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HEADER_CHECKS(5)              File Formats Manual             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 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 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.

              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 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.

              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,
                     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 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
              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, 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.

       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
              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)