postfix/postfix/man/man5/header_checks.5

.TH HEADER_CHECKS 5 
.ad
.fi
.SH NAME
header_checks
\-
Postfix built-in content inspection
.SH "SYNOPSIS"
.na
.nf
\fBheader_checks = pcre:/etc/postfix/header_checks\fR
.br
\fBmime_header_checks = pcre:/etc/postfix/mime_header_checks\fR
.br
\fBnested_header_checks = pcre:/etc/postfix/nested_header_checks\fR
.br
\fBbody_checks = pcre:/etc/postfix/body_checks\fR
.sp
\fBpostmap -fq "\fIstring\fB" pcre:/etc/postfix/\fIfilename\fR
.br
\fBpostmap -fq - pcre:/etc/postfix/\fIfilename\fR <\fIinputfile\fR
.SH DESCRIPTION
.ad
.fi
This document describes access control on the content of
message headers and message body lines; it is implemented
by the Postfix cleanup(8) server before mail is queued.
See \fBaccess\fR(5) 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 body_checks 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 section if you need more sophisticated content analysis.

Postfix supports four built-in content inspection classes:
.IP \fBheader_checks\fR
These are applied to initial message headers (except for
the headers that are processed with \fBmime_header_checks\fR).
.IP "\fBmime_header_checks\fR (default: \fB$header_checks\fR)"
These are applied to MIME related message headers only.
.sp
This feature is available in Postfix 2.0 and later.
.IP "\fBnested_header_checks\fR (default: \fB$header_checks\fR)"
These are applied to message headers of attached email
messages (except for the headers that are processed with
\fBmime_header_checks\fR).
.sp
This feature is available in Postfix 2.0 and later.
.IP \fBbody_checks\fR
These are applied to all other content, including multi-part
message boundaries.
.sp
With Postfix versions before 2.0, all content after the initial
message headers is treated as body content.
.PP
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.
.SH "TABLE FORMAT"
.na
.nf
.ad
.fi
This document assumes that header and body_checks rules are specified
in the form of Postfix regular expression lookup tables. Usually the
best performance is obtained with \fBpcre\fR (Perl Compatible Regular
Expression) tables, but the slower \fBregexp\fR (POSIX regular
expressions) support is more widely available.
Use the command "\fBpostconf -m\fR" 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 \fBpcre_table\fR(5) or \fBregexp_table\fR(5), respectively.
.IP "\fB/\fIpattern\fB/\fIflags action\fR"
When \fIpattern\fR matches the input string, execute
the corresponding \fIaction\fR. See below for a list
of possible actions.
.IP "\fB!/\fIpattern\fB/\fIflags action\fR"
When \fIpattern\fR does \fBnot\fR match the input string,
execute the corresponding \fIaction\fR.
.IP "\fBif /\fIpattern\fB/\fIflags\fR"
.IP "\fBendif\fR"
Match the input string against the patterns between \fBif\fR
and \fBendif\fR, if and only if the same input string also
matches \fIpattern\fR. The \fBif\fR..\fBendif\fR can nest.
.sp
Note: do not prepend whitespace to patterns inside
\fBif\fR..\fBendif\fR.
.IP "\fBif !/\fIpattern\fB/\fIflags\fR"
.IP "\fBendif\fR"
Match the input string against the patterns between \fBif\fR
and \fBendif\fR, if and only if the same input string does
\fBnot\fR match \fIpattern\fR. The \fBif\fR..\fBendif\fR
can nest.
.IP "blank lines and comments"
Empty lines and whitespace-only lines are ignored, as
are lines whose first non-whitespace character is a `#'.
.IP "multi-line text"
A pattern/action line starts with non-whitespace text. A line that
starts with whitespace continues a logical line.
.SH "TABLE SEARCH ORDER"
.na
.nf
.ad
.fi
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.
.SH "TEXT SUBSTITUTION"
.na
.nf
.ad
.fi
Substitution of substrings from the matched expression into the
\fIaction\fR
string is possible using the conventional Perl syntax
(\fB$1\fR, \fB$2\fR, etc.).
The macros in the result string may need to be written as \fB${n}\fR
or \fB$(n)\fR if they aren't followed by whitespace.

Note: since negated patterns (those preceded by \fB!\fR) return a
result when the expression does not match, substitutions are not
available for negated patterns.
.SH "ACTIONS"
.na
.nf
.ad
.fi
Action names are case insensitive. They are shown in upper case
for consistency with other Postfix documentation.
.IP "\fBDISCARD \fIoptional text...\fR"
Claim successful delivery and silently discard the message.
Log the optional text if specified, otherwise log a generic
message.
.sp
Note: this action disables further header or body_checks inspection
of the current message and affects all recipients.
To discard only one recipient without discarding the entire message,
use the transport(5) table to direct mail to the discard(8) service.
.sp
This feature is available in Postfix 2.0 and later.
.IP \fBDUNNO\fR
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.
.sp
For backwards compatibility reasons, Postfix also accepts
\fBOK\fR but it is (and always has been) treated as \fBDUNNO\fR.
.sp
This feature is available in Postfix 2.1 and later.
.IP "\fBFILTER \fItransport:destination\fR"
Write a content filter request to the queue file, and
inspect the next input line.
After the complete message is received it will be sent through
the specified external content filter.  More information about
external content filters is in the Postfix FILTER_README file.
.sp
Note: this action overrides the \fBcontent_filter\fR setting,
and affects all recipients of the message. In the case that multiple
\fBFILTER\fR actions fire, only the last one is executed.
.sp
This feature is available in Postfix 2.0 and later.
.IP "\fBHOLD \fIoptional text...\fR"
Arrange for the message to be placed on the \fBhold\fR queue,
and inspect the next input line.  The message remains on \fBhold\fR
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
\fBpostcat\fR(1) command, and can be destroyed or released with
the \fBpostsuper\fR(1) command.
.sp
Note: use "\fBpostsuper -r\fR" to release mail that was kept on
hold for a significant fraction of \fB$maximal_queue_lifetime\fR
or \fB$bounce_queue_lifetime\fR, or longer. Use "\fBpostsuper -H\fR"
only for mail that will not expire within a few delivery attempts.
.sp
Note: this action affects all recipients of the message.
.sp
This feature is available in Postfix 2.0 and later.
.IP \fBIGNORE\fR
Delete the current line from the input, and inspect
the next input line.
.IP "\fBPREPEND \fItext...\fR"
Prepend one line with the specified text, and inspect the next
input line.
.sp
Notes:
.RS
.IP \(bu
The prepended text is output on a separate line, immediately
before the input that triggered the \fBPREPEND\fR action.
.IP \(bu
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.
.IP \(bu
When prepending text before a message header line, the prepended
text must begin with a valid message header label.
.IP \(bu
This action cannot be used to prepend multi-line text.
.RE
.IP
This feature is available in Postfix 2.1 and later.
.IP "\fBREDIRECT \fIuser@domain\fR"
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).
.sp
Note: this action overrides the \fBFILTER\fR action, and affects
all recipients of the message. If multiple \fBREDIRECT\fR actions
fire, only the last one is executed.
.sp
This feature is available in Postfix 2.1 and later.
.IP "\fBREPLACE \fItext...\fR"
Replace the current line with the specified text, and inspect the next
input line.
.sp
This feature is available in Postfix 2.2 and later. The
description below applies to Postfix 2.2.2 and later.
.sp
Notes:
.RS
.IP \(bu
When replacing a message header line, the replacement text
must begin with a valid header label.
.IP \(bu
The replaced text remains part of the input stream. Unlike
the result from the \fBPREPEND\fR action, a replaced message
header may be subject to address rewriting and may affect
the way that Postfix adds missing message headers.
.RE
.IP "\fBREJECT \fIoptional text...\fR
Reject the entire message. Reply with \fIoptional text...\fR when
the optional text is specified, otherwise reply with a generic error
message.
.sp
Note: this action disables further header or body_checks inspection
of the current message and affects all recipients.
.sp
Postfix version 2.3 and later support enhanced status codes.
When no code is specified at the beginning of \fIoptional
text...\fR, Postfix inserts a default enhanced status code of
"5.7.1".
.IP "\fBWARN \fIoptional text...\fR
Log a warning with the \fIoptional text...\fR (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.
.SH BUGS
.ad
.fi
Many people overlook the main limitations of header and body_checks
rules.
.IP \(bu
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.
.IP \(bu
If text in the message body is encoded
(RFC 2045) then the rules have to specified for the encoded
form.
.IP \(bu
Likewise, when message headers are encoded (RFC
2047) then the rules need to be specified for the encoded
form.
.PP
Message headers added by the \fBcleanup\fR(8) daemon itself
are excluded from inspection. Examples of such message headers
are \fBFrom:\fR, \fBTo:\fR, \fBMessage-ID:\fR, \fBDate:\fR.

Message headers deleted by the \fBcleanup\fR(8) daemon will
be examined before they are deleted. Examples are: \fBBcc:\fr,
\fBContent-Length:\fR, \fBReturn-Path:\fR.
.SH "CONFIGURATION PARAMETERS"
.na
.nf
.ad
.fi
.IP \fBbody_checks\fR
Lookup tables with content filter rules for message body lines.
These filters see one physical line at a time, in chunks of
at most \fB$line_length_limit\fR bytes.
.IP \fBbody_checks_size_limit\fP
The amount of content per message body segment (attachment) that is
subjected to \fB$body_checks\fR filtering.
.IP \fBheader_checks\fR
.IP "\fBmime_header_checks\fR (default: \fB$header_checks\fR)"
.IP "\fBnested_header_checks\fR (default: \fB$header_checks\fR)"
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 message, and to the initial headers of attached messages.
.sp
Note: these filters see one logical message header at a time, even
when a message header spans multiple lines. Message headers that
are longer than \fB$header_size_limit\fR characters are truncated.
.IP \fBdisable_mime_input_processing\fR
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
\fBheader_checks\fR is applied to all the initial message headers,
and that \fBbody_checks\fR is applied to the remainder of the
message.
.sp
Note: when used in this manner, \fBbody_checks\fR will process
a multi-line message header one line at a time.
.SH "EXAMPLES"
.na
.nf
.ad
.fi
Header pattern to block attachments with bad file name extensions.

.na
.nf
/etc/postfix/main.cf:
.ti +4
header_checks = regexp:/etc/postfix/header_checks

/etc/postfix/header_checks:
.ti +4
/^content-(type|disposition):.*name[[:space:]]*=.*\\.(exe|vbs)/
.ti +8
REJECT Bad attachment file name extension: $2

.ad
.fi
Body pattern to stop a specific HTML browser vulnerability exploit.

.na
.nf
/etc/postfix/main.cf:
.ti +4
body_checks = regexp:/etc/postfix/body_checks

/etc/postfix/body_checks:
.ti +4
/^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
.ti +8
REJECT IFRAME vulnerability exploit
.SH "SEE ALSO"
.na
.nf
cleanup(8), canonicalize and enqueue Postfix message
pcre_table(5), format of PCRE lookup tables
regexp_table(5), format of POSIX regular expression tables
postconf(1), Postfix configuration utility
postmap(1), Postfix lookup table management
postsuper(1), Postfix janitor
postcat(1), show Postfix queue file contents
RFC 2045, base64 and quoted-printable encoding rules
RFC 2047, message header encoding for non-ASCII text
.SH "README FILES"
.na
.nf
.ad
.fi
Use "\fBpostconf readme_directory\fR" or
"\fBpostconf html_directory\fR" to locate this information.
.na
.nf
DATABASE_README, Postfix lookup table overview
CONTENT_INSPECTION_README, Postfix content inspection overview
BUILTIN_FILTER_README, Postfix built-in content inspection
BACKSCATTER_README, blocking returned forged mail
.SH "LICENSE"
.na
.nf
.ad
.fi
The Secure Mailer license must be distributed with this software.
.SH "AUTHOR(S)"
.na
.nf
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA