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    Command:

    header_checks

    
    
    

    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.
    
           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.
    
                  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:
    
                  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:
    
                  ?      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".
    
           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

           ?      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.
           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
    
                                                                  HEADER_CHECKS(5)
    
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