.TH POSTSCREEN 8
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.SH NAME
postscreen
\-
Postfix zombie blocker
.SH "SYNOPSIS"
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.nf
\fBpostscreen\fR [generic Postfix daemon options]
.SH DESCRIPTION
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The Postfix \fBpostscreen\fR(8) server performs triage on
multiple inbound SMTP connections at the same time. While
a single \fBpostscreen\fR(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,
\fBpostscreen\fR(8) is used on the "port 25" service, while
MUA clients submit mail via the \fBsubmission\fR service,
or via a "port 25" server that provides no MX service (i.e.
a dedicated server that provides \fBsubmission\fR service
on port 25).
\fBpostscreen\fR(8) maintains a temporary whitelist for
clients that have passed a number of tests. When an SMTP
client IP address is whitelisted, \fBpostscreen\fR(8) hands
off the connection immediately to a Postfix SMTP server
process. This minimizes the overhead for legitimate mail.
By default, \fBpostscreen\fR(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, \fBpostscreen\fR(8) is
configured to reject mail from clients that fail one or
more tests. \fBpostscreen\fR(8) logs rejected mail with the
client address, helo, sender and recipient information.
\fBpostscreen\fR(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.
.SH "SECURITY"
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.nf
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The \fBpostscreen\fR(8) server is moderately security-sensitive.
It talks to untrusted clients on the network. The process
can be run chrooted at fixed low privilege.
.SH "STANDARDS"
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.nf
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)
.SH DIAGNOSTICS
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Problems and transactions are logged to \fBsyslogd\fR(8).
.SH BUGS
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The \fBpostscreen\fR(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
\fBpostscreen\fR(8)'s built-in SMTP protocol engine. When
these tests succeed, \fBpostscreen\fR(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, \fBpostscreen\fR(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. \fBpostscreen\fR(8) mitigates
the impact of this limitation by giving such tests a long
expiration time.
.SH "CONFIGURATION PARAMETERS"
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.nf
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Changes to main.cf are not picked up automatically, as
\fBpostscreen\fR(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
\fBpostconf\fR(5) for more details including examples.
NOTE: Some \fBpostscreen\fR(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 $\fIname\fR
of an smtpd parameter with a stress-dependent default).
Other parameters always evaluate as if the \fBstress\fR
parameter value is the empty string.
.SH "COMPATIBILITY CONTROLS"
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.nf
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.IP "\fBpostscreen_command_filter ($smtpd_command_filter)\fR"
A mechanism to transform commands from remote SMTP clients.
.IP "\fBpostscreen_discard_ehlo_keyword_address_maps ($smtpd_discard_ehlo_keyword_address_maps)\fR"
Lookup tables, indexed by the remote SMTP client address, with
case insensitive lists of EHLO keywords (pipelining, starttls, auth,
etc.) that the \fBpostscreen\fR(8) server will not send in the EHLO response
to a remote SMTP client.
.IP "\fBpostscreen_discard_ehlo_keywords ($smtpd_discard_ehlo_keywords)\fR"
A case insensitive list of EHLO keywords (pipelining, starttls,
auth, etc.) that the \fBpostscreen\fR(8) server will not send in the EHLO
response to a remote SMTP client.
.SH "TROUBLE SHOOTING CONTROLS"
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.nf
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.IP "\fBpostscreen_expansion_filter (see 'postconf -d' output)\fR"
List of characters that are permitted in postscreen_reject_footer
attribute expansions.
.IP "\fBpostscreen_reject_footer ($smtpd_reject_footer)\fR"
Optional information that is appended after a 4XX or 5XX
\fBpostscreen\fR(8) server
response.
.IP "\fBsoft_bounce (no)\fR"
Safety net to keep mail queued that would otherwise be returned to
the sender.
.SH "PERMANENT WHITE/BLACKLIST TEST"
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.nf
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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.
.IP "\fBpostscreen_access_list (permit_mynetworks)\fR"
Permanent white/blacklist for remote SMTP client IP addresses.
.IP "\fBpostscreen_blacklist_action (ignore)\fR"
The action that \fBpostscreen\fR(8) takes when a remote SMTP client is
permanently blacklisted with the postscreen_access_list parameter.
.SH "MAIL EXCHANGER POLICY TESTS"
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.nf
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When a remote SMTP client is not on the permanent access
list, \fBpostscreen\fR(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,
\fBpostscreen\fR(8) can deny the temporary whitelist status
to clients that connect only to backup MX hosts.
.IP "\fBpostscreen_whitelist_interfaces (static:all)\fR"
A list of local \fBpostscreen\fR(8) server IP addresses where a
non-whitelisted remote SMTP client can obtain \fBpostscreen\fR(8)'s temporary
whitelist status.
.SH "BEFORE-GREETING TESTS"
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.nf
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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.
.IP "\fBdnsblog_service_name (dnsblog)\fR"
The name of the \fBdnsblog\fR(8) service entry in master.cf.
.IP "\fBpostscreen_dnsbl_action (ignore)\fR"
The action that \fBpostscreen\fR(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).
.IP "\fBpostscreen_dnsbl_reply_map (empty)\fR"
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.
.IP "\fBpostscreen_dnsbl_sites (empty)\fR"
Optional list of DNS white/blacklist domains, filters and weight
factors.
.IP "\fBpostscreen_dnsbl_threshold (1)\fR"
The inclusive lower bound for blocking a remote SMTP client, based on
its combined DNSBL score as defined with the postscreen_dnsbl_sites
parameter.
.IP "\fBpostscreen_greet_action (ignore)\fR"
The action that \fBpostscreen\fR(8) takes when a remote SMTP client speaks
before its turn within the time specified with the postscreen_greet_wait
parameter.
.IP "\fBpostscreen_greet_banner ($smtpd_banner)\fR"
The \fItext\fR in the optional "220-\fItext\fR..." server
response that
\fBpostscreen\fR(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).
.IP "\fBpostscreen_greet_wait (${stress?2}${stress:6}s)\fR"
The amount of time that \fBpostscreen\fR(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).
.IP "\fBsmtpd_service_name (smtpd)\fR"
The internal service that \fBpostscreen\fR(8) hands off allowed
connections to.
.SH "AFTER-GREETING TESTS"
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.nf
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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.
.IP "\fBpostscreen_bare_newline_action (ignore)\fR"
The action that \fBpostscreen\fR(8) takes when a remote SMTP client sends
a bare newline character, that is, a newline not preceded by carriage
return.
.IP "\fBpostscreen_bare_newline_enable (no)\fR"
Enable "bare newline" SMTP protocol tests in the \fBpostscreen\fR(8)
server.
.IP "\fBpostscreen_disable_vrfy_command ($disable_vrfy_command)\fR"
Disable the SMTP VRFY command in the \fBpostscreen\fR(8) daemon.
.IP "\fBpostscreen_forbidden_commands ($smtpd_forbidden_commands)\fR"
List of commands that the \fBpostscreen\fR(8) server considers in
violation of the SMTP protocol.
.IP "\fBpostscreen_helo_required ($smtpd_helo_required)\fR"
Require that a remote SMTP client sends HELO or EHLO before
commencing a MAIL transaction.
.IP "\fBpostscreen_non_smtp_command_action (drop)\fR"
The action that \fBpostscreen\fR(8) takes when a remote SMTP client sends
non-SMTP commands as specified with the postscreen_forbidden_commands
parameter.
.IP "\fBpostscreen_non_smtp_command_enable (no)\fR"
Enable "non-SMTP command" tests in the \fBpostscreen\fR(8) server.
.IP "\fBpostscreen_pipelining_action (enforce)\fR"
The action that \fBpostscreen\fR(8) takes when a remote SMTP client
sends
multiple commands instead of sending one command and waiting for
the server to respond.
.IP "\fBpostscreen_pipelining_enable (no)\fR"
Enable "pipelining" SMTP protocol tests in the \fBpostscreen\fR(8)
server.
.SH "CACHE CONTROLS"
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.nf
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.fi
.IP "\fBpostscreen_cache_cleanup_interval (12h)\fR"
The amount of time between \fBpostscreen\fR(8) cache cleanup runs.
.IP "\fBpostscreen_cache_map (btree:$data_directory/postscreen_cache)\fR"
Persistent storage for the \fBpostscreen\fR(8) server decisions.
.IP "\fBpostscreen_cache_retention_time (7d)\fR"
The amount of time that \fBpostscreen\fR(8) will cache an expired
temporary whitelist entry before it is removed.
.IP "\fBpostscreen_bare_newline_ttl (30d)\fR"
The amount of time that \fBpostscreen\fR(8) will use the result from
a successful "bare newline" SMTP protocol test.
.IP "\fBpostscreen_dnsbl_ttl (1h)\fR"
The amount of time that \fBpostscreen\fR(8) will use the result from
a successful DNS blocklist test.
.IP "\fBpostscreen_greet_ttl (1d)\fR"
The amount of time that \fBpostscreen\fR(8) will use the result from
a successful PREGREET test.
.IP "\fBpostscreen_non_smtp_command_ttl (30d)\fR"
The amount of time that \fBpostscreen\fR(8) will use the result from
a successful "non_smtp_command" SMTP protocol test.
.IP "\fBpostscreen_pipelining_ttl (30d)\fR"
The amount of time that \fBpostscreen\fR(8) will use the result from
a successful "pipelining" SMTP protocol test.
.SH "RESOURCE CONTROLS"
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.IP "\fBline_length_limit (2048)\fR"
Upon input, long lines are chopped up into pieces of at most
this length; upon delivery, long lines are reconstructed.
.IP "\fBpostscreen_client_connection_count_limit ($smtpd_client_connection_count_limit)\fR"
How many simultaneous connections any remote SMTP client is
allowed to have
with the \fBpostscreen\fR(8) daemon.
.IP "\fBpostscreen_command_count_limit (20)\fR"
The limit on the total number of commands per SMTP session for
\fBpostscreen\fR(8)'s built-in SMTP protocol engine.
.IP "\fBpostscreen_command_time_limit (${stress?10}${stress:300}s)\fR"
The time limit to read an entire command line with \fBpostscreen\fR(8)'s
built-in SMTP protocol engine.
.IP "\fBpostscreen_post_queue_limit ($default_process_limit)\fR"
The number of clients that can be waiting for service from a
real Postfix SMTP server process.
.IP "\fBpostscreen_pre_queue_limit ($default_process_limit)\fR"
The number of non-whitelisted clients that can be waiting for
a decision whether they will receive service from a real Postfix
SMTP server
process.
.IP "\fBpostscreen_watchdog_timeout (10s)\fR"
How much time a \fBpostscreen\fR(8) process may take to respond to
a remote SMTP client command or to perform a cache operation before it
is terminated by a built-in watchdog timer.
.SH "STARTTLS CONTROLS"
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.nf
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.IP "\fBpostscreen_tls_security_level ($smtpd_tls_security_level)\fR"
The SMTP TLS security level for the \fBpostscreen\fR(8) server; when
a non-empty value is specified, this overrides the obsolete parameters
postscreen_use_tls and postscreen_enforce_tls.
.IP "\fBtlsproxy_service_name (tlsproxy)\fR"
The name of the \fBtlsproxy\fR(8) service entry in master.cf.
.SH "OBSOLETE STARTTLS SUPPORT CONTROLS"
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.nf
.ad
.fi
These parameters are supported for compatibility with
\fBsmtpd\fR(8) legacy parameters.
.IP "\fBpostscreen_use_tls ($smtpd_use_tls)\fR"
Opportunistic TLS: announce STARTTLS support to remote SMTP clients,
but do not require that clients use TLS encryption.
.IP "\fBpostscreen_enforce_tls ($smtpd_enforce_tls)\fR"
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and
require that clients use TLS encryption.
.SH "MISCELLANEOUS CONTROLS"
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.nf
.ad
.fi
.IP "\fBconfig_directory (see 'postconf -d' output)\fR"
The default location of the Postfix main.cf and master.cf
configuration files.
.IP "\fBdelay_logging_resolution_limit (2)\fR"
The maximal number of digits after the decimal point when logging
sub-second delay values.
.IP "\fBcommand_directory (see 'postconf -d' output)\fR"
The location of all postfix administrative commands.
.IP "\fBmax_idle (100s)\fR"
The maximum amount of time that an idle Postfix daemon process waits
for an incoming connection before terminating voluntarily.
.IP "\fBprocess_id (read-only)\fR"
The process ID of a Postfix command or daemon process.
.IP "\fBprocess_name (read-only)\fR"
The process name of a Postfix command or daemon process.
.IP "\fBsyslog_facility (mail)\fR"
The syslog facility of Postfix logging.
.IP "\fBsyslog_name (see 'postconf -d' output)\fR"
The mail system name that is prepended to the process name in syslog
records, so that "smtpd" becomes, for example, "postfix/smtpd".
.SH "SEE ALSO"
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.nf
smtpd(8), Postfix SMTP server
tlsproxy(8), Postfix TLS proxy server
dnsblog(8), DNS black/whitelist logger
syslogd(8), system logging
.SH "README FILES"
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.nf
.ad
.fi
Use "\fBpostconf readme_directory\fR" or "\fBpostconf
html_directory\fR" to locate this information.
.nf
.na
POSTSCREEN_README, Postfix Postscreen Howto
.SH "LICENSE"
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.nf
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The Secure Mailer license must be distributed with this software.
.SH "HISTORY"
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This service was introduced with Postfix version 2.8.
Many ideas in \fBpostscreen\fR(8) were explored in earlier
work by Michael Tokarev, in OpenBSD spamd, and in MailChannels
Traffic Control.
.SH "AUTHOR(S)"
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.nf
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA