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Linux 2.6 - man page for postscreen (linux section 8postfix)

POSTSCREEN(8postfix)							     POSTSCREEN(8postfix)

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.

       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 connec-
       tion immediately to a Postfix SMTP server process. This minimizes the overhead for legiti-
       mate 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 use-
       ful 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 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  pro-
       tocol  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 includ-
       ing examples.

       NOTE:  Some  postscreen(8)  parameters  implement stress-dependent behavior.  This is sup-
       ported 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_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 expan-
	      sions.

       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 per-
       manently  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 permanently  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 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 an 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 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  com-
	      mand  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 connections to.

AFTER-GREETING TESTS
       These tests are executed after the remote SMTP client receives the "220 servername" greet-
       ing.  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 char-
	      acter, 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 an 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 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  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 PRE-
	      GREET 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 "pipe-
	      lining" 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  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  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 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 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 incom-
	      ing 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
       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(8postfix)


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