482 lines
18 KiB
Groff
482 lines
18 KiB
Groff
.TH POSTSCREEN 8
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.ad
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.fi
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.SH NAME
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postscreen
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\-
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Postfix zombie blocker
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.SH "SYNOPSIS"
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.na
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.nf
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\fBpostscreen\fR [generic Postfix daemon options]
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.SH DESCRIPTION
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.ad
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.fi
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The Postfix \fBpostscreen\fR(8) server provides additional
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protection against mail server overload. One \fBpostscreen\fR(8)
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process handles multiple inbound SMTP connections, and decides
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which clients may talk to a Postfix SMTP server process.
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By keeping spambots away, \fBpostscreen\fR(8) leaves more
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SMTP server processes available for legitimate clients, and
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delays the onset of server overload conditions.
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This program should not be used on SMTP ports that receive
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mail from end\-user clients (MUAs). In a typical deployment,
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\fBpostscreen\fR(8) handles the MX service on TCP port 25, and
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\fBsmtpd\fR(8) receives mail from MUAs on the \fBsubmission\fR
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service (TCP port 587) which requires client authentication.
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Alternatively, a site could set up a dedicated, non\-postscreen,
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"port 25" server that provides \fBsubmission\fR service and
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client authentication, but no MX service.
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\fBpostscreen\fR(8) maintains a temporary allowlist for
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clients that have passed a number of tests. When an SMTP
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client IP address is allowlisted, \fBpostscreen\fR(8) hands
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off the connection immediately to a Postfix SMTP server
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process. This minimizes the overhead for legitimate mail.
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By default, \fBpostscreen\fR(8) logs statistics and hands
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off each connection to a Postfix SMTP server process, while
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excluding clients in mynetworks from all tests (primarily,
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to avoid problems with non\-standard SMTP implementations
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in network appliances). This default mode blocks no clients,
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and is useful for non\-destructive testing.
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In a typical production setting, \fBpostscreen\fR(8) is
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configured to reject mail from clients that fail one or
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more tests. \fBpostscreen\fR(8) logs rejected mail with the
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client address, helo, sender and recipient information.
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\fBpostscreen\fR(8) is not an SMTP proxy; this is intentional.
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The purpose is to keep spambots away from Postfix SMTP
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server processes, while minimizing overhead for legitimate
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traffic.
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.SH "SECURITY"
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.na
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.nf
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.ad
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.fi
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The \fBpostscreen\fR(8) server is moderately security\-sensitive.
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It talks to untrusted clients on the network. The process
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can be run chrooted at fixed low privilege.
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.SH "STANDARDS"
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.na
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.nf
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RFC 821 (SMTP protocol)
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RFC 1123 (Host requirements)
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RFC 1652 (8bit\-MIME transport)
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RFC 1869 (SMTP service extensions)
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RFC 1870 (Message Size Declaration)
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RFC 1985 (ETRN command)
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RFC 2034 (SMTP Enhanced Status Codes)
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RFC 2821 (SMTP protocol)
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Not: RFC 2920 (SMTP Pipelining)
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RFC 3030 (CHUNKING without BINARYMIME)
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RFC 3207 (STARTTLS command)
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RFC 3461 (SMTP DSN Extension)
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RFC 3463 (Enhanced Status Codes)
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RFC 5321 (SMTP protocol, including multi\-line 220 banners)
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.SH DIAGNOSTICS
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.ad
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.fi
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Problems and transactions are logged to \fBsyslogd\fR(8)
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or \fBpostlogd\fR(8).
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.SH BUGS
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.ad
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.fi
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The \fBpostscreen\fR(8) built\-in SMTP protocol engine
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currently does not announce support for AUTH, XCLIENT or
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XFORWARD.
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If you need to make these services available
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on port 25, then do not enable the optional "after 220
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server greeting" tests.
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The optional "after 220 server greeting" tests may result in
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unexpected delivery delays from senders that retry email delivery
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from a different IP address. Reason: after passing these tests a
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new client must disconnect, and reconnect from the same IP
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address before it can deliver mail. See POSTSCREEN_README, section
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"Tests after the 220 SMTP server greeting", for a discussion.
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.SH "CONFIGURATION PARAMETERS"
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.na
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.nf
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.ad
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.fi
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Changes to main.cf are not picked up automatically, as
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\fBpostscreen\fR(8) processes may run for several hours.
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Use the command "postfix reload" after a configuration
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change.
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The text below provides only a parameter summary. See
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\fBpostconf\fR(5) for more details including examples.
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NOTE: Some \fBpostscreen\fR(8) parameters implement
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stress\-dependent behavior. This is supported only when the
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default parameter value is stress\-dependent (that is, it
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looks like ${stress?{X}:{Y}}, or it is the $\fIname\fR
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of an smtpd parameter with a stress\-dependent default).
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Other parameters always evaluate as if the \fBstress\fR
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parameter value is the empty string.
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.SH "COMPATIBILITY CONTROLS"
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.na
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.nf
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.ad
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.fi
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.IP "\fBpostscreen_command_filter ($smtpd_command_filter)\fR"
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A mechanism to transform commands from remote SMTP clients.
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.IP "\fBpostscreen_discard_ehlo_keyword_address_maps ($smtpd_discard_ehlo_keyword_address_maps)\fR"
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Lookup tables, indexed by the remote SMTP client address, with
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case insensitive lists of EHLO keywords (pipelining, starttls, auth,
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etc.) that the \fBpostscreen\fR(8) server will not send in the EHLO response
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to a remote SMTP client.
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.IP "\fBpostscreen_discard_ehlo_keywords ($smtpd_discard_ehlo_keywords)\fR"
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A case insensitive list of EHLO keywords (pipelining, starttls,
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auth, etc.) that the \fBpostscreen\fR(8) server will not send in the EHLO
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response to a remote SMTP client.
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.PP
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Available in Postfix version 3.1 and later:
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.IP "\fBdns_ncache_ttl_fix_enable (no)\fR"
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Enable a workaround for future libc incompatibility.
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.PP
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Available in Postfix version 3.4 and later:
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.IP "\fBpostscreen_reject_footer_maps ($smtpd_reject_footer_maps)\fR"
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Optional lookup table for information that is appended after a 4XX
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or 5XX \fBpostscreen\fR(8) server response.
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.PP
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Available in Postfix 3.6 and later:
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.IP "\fBrespectful_logging (see 'postconf -d' output)\fR"
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Avoid logging that implies white is better than black.
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.SH "TROUBLE SHOOTING CONTROLS"
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.na
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.nf
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.ad
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.fi
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.IP "\fBpostscreen_expansion_filter (see 'postconf -d' output)\fR"
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List of characters that are permitted in postscreen_reject_footer
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attribute expansions.
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.IP "\fBpostscreen_reject_footer ($smtpd_reject_footer)\fR"
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Optional information that is appended after a 4XX or 5XX
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\fBpostscreen\fR(8) server
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response.
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.IP "\fBsoft_bounce (no)\fR"
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Safety net to keep mail queued that would otherwise be returned to
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the sender.
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.SH "BEFORE-POSTSCREEN PROXY AGENT"
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.na
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.nf
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.ad
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.fi
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Available in Postfix version 2.10 and later:
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.IP "\fBpostscreen_upstream_proxy_protocol (empty)\fR"
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The name of the proxy protocol used by an optional before\-postscreen
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proxy agent.
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.IP "\fBpostscreen_upstream_proxy_timeout (5s)\fR"
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The time limit for the proxy protocol specified with the
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postscreen_upstream_proxy_protocol parameter.
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.SH "PERMANENT ALLOW/DENYLIST TEST"
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.na
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.nf
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.ad
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.fi
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This test is executed immediately after a remote SMTP client
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connects. If a client is permanently allowlisted, the client
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will be handed off immediately to a Postfix SMTP server
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process.
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.IP "\fBpostscreen_access_list (permit_mynetworks)\fR"
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Permanent allow/denylist for remote SMTP client IP addresses.
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.IP "\fBpostscreen_blacklist_action (ignore)\fR"
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Renamed to postscreen_denylist_action in Postfix 3.6.
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.SH "MAIL EXCHANGER POLICY TESTS"
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.na
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.nf
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.ad
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.fi
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When \fBpostscreen\fR(8) is configured to monitor all primary
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and backup MX addresses, it can refuse to allowlist clients
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that connect to a backup MX address only. For small sites,
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this requires configuring primary and backup MX addresses
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on the same MTA. Larger sites would have to share the
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\fBpostscreen\fR(8) cache between primary and backup MTAs,
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which would introduce a common point of failure.
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.IP "\fBpostscreen_allowlist_interfaces (static:all)\fR"
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A list of local \fBpostscreen\fR(8) server IP addresses where a
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non\-allowlisted remote SMTP client can obtain \fBpostscreen\fR(8)'s temporary
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allowlist status.
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.SH "BEFORE 220 GREETING TESTS"
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.na
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.nf
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.ad
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.fi
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These tests are executed before the remote SMTP client
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receives the "220 servername" greeting. If no tests remain
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after the successful completion of this phase, the client
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will be handed off immediately to a Postfix SMTP server
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process.
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.IP "\fBdnsblog_service_name (dnsblog)\fR"
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The name of the \fBdnsblog\fR(8) service entry in master.cf.
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.IP "\fBpostscreen_dnsbl_action (ignore)\fR"
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The action that \fBpostscreen\fR(8) takes when a remote SMTP client's combined
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DNSBL score is equal to or greater than a threshold (as defined
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with the postscreen_dnsbl_sites and postscreen_dnsbl_threshold
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parameters).
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.IP "\fBpostscreen_dnsbl_reply_map (empty)\fR"
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A mapping from an actual DNSBL domain name which includes a secret
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password, to the DNSBL domain name that postscreen will reply with
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when it rejects mail.
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.IP "\fBpostscreen_dnsbl_sites (empty)\fR"
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Optional list of patterns with DNS allow/denylist domains, filters
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and weight
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factors.
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.IP "\fBpostscreen_dnsbl_threshold (1)\fR"
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The inclusive lower bound for blocking a remote SMTP client, based on
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its combined DNSBL score as defined with the postscreen_dnsbl_sites
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parameter.
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.IP "\fBpostscreen_greet_action (ignore)\fR"
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The action that \fBpostscreen\fR(8) takes when a remote SMTP client speaks
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before its turn within the time specified with the postscreen_greet_wait
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parameter.
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.IP "\fBpostscreen_greet_banner ($smtpd_banner)\fR"
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The \fItext\fR in the optional "220\-\fItext\fR..." server
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response that
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\fBpostscreen\fR(8) sends ahead of the real Postfix SMTP server's "220
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text..." response, in an attempt to confuse bad SMTP clients so
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that they speak before their turn (pre\-greet).
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.IP "\fBpostscreen_greet_wait (normal: 6s, overload: 2s)\fR"
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The amount of time that \fBpostscreen\fR(8) will wait for an SMTP
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client to send a command before its turn, and for DNS blocklist
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lookup results to arrive (default: up to 2 seconds under stress,
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up to 6 seconds otherwise).
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.IP "\fBsmtpd_service_name (smtpd)\fR"
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The internal service that \fBpostscreen\fR(8) hands off allowed
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connections to.
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.PP
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Available in Postfix version 2.11 and later:
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.IP "\fBpostscreen_dnsbl_whitelist_threshold (0)\fR"
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Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.
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.PP
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Available in Postfix version 3.0 and later:
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.IP "\fBpostscreen_dnsbl_timeout (10s)\fR"
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The time limit for DNSBL or DNSWL lookups.
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.PP
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Available in Postfix version 3.6 and later:
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.IP "\fBpostscreen_denylist_action (ignore)\fR"
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The action that \fBpostscreen\fR(8) takes when a remote SMTP client is
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permanently denylisted with the postscreen_access_list parameter.
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.IP "\fBpostscreen_allowlist_interfaces (static:all)\fR"
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A list of local \fBpostscreen\fR(8) server IP addresses where a
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non\-allowlisted remote SMTP client can obtain \fBpostscreen\fR(8)'s temporary
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allowlist status.
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.IP "\fBpostscreen_dnsbl_allowlist_threshold (0)\fR"
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Allow a remote SMTP client to skip "before" and "after 220
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greeting" protocol tests, based on its combined DNSBL score as
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defined with the postscreen_dnsbl_sites parameter.
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.SH "AFTER 220 GREETING TESTS"
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.na
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.nf
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.ad
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.fi
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These tests are executed after the remote SMTP client
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receives the "220 servername" greeting. If a client passes
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all tests during this phase, it will receive a 4XX response
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to all RCPT TO commands. After the client reconnects, it
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will be allowed to talk directly to a Postfix SMTP server
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process.
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.IP "\fBpostscreen_bare_newline_action (ignore)\fR"
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The action that \fBpostscreen\fR(8) takes when a remote SMTP client sends
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a bare newline character, that is, a newline not preceded by carriage
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return.
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.IP "\fBpostscreen_bare_newline_enable (no)\fR"
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Enable "bare newline" SMTP protocol tests in the \fBpostscreen\fR(8)
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server.
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.IP "\fBpostscreen_disable_vrfy_command ($disable_vrfy_command)\fR"
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Disable the SMTP VRFY command in the \fBpostscreen\fR(8) daemon.
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.IP "\fBpostscreen_forbidden_commands ($smtpd_forbidden_commands)\fR"
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List of commands that the \fBpostscreen\fR(8) server considers in
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violation of the SMTP protocol.
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.IP "\fBpostscreen_helo_required ($smtpd_helo_required)\fR"
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Require that a remote SMTP client sends HELO or EHLO before
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commencing a MAIL transaction.
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.IP "\fBpostscreen_non_smtp_command_action (drop)\fR"
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The action that \fBpostscreen\fR(8) takes when a remote SMTP client sends
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non\-SMTP commands as specified with the postscreen_forbidden_commands
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parameter.
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.IP "\fBpostscreen_non_smtp_command_enable (no)\fR"
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Enable "non\-SMTP command" tests in the \fBpostscreen\fR(8) server.
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.IP "\fBpostscreen_pipelining_action (enforce)\fR"
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The action that \fBpostscreen\fR(8) takes when a remote SMTP client
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sends
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multiple commands instead of sending one command and waiting for
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the server to respond.
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.IP "\fBpostscreen_pipelining_enable (no)\fR"
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Enable "pipelining" SMTP protocol tests in the \fBpostscreen\fR(8)
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server.
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.SH "CACHE CONTROLS"
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.na
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.nf
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.ad
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.fi
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.IP "\fBpostscreen_cache_cleanup_interval (12h)\fR"
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The amount of time between \fBpostscreen\fR(8) cache cleanup runs.
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.IP "\fBpostscreen_cache_map (btree:$data_directory/postscreen_cache)\fR"
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Persistent storage for the \fBpostscreen\fR(8) server decisions.
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.IP "\fBpostscreen_cache_retention_time (7d)\fR"
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The amount of time that \fBpostscreen\fR(8) will cache an expired
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temporary allowlist entry before it is removed.
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.IP "\fBpostscreen_bare_newline_ttl (30d)\fR"
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The amount of time that \fBpostscreen\fR(8) remembers that a client
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IP address passed a "bare newline" SMTP protocol test, before it
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address is required to pass that test again.
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.IP "\fBpostscreen_dnsbl_max_ttl (${postscreen_dnsbl_ttl?{$postscreen_dnsbl_ttl}:{1}}h)\fR"
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The maximum amount of time that \fBpostscreen\fR(8) remembers that a
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client IP address passed a DNS\-based reputation test, before it is
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required to pass that test again.
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.IP "\fBpostscreen_dnsbl_min_ttl (60s)\fR"
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The minimum amount of time that \fBpostscreen\fR(8) remembers that a
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client IP address passed a DNS\-based reputation test, before it
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is required to pass that test again.
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.IP "\fBpostscreen_greet_ttl (1d)\fR"
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The amount of time that \fBpostscreen\fR(8) remembers that a client
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IP address passed a PREGREET test, before it is required to pass
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that test again.
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.IP "\fBpostscreen_non_smtp_command_ttl (30d)\fR"
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The amount of time that \fBpostscreen\fR(8) remembers that a client
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IP address passed a "non_smtp_command" SMTP protocol test, before
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it is required to pass that test again.
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.IP "\fBpostscreen_pipelining_ttl (30d)\fR"
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The amount of time that \fBpostscreen\fR(8) remembers that a client
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IP address passed a "pipelining" SMTP protocol test, before it is
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required to pass that test again.
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.SH "RESOURCE CONTROLS"
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.na
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.nf
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.ad
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.fi
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.IP "\fBline_length_limit (2048)\fR"
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Upon input, long lines are chopped up into pieces of at most
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this length; upon delivery, long lines are reconstructed.
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.IP "\fBpostscreen_client_connection_count_limit ($smtpd_client_connection_count_limit)\fR"
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How many simultaneous connections any remote SMTP client is
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allowed to have
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with the \fBpostscreen\fR(8) daemon.
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.IP "\fBpostscreen_command_count_limit (20)\fR"
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The limit on the total number of commands per SMTP session for
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\fBpostscreen\fR(8)'s built\-in SMTP protocol engine.
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.IP "\fBpostscreen_command_time_limit (normal: 300s, overload: 10s)\fR"
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The time limit to read an entire command line with \fBpostscreen\fR(8)'s
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built\-in SMTP protocol engine.
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.IP "\fBpostscreen_post_queue_limit ($default_process_limit)\fR"
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The number of clients that can be waiting for service from a
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real Postfix SMTP server process.
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.IP "\fBpostscreen_pre_queue_limit ($default_process_limit)\fR"
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The number of non\-allowlisted clients that can be waiting for
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a decision whether they will receive service from a real Postfix
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SMTP server
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process.
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.IP "\fBpostscreen_watchdog_timeout (10s)\fR"
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How much time a \fBpostscreen\fR(8) process may take to respond to
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a remote SMTP client command or to perform a cache operation before it
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is terminated by a built\-in watchdog timer.
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.SH "STARTTLS CONTROLS"
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.na
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.nf
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.ad
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.fi
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.IP "\fBpostscreen_tls_security_level ($smtpd_tls_security_level)\fR"
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The SMTP TLS security level for the \fBpostscreen\fR(8) server; when
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a non\-empty value is specified, this overrides the obsolete parameters
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postscreen_use_tls and postscreen_enforce_tls.
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.IP "\fBtlsproxy_service_name (tlsproxy)\fR"
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The name of the \fBtlsproxy\fR(8) service entry in master.cf.
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.SH "OBSOLETE STARTTLS SUPPORT CONTROLS"
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.na
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.nf
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.ad
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.fi
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These parameters are supported for compatibility with
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\fBsmtpd\fR(8) legacy parameters.
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.IP "\fBpostscreen_use_tls ($smtpd_use_tls)\fR"
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Opportunistic TLS: announce STARTTLS support to remote SMTP clients,
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but do not require that clients use TLS encryption.
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.IP "\fBpostscreen_enforce_tls ($smtpd_enforce_tls)\fR"
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Mandatory TLS: announce STARTTLS support to remote SMTP clients, and
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require that clients use TLS encryption.
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.SH "MISCELLANEOUS CONTROLS"
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.na
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.nf
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.ad
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.fi
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.IP "\fBconfig_directory (see 'postconf -d' output)\fR"
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The default location of the Postfix main.cf and master.cf
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configuration files.
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.IP "\fBdelay_logging_resolution_limit (2)\fR"
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The maximal number of digits after the decimal point when logging
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sub\-second delay values.
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.IP "\fBcommand_directory (see 'postconf -d' output)\fR"
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The location of all postfix administrative commands.
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.IP "\fBmax_idle (100s)\fR"
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The maximum amount of time that an idle Postfix daemon process waits
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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"
|
|
A prefix that is prepended to the process name in syslog
|
|
records, so that, for example, "smtpd" becomes "prefix/smtpd".
|
|
.PP
|
|
Available in Postfix 3.3 and later:
|
|
.IP "\fBservice_name (read\-only)\fR"
|
|
The master.cf service name of a Postfix daemon process.
|
|
.PP
|
|
Available in Postfix 3.5 and later:
|
|
.IP "\fBinfo_log_address_format (external)\fR"
|
|
The email address form that will be used in non\-debug logging
|
|
(info, warning, etc.).
|
|
.SH "SEE ALSO"
|
|
.na
|
|
.nf
|
|
smtpd(8), Postfix SMTP server
|
|
tlsproxy(8), Postfix TLS proxy server
|
|
dnsblog(8), DNS allow/denylist logger
|
|
postlogd(8), Postfix logging
|
|
syslogd(8), system logging
|
|
.SH "README FILES"
|
|
.na
|
|
.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"
|
|
.na
|
|
.nf
|
|
.ad
|
|
.fi
|
|
The Secure Mailer license must be distributed with this software.
|
|
.SH HISTORY
|
|
.ad
|
|
.fi
|
|
.ad
|
|
.fi
|
|
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)"
|
|
.na
|
|
.nf
|
|
Wietse Venema
|
|
IBM T.J. Watson Research
|
|
P.O. Box 704
|
|
Yorktown Heights, NY 10598, USA
|
|
|
|
Wietse Venema
|
|
Google, Inc.
|
|
111 8th Avenue
|
|
New York, NY 10011, USA
|