HEADER_CHECKS(5) File Formats Manual HEADER_CHECKS(5)
NAME
header_checks - Postfix built-in content inspection
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
milter_header_checks = pcre:/etc/postfix/milter_header_checks
smtp_header_checks = pcre:/etc/postfix/smtp_header_checks
smtp_mime_header_checks = pcre:/etc/postfix/smtp_mime_header_checks
smtp_nested_header_checks = pcre:/etc/postfix/smtp_nested_header_checks
smtp_body_checks = pcre:/etc/postfix/smtp_body_checks
postmap -q "string" pcre:/etc/postfix/filename
postmap -q - pcre:/etc/postfix/filename <inputfile
DESCRIPTION
This document describes access control on the content of message headers and message body lines; it is imple‐
mented by the Postfix cleanup(8) server before mail is queued. See access(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.
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.
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.
FILTERS WHILE RECEIVING MAIL
Postfix implements the following four built-in content inspection classes while receiving mail:
header_checks (default: empty)
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 pro‐
cessed with mime_header_checks).
This feature is available in Postfix 2.0 and later.
body_checks
FILTERS WHILE DELIVERING MAIL
Postfix supports all four content inspection classes while delivering mail via SMTP.
smtp_header_checks (default: empty)
smtp_mime_header_checks (default: empty)
smtp_nested_header_checks (default: empty)
smtp_body_checks (default: empty)
These features are available in Postfix 2.5 and later.
COMPATIBILITY
With Postfix version 2.2 and earlier specify "postmap -fq" to query a table that contains case sensitive pat‐
terns. By default, regexp: and pcre: patterns are case insensitive.
TABLE FORMAT
This document assumes that header and body_checks rules are specified in the form of Postfix regular expres‐
sion lookup tables. Usually the best performance is obtained with pcre (Perl Compatible Regular Expression)
tables. The regexp (POSIX regular expressions) tables are usually slower, but more widely available. Use the
command "postconf -m" 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 pcre_table(5) or regexp_table(5), respectively.
/pattern/flags action
When /pattern/ matches the input string, execute the corresponding action. See below for a list of pos‐
sible actions.
!/pattern/flags action
When /pattern/ does not match the input string, execute the corresponding action.
if /pattern/flags
endif Match the input string against the patterns between if and endif, if and only if the same 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 same 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
ACTIONS
Action names are case insensitive. They are shown in upper case for consistency with other Postfix documenta‐
tion.
DISCARD optional text...
Claim successful delivery and silently discard the message. Log the optional text if specified, other‐
wise log a generic message.
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 trans‐
port(5) table to direct mail to the discard(8) service.
This feature is available in Postfix 2.0 and later.
This feature is not supported with smtp header/body checks.
DUNNO 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 OK but it is (and always has been) treated as
DUNNO.
This feature is available in Postfix 2.1 and later.
FILTER transport:destination
After the message is queued, send the entire message through the specified external content filter. The
transport name specifies the first field of a mail delivery agent definition in master.cf; the syntax
of the next-hop destination is described in the manual page of the corresponding delivery agent. More
information about external content filters is in the Postfix FILTER_README file.
Note 1: do not use $number regular expression substitutions for transport or destination unless you
know that the information has a trusted origin.
Note 2: this action overrides the main.cf content_filter setting, and affects all recipients of the
message. In the case that multiple FILTER actions fire, only the last one is executed.
Note 3: the purpose of the FILTER command is to override message routing. To override the recipient's
transport but not the next-hop destination, specify an empty filter destination (Postfix 2.7 and
later), or specify a transport:destination that delivers through a different Postfix instance (Postfix
2.6 and earlier). Other options are using the recipient-dependent transport_maps or the sen‐
der-dependent sender_dependent_default_transport_maps features.
This feature is available in Postfix 2.0 and later.
This feature is not supported with smtp header/body checks.
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.
INFO optional text...
Log an "info:" record with the optional text... (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.
PREPEND text...
Prepend one line with the specified text, and inspect the next input line.
Notes:
· The prepended text is output on a separate line, immediately before the input that triggered the
PREPEND action.
· 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.
· When prepending text before a message header line, the prepended text must begin with a valid
message header label.
· 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 milter_header_checks.
REDIRECT user@domain
Write a message redirection request to the queue file, and inspect the next input line. After the mes‐
sage 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.
This feature is available in Postfix 2.1 and later.
This feature is not supported with smtp header/body checks.
REPLACE text...
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:
· When replacing a message header line, the replacement text must begin with a valid header label.
· The replaced text remains part of the input stream. Unlike the result from the PREPEND action, a
replaced message header may be subject to address rewriting and may affect the way that Postfix
adds missing message headers.
REJECT optional text...
Reject the entire message. Reply with optional text... when the optional text is specified, otherwise
line. This action is useful for debugging and for testing a pattern before applying more drastic
actions.
BUGS
Empty lines never match, because some map types mis-behave when given a zero-length search string. This limi‐
tation may be removed for regular expression tables in a future release.
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. 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 need to be specified for the encoded
form.
· Likewise, when message 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 filter‐
ing.
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 anywhere 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 $header_size_limit characters are truncated.
disable_mime_input_processing
While receiving mail, give no special treatment to MIME related message headers; all text after the
initial message headers is considered to be part of the message body. This means that header_checks is
applied to all the initial message headers, and that body_checks is applied to the remainder of the
message.
Note: when used in this manner, body_checks will process a multi-line message header one line at a
time.
inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
\{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x
REJECT Attachment name "$2" may not end with ".$4"
Body pattern to stop a specific HTML browser vulnerability exploit.
/etc/postfix/main.cf:
body_checks = regexp:/etc/postfix/body_checks
/etc/postfix/body_checks:
/^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
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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