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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,  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)
       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
              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  parame-
              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)
              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 con-
              nections 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 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 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  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

       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)
              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)

Mac OS X 10.8 - Generated Tue Sep 4 07:59:01 CDT 2012
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