    POSTSCREEN(8)                                                    POSTSCREEN(8)

    NAME
           postscreen - Postfix zombie blocker

    SYNOPSIS
           postscreen [generic Postfix daemon options]

    DESCRIPTION
           The Postfix postscreen(8) server performs triage on multi-
           ple inbound SMTP connections at the  same  time.  While  a
           single  postscreen(8)  process  keeps  spambots  away from
           Postfix SMTP server processes, more  Postfix  SMTP  server
           processes remain available for legitimate clients.

           This program should not be used on SMTP ports that receive
           mail from end-user clients (MUAs). In  a  typical  deploy-
           ment,  postscreen(8)  is  used  on  the "port 25" service,
           while MUA clients submit mail via the submission  service.

           postscreen(8)  maintains a temporary whitelist for clients
           that have passed a number of tests.  When an  SMTP  client
           IP  address  is  whitelisted,  postscreen(8) hands off the
           connection immediately to a Postfix SMTP  server  process.
           This minimizes the overhead for legitimate mail.

           By  default,  postscreen(8)  logs statistics and hands off
           every connection to a Postfix SMTP server  process,  while
           excluding clients in mynetworks from all tests (primarily,
           to avoid problems with non-standard  SMTP  implementations
           in  network  appliances).   This  mode  is useful for non-
           destructive testing.

           In a typical production setting, postscreen(8) is  config-
           ured  to  reject  mail  from clients that fail one or more
           tests. postscreen(8) logs rejected mail  with  the  client
           address, helo, sender and recipient information.

           postscreen(8)  is  not an SMTP proxy; this is intentional.
           The purpose is to keep spambots  away  from  Postfix  SMTP
           server processes, while minimizing overhead for legitimate
           traffic.

    SECURITY
           The postscreen(8) server is moderately security-sensitive.
           It  talks to untrusted clients on the network. The process
           can be run chrooted at fixed low privilege.

    STANDARDS
           RFC 821 (SMTP protocol)
           RFC 1123 (Host requirements)
           RFC 1652 (8bit-MIME transport)
           RFC 1869 (SMTP service extensions)
           RFC 1870 (Message Size Declaration)
           RFC 1985 (ETRN command)
           RFC 2034 (SMTP Enhanced Status Codes)
           RFC 2821 (SMTP protocol)
           RFC 2920 (SMTP Pipelining)
           RFC 3207 (STARTTLS command)
           RFC 3461 (SMTP DSN Extension)
           RFC 3463 (Enhanced Status Codes)
           RFC 5321 (SMTP protocol, including multi-line 220 banners)

    DIAGNOSTICS
           Problems and transactions are logged to syslogd(8).

    BUGS
           The postscreen(8) built-in SMTP protocol engine  currently
           does  not  announce support for AUTH, XCLIENT or XFORWARD.
           Support for AUTH may be added in the future.  In the  mean
           time, if you need to make these services available on port
           25, then do not enable  the  optional  "after  220  server
           greeting" tests.

           The  optional  "after  220  server greeting" tests involve
           postscreen(8)'s built-in SMTP protocol engine. When  these
           tests succeed, postscreen(8) adds the client to the tempo-
           rary whitelist but it cannot not hand off the "live"  con-
           nection  to a Postfix SMTP server process in the middle of
           a session.   Instead,  postscreen(8)  defers  attempts  to
           deliver  mail  with a 4XX status, and waits for the client
           to disconnect.  The next time a good client  connects,  it
           will  be  allowed to talk to a Postfix SMTP server process
           to deliver mail. postscreen(8)  mitigates  the  impact  of
           this  limitation  by  giving  such tests a long expiration
           time.

    CONFIGURATION PARAMETERS
           Changes to main.cf are not  picked  up  automatically,  as
           postscreen(8)  processes  may  run for several hours.  Use
           the command "postfix reload" after a configuration change.

           The  text  below  provides  only  a parameter summary. See
           postconf(5) for more details including examples.

           NOTE:  Some  postscreen(8)  parameters  implement  stress-
           dependent  behavior.   This  is  supported  only  when the
           default parameter value is stress-dependent (that  is,  it
           looks  like  ${stress?X}${stress:Y}, or it is the $name of
           an  smtpd  parameter  with  a  stress-dependent  default).
           Other  parameters always evaluate as if the stress parame-
           ter value is the empty string.

    COMPATIBILITY CONTROLS
           postscreen_command_filter ($smtpd_command_filter)
                  A mechanism to transform commands from remote  SMTP
                  clients.

           postscreen_discard_ehlo_keyword_address_maps  ($smtpd_dis-
           card_ehlo_keyword_address_maps)
                  Lookup  tables,  indexed  by the remote SMTP client
                  address, with case insensitive lists of  EHLO  key-
                  words  (pipelining,  starttls, auth, etc.) that the
                  postscreen(8) server will  not  send  in  the  EHLO
                  response to a remote SMTP client.

           postscreen_discard_ehlo_keywords ($smtpd_discard_ehlo_key-
           words)
                  A  case insensitive list of EHLO keywords (pipelin-
                  ing, starttls, auth, etc.) that  the  postscreen(8)
                  server  will  not  send  in  the EHLO response to a
                  remote SMTP client.

    TROUBLE SHOOTING CONTROLS
           postscreen_expansion_filter (see 'postconf -d' output)
                  List  of   characters   that   are   permitted   in
                  postscreen_reject_footer attribute expansions.

           postscreen_reject_footer ($smtpd_reject_footer)
                  Optional  information  that is appended after a 4XX
                  or 5XX server response.

           soft_bounce (no)
                  Safety net to keep mail queued that would otherwise
                  be returned to the sender.

    PERMANENT WHITE/BLACKLIST TEST
           This  test  is  executed  immediately  after a remote SMTP
           client connects. If a client is  permanently  whitelisted,
           the  client  will  be  handed off immediately to a Postfix
           SMTP server process.

           postscreen_access_list (permit_mynetworks)
                  Permanent white/blacklist for remote SMTP client IP
                  addresses.

           postscreen_blacklist_action (ignore)
                  The  action  that  postscreen(8) takes when an SMTP
                  client  is   permanently   blacklisted   with   the
                  postscreen_access_list parameter.

    BEFORE-GREETING TESTS
           These  tests  are  executed  before the remote SMTP client
           receives the "220 servername" greeting. If no tests remain
           after  the successful completion of this phase, the client
           will be handed off immediately to a  Postfix  SMTP  server
           process.

           dnsblog_service_name (dnsblog)
                  The  name  of  the dnsblog(8) service entry in mas-
                  ter.cf.

           postscreen_dnsbl_action (ignore)
                  The action that postscreen(8) takes  when  an  SMTP
                  client's  combined  DNSBL  score  is  equal  to  or
                  greater than  a  threshold  (as  defined  with  the
                  postscreen_dnsbl_sites and postscreen_dnsbl_thresh-
                  old parameters).

           postscreen_dnsbl_reply_map (empty)
                  A mapping  from  actual  DNSBL  domain  name  which
                  includes  a  secret  password,  to the DNSBL domain
                  name  that  postscreen  will  reply  with  when  it
                  rejects mail.

           postscreen_dnsbl_sites (empty)
                  Optional  list of DNS white/blacklist domains, fil-
                  ters and weight factors.

           postscreen_dnsbl_threshold (1)
                  The inclusive lower  bound  for  blocking  an  SMTP
                  client,  based  on  its  combined  DNSBL  score  as
                  defined with the postscreen_dnsbl_sites  parameter.

           postscreen_greet_action (ignore)
                  The  action  that  postscreen(8) takes when an SMTP
                  client speaks before its turn within the time spec-
                  ified with the postscreen_greet_wait parameter.

           postscreen_greet_banner ($smtpd_banner)
                  The  text  in  the  optional  "220-text..."  server
                  response that postscreen(8) sends ahead of the real
                  Postfix SMTP server's "220 text..." response, in an
                  attempt to confuse bad SMTP clients  so  that  they
                  speak before their turn (pre-greet).

           postscreen_greet_wait (${stress?2}${stress:6}s)
                  The amount of time that postscreen(8) will wait for
                  an SMTP client to send a command before  its  turn,
                  and  for  DNS  blocklist  lookup  results to arrive
                  (default: up to 2 seconds under  stress,  up  to  6
                  seconds otherwise).

           smtpd_service_name (smtpd)
                  The  internal  service  that postscreen(8) forwards
                  allowed connections to.

    AFTER-GREETING TESTS
           These tests are executed  after  the  remote  SMTP  client
           receives the "220 servername" greeting. If a client passes
           all tests  during  this  phase,  it  will  receive  a  4XX
           response  to  RCPT  TO commands until the client hangs up.
           After this, the client will be allowed to talk directly to
           a Postfix SMTP server process.

           postscreen_bare_newline_action (ignore)
                  The  action  that  postscreen(8) takes when an SMTP
                  client sends a bare newline character, that  is,  a
                  newline not preceded by carriage return.

           postscreen_bare_newline_enable (no)
                  Enable  "bare  newline"  SMTP protocol tests in the
                  postscreen(8) server.

           postscreen_disable_vrfy_command ($disable_vrfy_command)
                  Disable the SMTP VRFY command in the  postscreen(8)
                  daemon.

           postscreen_forbidden_commands ($smtpd_forbidden_commands)
                  List of commands that the postscreen(8) server con-
                  siders in violation of the SMTP protocol.

           postscreen_helo_required ($smtpd_helo_required)
                  Require that a remote SMTP  client  sends  HELO  or
                  EHLO before commencing a MAIL transaction.

           postscreen_non_smtp_command_action (drop)
                  The  action  that  postscreen(8) takes when an SMTP
                  client sends non-SMTP commands  as  specified  with
                  the postscreen_forbidden_commands parameter.

           postscreen_non_smtp_command_enable (no)
                  Enable    "non-SMTP    command"    tests   in   the
                  postscreen(8) server.

           postscreen_pipelining_action (enforce)
                  The action that postscreen(8) takes  when  an  SMTP
                  client  sends  multiple commands instead of sending
                  one command and waiting for the server to  respond.

           postscreen_pipelining_enable (no)
                  Enable  "pipelining"  SMTP  protocol  tests  in the
                  postscreen(8) server.

    CACHE CONTROLS
           postscreen_cache_cleanup_interval (12h)
                  The amount  of  time  between  postscreen(8)  cache
                  cleanup runs.

           postscreen_cache_map                   (btree:$data_direc-
           tory/postscreen_cache)
                  Persistent  storage  for  the  postscreen(8) server
                  decisions.

           postscreen_cache_retention_time (7d)
                  The amount of time that postscreen(8) will cache an
                  expired  temporary  whitelist  entry  before  it is
                  removed.

           postscreen_bare_newline_ttl (30d)
                  The amount of time that postscreen(8) will use  the
                  result from a successful "bare newline" SMTP proto-
                  col test.

           postscreen_dnsbl_ttl (1h)
                  The amount of time that postscreen(8) will use  the
                  result from a successful DNS blocklist test.

           postscreen_greet_ttl (1d)
                  The  amount of time that postscreen(8) will use the
                  result from a successful PREGREET test.

           postscreen_non_smtp_command_ttl (30d)
                  The amount of time that postscreen(8) will use  the
                  result  from  a  successful "non_smtp_command" SMTP
                  protocol test.

           postscreen_pipelining_ttl (30d)
                  The amount of time that postscreen(8) will use  the
                  result from a successful "pipelining" SMTP protocol
                  test.

    RESOURCE CONTROLS
           line_length_limit (2048)
                  Upon input, long lines are chopped up  into  pieces
                  of  at  most this length; upon delivery, long lines
                  are reconstructed.

           postscreen_client_connection_count_limit
           ($smtpd_client_connection_count_limit)
                  How many simultaneous  connections  any  client  is
                  allowed to have with the postscreen(8) daemon.

           postscreen_command_count_limit (20)
                  The  limit on the total number of commands per SMTP
                  session for postscreen(8)'s built-in SMTP  protocol
                  engine.

           postscreen_command_time_limit (${stress?10}${stress:300}s)
                  The  time limit to read an entire command line with
                  postscreen(8)'s built-in SMTP protocol engine.

           postscreen_post_queue_limit ($default_process_limit)
                  The number of clients that can be waiting for  ser-
                  vice from a real SMTP server process.

           postscreen_pre_queue_limit ($default_process_limit)
                  The  number  of non-whitelisted clients that can be
                  waiting for a decision whether  they  will  receive
                  service from a real SMTP server process.

           postscreen_watchdog_timeout (10s)
                  How  much  time a postscreen(8) process may take to
                  respond to an SMTP client command or to  perform  a
                  cache operation before it is terminated by a built-
                  in watchdog timer.

    STARTTLS CONTROLS
           postscreen_tls_security_level ($smtpd_tls_security_level)
                  The SMTP TLS security level for  the  postscreen(8)
                  server;  when  a non-empty value is specified, this
                  overrides       the       obsolete       parameters
                  postscreen_use_tls and postscreen_enforce_tls.

           tlsproxy_service_name (tlsproxy)
                  The  name  of the tlsproxy(8) service entry in mas-
                  ter.cf.

    OBSOLETE STARTTLS SUPPORT CONTROLS
           These parameters  are  supported  for  compatibility  with
           smtpd(8) legacy parameters.

           postscreen_use_tls ($smtpd_use_tls)
                  Opportunistic  TLS:  announce  STARTTLS  support to
                  SMTP clients, but do not require that  clients  use
                  TLS encryption.

           postscreen_enforce_tls ($smtpd_enforce_tls)
                  Mandatory  TLS:  announce  STARTTLS support to SMTP
                  clients, and require that clients use  TLS  encryp-
                  tion.

    MISCELLANEOUS CONTROLS
           config_directory (see 'postconf -d' output)
                  The  default  location  of  the Postfix main.cf and
                  master.cf configuration files.

           delay_logging_resolution_limit (2)
                  The maximal number  of  digits  after  the  decimal
                  point when logging sub-second delay values.

           command_directory (see 'postconf -d' output)
                  The  location  of  all  postfix administrative com-
                  mands.

           max_idle (100s)
                  The maximum amount of time  that  an  idle  Postfix
                  daemon  process  waits  for  an incoming connection
                  before terminating voluntarily.

           process_id (read-only)
                  The process ID  of  a  Postfix  command  or  daemon
                  process.

           process_name (read-only)
                  The  process  name  of  a Postfix command or daemon
                  process.

           syslog_facility (mail)
                  The syslog facility of Postfix logging.

           syslog_name (see 'postconf -d' output)
                  The mail system  name  that  is  prepended  to  the
                  process  name  in  syslog  records, so that "smtpd"
                  becomes, for example, "postfix/smtpd".

    SEE ALSO
           smtpd(8), Postfix SMTP server
           tlsproxy(8), Postfix TLS proxy server
           dnsblog(8), DNS black/whitelist logger
           syslogd(8), system logging

    README FILES
           POSTSCREEN_README, Postfix Postscreen Howto

    LICENSE
           The  Secure  Mailer  license must be distributed with this
           software.

    HISTORY
           This service was introduced with Postfix version 2.8.

           Many ideas in postscreen(8) were explored in earlier  work
           by  Michael Tokarev, in OpenBSD spamd, and in MailChannels
           Traffic Control.

    AUTHOR(S)
           Wietse Venema
           IBM T.J. Watson Research
           P.O. Box 704
           Yorktown Heights, NY 10598, USA

                                                                     POSTSCREEN(8)