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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 con-
       trol 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 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 content.

       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 sensi-tive sensitive
       tive 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  avail-able. available.
       able.   Use  the  command  "postconf -m" to find out what lookup table types your Postfix system sup-ports. supports.
       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 con-tinues continues
              tinues 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  speci-fied, specified,
              fied, otherwise 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 recipient 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 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 Postfix FILTER_README file.

              Note: this action overrides the content_filter setting, and affects all recipients of the mes-sage. message.
              sage. 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:

                    The  prepended  text  is  output  on a separate line, immediately before the input that
                     triggered the PREPEND action.

                    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.

                    When prepending text before a message header line, the prepended text must begin with a
                     valid message header label.

                    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 recip-ient(s). recipient(s).
              ient(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 description 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 inspection 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 message), and inspect the next input
              line. This action is useful for debugging and for testing a pattern before applying more dras-tic drastic
              tic 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.

             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.

             If  text in the message body is encoded (RFC 2045) then the rules need to be 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:, Content-Length:, Return-Path:.

CONFIGURATION PARAMETERS
       body_checks
              Lookup  tables with content filter rules for message body lines.  These filters see one physi-cal physical
              cal 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 anywhere
              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. truncated.
              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)

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