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. See STRIP for an alternative that logs the action.

       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.

       PASS optional text...
              Log a "pass:" record with the optional text... (or log a generic
              text),  and turn off header, body, and Milter inspection for the
              remainder of this message.

              Note: this feature relies on trust in information that  is  easy
              to forge.

              This feature is available in Postfix 3.2 and later.

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

       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.

       STRIP optional text...
              Log  a  "strip:"  record  with  the  optional  text... (or log a
              generic text), delete the input line from the input, and inspect
              the next input line. See IGNORE for a silent alternative.

              This feature is available in Postfix 3.2 and later.

       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

       Wietse Venema
       Google, Inc.
       111 8th Avenue
       New York, NY 10011, USA

                                                              HEADER_CHECKS(5)