.TH HEADER_CHECKS 5 .ad .fi .SH NAME header_checks \- Postfix built-in header/body inspection .SH SYNOPSIS .na .nf \fBheader_checks = pcre:/etc/postfix/header_checks\fR .br \fBmime_header_checks = pcre:/etc/postfix/mime_header_checks\fR .br \fBnested_header_checks = pcre:/etc/postfix/nested_header_checks\fR .sp \fBbody_checks = pcre:/etc/postfix/body_checks\fR .sp \fBpostmap -q "\fIstring\fB" pcre:/etc/postfix/\fIfilename\fR .br \fBpostmap -q - pcre:/etc/postfix/\fIfilename <\fIinputfile\fR .SH DESCRIPTION .ad .fi Postfix provides a simple built-in content inspection mechanism that examines incoming mail one message header or one message body line at a time. This feature is implemented by the Postfix \fBcleanup\fR(8) server. Postfix header or body_checks are designed to stop a flood of mail from worms or viruses. They are not meant to be a substitute for content filters that decode attachments and that do other sophisticated content analyses. Postfix supports four built-in content inspection classes: .IP \fBheader_checks\fR These are applied to each primary message header (except for the MIME related headers). .IP "\fBmime_header_checks\fR (default: \fB$header_checks\fR)" These are applied each MIME related message header only. .IP "\fBnested_header_checks\fR (default: \fB$header_checks\fR)" These are applied to each message header of attached email messages (except for the MIME related headers). .IP \fBbody_checks\fR These are applied to all other content, including multi-part message boundaries. .PP 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. .SH TABLE FORMAT .na .nf .ad .fi 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 \fBpcre\fR (Perl Compatible Regular Expression) tables, but the slower \fBregexp\fR (POSIX regular expressions) support is more widely available. Use the command \fBpostconf -m\fR 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 \fBpcre_table\fR(5) or \fBregexp_table\fR(5), respectively. .IP "\fB/\fIpattern\fB/\fIflags action\fR" When \fIpattern\fR matches the input string, execute the corresponding \fIaction\fR. See below for a list of possible actions. .IP "\fB!/\fIpattern\fB/\fIflags action\fR" When \fIpattern\fR does \fBnot\fR match the input string, execute the corresponding \fIaction\fR. .IP "\fBif /\fIpattern\fB/\fIflags\fR" .IP "\fBendif\fR" Match the input string against the patterns between \fBif\fR and \fBendif\fR, if and only if the input string also matches \fIpattern\fR. The \fBif\fR..\fBendif\fR can nest. .sp Note: do not prepend whitespace to patterns inside \fBif\fR..\fBendif\fR. .IP "\fBif !/\fIpattern\fB/\fIflags\fR" .IP "\fBendif\fR" Match the input string against the patterns between \fBif\fR and \fBendif\fR, if and only if the input string does \fBnot\fR match \fIpattern\fR. The \fBif\fR..\fBendif\fR can nest. .IP "blank lines and comments" Empty lines and whitespace-only lines are ignored, as are lines whose first non-whitespace character is a `#'. .IP "multi-line text" A pattern/action line starts with non-whitespace text. A line that starts with whitespace continues a logical line. .SH TABLE SEARCH ORDER .na .nf .ad .fi 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. .SH TEXT SUBSTITUTION .na .nf .ad .fi Substitution of substrings from the matched expression into the \fIaction\fR 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 \fB!\fR) return a result when the expression does not match, substitutions are not available for negated patterns. .SH ACTIONS .na .nf .ad .fi Action names are case insensitive. They are shown in upper case for consistency with other Postfix documentation. .IP "\fBDISCARD \fIoptional text...\fR Claim successful delivery and silently discard the message. Log the optional text if specified, otherwise log a generic message. .sp Note: this action disables further header or body_checks inspection of the current message and affects all recipients. .IP \fBDUNNO\fR 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. .sp For backwards compatibility reasons, Postfix also accepts \fBOK\fR but it is (and always has been) treated as \fBDUNNO\fR. .IP "\fBFILTER \fItransport:destination\fR" Write a content filter request to the queue file and inspect the next input line. After the complete message is received it will be sent through the specified external content filter. More information about external content filters is in the Postfix FILTER_README file. .sp Note: this action overrides the \fBmain.cf content_filter\fR setting, and affects all recipients of the message. In the case that multiple \fBFILTER\fR actions fire, only the last one is executed. .IP "\fBHOLD \fIoptional text...\fR" Arrange for the message to be placed on the \fBhold\fR queue, and inspect the next input line. The message remains on \fBhold\fR 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 \fBpostcat\fR(1) command, and can be destroyed or released with the \fBpostsuper\fR(1) command. .sp Note: this action affects all recipients of the message. .IP \fBIGNORE\fR Delete the current line from the input and inspect the next input line. .IP "\fBREDIRECT \fIuser@domain\fR" 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). .sp Note: this action overrides the \fBFILTER\fR action, and affects all recipients of the message. If multiple \fBREDIRECT\fR actions fire, only the last one is executed. .IP "\fBREJECT \fIoptional text...\fR Reject the entire message. Reply with \fIoptional text...\fR when the optional text is specified, otherwise reply with a generic error message. .sp Note: this action disables further header or body_checks inspection of the current message and affects all recipients. .IP "\fBWARN \fIoptional text...\fR Log a warning with the \fIoptional text...\fR (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. .SH BUGS .ad .fi 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. Message headers added by the \fBcleanup\fR(8) daemon itself are excluded from inspection. Examples of such message headers are \fBFrom:\fR, \fBTo:\fR, \fBMessage-ID:\fR, \fBDate:\fR. .SH CONFIGURATION PARAMETERS .na .nf .ad .fi .IP \fBbody_checks\fR Lookup tables with content filter rules for message body lines. These filters see one physical line at a time, in chunks of at most \fB$line_length_limit\fR bytes. .IP \fBbody_checks_size_limit\fP The amount of content per message body segment (attachment) that is subjected to \fB$body_checks\fR filtering. .IP \fBheader_checks\fR .IP "\fBmime_header_checks\fR (default: \fB$header_checks\fR)" .IP "\fBnested_header_checks\fR (default: \fB$header_checks\fR)" Lookup tables with content filter rules for message header lines: respectively, these are applied to the primary message headers (not including MIME headers), to the MIME headers anywhere in the message, and to the initial headers of attached messages. .sp Note: these filters see one logical message header at a time, even when a message header spans multiple lines. Message headers that are longer than \fB$header_size_limit\fR characters are truncated. .IP \fBdisable_mime_input_processing\fR 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 \fBheader_checks\fR is applied to all the primary message headers, and that \fBbody_checks\fR is applied to the remainder of the message. .sp Note: when used in this manner, \fBbody_checks\fR will process a multi-line message header one line at a time. .SH EXAMPLES .na .nf .ad .fi Header pattern to block attachments with bad file name extensions. .na .nf /^content-(type|disposition):.*name[[:space:]]*=.*\\.(exe|vbs)/ .ti +4 REJECT Bad attachment file name extension: $1 .ad .fi Body pattern to stop a specific HTML browser vulnerability exploit. .na .nf /^