HEADER_CHECKS(5) | HEADER_CHECKS(5) |
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.
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. |
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). |
body_checks |
These are applied to all other content, including multi-part message boundaries. |
milter_header_checks (default: empty) |
These are applied to headers that are added with Milter applications. |
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. |
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. |
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. |
Note: since negated patterns (those preceded by !) return a result when the expression does not match, substitutions are not available for negated patterns.
DISCARD optional text... |
Claim successful delivery and silently discard the message. Log the optional text if specified, otherwise log a generic message. |
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. |
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. |
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. |
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. |
PREPEND text... |
Prepend one line with the specified text, and inspect the next input line. |
• | The prepended text is output on a separate line, immediately before the input that triggered the PREPEND action. |
• | 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. |
• | When prepending text before a message header line, the prepended text must begin with a valid message header label. |
• | 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). |
REPLACE text... |
Replace the current line with the specified text, and inspect the next input line. |
• | When replacing a message header line, the replacement text must begin with a valid header label. |
• | 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. |
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. |
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. 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 need to be 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:, Content-Length:, Return-Path:.
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: $header_checks) |
Lookup tables with content filter rules for message header lines: respectively, these are applied to the initial message headers (not including MIME headers), to the MIME headers anywhere in the message, and to the initial headers of attached messages. |
disable_mime_input_processing |
While receiving mail, give no special treatment to MIME related message headers; all text after the initial message headers is considered to be part of the message body. This means that header_checks is applied to all the initial message headers, and that body_checks is applied to the remainder of the message. |
/etc/postfix/main.cf:
header_checks = pcre:/etc/postfix/header_checks.pcre
/etc/postfix/header_checks.pcre:
/^Content-(Disposition|Type).*name\s*=\s*"?(.*(\.|=2E)(
ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
hlp|ht[at]|
inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
\{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/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
August 25, 2011 |