SYNOPSIS
header_checks = pcre:/etc/postfix/header_checks
mime_header_checks = pcre:/etc/postfix/mime_header_checks
nested_header_checks = pcre:/etc/postfix/nested_header_checks
body_checks = pcre:/etc/postfix/body_checks
postmap -fq "string" pcre:/etc/postfix/filename
postmap -fq - pcre:/etc/postfix/filename <inputfile
DESCRIPTION
Postfix provides a simple built-in content inspection mechanism that
examines incoming mail one message header or one message body line at a
time. Each input is compared against a list of patterns, and when a
match is found the corresponding action is executed. This feature is
implemented by the Postfix cleanup(8) server.
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 sec-
tion if you need more sophisticated content analysis.
Postfix supports four built-in content inspection classes:
header_checks
These are applied to initial message headers (except for the
headers that are processed with mime_header_checks).
mime_header_checks (default: $header_checks)
These are applied to MIME related message headers only.
This feature is available in Postfix 2.0 and later.
nested_header_checks (default: $header_checks)
These are applied to message headers of attached email messages
(except for the headers that are processed with
mime_header_checks).
This feature is available in Postfix 2.0 and later.
body_checks
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.
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.
action. See below for a list of possible actions.
!/pattern/flags action
When pattern does not match the input string, execute the corre-
sponding action.
if /pattern/flags
endif Match the input string against the patterns between if and
endif, if and only if the input string also matches pattern. The
if..endif can nest.
Note: do not prepend whitespace to patterns inside if..endif.
if !/pattern/flags
endif Match the input string against the patterns between if and
endif, if and only if the input string does not match pattern.
The if..endif 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.
TABLE SEARCH ORDER
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.
TEXT SUBSTITUTION
Substitution of substrings from the matched expression into the action
string is possible using the conventional Perl syntax ($1, $2, etc.).
The macros in the result string may need to be written as ${n} or $(n)
if they aren't followed by whitespace.
Note: since negated patterns (those preceded by !) return a result when
the expression does not match, substitutions are not available for
negated patterns.
ACTIONS
Action names are case insensitive. They are shown in upper case for
consistency with other Postfix documentation.
DISCARD optional text...
Claim successful delivery and silently discard the message. Log
the optional text if specified, otherwise log a generic message.
FILTER transport:destination
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.
Note: this action overrides the main.cf content_filter setting,
and affects all recipients of the message. In the case that mul-
tiple FILTER actions fire, only the last one is executed.
This feature is available in Postfix 2.0 and later.
HOLD optional text...
Arrange for the message to be placed on the hold queue, and
inspect the next input line. The message remains on hold 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 postcat(1)
command, and can be destroyed or released with the postsuper(1)
command.
Note: this action affects all recipients of the message.
This feature is available in Postfix 2.0 and later.
IGNORE Delete the current line from the input and inspect the next
input line.
PREPEND text...
Prepend one line with the specified text and inspect the next
input line.
Note: the prepended text is output immediately before the input
that triggered the PREPEND action. A body action cannot prepend
a message header.
Note: this action cannot be used to prepend multi-line text.
This feature is available in Postfix 2.1 and later.
REDIRECT user@domain
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 FILTER action, and affects all
recipients of the message. If multiple REDIRECT actions fire,
only the last one is executed.
actions.
BUGS
Many people overlook the main limitations of header and body_checks
rules. These rules operate on one logical message header or one body
line at a time, and a decision made for one line is not carried over to
the next line. If text in the message body is encoded (RFC 2045) then
the rules have to specified for the encoded form. Likewise, when mes-
sage headers are encoded (RFC 2047) then the rules need to be specified
for the encoded form.
Message headers added by the cleanup(8) daemon itself are excluded from
inspection. Examples of such message headers are From:, To:, Message-
ID:, Date:.
Message headers deleted by the cleanup(8) daemon will be examined
before they are deleted. Examples are: Bcc:, Content-Length:, Return-
Path:.
CONFIGURATION PARAMETERS
body_checks
Lookup tables with content filter rules for message body lines.
These filters see one physical line at a time, in chunks of at
most $line_length_limit bytes.
body_checks_size_limit
The amount of content per message body segment (attachment) that
is subjected to $body_checks filtering.
header_checks
mime_header_checks (default: $header_checks)
nested_header_checks (default: $header_checks)
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 any-
where in the message, and to the initial headers of attached
messages.
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 $header_size_limit characters are trun-
cated.
disable_mime_input_processing
While receiving mail, give no special treatment to MIME related
message headers; all text after the initial message headers is
considered to be part of the message body. This means that
header_checks is applied to all the initial message headers, and
that body_checks is applied to the remainder of the message.
/etc/postfix/main.cf:
body_checks = regexp:/etc/postfix/body_checks
/etc/postfix/body_checks:
/^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
REJECT IFRAME vulnerability exploit
SEE ALSO
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
README FILES
Use "postconf readme_directory" or "postconf html_directory" to locate
this information.
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
LICENSE
The Secure Mailer license must be distributed with this software.
AUTHOR(S)
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
HEADER_CHECKS(5)
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