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HEADER_CHECKS(5)                                              HEADER_CHECKS(5)

NAME
       header_checks - Postfix built-in header/body 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 -fq "string" pcre:/etc/postfix/filename
       postmap -fq - pcre:/etc/postfix/filename <inputfile

DESCRIPTION
       Postfix  provides  a  simple  built-in  content inspection
       mechanism that examines incoming mail one  message  header
       or one message body line at a time. Each input is compared
       against a list of patterns, and when a match is found  the
       corresponding  action is executed.  This feature is imple-
       mented by the Postfix cleanup(8) server.

       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  attach-
       ments,  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:

       header_checks
              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 con-
              tent.

       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.

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  sup-
       ports.

       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 pos-
              sible actions.

       !/pattern/flags action
              When pattern does not match the input string,  exe-
              cute the corresponding action.

       if /pattern/flags

       endif  Match the input string against the patterns between
              if and endif, if and only if the input string  also
              matches pattern. 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 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.

       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,  oth-
              erwise 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 recip-
              ient 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.

       DUNNO  Pretend  that the input line did not match any pat-
              tern, 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
              Write a content filter request to  the  queue  file
              and  inspect  the  next input line.  After the com-
              plete message is received it will be  sent  through
              the specified external content filter.  More infor-
              mation about external content  filters  is  in  the
              Postfix FILTER_README file.

              Note:   this  action  overrides  the  main.cf  con-
              tent_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.

       HOLD optional text...
              Arrange  for  the  message to be placed on the hold
              queue, and inspect the next input line.   The  mes-
              sage  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 $maxi-
              mal_queue_lifetime  or  $bounce_queue_lifetime,  or
              longer.

              Note:  this  action  affects  all recipients of the
              message.

              This feature is available in Postfix 2.0 and later.

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

       PREPEND text...
              Prepend  one  line  with  the  specified  text  and
              inspect the next input line.

              Notes:

              o      The  prepended  text is output on a separate
                     line,  immediately  before  the  input  that
                     triggered the PREPEND action.

              o      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.

              o      When prepending text before a message header
                     line, the prepended text must begin  with  a
                     valid message header label.

              o      This action cannot be used to prepend multi-
                     line text.

              This feature is available in Postfix 2.1 and later.

       REDIRECT [email protected]
              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  exe-
              cuted.

              This feature is available in Postfix 2.1 and later.

       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:

              o      When  replacing  a  message header line, the
                     replacement text must  begin  with  a  valid
                     header label.

              o      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, other-
              wise 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 sta-
              tus codes.  When no code is specified at the begin-
              ning of optional text..., Postfix inserts a default
              enhanced status code of "5.7.1".

       WARN optional text...
              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
       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, and 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  have  to 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:, Con-
       tent-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 fil-
              tering.

       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 mes-
              sage, and to the initial headers of  attached  mes-
              sages.

              Note:  these filters see one logical message header
              at a time, even when a message header spans  multi-
              ple  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.

       /etc/postfix/main.cf:
           header_checks = regexp:/etc/postfix/header_checks

       /etc/postfix/header_checks:
           /^content-(type|disposition):.*name[[:space:]]*=.*\.(exe|vbs)/
               REJECT Bad attachment file name extension: $2

       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
       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)