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 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, 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
                  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)