HEADER_CHECKS(5) 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
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.
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 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:
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 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.
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, but
the slower regexp (POSIX regular expressions) support is
more widely available. Use the command "postconf -m" 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 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, exe-
cute 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, oth-
erwise 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 recip-
ient 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.
DUNNO 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 OK but it is (and always has been) treated
as DUNNO.
This feature is available in Postfix 2.1 and later.
FILTER transport:destination
Write a content filter request to the queue file,
and inspect the next input line. After the com-
plete message is received it will be sent through
the specified external content filter. More infor-
mation about external content filters is in the
Postfix FILTER_README file.
Note: this action overrides the content_filter set-
ting, and affects all recipients of the message. In
the case that multiple 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 mes-
sage 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 $maxi-
mal_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.
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.
Notes:
o The prepended text is output on a separate
line, immediately before the input that
triggered the PREPEND action.
o 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.
o When prepending text before a message header
line, the prepended text must begin with a
valid message header label.
o 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 exe-
cuted.
This feature is available in Postfix 2.1 and later.
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:
o When replacing a message header line, the
replacement text must begin with a valid
header label.
o 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, other-
wise 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 sta-
tus codes. When no code is specified at the begin-
ning of optional text..., Postfix inserts a default
enhanced status code of "5.7.1".
WARN optional text...
Log a warning with the optional text... (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.
BUGS
Many people overlook the main limitations of header and
body_checks rules.
o 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.
o If text in the message body is encoded (RFC 2045)
then the rules need to be specified for the encoded
form.
o 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:, Con-
tent-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 fil-
tering.
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 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
$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.
/etc/postfix/main.cf:
header_checks = regexp:/etc/postfix/header_checks
/etc/postfix/header_checks:
/^content-(type|disposition):.*name[[:space:]]*=.*\.(exe|vbs)/
REJECT Bad attachment file name extension: $2
Body pattern to stop a specific HTML browser vulnerability
exploit.
/etc/postfix/main.cf:
body_checks = regexp:/etc/postfix/body_checks
/etc/postfix/body_checks:
/^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
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
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)