Postfix manual - postscreen(8)

    POSTSCREEN(8)                                                    POSTSCREEN(8)

    NAME
           postscreen - Postfix zombie blocker

    SYNOPSIS
           postscreen [generic Postfix daemon options]

    DESCRIPTION
           The Postfix postscreen(8) server provides additional protection against
           mail  server  overload.  One  postscreen(8)  process  handles  multiple
           inbound SMTP connections, and decides which clients may talk to a Post-
           fix SMTP server  process.   By  keeping  spambots  away,  postscreen(8)
           leaves more SMTP server processes available for legitimate clients, and
           delays the onset of server overload conditions.

           This program should not be used on SMTP ports that  receive  mail  from
           end-user clients (MUAs). In a typical deployment, postscreen(8) handles
           the MX service on TCP port 25, and smtpd(8) receives mail from MUAs  on
           the submission service (TCP port 587) which requires client authentica-
           tion.  Alternatively, a site could set up a dedicated,  non-postscreen,
           "port  25" server that provides submission service and client authenti-
           cation, but no MX service.

           postscreen(8) maintains a temporary allowlist  for  clients  that  have
           passed  a  number  of  tests.   When  an  SMTP  client  IP  address  is
           allowlisted, 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 each 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 default mode blocks no
           clients, and 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 3030 (CHUNKING without BINARYMIME)
           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) or postlogd(8).

    BUGS
           The  postscreen(8)  built-in  SMTP  protocol  engine currently does not
           announce support for AUTH, XCLIENT or XFORWARD.  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 may result in unexpected
           delivery delays from senders that retry email delivery from a different
           IP address.  Reason: after passing these tests a new client  must  dis-
           connect,  and  reconnect from the same IP address before it can deliver
           mail. See POSTSCREEN_README, section "Tests after the 220  SMTP  server
           greeting", for a discussion.

    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}:{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.

           Available in Postfix version 3.1 and later:

           dns_ncache_ttl_fix_enable (no)
                  Enable a workaround for future libc incompatibility.

           Available in Postfix version 3.4 and later:

           postscreen_reject_footer_maps ($smtpd_reject_footer_maps)
                  Optional lookup table for information that is appended  after  a
                  4XX or 5XX postscreen(8) server response.

           Available in Postfix 3.6 and later:

           respectful_logging (see 'postconf -d' output)
                  Avoid logging that implies white is better than black.

    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.

    BEFORE-POSTSCREEN PROXY AGENT
           Available in Postfix version 2.10 and later:

           postscreen_upstream_proxy_protocol (empty)
                  The  name  of  the  proxy   protocol   used   by   an   optional
                  before-postscreen proxy agent.

           postscreen_upstream_proxy_timeout (5s)
                  The  time  limit  for  the  proxy  protocol  specified  with the
                  postscreen_upstream_proxy_protocol parameter.

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

           postscreen_access_list (permit_mynetworks)
                  Permanent allow/denylist for remote SMTP client IP addresses.

           postscreen_blacklist_action (ignore)
                  Renamed to postscreen_denylist_action in Postfix 3.6.

    MAIL EXCHANGER POLICY TESTS
           When postscreen(8) is configured to monitor all primary and  backup  MX
           addresses,  it can refuse to allowlist clients that connect to a backup
           MX address only. For small sites, this requires configuring primary and
           backup  MX  addresses on the same MTA. Larger sites would have to share
           the postscreen(8) cache between primary and backup  MTAs,  which  would
           introduce a common point of failure.

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

    BEFORE 220 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 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  an  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 patterns with DNS allow/denylist domains, fil-
                  ters 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 (normal: 6s, overload: 2s)
                  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  con-
                  nections to.

           Available in Postfix version 2.11 and later:

           postscreen_dnsbl_whitelist_threshold (0)
                  Renamed  to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.

           Available in Postfix version 3.0 and later:

           postscreen_dnsbl_timeout (10s)
                  The time limit for DNSBL or DNSWL lookups.

           Available in Postfix version 3.6 and later:

           postscreen_denylist_action (ignore)
                  The action that postscreen(8) takes when a remote SMTP client is
                  permanently  denylisted  with the postscreen_access_list parame-
                  ter.

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

           postscreen_dnsbl_allowlist_threshold (0)
                  Allow a remote SMTP client  to  skip  "before"  and  "after  220
                  greeting"  protocol  tests, based on its combined DNSBL score as
                  defined with the postscreen_dnsbl_sites parameter.

    AFTER 220 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  all  RCPT  TO  commands.  After  the
           client  reconnects,  it  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 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 a remote 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 a remote SMTP client
                  sends multiple commands instead of sending one command and wait-
                  ing 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 allowlist 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_max_ttl
           (${postscreen_dnsbl_ttl?{$postscreen_dnsbl_ttl}:{1}}h)
                  The maximum amount of  time  that  postscreen(8)  will  use  the
                  result  from  a  successful  DNS-based  reputation test before a
                  client IP address is required to pass that test again.

           postscreen_dnsbl_min_ttl (60s)
                  The minimum amount of  time  that  postscreen(8)  will  use  the
                  result  from  a  successful  DNS-based  reputation test before a
                  client IP address is required to pass that test again.

           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  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 (normal: 300s, overload: 10s)
                  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-allowlisted 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 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 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 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)
                  A  prefix  that  is  prepended  to  the  process  name in syslog
                  records, so that, for example, "smtpd" becomes "prefix/smtpd".

           Available in Postfix 3.3 and later:

           service_name (read-only)
                  The master.cf service name of a Postfix daemon process.

           Available in Postfix 3.5 and later:

           info_log_address_format (external)
                  The email address form that will be used  in  non-debug  logging
                  (info, warning, etc.).

    SEE ALSO
           smtpd(8), Postfix SMTP server
           tlsproxy(8), Postfix TLS proxy server
           dnsblog(8), DNS allow/denylist logger
           postlogd(8), Postfix logging
           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

           Wietse Venema
           Google, Inc.
           111 8th Avenue
           New York, NY 10011, USA

                                                                     POSTSCREEN(8)