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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 con-
       trol 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  attachments, 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 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 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 sensi-tive sensitive
       tive 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  avail-able. available.
       able.   Use  the  command  "postconf -m" to find out what lookup table types your Postfix system sup-ports. supports.
       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 con-tinues continues
              tinues 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  speci-fied, specified,
              fied, 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.

              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
              master.cf;  the syntax of the next-hop destination is described in the manual page of the cor-responding corresponding
              responding 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.

              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:

              •      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.

              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 recip-ient(s). recipient(s).
              ient(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 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".

              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.

       •      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  physi-cal physical
              cal 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 anywhere
              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. truncated.
              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 pur-pose purpose
       pose 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)