Postfix manual - header_checks(5)

    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 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.

           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.

           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.

    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 content.

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

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

                  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 Expres-
           sion) tables. The regexp (POSIX regular expressions) tables are usually
           slower, but 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.

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

           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 mas-
                  ter.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 set-
                  ting, and affects all recipients of the  message.  In  the  case
                  that  multiple  FILTER  actions  fire, only the last one is exe-
                  cuted.

                  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 (Post-
                  fix  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 trans-
                  port_maps  or  the  sender-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 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.

                  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, 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.

                  This feature is not supported with milter_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 executed.

                  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 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".

                  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:,  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.  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
           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)