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HEADER_CHECKS(5)					      HEADER_CHECKS(5)

NAME
       header_checks - Postfix built-in content inspection

SYNOPSIS
       header_checks = pcre:$config_directory/header_checks
       mime_header_checks = pcre:$config_directory/mime_header_checks
       nested_header_checks = pcre:$config_directory/nested_header_checks
       body_checks = pcre:$config_directory/body_checks

       milter_header_checks = pcre:$config_directory/milter_header_checks

       smtp_header_checks = pcre:$config_directory/smtp_header_checks
       smtp_mime_header_checks = pcre:$config_directory/smtp_mime_header_checks
       smtp_nested_header_checks = pcre:$config_directory/smtp_nested_header_checks
       smtp_body_checks = pcre:$config_directory/smtp_body_checks

       postmap -q "string" pcre:$config_directory/filename
       postmap -q - pcre:$config_directory/filename <inputfile

DESCRIPTION
       This  document describes access control on the content of message head‐
       ers 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 exam‐
       ined 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 sec‐
       tion 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 applica‐
	      tions.

	      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 Expres‐
       sion) 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 correspond‐
	      ing action. See below for a list of possible actions.

       !/pattern/flags action
	      When /pattern/ does not match the input string, execute the cor‐
	      responding 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 /pat‐
	      tern/. 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 /pat‐
	      tern/. 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, otherwise log a generic message.

	      Note: this action disables further header or body_checks inspec‐
	      tion of the current message and affects all recipients.  To dis‐
	      card only one recipient without discarding the  entire  message,
	      use the transport(5) table to direct mail to the discard(8) ser‐
	      vice.

	      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
	      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 mas‐
	      ter.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 set‐
	      ting, and affects all recipients of the  message.	 In  the  case
	      that  multiple  FILTER  actions  fire, only the last one is exe‐
	      cuted.

	      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 (Post‐
	      fix  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 trans‐
	      port_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, immedi‐
		     ately 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 descrip‐
	      tion 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. 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 inspec‐
	      tion 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 any‐
	      where 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 trun‐
	      cated.

       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.

       /usr/local/etc/postfix/main.cf:
	   header_checks = pcre:$config_directory/header_checks.pcre

       /usr/local/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.

       /usr/local/etc/postfix/main.cf:
	   body_checks = regexp:$config_directory/body_checks

       /usr/local/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

							      HEADER_CHECKS(5)
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