Provided by: postfix_2.8.5-2~build1_i386 bug

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 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,
                     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)