Provided by: postfix_2.7.0-1_i386 bug

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 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 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,  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
              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, 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
              After the message is queued, send the entire message 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 substitutions  for
              transport  or  destination  unless you know that the information
              has a trusted origin.

              Note  2:  this  action  overrides  the  main.cf   content_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  recipient’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  sender-dependent  sender_dependent_default_transport_maps features.

              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
              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
              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
              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.

       ·      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 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
              anywhere 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
              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 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.

       /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

       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)