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.

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 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
              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 content_filter set-
              ting, 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. 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
       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 have to 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.

       /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)