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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
       Postfix 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
       authentication.  Alternatively, a site could set up a dedicated, non-
       postscreen, "port 25" server that provides submission service and
       client authentication, 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
       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.  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
       Postfix 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
       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.

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
              parameter.

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
              temporary 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
       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.

       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
              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(8)