# HEADER_CHECKS(5) HEADER_CHECKS(5)
#
# NAME
# header_checks - Postfix built-in header/body 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
#
# postmap -fq "string" pcre:/etc/postfix/filename
# postmap -fq - pcre:/etc/postfix/filename <inputfile
#
# DESCRIPTION
# Postfix provides a simple built-in content inspection
# mechanism that examines incoming mail one message header
# or one message body line at a time. Each input is compared
# against a list of patterns, and when a match is found the
# corresponding action is executed. This feature is imple-
# mented by the Postfix cleanup(8) server.
#
# 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.
#
# Postfix supports four built-in content inspection classes:
#
# header_checks
# 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.
#
# 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.
#
# 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 pos-
# sible actions.
#
# !/pattern/flags action
# When pattern does not match the input string, exe-
# cute the corresponding action.
#
# if /pattern/flags
#
# endif Match the input string against the patterns between
# if and endif, if and only if the 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 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.
#
# This feature is available in Postfix 2.0 and later.
#
# 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
# Write a content filter request to the queue file
# and inspect the next input line. After the com-
# plete message is received it will be sent through
# the specified external content filter. More infor-
# mation about external content filters is in the
# Postfix FILTER_README file.
#
# Note: 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.
#
# This feature is available in Postfix 2.0 and later.
#
# 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: this action affects all recipients of the
# message.
#
# This feature is available in Postfix 2.0 and later.
#
# IGNORE Delete the current line from the input and inspect
# the next input line.
#
# PREPEND text...
# Prepend one line with the specified text and
# inspect the next input line.
#
# Note: the prepended text is output immediately
# before the input that triggered the PREPEND action.
# A body action cannot prepend a message header.
#
# Note: this action cannot be used to prepend multi-
# line text.
#
# This feature is available in Postfix 2.1 and later.
#
# 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.
#
# 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.
#
# WARN optional text...
# Log a warning with the optional text... (or log a
# generic message) and inspect the next input line.
# This action is useful for debugging and for testing
# a pattern before applying more drastic actions.
#
# BUGS
# 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, and 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 have to 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:, 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.
#
# /etc/postfix/main.cf:
# header_checks = regexp:/etc/postfix/header_checks
#
# /etc/postfix/header_checks:
# /^content-(type|disposition):.*name[[:space:]]*=.*\.(exe|vbs)/
# REJECT Bad attachment file name extension: $2
#
# Body pattern to stop a specific HTML browser vulnerability
# exploit.
#
# /etc/postfix/main.cf:
# body_checks = regexp:/etc/postfix/body_checks
#
# /etc/postfix/body_checks:
# /^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
# 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_direc-
# tory" 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)