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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>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 headers 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.
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 attach-
ments, and they do not unzip archives. See the documents
referenced below in the README FILES section if you need
more sophisticated content analysis.
Postfix supports four built-in content inspection classes:
<b><a href="postconf.5.html#header_checks">header_checks</a></b>
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 con-
tent.
Note: message headers are examined one logical header at a
time, even when a message header spans multiple lines.
Body lines are always examined one line at a time.
<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 Expression) tables, but
the slower <b>regexp</b> (POSIX regular expressions) support is
more widely available. Use the command "<b>postconf -m</b>" to
find out what lookup table types your Postfix system sup-
ports.
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 corresponding <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 corresponding <i>action</i>.
<b>if /</b><i>pattern</i><b>/</b><i>flags</i>
<b>endif</b> Match the input string against the patterns between
<b>if</b> and <b>endif</b>, if and only if the same input string
also matches /<i>pattern</i>/. 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> Match the input string against the patterns between
<b>if</b> and <b>endif</b>, if and only if the same input string
does <b>not</b> match /<i>pattern</i>/. 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>DISCARD</b> <i>optional text...</i>
Claim successful delivery and silently discard the
message. Log the optional text if specified, oth-
erwise log a generic message.
Note: this action disables further header or
<a href="postconf.5.html#body_checks">body_checks</a> inspection of the current message and
affects all recipients. To discard only one recip-
ient 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>
service.
This feature is available in Postfix 2.0 and later.
<b>DUNNO</b> Pretend that the input line did not match any pat-
tern, 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>
After the message is queued, send the entire mes-
sage 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>destination</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 $<i>number</i> regular expression sub-
stitutions for <i>transport</i> or <i>destination</i> unless you
know that the information has a trusted origin.
Note 2: this action overrides the <a href="postconf.5.html">main.cf</a> <b><a href="postconf.5.html#content_filter">con</a>-</b>
<b><a href="postconf.5.html#content_filter">tent_filter</a></b> setting, and affects all recipients of
the message. In the case that multiple <b>FILTER</b>
actions fire, only the last one is executed.
Note 3: the purpose of the FILTER command is to
override message routing. To override the recipi-
ent's <i>transport</i> but not the next-hop <i>destination</i>,
specify an empty filter <i>destination</i> (Postfix 2.7
and later), or specify a <i>transport:destination</i> that
delivers through a different Postfix instance
(Postfix 2.6 and earlier). Other options are using
the recipient-dependent <b><a href="postconf.5.html#transport_maps">transport_maps</a></b> or the sen-
der-dependent <b><a href="postconf.5.html#sender_dependent_default_transport_maps">sender_dependent_default_transport</a>-</b>
<b><a href="postconf.5.html#sender_dependent_default_transport_maps">_maps</a></b> features.
This feature is available in Postfix 2.0 and later.
<b>HOLD</b> <i>optional text...</i>
Arrange for the message to be placed on the <b>hold</b>
queue, and inspect the next input line. The mes-
sage remains on <b>hold</b> until someone either deletes
it or releases it for delivery. Log the optional
text if specified, otherwise log a generic message.
Mail that is placed on hold can be examined with
the <a href="postcat.1.html"><b>postcat</b>(1)</a> command, and can be destroyed or
released with the <a href="postsuper.1.html"><b>postsuper</b>(1)</a> command.
Note: use "<b>postsuper -r</b>" to release mail that was
kept on hold for a significant fraction of <b>$<a href="postconf.5.html#maximal_queue_lifetime">maxi</a>-</b>
<b><a href="postconf.5.html#maximal_queue_lifetime">mal_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.
<b>IGNORE</b> Delete the current line from the input, and inspect
the next input line.
<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, immediately 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.
<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 exe-
cuted.
This feature is available in Postfix 2.1 and later.
<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 description 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. Reply with <i>optional</i>
<i>text...</i> when the optional text is specified, other-
wise reply with a generic error message.
Note: this action disables further header or
<a href="postconf.5.html#body_checks">body_checks</a> inspection of the current message and
affects all recipients.
Postfix version 2.3 and later support enhanced sta-
tus codes. When no code is specified at the begin-
ning of <i>optional text...</i>, Postfix inserts a default
enhanced status code of "5.7.1".
<b>WARN</b> <i>optional text...</i>
Log a warning with the <i>optional text...</i> (or log a
generic message), 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="http://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="http://tools.ietf.org/html/rfc2047">RFC</a>
<a href="http://tools.ietf.org/html/rfc2047">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>Message-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:, Con-</b>
<b>tent-Length:</b>, <b>Return-Path:</b>.
<b>CONFIGURATION PARAMETERS</b>
<b><a href="postconf.5.html#body_checks">body_checks</a></b>
Lookup tables with content filter rules for message
body lines. These filters see one physical line at
a time, in chunks of at most <b>$<a href="postconf.5.html#line_length_limit">line_length_limit</a></b>
bytes.
<b><a href="postconf.5.html#body_checks_size_limit">body_checks_size_limit</a></b>
The amount of content per message body segment
(attachment) that is subjected to <b>$<a href="postconf.5.html#body_checks">body_checks</a></b> fil-
tering.
<b><a href="postconf.5.html#header_checks">header_checks</a></b>
<b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
<b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
Lookup tables with content filter rules for message
header lines: respectively, these are applied to
the initial message headers (not including MIME
headers), to the MIME headers anywhere in the mes-
sage, and to the initial headers of attached mes-
sages.
Note: these filters see one logical message header
at a time, even when a message header spans multi-
ple lines. Message headers that are longer than
<b>$<a href="postconf.5.html#header_size_limit">header_size_limit</a></b> characters are truncated.
<b><a href="postconf.5.html#disable_mime_input_processing">disable_mime_input_processing</a></b>
While receiving mail, give no special treatment to
MIME related message headers; all text after the
initial message headers is considered to be part of
the message body. This means that <b><a href="postconf.5.html#header_checks">header_checks</a></b> is
applied to all the initial message headers, and
that <b><a href="postconf.5.html#body_checks">body_checks</a></b> is applied to the remainder of the
message.
Note: when used in this manner, <b><a href="postconf.5.html#body_checks">body_checks</a></b> will
process a multi-line message header one line at a
time.
<b>EXAMPLES</b>
Header pattern to block attachments with bad file name
extensions. For convenience, the PCRE /x flag is speci-
fied, 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="http://tools.ietf.org/html/rfc2045">RFC 2045</a>, base64 and quoted-printable encoding rules
<a href="http://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
HEADER_CHECKS(5)
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