POSTSCREEN(8)               System Manager's Manual              POSTSCREEN(8)

NAME
       postscreen - Postfix zombie blocker

SYNOPSIS
       postscreen [generic Postfix daemon options]

DESCRIPTION
       The Postfix postscreen(8) server provides additional protection against
       mail  server  overload.  One  postscreen(8)  process  handles  multiple
       inbound SMTP connections, and decides which clients may talk to a Post‐
       fix SMTP server  process.   By  keeping  spambots  away,  postscreen(8)
       leaves more SMTP server processes available for legitimate clients, and
       delays the onset of server overload conditions.

       This program should not be used on SMTP ports that  receive  mail  from
       end-user clients (MUAs). In a typical deployment, postscreen(8) handles
       the MX service on TCP port 25, while MUA clients submit  mail  via  the
       submission  service  on  TCP port 587 which requires client authentica‐
       tion.  Alternatively, a site could set up a dedicated,  non-postscreen,
       "port  25" server that provides submission service and client authenti‐
       cation, but no MX 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.  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.  When the  client
       connects  again, postscreen(8) will allow the client to talk to a Post‐
       fix SMTP server process (provided that the  whitelist  status  has  not
       expired).   postscreen(8)  mitigates  the  impact of this limitation by
       giving the "after 220 server greeting" 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.

BEFORE-POSTSCREEN PROXY AGENT
       Available in Postfix version 2.10 and later:

       postscreen_upstream_proxy_protocol (empty)
              The  name  of  the  proxy  protocol  used by an optional before-
              postscreen proxy agent.

       postscreen_upstream_proxy_timeout (5s)
              The time  limit  for  the  proxy  protocol  specified  with  the
              postscreen_upstream_proxy_protocol parameter.

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  postscreen(8)  is configured to monitor all primary and backup MX
       addresses, it can refuse to whitelist clients that connect to a  backup
       MX address only. For small sites, this requires configuring primary and
       backup MX addresses on the same MTA. Larger sites would have  to  share
       the  postscreen(8)  cache  between primary and backup MTAs, which would
       introduce a common point of failure.

       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 220 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.

       Available in Postfix version 2.11 and later:

       postscreen_dnsbl_whitelist_threshold (0)
              Allow a remote SMTP client  to  skip  "before"  and  "after  220
              greeting"  protocol  tests, based on its combined DNSBL score as
              defined with the postscreen_dnsbl_sites parameter.

AFTER 220 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  all  RCPT  TO  commands.  After  the
       client  reconnects,  it  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
       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)