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  4. header_checks(5)

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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

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 attachments, 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:

These are applied to initial message headers (except for the headers that are processed with mime_header_checks).
These are applied to MIME related message headers only.

This feature is available in Postfix 2.0 and later.

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.

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.

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
    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.

  

  

  

  
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.

  

  

  

  

  
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.

  

  
Empty lines and whitespace-only lines are ignored, as are lines whose
      first non-whitespace character is a `#'.

  

  
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.



  

  
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.

  

  

  
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.

  

  

  
Write a content filter request to the queue file, and inspect the next
      input line. After the complete message is received it will be sent through
      the specified external content filter. More information about external
      content filters is in the Postfix FILTER_README file.
    
Note: this action overrides the content_filter setting,
        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.

  

  

  
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.

  

  

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

  

  
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.

  

  
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.

  

  

  
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 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".

  

  

  
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




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





  

  
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.

  

  
The amount of content per message body segment (attachment) that is
      subjected to $body_checks filtering.

  

  

  

  

  

  
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.

  

  

  
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