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<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html> <head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link rel='stylesheet' type='text/css' href='postfix-doc.css'>
<title> Postfix manual - header_checks(5) </title>
</head> <body> <pre>
HEADER_CHECKS(5) HEADER_CHECKS(5)
<b>NAME</b>
<a href="postconf.5.html#header_checks">header_checks</a> - Postfix built-in content inspection
<b>SYNOPSIS</b>
<b><a href="postconf.5.html#header_checks">header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/header_checks</b>
<b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/mime_header_checks</b>
<b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/nested_header_checks</b>
<b><a href="postconf.5.html#body_checks">body_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/body_checks</b>
<b><a href="postconf.5.html#milter_header_checks">milter_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/<a href="postconf.5.html#milter_header_checks">milter_header_checks</a></b>
<b><a href="postconf.5.html#smtp_header_checks">smtp_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/smtp_header_checks</b>
<b><a href="postconf.5.html#smtp_mime_header_checks">smtp_mime_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/smtp_mime_header_checks</b>
<b><a href="postconf.5.html#smtp_nested_header_checks">smtp_nested_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/smtp_nested_header_checks</b>
<b><a href="postconf.5.html#smtp_body_checks">smtp_body_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/smtp_body_checks</b>
<b>postmap -q "</b><i>string</i><b>" <a href="pcre_table.5.html">pcre</a>:/etc/postfix/</b><i>filename</i>
<b>postmap -q - <a href="pcre_table.5.html">pcre</a>:/etc/postfix/</b><i>filename</i> <<i>inputfile</i>
<b>DESCRIPTION</b>
This document describes access control on the content of message head-
ers and message body lines; it is implemented by the Postfix <a href="cleanup.8.html"><b>cleanup</b>(8)</a>
server before mail is queued. See <a href="access.5.html"><b>access</b>(5)</a> for access control on
remote SMTP client information.
Each message header or message body line is compared against a list of
patterns. When a match is found the corresponding action is executed,
and the matching process is repeated for the next message header or
message body line.
Note: message headers are examined one logical header at a time, even
when a message header spans multiple lines. Body lines are always exam-
ined one line at a time.
For examples, see the EXAMPLES section at the end of this manual page.
Postfix header or <a href="postconf.5.html#body_checks">body_checks</a> are designed to stop a flood of mail from
worms or viruses; they do not decode attachments, and they do not unzip
archives. See the documents referenced below in the README FILES sec-
tion if you need more sophisticated content analysis.
<b>FILTERS WHILE RECEIVING MAIL</b>
Postfix implements the following four built-in content inspection
classes while receiving mail:
<b><a href="postconf.5.html#header_checks">header_checks</a></b> (default: empty)
These are applied to initial message headers (except for the
headers that are processed with <b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b>).
<b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
These are applied to MIME related message headers only.
This feature is available in Postfix 2.0 and later.
<b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
These are applied to message headers of attached email messages
(except for the headers that are processed with
<b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b>).
This feature is available in Postfix 2.0 and later.
<b><a href="postconf.5.html#body_checks">body_checks</a></b>
These are applied to all other content, including multi-part
message boundaries.
With Postfix versions before 2.0, all content after the initial
message headers is treated as body content.
<b>FILTERS AFTER RECEIVING MAIL</b>
Postfix supports a subset of the built-in content inspection classes
after the message is received:
<b><a href="postconf.5.html#milter_header_checks">milter_header_checks</a></b> (default: empty)
These are applied to headers that are added with Milter applica-
tions.
This feature is available in Postfix 2.7 and later.
<b>FILTERS WHILE DELIVERING MAIL</b>
Postfix supports all four content inspection classes while delivering
mail via SMTP.
<b><a href="postconf.5.html#smtp_header_checks">smtp_header_checks</a></b> (default: empty)
<b><a href="postconf.5.html#smtp_mime_header_checks">smtp_mime_header_checks</a></b> (default: empty)
<b><a href="postconf.5.html#smtp_nested_header_checks">smtp_nested_header_checks</a></b> (default: empty)
<b><a href="postconf.5.html#smtp_body_checks">smtp_body_checks</a></b> (default: empty)
These features are available in Postfix 2.5 and later.
<b>COMPATIBILITY</b>
With Postfix version 2.2 and earlier specify "<b>postmap -fq</b>" to query a
table that contains case sensitive patterns. By default, <a href="regexp_table.5.html">regexp</a>: and
<a href="pcre_table.5.html">pcre</a>: patterns are case insensitive.
<b>TABLE FORMAT</b>
This document assumes that header and <a href="postconf.5.html#body_checks">body_checks</a> rules are specified
in the form of Postfix regular expression lookup tables. Usually the
best performance is obtained with <b>pcre</b> (Perl Compatible Regular Expres-
sion) tables. The <b>regexp</b> (POSIX regular expressions) tables are usually
slower, but more widely available. Use the command "<b>postconf -m</b>" to
find out what lookup table types your Postfix system supports.
The general format of Postfix regular expression tables is given below.
For a discussion of specific pattern or flags syntax, see <a href="pcre_table.5.html"><b>pcre_table</b>(5)</a>
or <a href="regexp_table.5.html"><b>regexp_table</b>(5)</a>, respectively.
<b>/</b><i>pattern</i><b>/</b><i>flags action</i>
When /<i>pattern</i>/ matches the input string, execute the correspond-
ing <i>action</i>. See below for a list of possible actions.
<b>!/</b><i>pattern</i><b>/</b><i>flags action</i>
When /<i>pattern</i>/ does <b>not</b> match the input string, execute the cor-
responding <i>action</i>.
<b>if /</b><i>pattern</i><b>/</b><i>flags</i>
<b>endif</b> If the input string matches /<i>pattern</i>/, then match that input
string against the patterns between <b>if</b> and <b>endif</b>. The <b>if</b>..<b>endif</b>
can nest.
Note: do not prepend whitespace to patterns inside <b>if</b>..<b>endif</b>.
<b>if !/</b><i>pattern</i><b>/</b><i>flags</i>
<b>endif</b> If the input string does not match /<i>pattern</i>/, then match that
input string against the patterns between <b>if</b> and <b>endif</b>. The
<b>if</b>..<b>endif</b> can nest.
blank lines and comments
Empty lines and whitespace-only lines are ignored, as are lines
whose first non-whitespace character is a `#'.
multi-line text
A pattern/action line starts with non-whitespace text. A line
that starts with whitespace continues a logical line.
<b>TABLE SEARCH ORDER</b>
For each line of message input, the patterns are applied in the order
as specified in the table. When a pattern is found that matches the
input line, the corresponding action is executed and then the next
input line is inspected.
<b>TEXT SUBSTITUTION</b>
Substitution of substrings from the matched expression into the <i>action</i>
string is possible using the conventional Perl syntax (<b>$1</b>, <b>$2</b>, etc.).
The macros in the result string may need to be written as <b>${n}</b> or <b>$(n)</b>
if they aren't followed by whitespace.
Note: since negated patterns (those preceded by <b>!</b>) return a result when
the expression does not match, substitutions are not available for
negated patterns.
<b>ACTIONS</b>
Action names are case insensitive. They are shown in upper case for
consistency with other Postfix documentation.
<b>BCC</b> <i>user@domain</i>
Add the specified address as a BCC recipient, and inspect the
next input line. The address must have a local part and domain
part. The number of BCC addresses that can be added is limited
only by the amount of available storage space.
Note 1: the BCC address is added as if it was specified with
NOTIFY=NONE. The sender will not be notified when the BCC
address is undeliverable, as long as all down-stream software
implements <a href="https://tools.ietf.org/html/rfc3461">RFC 3461</a>.
Note 2: this ignores duplicate addresses (with the same delivery
status notification options).
This feature is available in Postfix 3.0 and later.
This feature is not supported with smtp header/body checks.
<b>DISCARD</b> <i>optional text...</i>
Claim successful delivery and silently discard the message. Do
not inspect the remainder of the input message. Log the
optional text if specified, otherwise log a generic message.
Note: this action disables further header or <a href="postconf.5.html#body_checks">body_checks</a> inspec-
tion of the current message and affects all recipients. To dis-
card only one recipient without discarding the entire message,
use the <a href="transport.5.html">transport(5)</a> table to direct mail to the <a href="discard.8.html">discard(8)</a> ser-
vice.
This feature is available in Postfix 2.0 and later.
This feature is not supported with smtp header/body checks.
<b>DUNNO</b> Pretend that the input line did not match any pattern, and
inspect the next input line. This action can be used to shorten
the table search.
For backwards compatibility reasons, Postfix also accepts <b>OK</b> but
it is (and always has been) treated as <b>DUNNO</b>.
This feature is available in Postfix 2.1 and later.
<b>FILTER</b> <i>transport:destination</i>
Override the <a href="postconf.5.html#content_filter">content_filter</a> parameter setting, and inspect the
next input line. After the message is queued, send the entire
message through the specified external content filter. The
<i>transport</i> name specifies the first field of a mail delivery
agent definition in <a href="master.5.html">master.cf</a>; the syntax of the next-hop <i>desti-</i>
<i>nation</i> is described in the manual page of the corresponding
delivery agent. More information about external content filters
is in the Postfix <a href="FILTER_README.html">FILTER_README</a> file.
Note 1: do not use $<i>number</i> regular expression substitutions for
<i>transport</i> or <i>destination</i> unless you know that the information
has a trusted origin.
Note 2: this action overrides the <a href="postconf.5.html">main.cf</a> <b><a href="postconf.5.html#content_filter">content_filter</a></b> set-
ting, and affects all recipients of the message. In the case
that multiple <b>FILTER</b> actions fire, only the last one is exe-
cuted.
Note 3: the purpose of the FILTER command is to override message
routing. To override the recipient's <i>transport</i> but not the
next-hop <i>destination</i>, specify an empty filter <i>destination</i> (Post-
fix 2.7 and later), or specify a <i>transport:destination</i> that
delivers through a different Postfix instance (Postfix 2.6 and
earlier). Other options are using the recipient-dependent <b><a href="postconf.5.html#transport_maps">trans</a>-</b>
<b><a href="postconf.5.html#transport_maps">port_maps</a></b> or the sender-dependent <b><a href="postconf.5.html#sender_dependent_default_transport_maps">sender_dependent_default-</b>
<b>_transport_maps</a></b> features.
This feature is available in Postfix 2.0 and later.
This feature is not supported with smtp header/body checks.
<b>HOLD</b> <i>optional text...</i>
Arrange for the message to be placed on the <b>hold</b> queue, and
inspect the next input line. The message remains on <b>hold</b> until
someone either deletes it or releases it for delivery. Log the
optional text if specified, otherwise log a generic message.
Mail that is placed on hold can be examined with the <a href="postcat.1.html"><b>postcat</b>(1)</a>
command, and can be destroyed or released with the <a href="postsuper.1.html"><b>postsuper</b>(1)</a>
command.
Note: use "<b>postsuper -r</b>" to release mail that was kept on hold
for a significant fraction of <b>$<a href="postconf.5.html#maximal_queue_lifetime">maximal_queue_lifetime</a></b> or
<b>$<a href="postconf.5.html#bounce_queue_lifetime">bounce_queue_lifetime</a></b>, or longer. Use "<b>postsuper -H</b>" only for
mail that will not expire within a few delivery attempts.
Note: this action affects all recipients of the message.
This feature is available in Postfix 2.0 and later.
This feature is not supported with smtp header/body checks.
<b>IGNORE</b> Delete the current line from the input, and inspect the next
input line. See <b>STRIP</b> for an alternative that logs the action.
<b>INFO</b> <i>optional text...</i>
Log an "info:" record with the <i>optional text...</i> (or log a
generic text), and inspect the next input line. This action is
useful for routine logging or for debugging.
This feature is available in Postfix 2.8 and later.
<b>PASS</b> <i>optional text...</i>
Log a "pass:" record with the <i>optional text...</i> (or log a generic
text), and turn off header, body, and Milter inspection for the
remainder of this message.
Note: this feature relies on trust in information that is easy
to forge.
This feature is available in Postfix 3.2 and later.
This feature is not supported with smtp header/body checks.
<b>PREPEND</b> <i>text...</i>
Prepend one line with the specified text, and inspect the next
input line.
Notes:
<b>o</b> The prepended text is output on a separate line, immedi-
ately before the input that triggered the <b>PREPEND</b> action.
<b>o</b> The prepended text is not considered part of the input
stream: it is not subject to header/body checks or
address rewriting, and it does not affect the way that
Postfix adds missing message headers.
<b>o</b> When prepending text before a message header line, the
prepended text must begin with a valid message header
label.
<b>o</b> This action cannot be used to prepend multi-line text.
This feature is available in Postfix 2.1 and later.
This feature is not supported with <a href="postconf.5.html#milter_header_checks">milter_header_checks</a>.
<b>REDIRECT</b> <i>user@domain</i>
Write a message redirection request to the queue file, and
inspect the next input line. After the message is queued, it
will be sent to the specified address instead of the intended
recipient(s).
Note: this action overrides the <b>FILTER</b> action, and affects all
recipients of the message. If multiple <b>REDIRECT</b> actions fire,
only the last one is executed.
This feature is available in Postfix 2.1 and later.
This feature is not supported with smtp header/body checks.
<b>REPLACE</b> <i>text...</i>
Replace the current line with the specified text, and inspect
the next input line.
This feature is available in Postfix 2.2 and later. The descrip-
tion below applies to Postfix 2.2.2 and later.
Notes:
<b>o</b> When replacing a message header line, the replacement
text must begin with a valid header label.
<b>o</b> The replaced text remains part of the input stream.
Unlike the result from the <b>PREPEND</b> action, a replaced
message header may be subject to address rewriting and
may affect the way that Postfix adds missing message
headers.
<b>REJECT</b> <i>optional text...</i>
Reject the entire message. Do not inspect the remainder of the
input message. Reply with <i>optional text...</i> when the optional
text is specified, otherwise reply with a generic error message.
Note: this action disables further header or <a href="postconf.5.html#body_checks">body_checks</a> inspec-
tion of the current message and affects all recipients.
Postfix version 2.3 and later support enhanced status codes.
When no code is specified at the beginning of <i>optional text...</i>,
Postfix inserts a default enhanced status code of "5.7.1".
This feature is not supported with smtp header/body checks.
<b>STRIP</b> <i>optional text...</i>
Log a "strip:" record with the <i>optional text...</i> (or log a
generic text), delete the input line from the input, and inspect
the next input line. See <b>IGNORE</b> for a silent alternative.
This feature is available in Postfix 3.2 and later.
<b>WARN</b> <i>optional text...</i>
Log a "warning:" record with the <i>optional text...</i> (or log a
generic text), and inspect the next input line. This action is
useful for debugging and for testing a pattern before applying
more drastic actions.
<b>BUGS</b>
Empty lines never match, because some map types mis-behave when given a
zero-length search string. This limitation may be removed for regular
expression tables in a future release.
Many people overlook the main limitations of header and <a href="postconf.5.html#body_checks">body_checks</a>
rules.
<b>o</b> These rules operate on one logical message header or one body
line at a time. A decision made for one line is not carried over
to the next line.
<b>o</b> If text in the message body is encoded (<a href="https://tools.ietf.org/html/rfc2045">RFC 2045</a>) then the rules
need to be specified for the encoded form.
<b>o</b> Likewise, when message headers are encoded (<a href="https://tools.ietf.org/html/rfc2047">RFC 2047</a>) then the
rules need to be specified for the encoded form.
Message headers added by the <a href="cleanup.8.html"><b>cleanup</b>(8)</a> daemon itself are excluded from
inspection. Examples of such message headers are <b>From:</b>, <b>To:</b>, <b>Mes-</b>
<b>sage-ID:</b>, <b>Date:</b>.
Message headers deleted by the <a href="cleanup.8.html"><b>cleanup</b>(8)</a> daemon will be examined
before they are deleted. Examples are: <b>Bcc:</b>, <b>Content-Length:</b>,
<b>Return-Path:</b>.
<b>CONFIGURATION PARAMETERS</b>
<b><a href="postconf.5.html#body_checks">body_checks</a> (empty)</b>
Optional lookup tables for content inspection as specified in
the <a href="header_checks.5.html"><b>body_checks</b>(5)</a> manual page.
<b><a href="postconf.5.html#body_checks_size_limit">body_checks_size_limit</a> (51200)</b>
How much text in a message body segment (or attachment, if you
prefer to use that term) is subjected to <a href="postconf.5.html#body_checks">body_checks</a> inspection.
<b><a href="postconf.5.html#header_checks">header_checks</a> (empty)</b>
Optional lookup tables for content inspection of primary
non-MIME message headers, as specified in the <a href="header_checks.5.html"><b>header_checks</b>(5)</a>
manual page.
<b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a> ($<a href="postconf.5.html#header_checks">header_checks</a>)</b>
Optional lookup tables for content inspection of MIME related
message headers, as described in the <a href="header_checks.5.html"><b>header_checks</b>(5)</a> manual
page.
<b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a> ($<a href="postconf.5.html#header_checks">header_checks</a>)</b>
Optional lookup tables for content inspection of non-MIME mes-
sage headers in attached messages, as described in the
<a href="header_checks.5.html"><b>header_checks</b>(5)</a> manual page.
<b><a href="postconf.5.html#disable_mime_input_processing">disable_mime_input_processing</a> (no)</b>
Turn off MIME processing while receiving mail.
<b>EXAMPLES</b>
Header pattern to block attachments with bad file name extensions. For
convenience, the PCRE /x flag is specified, so that there is no need to
collapse the pattern into a single line of text. The purpose of the
[[:xdigit:]] sub-expressions is to recognize Windows CLSID strings.
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#header_checks">header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/header_checks.pcre
/etc/postfix/header_checks.<a href="pcre_table.5.html">pcre</a>:
/^Content-(Disposition|Type).*name\s*=\s*"?([^;]*(\.|=2E)(
ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
hlp|ht[at]|
inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
\{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x
REJECT Attachment name "$2" may not end with ".$4"
Body pattern to stop a specific HTML browser vulnerability exploit.
/etc/postfix/<a href="postconf.5.html">main.cf</a>:
<a href="postconf.5.html#body_checks">body_checks</a> = <a href="regexp_table.5.html">regexp</a>:/etc/postfix/body_checks
/etc/postfix/body_checks:
/^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
REJECT IFRAME vulnerability exploit
<b>SEE ALSO</b>
<a href="cleanup.8.html">cleanup(8)</a>, canonicalize and enqueue Postfix message
<a href="pcre_table.5.html">pcre_table(5)</a>, format of PCRE lookup tables
<a href="regexp_table.5.html">regexp_table(5)</a>, format of POSIX regular expression tables
<a href="postconf.1.html">postconf(1)</a>, Postfix configuration utility
<a href="postmap.1.html">postmap(1)</a>, Postfix lookup table management
<a href="postsuper.1.html">postsuper(1)</a>, Postfix janitor
<a href="postcat.1.html">postcat(1)</a>, show Postfix queue file contents
<a href="https://tools.ietf.org/html/rfc2045">RFC 2045</a>, base64 and quoted-printable encoding rules
<a href="https://tools.ietf.org/html/rfc2047">RFC 2047</a>, message header encoding for non-ASCII text
<b>README FILES</b>
<a href="DATABASE_README.html">DATABASE_README</a>, Postfix lookup table overview
<a href="CONTENT_INSPECTION_README.html">CONTENT_INSPECTION_README</a>, Postfix content inspection overview
<a href="BUILTIN_FILTER_README.html">BUILTIN_FILTER_README</a>, Postfix built-in content inspection
<a href="BACKSCATTER_README.html">BACKSCATTER_README</a>, blocking returned forged mail
<b>LICENSE</b>
The Secure Mailer license must be distributed with this software.
<b>AUTHOR(S)</b>
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
HEADER_CHECKS(5)
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