Updated: 2022/Sep/29
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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 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 If the input string matches /pattern/, then match that input
string against the patterns between if and endif. The if..endif
can nest.
Note: do not prepend whitespace to patterns inside if..endif.
if !/pattern/flags
endif If the input string does not match /pattern/, then match that
input string against the patterns between if and endif. 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.
BCC user@domain
Add the specified address as a BCC recipient, and inspect the
next input line. The address must have a local part and domain
part. The number of BCC addresses that can be added is limited
only by the amount of available storage space.
Note 1: the BCC address is added as if it was specified with
NOTIFY=NONE. The sender will not be notified when the BCC
address is undeliverable, as long as all down-stream software
implements RFC 3461.
Note 2: this ignores duplicate addresses (with the same delivery
status notification options).
This feature is available in Postfix 3.0 and later.
This feature is not supported with smtp header/body checks.
DISCARD optional text...
Claim successful delivery and silently discard the message. Do
not inspect the remainder of the input 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
Override the content_filter parameter setting, and inspect the
next input line. 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. See STRIP for an alternative that logs the action.
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.
PASS optional text...
Log a "pass:" record with the optional text... (or log a generic
text), and turn off header, body, and Milter inspection for the
remainder of this message.
Note: this feature relies on trust in information that is easy
to forge.
This feature is available in Postfix 3.2 and later.
This feature is not supported with smtp header/body checks.
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. Do not inspect the remainder of the
input 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.
STRIP optional text...
Log a "strip:" record with the optional text... (or log a
generic text), delete the input line from the input, and inspect
the next input line. See IGNORE for a silent alternative.
This feature is available in Postfix 3.2 and later.
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
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA
HEADER_CHECKS(5)