POSTSCREEN(8)                                                    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}:{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 (normal: 6s, overload: 2s)
              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.

       Available in Postfix version 2.11 and later:

       postscreen_dnsbl_timeout (10s)
              The time limit for DNSBL or DNSWL lookups.

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 (normal: 300s, overload: 10s)
              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)