Provided by: postfix_2.9.1-4_amd64 bug

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, 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 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
       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  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 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
              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 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 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 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  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_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
              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 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  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 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 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, "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)