POSTSCREEN(8)                                                    POSTSCREEN(8)



NAME
       postscreen - Postfix zombie blocker

SYNOPSIS
       postscreen [generic Postfix daemon options]

DESCRIPTION
       The  Postfix  postscreen(8)  server performs triage on multiple 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 deployment, postscreen(8) is used
       on the "port 25" service, while MUA clients submit mail via the submis-
       sion 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 connec-
       tion to a Postfix SMTP  server  process,  while  excluding  clients  in
       mynetworks  from all tests (primarily, to avoid problems with non-stan-
       dard SMTP implementations in network appliances).  This mode is  useful
       for non-destructive testing.

       In  a typical production setting, postscreen(8) is configured 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 min-
       imizing 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)
       Not: 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, and do not use DNSBLs that  reject  traffic
       from dial-up and residential networks.

       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 temporary whitelist but it cannot not hand off
       the "live" connection 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  behav-
       ior.   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  parameter  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_discard_ehlo_key-
       word_address_maps)
              Lookup  tables,  indexed by the remote SMTP client address, with
              case insensitive lists of EHLO keywords  (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_keywords)
              A case insensitive list of EHLO keywords (pipelining,  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 per-
              manently 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 com-
       pletion  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 master.cf.

       postscreen_dnsbl_action (ignore)
              The action that postscreen(8) takes when an SMTP  client's  com-
              bined  DNSBL  score  is equal to or greater than a threshold (as
              defined      with      the      postscreen_dnsbl_sites       and
              postscreen_dnsbl_threshold 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, filters 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   specified  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 connec-
              tions 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  car-
              riage 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 considers in vio-
              lation of the SMTP protocol.

       postscreen_helo_required ($smtpd_helo_required)
              Require that a remote SMTP client sends HELO or EHLO before com-
              mencing 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_forbid-
              den_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_directory/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 tem-
              porary 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 protocol 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_connec-
       tion_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 service 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 param-
              eters postscreen_use_tls and postscreen_enforce_tls.

       tlsproxy_service_name (tlsproxy)
              The name of the tlsproxy(8) service entry in master.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 encryption.

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

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

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

       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,  "post-
              fix/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
       Use  "postconf readme_directory" or "postconf html_directory" to locate
       this information.
       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)

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