1HEADER_CHECKS(5) File Formats Manual HEADER_CHECKS(5)
2
6 header_checks - Postfix built-in content inspection
7
9 header_checks = pcre:/etc/postfix/header_checks
10 mime_header_checks = pcre:/etc/postfix/mime_header_checks
11 nested_header_checks = pcre:/etc/postfix/nested_header_checks
12 body_checks = pcre:/etc/postfix/body_checks
13 milter_header_checks = pcre:/etc/postfix/milter_header_checks
15 smtp_header_checks = pcre:/etc/postfix/smtp_header_checks
17 smtp_mime_header_checks = pcre:/etc/postfix/smtp_mime_header_checks
18 smtp_nested_header_checks = pcre:/etc/postfix/smtp_nested_header_checks
19 smtp_body_checks = pcre:/etc/postfix/smtp_body_checks
20 postmap -q "string" pcre:/etc/postfix/filename
22 postmap -q - pcre:/etc/postfix/filename <inputfile
23
25 This document describes access control on the content of message head‐
26 ers and message body lines; it is implemented by the Postfix cleanup(8)
27 server before mail is queued. See access(5) for access control on
28 remote SMTP client information.
29 Each message header or message body line is compared against a list of
31 patterns. When a match is found the corresponding action is executed,
32 and the matching process is repeated for the next message header or
33 message body line.
34 Note: message headers are examined one logical header at a time, even
36 when a message header spans multiple lines. Body lines are always exam‐
37 ined one line at a time.
38 For examples, see the EXAMPLES section at the end of this manual page.
40 Postfix header or body_checks are designed to stop a flood of mail from
42 worms or viruses; they do not decode attachments, and they do not unzip
43 archives. See the documents referenced below in the README FILES sec‐
44 tion if you need more sophisticated content analysis.
45
47 Postfix implements the following four built-in content inspection
48 classes while receiving mail:
49 header_checks (default: empty)
51 These are applied to initial message headers (except for the
52 headers that are processed with mime_header_checks).
53 mime_header_checks (default: $header_checks)
55 These are applied to MIME related message headers only.
56 This feature is available in Postfix 2.0 and later.
58 nested_header_checks (default: $header_checks)
60 These are applied to message headers of attached email messages
61 (except for the headers that are processed with
62 mime_header_checks).
63 This feature is available in Postfix 2.0 and later.
65 body_checks
67 These are applied to all other content, including multi-part
68 message boundaries.
69 With Postfix versions before 2.0, all content after the initial
71 message headers is treated as body content.
72
74 Postfix supports a subset of the built-in content inspection classes
75 after the message is received:
76 milter_header_checks (default: empty)
78 These are applied to headers that are added with Milter applica‐
79 tions.
80 This feature is available in Postfix 2.7 and later.
82
84 Postfix supports all four content inspection classes while delivering
85 mail via SMTP.
86 smtp_header_checks (default: empty)
88 smtp_mime_header_checks (default: empty)
90 smtp_nested_header_checks (default: empty)
92 smtp_body_checks (default: empty)
94 These features are available in Postfix 2.5 and later.
95
97 With Postfix version 2.2 and earlier specify "postmap -fq" to query a
98 table that contains case sensitive patterns. By default, regexp: and
99 pcre: patterns are case insensitive.
100
102 This document assumes that header and body_checks rules are specified
103 in the form of Postfix regular expression lookup tables. Usually the
104 best performance is obtained with pcre (Perl Compatible Regular Expres‐
105 sion) tables. The regexp (POSIX regular expressions) tables are usually
106 slower, but more widely available. Use the command "postconf -m" to
107 find out what lookup table types your Postfix system supports.
108 The general format of Postfix regular expression tables is given below.
110 For a discussion of specific pattern or flags syntax, see pcre_table(5)
111 or regexp_table(5), respectively.
112 /pattern/flags action
114 When /pattern/ matches the input string, execute the correspond‐
115 ing action. See below for a list of possible actions.
116 !/pattern/flags action
118 When /pattern/ does not match the input string, execute the cor‐
119 responding action.
120 if /pattern/flags
122 endif Match the input string against the patterns between if and
124 endif, if and only if the same input string also matches /pat‐
125 tern/. The if..endif can nest.
126 Note: do not prepend whitespace to patterns inside if..endif.
128 if !/pattern/flags
130 endif Match the input string against the patterns between if and
132 endif, if and only if the same input string does not match /pat‐
133 tern/. The if..endif can nest.
134 blank lines and comments
136 Empty lines and whitespace-only lines are ignored, as are lines
137 whose first non-whitespace character is a `#'.
138 multi-line text
140 A pattern/action line starts with non-whitespace text. A line
141 that starts with whitespace continues a logical line.
142
144 For each line of message input, the patterns are applied in the order
145 as specified in the table. When a pattern is found that matches the
146 input line, the corresponding action is executed and then the next
147 input line is inspected.
148
150 Substitution of substrings from the matched expression into the action
151 string is possible using the conventional Perl syntax ($1, $2, etc.).
152 The macros in the result string may need to be written as ${n} or $(n)
153 if they aren't followed by whitespace.
154 Note: since negated patterns (those preceded by !) return a result when
156 the expression does not match, substitutions are not available for
157 negated patterns.
158
160 Action names are case insensitive. They are shown in upper case for
161 consistency with other Postfix documentation.
162 DISCARD optional text...
164 Claim successful delivery and silently discard the message. Log
165 the optional text if specified, otherwise log a generic message.
166 Note: this action disables further header or body_checks inspec‐
168 tion of the current message and affects all recipients. To dis‐
169 card only one recipient without discarding the entire message,
170 use the transport(5) table to direct mail to the discard(8) ser‐
171 vice.
172 This feature is available in Postfix 2.0 and later.
174 This feature is not supported with smtp header/body checks.
176 DUNNO Pretend that the input line did not match any pattern, and
178 inspect the next input line. This action can be used to shorten
179 the table search.
180 For backwards compatibility reasons, Postfix also accepts OK but
182 it is (and always has been) treated as DUNNO.
183 This feature is available in Postfix 2.1 and later.
185 FILTER transport:destination
187 After the message is queued, send the entire message through the
188 specified external content filter. The transport name specifies
189 the first field of a mail delivery agent definition in mas‐
190 ter.cf; the syntax of the next-hop destination is described in
191 the manual page of the corresponding delivery agent. More
192 information about external content filters is in the Postfix
193 FILTER_README file.
194 Note 1: do not use $number regular expression substitutions for
196 transport or destination unless you know that the information
197 has a trusted origin.
198 Note 2: this action overrides the main.cf content_filter set‐
200 ting, and affects all recipients of the message. In the case
201 that multiple FILTER actions fire, only the last one is exe‐
202 cuted.
203 Note 3: the purpose of the FILTER command is to override message
205 routing. To override the recipient's transport but not the
206 next-hop destination, specify an empty filter destination (Post‐
207 fix 2.7 and later), or specify a transport:destination that
208 delivers through a different Postfix instance (Postfix 2.6 and
209 earlier). Other options are using the recipient-dependent trans‐
210 port_maps or the sender-dependent sender_dependent_default‐
211 _transport_maps features.
212 This feature is available in Postfix 2.0 and later.
214 This feature is not supported with smtp header/body checks.
216 HOLD optional text...
218 Arrange for the message to be placed on the hold queue, and
219 inspect the next input line. The message remains on hold until
220 someone either deletes it or releases it for delivery. Log the
221 optional text if specified, otherwise log a generic message.
222 Mail that is placed on hold can be examined with the postcat(1)
224 command, and can be destroyed or released with the postsuper(1)
225 command.
226 Note: use "postsuper -r" to release mail that was kept on hold
228 for a significant fraction of $maximal_queue_lifetime or
229 $bounce_queue_lifetime, or longer. Use "postsuper -H" only for
230 mail that will not expire within a few delivery attempts.
231 Note: this action affects all recipients of the message.
233 This feature is available in Postfix 2.0 and later.
235 This feature is not supported with smtp header/body checks.
237 IGNORE Delete the current line from the input, and inspect the next
239 input line.
240 INFO optional text...
242 Log an "info:" record with the optional text... (or log a
243 generic text), and inspect the next input line. This action is
244 useful for routine logging or for debugging.
245 This feature is available in Postfix 2.8 and later.
247 PREPEND text...
249 Prepend one line with the specified text, and inspect the next
250 input line.
251 Notes:
253 · The prepended text is output on a separate line, immedi‐
255 ately before the input that triggered the PREPEND action.
256 · The prepended text is not considered part of the input
258 stream: it is not subject to header/body checks or
259 address rewriting, and it does not affect the way that
260 Postfix adds missing message headers.
261 · When prepending text before a message header line, the
263 prepended text must begin with a valid message header
264 label.
265 · This action cannot be used to prepend multi-line text.
267 This feature is available in Postfix 2.1 and later.
269 This feature is not supported with milter_header_checks.
271 REDIRECT user@domain
273 Write a message redirection request to the queue file, and
274 inspect the next input line. After the message is queued, it
275 will be sent to the specified address instead of the intended
276 recipient(s).
277 Note: this action overrides the FILTER action, and affects all
279 recipients of the message. If multiple REDIRECT actions fire,
280 only the last one is executed.
281 This feature is available in Postfix 2.1 and later.
283 This feature is not supported with smtp header/body checks.
285 REPLACE text...
287 Replace the current line with the specified text, and inspect
288 the next input line.
289 This feature is available in Postfix 2.2 and later. The descrip‐
291 tion below applies to Postfix 2.2.2 and later.
292 Notes:
294 · When replacing a message header line, the replacement
296 text must begin with a valid header label.
297 · The replaced text remains part of the input stream.
299 Unlike the result from the PREPEND action, a replaced
300 message header may be subject to address rewriting and
301 may affect the way that Postfix adds missing message
302 headers.
303 REJECT optional text...
305 Reject the entire message. Reply with optional text... when the
306 optional text is specified, otherwise reply with a generic error
307 message.
308 Note: this action disables further header or body_checks inspec‐
310 tion of the current message and affects all recipients.
311 Postfix version 2.3 and later support enhanced status codes.
313 When no code is specified at the beginning of optional text...,
314 Postfix inserts a default enhanced status code of "5.7.1".
315 This feature is not supported with smtp header/body checks.
317 WARN optional text...
319 Log a "warning:" record with the optional text... (or log a
320 generic text), and inspect the next input line. This action is
321 useful for debugging and for testing a pattern before applying
322 more drastic actions.
323
325 Empty lines never match, because some map types mis-behave when given a
326 zero-length search string. This limitation may be removed for regular
327 expression tables in a future release.
328 Many people overlook the main limitations of header and body_checks
330 rules.
331 · These rules operate on one logical message header or one body
333 line at a time. A decision made for one line is not carried over
334 to the next line.
335 · If text in the message body is encoded (RFC 2045) then the rules
337 need to be specified for the encoded form.
338 · Likewise, when message headers are encoded (RFC 2047) then the
340 rules need to be specified for the encoded form.
341 Message headers added by the cleanup(8) daemon itself are excluded from
343 inspection. Examples of such message headers are From:, To:, Message-
344 ID:, Date:.
345 Message headers deleted by the cleanup(8) daemon will be examined
347 before they are deleted. Examples are: Bcc:, Content-Length:, Return-
348 Path:.
349
351 body_checks
352 Lookup tables with content filter rules for message body lines.
353 These filters see one physical line at a time, in chunks of at
354 most $line_length_limit bytes.
355 body_checks_size_limit
357 The amount of content per message body segment (attachment) that
358 is subjected to $body_checks filtering.
359 header_checks
361 mime_header_checks (default: $header_checks)
363 nested_header_checks (default: $header_checks)
365 Lookup tables with content filter rules for message header
366 lines: respectively, these are applied to the initial message
367 headers (not including MIME headers), to the MIME headers any‐
368 where in the message, and to the initial headers of attached
369 messages.
370 Note: these filters see one logical message header at a time,
372 even when a message header spans multiple lines. Message headers
373 that are longer than $header_size_limit characters are trun‐
374 cated.
375 disable_mime_input_processing
377 While receiving mail, give no special treatment to MIME related
378 message headers; all text after the initial message headers is
379 considered to be part of the message body. This means that
380 header_checks is applied to all the initial message headers, and
381 that body_checks is applied to the remainder of the message.
382 Note: when used in this manner, body_checks will process a
384 multi-line message header one line at a time.
385
387 Header pattern to block attachments with bad file name extensions. For
388 convenience, the PCRE /x flag is specified, so that there is no need to
389 collapse the pattern into a single line of text. The purpose of the
390 [[:xdigit:]] sub-expressions is to recognize Windows CLSID strings.
391 /etc/postfix/main.cf:
393 header_checks = pcre:/etc/postfix/header_checks.pcre
394 /etc/postfix/header_checks.pcre:
396 /^Content-(Disposition|Type).*name\s*=\s*"?(.*(\.|=2E)(
397 ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
398 hlp|ht[at]|
399 inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
400 \{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
401 ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
402 vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x
403 REJECT Attachment name "$2" may not end with ".$4"
404 Body pattern to stop a specific HTML browser vulnerability exploit.
406 /etc/postfix/main.cf:
408 body_checks = regexp:/etc/postfix/body_checks
409 /etc/postfix/body_checks:
411 /^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
412 REJECT IFRAME vulnerability exploit
413
415 cleanup(8), canonicalize and enqueue Postfix message
416 pcre_table(5), format of PCRE lookup tables
417 regexp_table(5), format of POSIX regular expression tables
418 postconf(1), Postfix configuration utility
419 postmap(1), Postfix lookup table management
420 postsuper(1), Postfix janitor
421 postcat(1), show Postfix queue file contents
422 RFC 2045, base64 and quoted-printable encoding rules
423 RFC 2047, message header encoding for non-ASCII text
424
426 Use "postconf readme_directory" or "postconf html_directory" to locate
427 this information.
428 DATABASE_README, Postfix lookup table overview
429 CONTENT_INSPECTION_README, Postfix content inspection overview
430 BUILTIN_FILTER_README, Postfix built-in content inspection
431 BACKSCATTER_README, blocking returned forged mail
432
434 The Secure Mailer license must be distributed with this software.
435
437 Wietse Venema
438 IBM T.J. Watson Research
439 P.O. Box 704
440 Yorktown Heights, NY 10598, USA
441 HEADER_CHECKS(5)