HEADER_CHECKS(5)                                              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 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.

       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.

       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 content.

       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.

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.

       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
              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  Post-
              fix 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.

       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.

       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, immedi-
                     ately 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 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.

       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:

              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, 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".

       WARN optional text...
              Log  a  warning with the optional text... (or log a generic mes-
              sage), 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.

       o      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.

       o      If text in the message body is encoded (RFC 2045) then the rules
              need to be specified for the encoded form.

       o      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.

       /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



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