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 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,
           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  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)
           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 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 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  mas-
                  ter.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_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 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 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 a remote
                  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  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_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  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 ser-
                  vice 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 per-
                  form 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
                  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  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)