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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  submission  service,  or  via  a "port 25" server that provides no MX service (i.e.  a dedicated
       server that provides submission service on port 25).

       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 nonstandard
       standard SMTP implementations in network appliances).  This mode is useful for non-destructive  test-ing. testing.
       ing.

       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 recipi-ent recipient
       ent 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 net-work. network.
       work. 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 discon-nect. disconnect.
       nect.  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 exam-ples. examples.
       ples.

       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  evalu-ate evaluate
       ate 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_keyword_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 postscreen(8) 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 a remote SMTP client is permanently blacklisted with
              the postscreen_access_list parameter.

MAIL EXCHANGER POLICY TESTS
       When a remote SMTP client is not on the permanent access list, postscreen(8) can implement  a  number
       of whitelist tests before it grants the client a temporary whitelist status to talk to a Postfix SMTP
       server process.

       By listening on both primary and backup MX addresses, postscreen(8) can deny the temporary  whitelist
       status to clients that connect only to backup MX hosts.

       postscreen_whitelist_interfaces (static:all)
              A  list  of local postscreen(8) server IP addresses where a non-whitelisted remote SMTP client
              can obtain postscreen(8)'s temporary whitelist status.

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 master.cf.

       postscreen_dnsbl_action (ignore)
              The action that postscreen(8) takes when a remote SMTP client's combined 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 a remote 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 a remote 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) hands off 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 a remote 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 considers 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 a remote SMTP client sends non-SMTP commands as spec-ified specified
              ified 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 a remote 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 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 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_connection_count_limit)
              How  many  simultaneous  connections  any  remote  SMTP  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 Postfix 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 Postfix SMTP server process.

       postscreen_watchdog_timeout (10s)
              How much time a postscreen(8) process may take to respond to a remote 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 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 remote SMTP clients, but do not  require  that
              clients use TLS encryption.

       postscreen_enforce_tls ($smtpd_enforce_tls)
              Mandatory  TLS: announce STARTTLS support to remote 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 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 commands.

       max_idle (100s)
              The maximum amount of time that an idle Postfix daemon process waits for an  incoming  connec-tion connection
              tion 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
       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)