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HEADER_CHECKS(5) | File Formats Manual | HEADER_CHECKS(5) |
NAME¶
header_checks - Postfix built-in content inspectionSYNOPSIS¶
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_checksmilter_header_checks = pcre:/etc/postfix/milter_header_checkssmtp_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_checkspostmap -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.
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.
- 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).
- body_checks
- These are applied to all other content, including
multi-part message boundaries.
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.
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, 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 supports.
- /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.
- 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.
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.
- 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.
- 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.
- 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.
- 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.
- PREPEND text...
- Prepend one line with the specified text, and inspect the
next input line.
- •
- 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.
- 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).
- REPLACE text...
- Replace the current line with the specified text, and
inspect the next input line.
- •
- 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.
- 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.
- •
- 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.
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: $header_checks)
- Lookup tables with content filter rules for message header
lines: respectively, these are applied to the initial message headers (not
including MIME headers), to the MIME headers anywhere in the message, and
to the initial headers of attached messages.
- disable_mime_input_processing
- While receiving mail, give no special treatment to MIME
related message headers; all text after the initial message headers is
considered to be part of the message body. This means that
header_checks is applied to all the initial message headers, and
that body_checks is applied to the remainder of the message.
EXAMPLES¶
Header pattern to block attachments with bad file name extensions. For convenience, the PCRE /x flag is specified, so that there is no need to collapse the pattern into a single line of text. The purpose of the [[:xdigit:]] sub-expressions is to recognize Windows CLSID strings.
/etc/postfix/main.cf: header_checks = pcre:/etc/postfix/header_checks.pcre /etc/postfix/header_checks.pcre: /^Content-(Disposition|Type).*name\s*=\s*"?(.*(\.|=2E)( ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe| hlp|ht[at]| inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws| \{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}| ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf| vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x REJECT Attachment name "$2" may not end with ".$4"
/etc/postfix/main.cf: body_checks = regexp:/etc/postfix/body_checks /etc/postfix/body_checks: /^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/ 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