Postfix manual - header_checks(5)

    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 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 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.

    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  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
                  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.

                  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 /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
                  After the message is queued, send the  entire  mes-
                  sage 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.

                  Note  1: do not use $number regular expression sub-
                  stitutions for transport or destination unless  you
                  know that the information has a trusted origin.

                  Note  2:  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.

                  Note 3: the purpose of the  FILTER  command  is  to
                  override  message routing.  To override the recipi-
                  ent's transport but not the  next-hop  destination,
                  specify  an  empty  filter destination (Postfix 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 transport_maps or the sen-
                  der-dependent   sender_dependent_default_transport-
                  _maps features.

                  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. 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,  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 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  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
           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:, 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.  For convenience, the PCRE /x flag  is  speci-
           fied,  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
           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)