This is the default example configuration of header_checks provided by Postfix. This config file was generated by Postfix running on Alpine 3.
It is located under: /etc/postfix/header_checks
# 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 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 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.
#
# 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 con-
# tent.
#
# FILTERS AFTER RECEIVING MAIL
# Postfix supports a subset of the built-in content inspec-
# tion 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. The
# regexp (POSIX regular expressions) tables are usually
# slower, but more widely available. Use the command "post-
# conf -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 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 spec-
# ified 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, oth-
# erwise 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 recip-
# ient 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 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
# Override the content_filter parameter setting, and
# inspect the next input line. After the message is
# queued, send the entire message through the speci-
# fied 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 sub-
# stitutions for transport or destination unless you
# know that the information has a trusted origin.
#
# Note 2: 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.
#
# Note 3: the purpose of the FILTER command is to
# override message routing. To override the recipi-
# ent'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 sen-
# der-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 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: use "postsuper -r" to release mail that was
# kept on hold for a significant fraction of $maxi-
# mal_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 mes-
# sage.
#
# 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, 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 mil-
# ter_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 exe-
# cuted.
#
# 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. Do not inspect the
# remainder of the input message. Reply with
# optional text... when the optional text is speci-
# fied, 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 sta-
# tus codes. When no code is specified at the begin-
# ning 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:, 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
# $header_size_limit characters are truncated.
#
# disable_mime_input_processing
# While receiving mail, give no special treatment to
# MIME related message headers; all text after the
# initial message headers is considered to be part of
# the message body. This means that header_checks is
# applied to all the initial message headers, and
# that body_checks is applied to the remainder of the
# message.
#
# Note: when used in this manner, body_checks will
# process a multi-line message header one line at a
# time.
#
# EXAMPLES
# Header pattern to block attachments with bad file name
# extensions. For convenience, the PCRE /x flag is speci-
# fied, so that there is no need to collapse the pattern
# into a single line of text. The purpose of the
# [[:xdigit:]] sub-expressions is to recognize Windows CLSID
# strings.
#
# /etc/postfix/main.cf:
# header_checks = pcre:/etc/postfix/header_checks.pcre
#
# /etc/postfix/header_checks.pcre:
# /^Content-(Disposition|Type).*name\s*=\s*"?([^;]*(\.|=2E)(
# ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
# hlp|ht[at]|
# inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
# \{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
# ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
# vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/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_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
#
# Wietse Venema
# Google, Inc.
# 111 8th Avenue
# New York, NY 10011, USA
#
# HEADER_CHECKS(5)
Download the raw file with wget or curl
wget -O header_checks.example http://exampleconfig.com/static/raw/postfix/alpine3/etc/postfix/header_checks
curl http://exampleconfig.com/static/raw/postfix/alpine3/etc/postfix/header_checks > header_checks.example
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