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

       milter_header_checks = pcre:/etc/postfix/milter_header_checks

       smtp_header_checks = pcre:/etc/postfix/smtp_header_checks
       smtp_mime_header_checks = pcre:/etc/postfix/smtp_mime_header_checks
       smtp_nested_header_checks = pcre:/etc/postfix/smtp_nested_header_checks
       smtp_body_checks = pcre:/etc/postfix/smtp_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.

       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.

       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.

FILTERS WHILE RECEIVING MAIL
       Postfix implements the  following  four  built-in  content
       inspection classes while receiving mail:

       header_checks (default: empty)
              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.

FILTERS AFTER RECEIVING MAIL
       Postfix  supports a subset of the built-in content inspec-
       tion classes after the message is received:

       milter_header_checks (default: empty)
              These are applied to headers that  are  added  with
              Milter applications.

              This feature is available in Postfix 2.7 and later.

FILTERS WHILE DELIVERING MAIL
       Postfix supports all four content inspection classes while
       delivering mail via SMTP.

       smtp_header_checks (default: empty)

       smtp_mime_header_checks (default: empty)

       smtp_nested_header_checks (default: empty)

       smtp_body_checks (default: empty)
              These  features  are  available  in Postfix 2.5 and
              later.

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 Expression) tables. The
       regexp  (POSIX  regular  expressions)  tables  are usually
       slower, but more widely available.  Use the command "post-
       conf  -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 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  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.

              This feature is not supported with smtp header/body
              checks.

       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
              After the message is queued, send the  entire  mes-
              sage through the specified external content filter.
              The transport name specifies the first field  of  a
              mail  delivery  agent  definition in master.cf; the
              syntax of the next-hop destination is described  in
              the  manual  page  of  the  corresponding  delivery
              agent.  More  information  about  external  content
              filters is in the Postfix FILTER_README file.

              Note  1: do not use $number regular expression sub-
              stitutions for transport or destination unless  you
              know that the information has a trusted origin.

              Note  2:  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.

              Note 3: the purpose of the  FILTER  command  is  to
              override  message routing.  To override the recipi-
              ent's transport but not the  next-hop  destination,
              specify  an  empty  filter destination (Postfix 2.7
              and later), or specify a transport:destination that
              delivers   through  a  different  Postfix  instance
              (Postfix 2.6 and earlier). Other options are  using
              the  recipient-dependent transport_maps or the sen-
              der-dependent   sender_dependent_default_transport-
              _maps features.

              This feature is available in Postfix 2.0 and later.

              This feature is not supported with smtp header/body
              checks.

       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.

              This feature is not supported with smtp header/body
              checks.

       IGNORE Delete the current line from the input, and inspect
              the next input line.

       INFO optional text...
              Log an "info:" record with the optional text... (or
              log  a  generic  text),  and inspect the next input
              line. This action is useful for routine logging  or
              for debugging.

              This feature is available in Postfix 2.8 and later.

       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.

              This   feature   is   not   supported   with   mil-
              ter_header_checks.

       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.

              This feature is not supported with smtp header/body
              checks.

       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".

              This feature is not supported with smtp header/body
              checks.

       WARN optional text...
              Log a "warning:" record with the  optional  text...
              (or log a generic text), 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:, 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.  For convenience, the PCRE /x flag  is  speci-
       fied,  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.

       /etc/postfix/main.cf:
           header_checks = pcre:/etc/postfix/header_checks.pcre

       /etc/postfix/header_checks.pcre:
           /^Content-(Disposition|Type).*name\s*=\s*"?(.*(\.|=2E)(
             ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
             hlp|ht[at]|
             inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
             \{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
             ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
             vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x
               REJECT Attachment name "$2" may not end with ".$4"

       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)