header_checks (5)

Contents

NAME
SYNOPSIS
DESCRIPTION
FILTERS WHILE RECEIVING MAIL
FILTERS AFTER RECEIVING MAIL
FILTERS WHILE DELIVERING MAIL
COMPATIBILITY
TABLE FORMAT
TABLE SEARCH ORDER
TEXT SUBSTITUTION
ACTIONS
BUGS
CONFIGURATION PARAMETERS
EXAMPLES
SEE ALSO
README FILES
LICENSE
AUTHOR(S)

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 implemented 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 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.




FILTERS AFTER RECEIVING MAIL






Postfix supports a subset of the built-in content inspection
classes after the message is received:



milter_header_checks (default: empty)

These are applied to headers that are added with Milter
applications.


This feature is available in Postfix 2.7 and later.




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
patterns. 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 expression 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 possible 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






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.


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.


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 sender-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.


Note: use "postsuper -r" to release mail that was kept on
hold for a significant fraction of $maximal_queue_lifetime
or $bounce_queue_lifetime, or longer. Use "postsuper -H"
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.

IGNORE

Delete the current line from the input, and inspect
the next input line.

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 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.


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 reply with a generic error
message.


Note: this action disables further header or body_checks inspection
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 optional
text..., Postfix inserts a default enhanced status code of
"5.7.1".


This feature is not supported with smtp header/body checks.

WARN optional text...

Log a "warning:" record with the optional text... (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.




BUGS




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 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 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 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.




EXAMPLES






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/main.cf:
    header_checks = pcre:/etc/postfix/header_checks.pcre

/etc/postfix/header_checks.pcre:
    /^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/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