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HEADER_CHECKS(5)              File Formats Manual             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.

       /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
       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)