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  If  the  input  string  matches /pattern/, then match that input
                  string against the patterns between if and endif.  The if..endif
                  can nest.

                  Note: do not prepend whitespace to patterns inside if..endif.

           if !/pattern/flags

           endif  If  the  input  string does not match /pattern/, then match that
                  input string against the patterns  between  if  and  endif.  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.

           BCC user@domain
                  Add the specified address as a BCC recipient,  and  inspect  the
                  next  input  line. The address must have a local part and domain
                  part. The number of BCC addresses that can be added  is  limited
                  only by the amount of available storage space.

                  Note  1:  the  BCC  address is added as if it was specified with
                  NOTIFY=NONE. The sender  will  not  be  notified  when  the  BCC
                  address  is  undeliverable,  as long as all down-stream software
                  implements RFC 3461.

                  Note 2: this ignores duplicate addresses (with the same delivery
                  status notification options).

                  This feature is available in Postfix 3.0 and later.

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

           DISCARD optional text...
                  Claim  successful delivery and silently discard the message.  Do
                  not inspect  the  remainder  of  the  input  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
                  Override the content_filter parameter setting, and  inspect  the
                  next  input  line.  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 desti-
                  nation  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. Do not inspect the remainder  of  the
                  input  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:, Mes-
           sage-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)