                            Courier IMAP/POP3 proxy

   Courier's IMAP and POP3 servers now include a built-in proxy aggregator.
   With a proxy aggregator, the mail accounts are split between multiple,
   independent servers, with an IMAP/POP3 server running on each individual
   server. A separate, proxy server sits in front and accepts ordinary IMAP
   and POP3 connections. It reads the login ID, determines which server the
   account is located on, connects to the server, and logs in. Then, for the
   lifetime on the login session the front-end server takes itself out of the
   loop, and forwards all session traffic between the IMAP/POP3 client, and
   the back-end server.

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Requirements

   Courier can operate in IMAP/POP3 proxy mode only when the Courier
   Authentication Library uses the userdb, LDAP, MySQL, or the PostgreSQL
   authentication module. Challenge-Response (CRAM) authentication will also
   work with the LDAP, MySQL, or the PostgreSQL authentication module. Yes,
   CRAM authentication will work (except for userdb).

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Configuration

   Follow the regular installation instructions to set up Courier on servers
   with the actual mail accounts. The proxy feature uses the "account
   options" feature of the Courier Authentication Library, specifically an
   option called "mailhost". Account option configuration process depends on
   the authentication module. With userdb, account options are specified by
   the "options" userdb attribute:

   userdb user@example.com set options=mailhost=servera.example.com           

   Instructions for setting up account options with LDAP, MySQL, or
   PostgreSQL, may be found in the appropriate configuration file. Briefly:

     * In authldaprc, put "LDAP_AUXOPTIONS<TAB>mailhost=mailhost", then
       populate the "mailhost" LDAP attribute (this may entail modifications
       of the LDAP schema).

     * In authmysqlrc, put
       "MYSQL_AUXOPTIONS<TAB>CONCAT("mailhost=",mailhost)" (or modify the
       existing MYSQL_AUXOPTIONS setting accordingly), then create a
       "mailhost" column in the account table.

     * In authpgsqlrc, put "PGSQL_AUXOPTIONS<TAB>'mailhost=' || mailhost" (or
       append ",mailhost=" || mailhost to an existing setting), then create a
       "mailhost" column in the account table.

   The "mailhost" option for each account should be the name of the server
   where that account is located. If possible, this should match, exactly,
   the PROXY_HOSTNAME environment variable or the value returned by the
   "gethostname" on the server.

   The final step is to set "IMAP_PROXY" and/or "POP3_PROXY" to "1" in the
   imapd and/or the pop3d configuration file, in the Courier configuration
   file directory on the proxy server.

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  Using the same configuration files on all servers

   It is possible to have both the proxy server, and the back-end servers
   with the actual accounts, read the same configuration file that enables
   proxying. Ordinarily, if the back-end server also has the proxy setting
   turned on, it will also attempt to establish a proxy connection (to
   itself; lather, rinse, repeat until the server runs out of sockets).

   However, if the "mailhost" option matches the server's hostname, as
   returned by "gethostname", no proxying takes place. Therefore, if specific
   attention and care is made, when setting up the server names and account
   options, all servers can boot off the same configuration file.

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  Alternative configurations

   If the server names are set up properly, it's possible to set things up
   without a dedicated front-end proxy aggregator server. All mail accounts
   are divided between a pool of servers, who are just one, big, happy
   family. IMAP and POP3 clients can connect to any server, at random. If
   they try to log into an account that happens to reside on the same box,
   then everything will be ready to go. If not, the server automatically
   opens a proxy connection to the right box, and everything will be ready to
   go as well.

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  Homogenous environments

   Both servers involved in a proxy connections should be running the same
   version of the Courier IMAP/POP3 server. The proxy code included in the
   Courier-IMAP package tarball will talk to the server from the Courier-MTA
   package tarball that includes the same build of the IMAP daemon, and
   vice-versa. Run "imapd --version" to determine the build of the IMAP
   daemon.

   All servers MUST use the same identical imapd and pop3d configuration
   files (with the possible exception of the proxy flag). The next section
   explains why.

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  Heterogenous environments

   It should generally be possible to have the Courier IMAP/POP3 server
   establish a proxy connection to some other third party, non-Courier, IMAP
   or POP3 server. Of course, the Courier Authentication Library running on
   the proxy server must have the same understanding of the account names and
   passwords as the other IMAP/POP3 server. The main issue is the different
   levels of protocol implementations.

   Both the IMAP and POP3 protocols have optional features that different
   servers may or may not implement. Some servers will implement certain
   optional features of the IMAP or POP3 protocol; other servers will
   implement different features parts.

   When the IMAP/POP3 client connects to the server, the client typically
   obtains the list of available optional features. After logging in, the
   client will have no reason to expect that it's now talking to a different
   server with a different set of protocol features. Therefore, it may not be
   possible to use a Courier proxy with some other IMAP/POP3 server that
   implements a widely different set of features. This may work with some
   clients, that don't make use of optional features; while other clients
   will report strange, or unpredictable errors.

   In some cases, setting the IMAP_PROXY_FOREIGN flag, in the imapd
   configuration file, may help. This command will send a message to the IMAP
   client explicitly informing the client that the list of available protocol
   features has changed; however some clients may ignore or not implement
   this particular message. There is no equivalent POP3 command.

   NOTE: As previously mentioned the IMAP/POP3 clients may use any supported  
         authentication method, including CRAM authentication (in most        
         cases), with or without encryption, to log in. However, Courier will 
         always use plain userid/password authentication, without encryption, 
         to establish proxy connections. When using a different server, that  
         server must be configured to allow plain userid/password             
         authentication.                                                      
                                                                              
         Note that the default configuration of the UW-IMAP server requires   
         encryption, and refuses non-encrypted connections. Proxy connections 
         are presumably carried over a private network, and there is no       
         reason to use encryption. Therefore, the UW-IMAP server will have to 
         be re-configured to allow non-encrypted connections, if it's to be   
         used with Courier in proxy mode.                                     
