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Editing: 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 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, 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 # After the message is queued, send the entire mes- # sage 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 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. # # 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 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. 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. # # 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. # # 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 # # HEADER_CHECKS(5)
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