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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 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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