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    Command:

    postscreen

    
    
    

    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,  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 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)
    
           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
                  postscreen(8) server response.
    
           soft_bounce (no)
                  ter.
    
    
    

    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 tempo-
                  rary 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 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 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)
    
    
    

    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 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.
    
                  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 (${stress?10}${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 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

                  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
    
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