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.
- 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. 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.
- 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