Postfix manual - header_checks(5)

    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.

                  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 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.

           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)