sufficiently
familiar with the SMTP protocol to be able to make full use of -bh, and
sometimes you just want to answer the question "Does this address have
access?" without bothering with any further details.

The "exim_checkaccess" utility is a 'packaged' version of -bh. It takes two
arguments, an IP address and an email address:

  exim_checkaccess 10.9.8.7 A.User@a.domain.example

The utility runs a call to Exim with the -bh option, to test whether the given
email address would be accepted in a RCPT command in a TCP/IP connection from
the host with the given IP address. The output of the utility is either the
word 'accepted', or the SMTP error response, for example:

  Rejected:
    550 Relay not permitted

When running this test, the utility uses "<>" as the envelope sender address
for the MAIL command, but you can change this by providing additional options.
These are passed directly to the Exim command. For example, to specify that
the test is to be run with the sender address "himself@there.example" you can
use:

  exim_checkaccess 10.9.8.7 A.User@a.domain.example \
                   -f himself@there.example

Note that these additional Exim command line items must be given after the two
mandatory arguments.

Because the "exim_checkaccess" uses -bh, it does not perform callouts while    |
running its checks. You can run checks that include callouts by using -bhc,    |
but this is not yet available in a 'packaged' form.                            |


46.8 Making DBM files (exim_dbmbuild)

The "exim_dbmbuild" program reads an input file containing keys and data in
the format used by the lsearch lookup (see section 9.2). It writes a DBM file
using the lower-cased alias names as keys and the remainder of the information
as data. The lower-casing can be prevented by calling the program with the
-nolc option.

A terminating zero is included as part of the key string. This is expected by
the dbm lookup type. However, if the option -nozero is given, "exim_dbmbuild"
creates files without terminating zeroes in either the key strings or the data
strings. The dbmnz lookup type can be used with such files.

The program requires two arguments: the name of the input file (which can be a
single hyphen to indicate the standard input), and the name of the output
file. It creates the output under a temporary name, and then renames it if all
went well. If the native DB interface is in use (USE_DB is set in a compile-
time configuration file - this is common in free versions of Unix) the two
file names must be different, because in this mode the Berkeley DB functions
create a single output file using exactly the name given. For example,

  exim_dbmbuild /etc/aliases /etc/aliases.db

reads the system alias file and creates a DBM version of it in
/etc/aliases.db.

In systems that use the "ndbm" routines (mostly proprietary versions of Unix),
two files are used, with the suffixes .dir and .pag. In this environment, the
suffixes are added to the second argument of "exim_dbmbuild", so it can be the
same as the first. This is also the case when the Berkeley functions are used
in compatibility mode (though this is not recommended), because in that case
it adds a .db suffix to the file name.

If a duplicate key is encountered, the program outputs a warning, and when it
finishes, its return code is 1 rather than zero, unless the -noduperr option
is used. By default, only the first of a set of duplicates is used - this
makes it compatible with lsearch lookups. There is an option -lastdup which
causes it to use the data for the last duplicate instead. There is also an
option -nowarn, which stops it listing duplicate keys to "stderr". For other
errors, where it doesn't actually make a new file, the return code is 2.


46.9 Finding individual retry times (exinext)

A utility called "exinext" (mostly a Perl script) provides the ability to fish
specific information out of the retry database. Given a mail domain (or a
complete address), it looks up the hosts for that domain, and outputs any
retry information for the hosts or for the domain. At present, the retry
information is obtained by running "exim_dumpdb" (see below) and post-
processing the output. For example:

  exinext piglet@milne.fict.example
  kanga.milne.fict.example:192.168.8.1 error 146: Connection refused
    first failed: 21-Feb-1996 14:57:34
    last tried:   21-Feb-1996 14:57:34
    next try at:  21-Feb-1996 15:02:34
  roo.milne.fict.example:192.168.8.3 error 146: Connection refused
    first failed: 20-Jan-1996 13:12:08
    last tried:   21-Feb-1996 11:42:03
    next try at:  21-Feb-1996 19:42:03
    past final cutoff time

You can also give "exinext" a local part, without a domain, and it will give
any retry information for that local part in your default domain. A message id
can be used to obtain retry information pertaining to a specific message. This
exists only when an attempt to deliver a message to a remote host suffers a
message-specific error (see section 43.2). "exinext" is not particularly
efficient, but then it isn't expected to be run very often.


46.10 Hints database maintenance (exim_dumpdb, exim_fixdb,
exim_tidydb)

Three utility programs are provided for maintaining the DBM files that Exim
uses to contain its delivery hint information. Each program requires two
arguments. The first specifies the name of Exim's spool directory, and the
second is the name of the database it is to operate on. These are as follows:

 .   "retry": the database of retry information

 .   "wait-"<transport name>: databases of information about messages waiting
     for remote hosts

 .   "misc": other hints data (for example, for serializing ETRN runs)

The entire contents of a database are written to the standard output by the
"exim_dumpdb" program, which has no options or arguments other than the spool
and database names. For example, to dump the retry database:

  exim_dumpdb /var/spool/exim retry

Two lines of output are produced for each entry:

    T:mail.ref.example:192.168.242.242 146 77 Connection refused
  31-Oct-1995 12:00:12  02-Nov-1995 12:21:39  02-Nov-1995 20:21:39 *

The first item on the first line is the key of the record. It starts with one
of the letters R, or T, depending on whether it refers to a routing or
transport retry. For a local delivery, the next part is the local address; for
a remote delivery it is the name of the remote host, followed by its failing
IP address (unless "no_retry_include_ip_address" is set on the smtp trans-
port). Then there follows an error code, an additional error code, and a
textual description of the error.

The three times on the second line are the time of first failure, the time of
the last delivery attempt, and the computed time for the next attempt. The
line ends with an asterisk if the cutoff time for the last retry rule has been
exceeded.

Each output line from "exim_dumpdb" for the "wait-"xxx databases consists of a
host name followed by a list of ids for messages that are or were waiting to
be delivered to that host. If there are a very large number for any one host,
continuation records, with a sequence number added to the host name, may be
seen. The data in these records is often out of date, because a message may be
routed to several alternative hosts, and Exim makes no effort to keep cross-
references.

The "exim_tidydb" utility program is used to tidy up the contents of the hints
databases. If run with no options, it removes all records from a database that
are more than 30 days old. The cutoff date can be altered by means of the -t
option, which must be followed by a time. For example, to remove all records
older than a week from the retry database:

  exim_tidydb -t 7d /var/spool/exim retry

Both the "wait-"xxx and "retry" databases contain items that involve message
ids. In the former these appear as data in records keyed by host - they were
messages that were waiting for that host - and in the latter they are the keys
for retry information for messages that have suffered certain types of error.
When "exim_tidydb" is run, a check is made to ensure that message ids in
database records are those of messages that are still on the queue. Message
ids for messages that no longer exist are removed from "wait-"xxx records, and
if this leaves any records empty, they are deleted. For the "retry" database,
records whose keys are non-existent message ids are removed. The "exim_tidydb"
utility outputs comments on the standard output whenever it removes infor-
mation from the database.

Removing records from a DBM file does not normally make the file smaller, but
all the common DBM libraries are able to re-use the space that is released. It
is therefore suggested that "exim_tidydb" be run periodically on all the hints
databases, but at a quiet time of day, because it requires a database to be
locked (and therefore inaccessible to Exim) while it does its work.

The "exim_fixdb" program is a utility for interactively modifying databases.
Its main use is for testing Exim, but it might also be occasionally useful for
getting round problems in a live system. It has no options, and its interface
is somewhat crude. On entry, it prompts for input with a right angle-bracket.
A key of a database record can then be entered, and the data for that record
is displayed.

If 'd' is typed at the next prompt, the entire record is deleted. For all
except the "retry" database, that is the only operation that can be carried
out. For the "retry" database, each field is output preceded by a number, and
data for individual fields can be changed by typing the field number followed
by new data, for example:

  > 4 951102:1000

resets the time of the next delivery attempt. Time values are given as a
sequence of digit pairs for year, month, day, hour, and minute. Colons can be
used as optional separators.


46.11 Mailbox maintenance (exim_lock)

The "exim_lock" utility locks a mailbox file using the same algorithm as Exim.
For a discussion of locking issues, see section 26.3. "Exim_lock" can be used
to prevent any modification of a mailbox by Exim or a user agent while
investigating a problem. The utility requires the name of the file as its
first argument. If the locking is successful, the second argument is run as a
command (using C's "system()" function); if there is no second argument, the
value of the SHELL environment variable is used; if this is unset or empty,
/bin/sh is run. When the command finishes, the mailbox is unlocked and the
utility ends. The following options are available:

-fcntl Use "fcntl()" locking on the open mailbox.

-flock Use "flock()" locking on the open mailbox, provided the operating
       system supports it.

-interval
       This must be followed by a number, which is a number of seconds; it
       sets the interval to sleep between retries (default 3).

-lockfile
       Create a lock file before opening the mailbox.

-mbx   Lock the mailbox using MBX rules.

-q     Suppress verification output.

-retries
       This must be followed by a number; it sets the number of times to try
       to get the lock (default 10).

-restore_time
       This option causes "exim_lock" to restore the modified and read times
       to the the locked file before exiting. This allows you to access a
       locked mailbox (for example, to take a backup copy) without disturbing
       the times that the user subsequently sees.

-timeout
       This must be followed by a number, which is a number of seconds; it
       sets a timeout to be used with a blocking "fcntl()" lock. If it is not
       set (the default), a non-blocking call is used.

-v     Generate verbose output.

If none of -fcntl, -flock, -lockfile or -mbx are given, the default is to
create a lock file and also to use "fcntl()" locking on the mailbox, which is
the same as Exim's default. The use of -flock or -fcntl requires that the file
be writeable; the use of -lockfile requires that the directory containing the
file be writeable. Locking by lock file does not last for ever; Exim assumes
that a lock file is expired if it is more than 30 minutes old.

The -mbx option can be used with either or both of -fcntl or -flock. It
assumes -fcntl by default. MBX locking causes a shared lock to be taken out on
the open mailbox, and an exclusive lock on the file /tmp/.n.m where n and m
are the device number and inode number of the mailbox file. When the locking
is released, if an exclusive lock can be obtained for the mailbox, the file in
/tmp is deleted.

The default output contains verification of the locking that takes place. The
-v option causes some additional information to be given. The -q option
suppresses all output except error messages.

A command such as

  exim_lock /var/spool/mail/spqr

runs an interactive shell while the file is locked, whereas

  exim_lock -q /var/spool/mail/spqr <<End
  <some commands>
  End

runs a specific non-interactive sequence of commands while the file is locked,
suppressing all verification output. A single command can be run by a command
such as

  exim_lock -q /var/spool/mail/spqr \
    "cp /var/spool/mail/spqr /some/where"

Note that if a command is supplied, it must be entirely contained within the
second argument - hence the quotes.



                             47. THE EXIM MONITOR


The Exim monitor is an application which displays in an X window information
about the state of Exim's queue and what Exim is doing. An admin user can
perform certain operations on messages from this GUI interface; however all
such facilities are also available from the command line, and indeed, the
monitor itself makes use of the command line to perform any actions requested.


47.1 Running the monitor

The monitor is started by running the script called "eximon". This is a shell
script that sets up a number of environment variables, and then runs the
binary called eximon.bin. The default appearance of the monitor window can
be changed by editing the Local/eximon.conf file created by editing
exim_monitor/EDITME. Comments in that file describe what the various par-
ameters are for.

The parameters that get built into the "eximon" script can be overridden for a
particular invocation by setting up environment variables of the same names,
preceded by 'EXIMON_'. For example, a shell command such as

  EXIMON_LOG_DEPTH=400 eximon

(in a Bourne-compatible shell) runs "eximon" with an overriding setting of the
LOG_DEPTH parameter. If EXIMON_LOG_FILE_PATH is set in the environment, it
overrides the Exim log file configuration. This makes it possible to have
"eximon" tailing log data that is written to syslog, provided that MAIL.INFO
syslog messages are routed to a file on the local host.

X resources can be used to change the appearance of the window in the normal
way. For example, a resource setting of the form

  Eximon*background: gray94

changes the colour of the background to light grey rather than white. The
stripcharts are drawn with both the data lines and the reference lines in
black. This means that the reference lines are not visible when on top of the
data. However, their colour can be changed by setting a resource called
'highlight' (an odd name, but that's what the Athena stripchart widget uses).
For example, if your X server is running Unix, you could set up lighter
reference lines in the stripcharts by obeying

  xrdb -merge <<End
  Eximon*highlight: gray
  End

In order to see the contents of messages on the queue, and to operate on them,
"eximon" must either be run as root or by an admin user.

The monitor's window is divided into three parts. The first contains one or
more stripcharts and two action buttons, the second contains a 'tail' of the
main log file, and the third is a display of the queue of messages awaiting
delivery, with two more action buttons. The following sections describe these
different parts of the display.


47.2 The stripcharts

The first stripchart is always a count of messages on the queue. Its name can
be configured by setting QUEUE_STRIPCHART_NAME in the Local/eximon.conf file.
The remaining stripcharts are defined in the configuration script by regular
expression matches on log file entries, making it possible to display, for
example, counts of messages delivered to certain hosts or using certain
transports. The supplied defaults display counts of received and delivered
messages, and of local and SMTP deliveries. The default period between
stripchart updates is one minute; this can be adjusted by a parameter in the
Local/eximon.conf file.

The stripchart displays rescale themselves automatically as the value they are
displaying changes. There are always 10 horizontal lines in each chart; the
title string indicates the value of each division when it is greater than one.
For example, 'x2' means that each division represents a value of 2.

It is also possible to have a stripchart which shows the percentage fullness
of a particular disk partition, which is useful when local deliveries are
confined to a single partition. This relies on the availability of the
"statvfs()" function or equivalent in the operating system. Most, but not all
versions of Unix that support Exim have this. For this particular stripchart,
the top of the chart always represents 100%, and the scale is given as 'x10%'.
This chart is configured by setting SIZE_STRIPCHART and (optionally)
SIZE_STRIPCHART_NAME in the Local/eximon.conf file.


47.3 Main action buttons

Below the stripcharts there is an action button for quitting the monitor. Next
to this is another button marked 'Size'. They are placed here so that
shrinking the window to its default minimum size leaves just the queue count
stripchart and these two buttons visible. Pressing the 'Size' button causes
the window to expand to its maximum size, unless it is already at the maximum,
in which case it is reduced to its minimum.

When expanding to the maximum, if the window cannot be fully seen where it
currently is, it is moved back to where it was the last time it was at full
size. When it is expanding from its minimum size, the old position is
remembered, and next time it is reduced to the minimum it is moved back there.

The idea is that you can keep a reduced window just showing one or two
stripcharts at a convenient place on your screen, easily expand it to show the
full window when required, and just as easily put it back to what it was. The
idea is copied from what the "twm" window manager does for its "f.fullzoom"
action. The minimum size of the window can be changed by setting the
MIN_HEIGHT and MIN_WIDTH values in Local/eximon.conf.

Normally, the monitor starts up with the window at its full size, but it can
be built so that it starts up with the window at its smallest size, by setting
START_SMALL=yes in Local/eximon.conf.


47.4 The log display

The second section of the window is an area in which a display of the tail of
the main log is maintained. To save space on the screen, the timestamp on each
log line is shortened by removing the date and, if "log_timezone" is set, the
timezone. The log tail is not available when the only destination for logging
data is syslog, unless the syslog lines are routed to a local file whose name
is passed to "eximon" via the EXIMON_LOG_FILE_PATH environment variable.

The log sub-window has a scroll bar at its lefthand side which can be used to
move back to look at earlier text, and the up and down arrow keys also have a
scrolling effect. The amount of log that is kept depends on the setting of
LOG_BUFFER in Local/eximon.conf, which specifies the amount of memory to use.
When this is full, the earlier 50% of data is discarded - this is much more
efficient than throwing it away line by line. The sub-window also has a
horizontal scroll bar for accessing the ends of long log lines. This is the
only means of horizontal scrolling; the right and left arrow keys are not
available. Text can be cut from this part of the window using the mouse in the
normal way. The size of this subwindow is controlled by parameters in the
configuration file Local/eximon.conf.

Searches of the text in the log window can be carried out by means of the ^R
and ^S keystrokes, which default to a reverse and a forward search, respect-
ively. The search covers only the text that is displayed in the window. It
cannot go further back up the log.

The point from which the search starts is indicated by a caret marker. This is
normally at the end of the text in the window, but can be positioned
explicitly by pointing and clicking with the left mouse button, and is moved
automatically by a successful search. If new text arrives in the window when
it is scrolled back, the caret remains where it is, but if the window is not
scrolled back, the caret is moved to the end of the new text.

Pressing ^R or ^S pops up a window into which the search text can be typed.
There are buttons for selecting forward or reverse searching, for carrying out
the search, and for cancelling. If the 'Search' button is pressed, the search
happens and the window remains so that further searches can be done. If the
'Return' key is pressed, a single search is done and the window is closed. If
^C is typed the search is cancelled.

The searching facility is implemented using the facilities of the Athena text
widget. By default this pops up a window containing both 'search' and
'replace' options. In order to suppress the unwanted 'replace' portion for
eximon, a modified version of the "TextPop" widget is distributed with Exim.
However, the linkers in BSDI and HP-UX seem unable to handle an externally
provided version of "TextPop" when the remaining parts of the text widget come
from the standard libraries. The compile-time option EXIMON_TEXTPOP can be
unset to cut out the modified "TextPop", making it possible to build Eximon on
these systems, at the expense of having unwanted items in the search popup
window.


47.5 The queue display

The bottom section of the monitor window contains a list of all messages that
are on the queue, which includes those currently being received or delivered,
as well as those awaiting delivery. The size of this subwindow is controlled
by parameters in the configuration file Local/eximon.conf, and the frequency
at which it is updated is controlled by another parameter in the same file -
the default is 5 minutes, since queue scans can be quite expensive. However,
there is an 'Update' action button just above the display which can be used to
force an update of the queue display at any time.

When a host is down for some time, a lot of pending mail can build up for it,
and this can make it hard to deal with other messages on the queue. To help
with this situation there is a button next to 'Update' called 'Hide'. If
pressed, a dialogue box called 'Hide addresses ending with' is put up. If you
type anything in here and press 'Return', the text is added to a chain of such
texts, and if every undelivered address in a message matches at least one of
the texts, the message is not displayed.

If there is an address that does not match any of the texts, all the addresses
are displayed as normal. The matching happens on the ends of addresses so,
for example, "cam.ac.uk" specifies all addresses in Cambridge, while
"xxx@foo.com.example" specifies just one specific address. When any hiding has
been set up, a button called 'Unhide' is displayed. If pressed, it cancels all
hiding. Also, to ensure that hidden messages do not get forgotten, a hide
request is automatically cancelled after one hour.

While the dialogue box is displayed, you can't press any buttons or do
anything else to the monitor window. For this reason, if you want to cut text
from the queue display to use in the dialogue box, you have to do the cutting
before pressing the 'Hide' button.

The queue display contains, for each unhidden queued message, the length of
time it has been on the queue, the size of the message, the message id, the
message sender, and the first undelivered recipient, all on one line. If it is
a bounce message, the sender is shown as '<>'. If there is more than one
recipient to which the message has not yet been delivered, subsequent ones are
listed on additional lines, up to a maximum configured number, following which
an ellipsis is displayed. Recipients that have already received the message
are not shown. If a message is frozen, an asterisk is displayed at the left-
hand side.

The queue display has a vertical scroll bar, and can also be scrolled by means
of the arrow keys. Text can be cut from it using the mouse in the normal way.
The text searching facilities, as described above for the log window, are also
available, but the caret is always moved to the end of the text when the queue
display is updated.


47.6 The queue menu

If the "shift" key is held down and the left button is clicked when the mouse
pointer is over the text for any message, an action menu pops up, and the
first line of the queue display for the message is highlighted. This does not
affect any selected text.

If you want to use some other event for popping up the menu, you can set the
MENU_EVENT parameter in Local/eximon.conf to change the default, or set
EXIMON_MENU_EVENT in the environment before starting the monitor. The value
set in this parameter is a standard X event description. For example, to run
eximon using "ctrl" rather than "shift" you could use

  EXIMON_MENU_EVENT='Ctrl<Btn1Down>' eximon

The title of the menu is the message id, and it contains entries which act as
follows:

 .   "message log": The contents of the message log for the message are
     displayed in a new text window.

 .   "headers": Information from the spool file that contains the envelope
     information and headers is displayed in a new text window. See chapter 49
     for a description of the format of spool files.

 .   "body": The contents of the spool file containing the body of the message
     are displayed in a new text window. There is a default limit of 20,000
     bytes to the amount of data displayed. This can be changed by setting the
     BODY_MAX option at compile time, or the EXIMON_BODY_MAX option at run
     time.

 .   "deliver message": A call to Exim is made using the -M option to request
     delivery of the message. This causes an automatic thaw if the message is
     frozen. The -v option is also set, and the output from Exim is displayed
     in a new text window. The delivery is run in a separate process, to avoid
     holding up the monitor while the delivery proceeds.

 .   "freeze message": A call to Exim is made using the -Mf option to request
     that the message be frozen.

 .   "thaw message": A call to Exim is made using the -Mt option to request
     that the message be thawed.

 .   "give up on msg": A call to Exim is made using the -Mg option to request
     that Exim gives up trying to deliver the message. A bounce message is
     generated for any remaining undelivered addresses.

 .   "remove message": A call to Exim is made using the -Mrm option to request
     that the message be deleted from the system without generating a bounce
     message.

 .   "add recipient": A dialog box is displayed into which a recipient address
     can be typed. If the address is not qualified and the QUALIFY_DOMAIN
     parameter is set in Local/eximon.conf, the address is qualified with that
     domain. Otherwise it must be entered as a fully qualified address.
     Pressing RETURN causes a call to Exim to be made using the -Mar option to
     request that an additional recipient be added to the message, unless the
     entry box is empty, in which case no action is taken.

 .   "mark delivered": A dialog box is displayed into which a recipient
     address can be typed. If the address is not qualified and the
     QUALIFY_DOMAIN parameter is set in Local/eximon.conf, the address is
     qualified with that domain. Otherwise it must be entered as a fully
     qualified address. Pressing RETURN causes a call to Exim to be made using
     the -Mmd option to mark the given recipient address as already delivered,
     unless the entry box is empty, in which case no action is taken.

 .   "mark all delivered": A call to Exim is made using the -Mmad option to
     mark all recipient addresses as already delivered.

 .   "edit sender": A dialog box is displayed initialized with the current
     sender's address. Pressing RETURN causes a call to Exim to be made using
     the -Mes option to replace the sender address, unless the entry box is
     empty, in which case no action is taken. If you want to set an empty
     sender (as in bounce messages), you must specify it as '<>'. Otherwise,
     if the address is not qualified and the QUALIFY_DOMAIN parameter is set
     in Local/eximon.conf, the address is qualified with that domain.

When a delivery is forced, a window showing the -v output is displayed. In
other cases when a call to Exim is made, if there is any output from Exim (in
particular, if the command fails) a window containing the command and the
output is displayed. Otherwise, the results of the action are normally
apparent from the log and queue displays. However, if you set
ACTION_OUTPUT=yes in Local/eximon.conf, a window showing the Exim command is
always opened, even if no output is generated.

The queue display is automatically updated for actions such as freezing and
thawing, unless ACTION_QUEUE_UPDATE=no has been set in Local/eximon.conf. In
this case the 'Update' button has to be used to force an update of the display
after one of these actions.

In any text window that is displayed as result of a menu action, the normal
cut-and-paste facility is available, and searching can be carried out using ^R
and ^S, as described above for the log tail window.



                         48. SECURITY CONSIDERATIONS


This chapter discusses a number of issues concerned with security, some of
which are also covered in other parts of this manual.

For reasons that this author does not understand, some people have promoted
Exim as a 'particularly secure' mailer. Perhaps it is because of the existence
of this chapter in the documentation. However, the intent of the chapter is
simply to describe the way Exim works in relation to certain security
concerns, not to make any specific claims about the effectiveness of its
security as compared with other MTAs.

What follows is a description of the way Exim is supposed to be. Best efforts
have been made to try to ensure that the code agrees with the theory, but an
absence of bugs can never be guaranteed. Any that are reported will get fixed
as soon as possible.

                                                                               |
48.1 Building a more 'hardened' Exim                                           |
                                                                               |
There are a number of build-time options that can be set in Local/Makefile to  |
create Exim binaries that are 'harder' to attack, in particular by a rogue     |
Exim administrator who does not have the root password, or by someone who has  |
penetrated the Exim (but not the root) account. These options are as follows:  |
                                                                               |
 .   ALT_CONFIG_PREFIX can be set to a string that is required to match the    |
     start of any file names used with the -C option. When it is set, these    |
     file names are also not allowed to contain the sequence '/../'. (However, |
     if the value of the -C option is identical to the value of CONFIGURE_FILE |
     in Local/Makefile, Exim ignores -C and proceeds as usual.) There is no    |
     default setting for "ALT_CONFIG_PREFIX".                                  |
                                                                               |
     If the permitted configuration files are confined to a directory to which |
     only root has access, this guards against someone who has broken into the |
     Exim account from running a privileged Exim with an arbitrary configur-   |
     ation file, and using it to break into other accounts.                    |
                                                                               |
 .   If ALT_CONFIG_ROOT_ONLY is defined, root privilege is retained for -C and |
     -D only if the caller of Exim is root. Without it, the Exim user may also |
     use -C and -D and retain privilege. Setting this option locks out the     |
     possibility of testing a configuration using -C right through message     |
     reception and delivery, even if the caller is root. The reception works,  |
     but by that time, Exim is running as the Exim user, so when it re-execs   |
     to regain privilege for the delivery, the use of -C causes privilege to   |
     be lost. However, root can test reception and delivery using two separate |
     commands. ALT_CONFIG_ROOT_ONLY is not set by default.                     |
                                                                               |
 .   If DISABLE_D_OPTION is defined, the use of the -D command line option is  |
     disabled.                                                                 |
                                                                               |
 .   FIXED_NEVER_USERS can be set to a colon-separated list of users that are  |
     never to be used for any deliveries. This is like the "never_users"       |
     runtime option, but it cannot be overridden; the runtime option adds      |
     additional users to the list. The default setting is 'root'; this         |
     prevents a non-root user who is permitted to modify the runtime file from |
     using Exim as a way to get root.                                          |
                                                                               |

48.2 Root privilege

The Exim binary is normally setuid to root, which means that it gains root
privilege (runs as root) when it starts execution. In some special cases (for
example, when the daemon is not in use and there are no local deliveries), it
may be possible to run Exim setuid to some user other than root. This is
discussed in the next section. However, in most installations, root privilege
is required for two things:

 .   To set up a socket connected to the standard SMTP port (25) when
     initialising the listening daemon. If Exim is run from "inetd", this
     privileged action is not required.

 .   To be able to change uid and gid in order to read users' .forward files
     and perform local deliveries as the receiving user or as specified in the
     configuration.

It is not necessary to be root to do any of the other things Exim does, such
as receiving messages and delivering them externally over SMTP, and it is
obviously more secure if Exim does not run as root except when necessary.
For this reason, a user and group for Exim to use must be defined in
Local/Makefile. These are known as 'the Exim user' and 'the Exim group'. Their
values can be changed by the run time configuration, though this is not
recommended. Often a user called "exim" is used, but some sites use "mail" or
another user name altogether.

Exim uses "setuid()" whenever it gives up root privilege. This is a permanent
abdication; the process cannot regain root afterwards. Prior to release 4.00,
"seteuid()" was used in some circumstances, but this is no longer the case.

After a new Exim process has interpreted its command line options, it changes
uid and gid in the following cases:

 .   If the -C option is used to specify an alternate configuration file, or
     if the -D option is used to define macro values for the configuration,
     and the calling process is not running as root or the Exim user, the uid
     and gid are changed to those of the calling process. However, if          |
     ALT_CONFIG_ROOT_ONLY is defined in Local/Makefile, only root callers may  |
     use -C and -D without losing privilege, and if DISABLE_D_OPTION is set,   |
     the -D option may not be used at all.                                     |

 .   If the expansion test option (-be) or one of the filter testing options
     (-bf or -bF) are used, the uid and gid are changed to those of the
     calling process.

 .   If the process is not a daemon process or a queue runner process or a
     delivery process or a process for testing address routing (started with
     -bt), the uid and gid are changed to the Exim user and group. This means
     that Exim always runs under its own uid and gid when receiving messages.
     This also applies when testing address verification (the -bv option) and
     testing incoming message policy controls (the -bh option).

 .   For a daemon, queue runner, delivery, or address testing process, the uid
     remains as root at this stage, but the gid is changed to the Exim group.

The processes that initially retain root privilege behave as follows:

 .   A daemon process changes the gid to the Exim group and the uid to the     |
     Exim user after setting up one or more listening sockets. The             |
     "initgroups()" function is called, so that if the Exim user is in any     |
     additional groups, they will be used during message reception.            |

 .   A queue runner process retains root privilege throughout its execution.
     Its job is to fork a controlled sequence of delivery processes.

 .   A delivery process retains root privilege throughout most of its
     execution, but any actual deliveries (that is, the transports themselves)
     are run in subprocesses which always change to a non-root uid and gid.
     For local deliveries this is typically the uid and gid of the owner of
     the mailbox; for remote deliveries, the Exim uid and gid are used. Once
     all the delivery subprocesses have been run, a delivery process changes
     to the Exim uid and gid while doing post-delivery tidying up such as
     updating the retry database and generating bounce and warning messages.

     While the recipient addresses in a message are being routed, the delivery
     process runs as root. However, if a user's filter file has to be
     processed, this is done in a subprocess that runs under the individual
     user's uid and gid. A system filter is run as root unless
     "system_filter_user" is set.

 .   A process that is testing addresses (the -bt option) runs as root so that
     the routing is done in the same environment as a message delivery.


48.3 Running Exim without privilege

Some installations like to run Exim in an unprivileged state for more of its
operation, for added security. Support for this mode of operation is provided
by the global option "deliver_drop_privilege". When this is set, the uid and
gid are changed to the Exim user and group at the start of a delivery process
(and also queue runner and address testing processes). This means that address
routing is no longer run as root, and the deliveries themselves cannot change
to any other uid.

Leaving the binary setuid to root, but setting "deliver_drop_privilege" means
that the daemon can still be started in the usual way, and it can respond
correctly to SIGHUP because the re-invocation regains root privilege.

An alternative approach is to make Exim setuid to the Exim user and also       |
setgid to the Exim group. If you do this, the daemon must be started from a    |
root process. (Calling Exim from a root process makes it behave in the way it
does when it is setuid root.) However, the daemon cannot restart itself after
a SIGHUP signal because it cannot regain privilege.

It is still useful to set "deliver_drop_privilege" in this case, because it
stops Exim from trying to re-invoke itself to do a delivery after a message
has been received. Such a re-invocation is a waste of resources because it has
no effect.

If restarting the daemon is not an issue (for example, if "inetd" is being
used instead of a daemon), having the binary setuid to the Exim user seems a
clean approach, but there is one complication:

In this style of operation, Exim is running with the real uid and gid set to
those of the calling process, and the effective uid/gid set to Exim's values.
Ideally, any association with the calling process' uid/gid should be dropped,
that is, the real uid/gid should be reset to the effective values so as to
discard any privileges that the caller may have. While some operating systems
have a function that permits this action for a non-root effective uid, quite a
number of them do not. Because of this lack of standardization, Exim does not
address this problem at this time.

For this reason, the recommended approach for 'mostly unprivileged' running is
to keep the Exim binary setuid to root, and to set "deliver_drop_privilege".
This also has the advantage of allowing a daemon to be used in the most
straightforward way.

If you configure Exim not to run delivery processes as root, there are a
number of restrictions on what you can do:

 .   You can deliver only as the Exim user/group. You should explicitly use
     the "user" and "group" options to override routers or local transports
     that normally deliver as the recipient. This makes sure that configur-
     ations that work in this mode function the same way in normal mode. Any
     implicit or explicit specification of another user causes an error.

 .   Use of .forward files is severely restricted, such that it is usually not
     worthwhile to include them in the configuration.

 .   Users who wish to use .forward would have to make their home directory
     and the file itself accessible to the Exim user. Pipe and append-to-file
     entries, and their equivalents in Exim filters, cannot be used. While
     they could be enabled in the Exim user's name, that would be insecure and
     not very useful.

 .   Unless the local user mailboxes are all owned by the Exim user (possible
     in some POP3 or IMAP-only environments):

      *   They must be owned by the Exim group and be writable by that group.
          This implies you must set "mode" in the appendfile configuration, as
          well as the mode of the mailbox files themselves.

      *   You must set "no_check_owner", since most or all of the files will
          not be owned by the Exim user.

      *   You must set "file_must_exist", because Exim cannot set the owner
          correctly on a newly created mailbox when unprivileged. This also
          implies that new mailboxes need to be created manually.

These restrictions severely restrict what can be done in local deliveries.
However, there are no restrictions on remote deliveries. If you are running a
gateway host that does no local deliveries, setting "deliver_drop_privilege"
gives more security at essentially no cost.


48.4 Delivering to local files

Full details of the checks applied by appendfile before it writes to a file
are given in chapter 26.


48.5 IPv4 source routing

Many operating systems suppress IP source-routed packets in the kernel, but
some cannot be made to do this, so Exim does its own check. It logs incoming
IPv4 source-routed TCP calls, and then drops them. Things are all different in
IPv6. No special checking is currently done.


48.6 The VRFY, EXPN, and ETRN commands in SMTP

Support for these SMTP commands is disabled by default. If required, they can
be enabled by defining suitable ACLs.


48.7 Privileged users

Exim recognises two sets of users with special privileges. Trusted users are
able to submit new messages to Exim locally, but supply their own sender
addresses and information about a sending host. For other users submitting
local messages, Exim sets up the sender address from the uid, and doesn't
permit a remote host to be specified.

However, an untrusted user is permitted to use the -f command line option in
the special form -f <> to indicate that a delivery failure for the message
should not cause an error report. This affects the message's envelope, but it
does not affect the Sender: header. Untrusted users may also be permitted
to use specific forms of address with the -f option by setting the
"untrusted_set_sender" option.

Trusted users are used to run processes that receive mail messages from some
other mail domain and pass them on to Exim for delivery either locally, or
over the Internet. Exim trusts a caller that is running as root, as the Exim
user, as any user listed in the "trusted_users" configuration option, or under
any group listed in the "trusted_groups" option.

Admin users are permitted to do things to the messages on Exim's queue. They
can freeze or thaw messages, cause them to be returned to their senders,
remove them entirely, or modify them in various ways. In addition, admin users
can run the Exim monitor and see all the information it is capable of
providing, which includes the contents of files on the spool.

By default, the use of the -M and -q options to cause Exim to attempt delivery
of messages on its queue is restricted to admin users. This restriction can be
relaxed by setting the "no_prod_requires_admin" option. Similarly, the use of
-bp (and its variants) to list the contents of the queue is also restricted
to admin users. This restriction can be relaxed by setting
"no_queue_list_requires_admin".

Exim recognises an admin user if the calling process is running as root or as
the Exim user or if any of the groups associated with the calling process is
the Exim group. It is not necessary actually to be running under the Exim
group. However, if admin users who are not root or the Exim user are to access
the contents of files on the spool via the Exim monitor (which runs
unprivileged), Exim must be built to allow group read access to its spool
files.


48.8 Spool files

Exim's spool directory and everything it contains is owned by the Exim user
and set to the Exim group. The mode for spool files is defined in the
Local/Makefile configuration file, and defaults to 0640. This means that any
user who is a member of the Exim group can access these files.


48.9 Use of argv[0]

Exim examines the last component of "argv[0]", and if it matches one of a set
of specific strings, Exim assumes certain options. For example, calling Exim
with the last component of "argv[0]" set to 'rsmtp' is exactly equivalent to
calling it with the option -bS. There are no security implications in this.


48.10 Use of %f formatting

The only use made of '%f' by Exim is in formatting load average values. These
are actually stored in integer variables as 1000 times the load average.
Consequently, their range is limited and so therefore is the length of the
converted output.


48.11 Embedded Exim path

Exim uses its own path name, which is embedded in the code, only when it needs
to re-exec in order to regain root privilege. Therefore, it is not root when
it does so. If some bug allowed the path to get overwritten, it would lead to
an arbitrary program's being run as exim, not as root.


48.12 Use of sprintf()

A large number of occurrences of 'sprintf' in the code are actually calls to
"string_sprintf()", a function that returns the result in malloc'd store. The
intermediate formatting is done into a large fixed buffer by a function that
runs through the format string itself, and checks the length of each
conversion before performing it, thus preventing buffer overruns.

The remaining uses of "sprintf()" happen in controlled circumstances where the
output buffer is known to be sufficiently long to contain the converted
string.


48.13 Use of debug_printf() and log_write()

Arbitrary strings are passed to both these functions, but they do their
formatting by calling the function "string_vformat()", which runs through the
format string itself, and checks the length of each conversion.


48.14 Use of strcat() and strcpy()

These are used only in cases where the output buffer is known to be large
enough to hold the result.



                          49. FORMAT OF SPOOL FILES


A message on Exim's queue consists of two files, whose names are the message
id followed by -D and -H, respectively. The data portion of the message is
kept in the -D file on its own. The message's envelope, status, and headers
are all kept in the -H file, whose format is described in this chapter. Each
of these two files contains the final component of its own name as its first
line. This is insurance against disk crashes where the directory is lost but
the files themselves are recoverable.

Files whose names end with -J may also be seen in the input directory (or its
subdirectories when "split_spool_directory" is set). These are journal files,
used to record addresses to which the message has been delivered during the
course of a delivery run. At the end of the run, the -H file is updated, and
the -J file is deleted.

The second line of the -H file contains the login name for the uid of the
process that called Exim to read the message, followed by the numerical uid
and gid. For a locally generated message, this is normally the user who sent
the message. For a message received over TCP/IP, it is normally the Exim user.

The third line of the file contains the address of the message's sender as
transmitted in the envelope, contained in angle brackets. The sender address
is empty for bounce messages. For incoming SMTP mail, the sender address is
given in the MAIL command. For locally generated mail, the sender address is
created by Exim from the login name of the current user and the configured
"qualify_domain". However, this can be overridden by the -f option or a
leading 'From' line if the caller is trusted, or if the supplied address is
'<>' or an address that matches "untrusted_set_senders".

The fourth line contains two numbers. The first is the time that the message
was received, in the conventional Unix form - the number of seconds since the
start of the epoch. The second number is a count of the number of messages
warning of delayed delivery that have been sent to the sender.

There follow a number of lines starting with a hyphen. These can appear in any
order, and are omitted when not relevant:

 .   -auth_id <text>: The id information for a message received on an
     authenticated SMTP connection - the value of the $authenticated_id
     variable.

 .   -auth_sender <address>: The address of an authenticated sender - the
     value of the $authenticated_sender variable.

 .   -body_linecount <number>: This records the number of lines in the body of
     the message, and is always present.

 .   -deliver_firsttime: This is written when a new message is first added to
     the spool. When the spool file is updated after a deferral, it is
     omitted.

 .   -frozen <time>: The message is frozen, and the freezing happened at
     <time>.

 .   -helo_name <text>: This records the host name as specified by a remote
     host in a HELO or EHLO command.

 .   -host_address <address>.<port>: This records the IP address of the host
     from which the message was received and the remote port number that was
     used. It is omitted for locally generated messages.

 .   -host_auth <text>: If the message was received on an authenticated SMTP
     connection, this records the name of the authenticator - the value of the
     $sender_host_authenticated variable.

 .   -host_lookup_failed: This is present if an attempt to look up the sending
     host's name from its IP address failed. It corresponds to the
     $host_lookup_failed variable.

 .   -host_name <text>: This records the name of the remote host from which
     the message was received, if the host name was looked up from the IP
     address when the message was being received. It is not present if no
     reverse lookup was done.

 .   -ident <text>: For locally submitted messages, this records the login of
     the originating user, unless it was a trusted user and the -oMt option
     was used to specify an ident value. For messages received over TCP/IP,
     this records the ident string supplied by the remote host, if any.

 .   -interface_address <address>.<port>: This records the IP address of the
     local interface and the port number through which a message was received
     from a remote host. It is omitted for locally generated messages.

 .   -local: The message is from a local sender.

 .   -localerror: The message is a locally-generated bounce message.

 .   -local_scan <string>: This records the data string that was returned by
     the "local_scan()" function when the message was received - the value of
     the $local_scan_data variable. It is omitted if no data was returned.

 .   -manual_thaw: The message was frozen but has been thawed manually, that
     is, by an explicit Exim command rather than via the auto-thaw process.

 .   -N: A testing delivery process was started using the -N option to
     suppress any actual deliveries, but delivery was deferred. At any further
     delivery attempts, -N is assumed.

 .   -received_protocol: This records the value of the $received_protocol
     variable, which contains the name of the protocol by which the message
     was received.

 .   -sender_set_untrusted: The envelope sender of this message was set by an
     untrusted local caller (used to ensure that the caller is displayed in
     queue listings).

 .   -tls_certificate_verified: A TLS certificate was received from the client
     that sent this message, and the certificate was verified by the server.

 .   -tls_cipher <cipher name>: When the message was received over an
     encrypted connection, this records the name of the cipher suite that was
     used.

 .   -tls_peerdn <peer DN>: When the message was received over an encrypted
     connection, and a certificate was received from the client, this records
     the Distinguished Name from that certificate.

Following the options there is a list of those addresses to which the message
is not to be delivered. This set of addresses is initialized from the command
line when the -t option is used and "extract_addresses_remove_arguments" is
set; otherwise it starts out empty. Whenever a successful delivery is made,
the address is added to this set. The addresses are kept internally as a
balanced binary tree, and it is a representation of that tree which is written
to the spool file. If an address is expanded via an alias or forward file, the
original address is added to the tree when deliveries to all its child
addresses are complete.

If the tree is empty, there is a single line in the spool file containing just
the text 'XX'. Otherwise, each line consists of two letters, which are either
Y or N, followed by an address. The address is the value for the node of the
tree, and the letters indicate whether the node has a left branch and/or a
right branch attached to it, respectively. If branches exist, they immediately
follow. Here is an example of a three-node tree:

  YY darcy@austen.fict.example
  NN alice@wonderland.fict.example
  NN editor@thesaurus.ref.example

After the non-recipients tree, there is a list of the message's recipients.
This is a simple list, preceded by a count. It includes all the original
recipients of the message, including those to whom the message has already
been delivered. In the simplest case, the list contains one address per line.
For example:

  4
  editor@thesaurus.ref.example
  darcy@austen.fict.example
  rdo@foundation
  alice@wonderland.fict.example

However, when a child address has been added to the top-level addresses as a
result of the use of the "one_time" option on a redirect router, each line is
of the following form:

  <top-level address> <errors_to address> <length>,<parent number>#<flag bits>

The 01 flag bit indicates the presence of the three other fields that follow
the top-level address. Other bits may be used in future to support additional
fields. The <parent number> is the offset in the recipients list of the
original parent of the 'one time' address. The first two fields are the
envelope sender that is associated with this address and its length. If the
length is zero, there is no special envelope sender (there are then two space
characters in the line). A non-empty field can arise from a redirect router
that has an "errors_to" setting.

A blank line separates the envelope and status information from the headers
which follow. A header may occupy several lines of the file, and to save
effort when reading it in, each header is preceded by a number and an
identifying character. The number is the number of characters in the header,
including any embedded newlines and the terminating newline. The character is
one of the following:

  <blank>  header in which Exim has no special interest
   B       Bcc: header
   C       Cc: header
   F       From: header
   I       Message-id: header
   P       Received: header - P for 'postmark'
   R       Reply-To: header
   S       Sender: header
   T       To: header
   *       replaced or deleted header

Deleted or replaced (rewritten) headers remain in the spool file for debugging
purposes. They are not transmitted when the message is delivered. Here is a
typical set of headers:

  111P Received: by hobbit.fict.example with local (Exim 4.00)
          id 14y9EI-00026G-00; Fri, 11 May 2001 10:28:59 +0100
  049  Message-Id: <E14y9EI-00026G-00@hobbit.fict.example>
  038* X-rewrote-sender: bb@hobbit.fict.example
  042* From: Bilbo Baggins <bb@hobbit.fict.example>
  049F From: Bilbo Baggins <B.Baggins@hobbit.fict.example>
  099* To: alice@wonderland.fict.example, rdo@foundation,
   darcy@austen.fict.example, editor@thesaurus.ref.example
  109T To: alice@wonderland.fict.example, rdo@foundation.fict.example,
   darcy@austen.fict.example, editor@thesaurus.ref.example
  038  Date: Fri, 11 May 2001 10:28:59 +0100

The asterisked headers indicate that the envelope sender, From: header, and
To: header have been rewritten, the last one because routing expanded the
unqualified domain "foundation".



                    50. ADDING NEW DRIVERS OR LOOKUP TYPES


The following actions have to be taken in order to add a new router,
transport, authenticator, or lookup type to Exim:

(1)  Choose a name for the driver or lookup type that does not conflict with
     any existing name; I will use 'newdriver' in what follows.

(2)  Add to src/EDITME the line

       <type>_NEWDRIVER=yes

     where <type> is ROUTER, TRANSPORT, AUTH, or LOOKUP. If the code is not to
     be included in the binary by default, comment this line out. You should
     also add any relevant comments about the driver or lookup type.

(3)  Add to src/config.h.defaults the line

       #define <type>_NEWDRIVER

(4)  Edit src/drtables.c, adding conditional code to pull in the private
     header and create a table entry as is done for all the other drivers and
     lookup types.

(5)  Edit Makefile in the appropriate sub-directory (src/routers,
     src/transports, src/auths, or src/lookups); add a line for the new driver
     or lookup type and add it to the definition of OBJ.

(6)  Create newdriver.h and newdriver.c in the appropriate sub-directory of
     src.

(7)  Edit scripts/MakeLinks and add commands to link the .h and .c files as
     for other drivers and lookups.

Then all you need to do is write the code! A good way to start is to make a
proforma by copying an existing module of the same type, globally changing all
occurrences of the name, and cutting out most of the code. Note that any
options you create must be listed in alphabetical order, because the tables
are searched using a binary chop procedure.

There is a README file in each of the sub-directories of src describing the
interface that is expected.

