1HEADER_CHECKS(5)              File Formats Manual             HEADER_CHECKS(5)
2
3
4

NAME

6       header_checks - Postfix built-in content inspection
7

SYNOPSIS

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
14       milter_header_checks = pcre:/etc/postfix/milter_header_checks
15
16       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
21       postmap -q "string" pcre:/etc/postfix/filename
22       postmap -q - pcre:/etc/postfix/filename <inputfile
23

DESCRIPTION

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
30       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
35       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
39       For examples, see the EXAMPLES section at the end of this manual page.
40
41       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

FILTERS WHILE RECEIVING MAIL

47       Postfix implements  the  following  four  built-in  content  inspection
48       classes while receiving mail:
49
50       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
54       mime_header_checks (default: $header_checks)
55              These are applied to MIME related message headers only.
56
57              This feature is available in Postfix 2.0 and later.
58
59       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
64              This feature is available in Postfix 2.0 and later.
65
66       body_checks
67              These are applied to all  other  content,  including  multi-part
68              message boundaries.
69
70              With  Postfix versions before 2.0, all content after the initial
71              message headers is treated as body content.
72

FILTERS AFTER RECEIVING MAIL

74       Postfix supports a subset of the built-in  content  inspection  classes
75       after the message is received:
76
77       milter_header_checks (default: empty)
78              These are applied to headers that are added with Milter applica‐
79              tions.
80
81              This feature is available in Postfix 2.7 and later.
82

FILTERS WHILE DELIVERING MAIL

84       Postfix supports all four content inspection classes  while  delivering
85       mail via SMTP.
86
87       smtp_header_checks (default: empty)
88
89       smtp_mime_header_checks (default: empty)
90
91       smtp_nested_header_checks (default: empty)
92
93       smtp_body_checks (default: empty)
94              These features are available in Postfix 2.5 and later.
95

COMPATIBILITY

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

TABLE FORMAT

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
109       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
113       /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
117       !/pattern/flags action
118              When /pattern/ does not match the input string, execute the cor‐
119              responding action.
120
121       if /pattern/flags
122
123       endif  If the input string matches /pattern/,  then  match  that  input
124              string against the patterns between if and endif.  The if..endif
125              can nest.
126
127              Note: do not prepend whitespace to patterns inside if..endif.
128
129       if !/pattern/flags
130
131       endif  If the input string does not match /pattern/,  then  match  that
132              input  string  against  the  patterns  between if and endif. The
133              if..endif can nest.
134
135       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
139       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

TABLE SEARCH ORDER

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

TEXT SUBSTITUTION

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
155       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

ACTIONS

160       Action names are case insensitive. They are shown  in  upper  case  for
161       consistency with other Postfix documentation.
162
163       BCC user@domain
164              Add  the  specified  address as a BCC recipient, and inspect the
165              next input line. The address must have a local part  and  domain
166              part.  The  number of BCC addresses that can be added is limited
167              only by the amount of available storage space.
168
169              Note 1: the BCC address is added as if  it  was  specified  with
170              NOTIFY=NONE.  The  sender  will  not  be  notified  when the BCC
171              address is undeliverable, as long as  all  down-stream  software
172              implements RFC 3461.
173
174              Note 2: this ignores duplicate addresses (with the same delivery
175              status notification options).
176
177              This feature is available in Postfix 3.0 and later.
178
179              This feature is not supported with smtp header/body checks.
180
181       DISCARD optional text...
182              Claim successful delivery and silently discard the message.   Do
183              not  inspect  the  remainder  of  the  input  message.   Log the
184              optional text if specified, otherwise log a generic message.
185
186              Note: this action disables further header or body_checks inspec‐
187              tion of the current message and affects all recipients.  To dis‐
188              card only one recipient without discarding the  entire  message,
189              use the transport(5) table to direct mail to the discard(8) ser‐
190              vice.
191
192              This feature is available in Postfix 2.0 and later.
193
194              This feature is not supported with smtp header/body checks.
195
196       DUNNO  Pretend that the input line  did  not  match  any  pattern,  and
197              inspect  the next input line. This action can be used to shorten
198              the table search.
199
200              For backwards compatibility reasons, Postfix also accepts OK but
201              it is (and always has been) treated as DUNNO.
202
203              This feature is available in Postfix 2.1 and later.
204
205       FILTER transport:destination
206              Override  the  content_filter parameter setting, and inspect the
207              next input line.  After the message is queued, send  the  entire
208              message  through  the  specified  external  content  filter. The
209              transport name specifies the first  field  of  a  mail  delivery
210              agent definition in master.cf; the syntax of the next-hop desti‐
211              nation is described in the  manual  page  of  the  corresponding
212              delivery agent.  More information about external content filters
213              is in the Postfix FILTER_README file.
214
215              Note 1: do not use $number regular expression substitutions  for
216              transport  or  destination  unless you know that the information
217              has a trusted origin.
218
219              Note 2: this action overrides the  main.cf  content_filter  set‐
220              ting,  and  affects  all  recipients of the message. In the case
221              that multiple FILTER actions fire, only the  last  one  is  exe‐
222              cuted.
223
224              Note 3: the purpose of the FILTER command is to override message
225              routing.  To override the  recipient's  transport  but  not  the
226              next-hop destination, specify an empty filter destination (Post‐
227              fix 2.7 and later),  or  specify  a  transport:destination  that
228              delivers  through  a different Postfix instance (Postfix 2.6 and
229              earlier). Other options are using the recipient-dependent trans‐
230              port_maps   or  the  sender-dependent  sender_dependent_default‐
231              _transport_maps features.
232
233              This feature is available in Postfix 2.0 and later.
234
235              This feature is not supported with smtp header/body checks.
236
237       HOLD optional text...
238              Arrange for the message to be placed  on  the  hold  queue,  and
239              inspect  the next input line.  The message remains on hold until
240              someone either deletes it or releases it for delivery.  Log  the
241              optional text if specified, otherwise log a generic message.
242
243              Mail  that is placed on hold can be examined with the postcat(1)
244              command, and can be destroyed or released with the  postsuper(1)
245              command.
246
247              Note:  use  "postsuper -r" to release mail that was kept on hold
248              for  a  significant  fraction  of   $maximal_queue_lifetime   or
249              $bounce_queue_lifetime,  or  longer. Use "postsuper -H" only for
250              mail that will not expire within a few delivery attempts.
251
252              Note: this action affects all recipients of the message.
253
254              This feature is available in Postfix 2.0 and later.
255
256              This feature is not supported with smtp header/body checks.
257
258       IGNORE Delete the current line from the input,  and  inspect  the  next
259              input line. See STRIP for an alternative that logs the action.
260
261       INFO optional text...
262              Log  an  "info:"  record  with  the  optional  text... (or log a
263              generic text), and inspect the next input line. This  action  is
264              useful for routine logging or for debugging.
265
266              This feature is available in Postfix 2.8 and later.
267
268       PASS optional text...
269              Log a "pass:" record with the optional text... (or log a generic
270              text), and turn off header, body, and Milter inspection for  the
271              remainder of this message.
272
273              Note:  this  feature relies on trust in information that is easy
274              to forge.
275
276              This feature is available in Postfix 3.2 and later.
277
278              This feature is not supported with smtp header/body checks.
279
280       PREPEND text...
281              Prepend one line with the specified text, and inspect  the  next
282              input line.
283
284              Notes:
285
286              ·      The  prepended text is output on a separate line, immedi‐
287                     ately before the input that triggered the PREPEND action.
288
289              ·      The prepended text is not considered part  of  the  input
290                     stream:  it  is  not  subject  to  header/body  checks or
291                     address rewriting, and it does not affect  the  way  that
292                     Postfix adds missing message headers.
293
294              ·      When  prepending  text  before a message header line, the
295                     prepended text must begin with  a  valid  message  header
296                     label.
297
298              ·      This action cannot be used to prepend multi-line text.
299
300              This feature is available in Postfix 2.1 and later.
301
302              This feature is not supported with milter_header_checks.
303
304       REDIRECT user@domain
305              Write  a  message  redirection  request  to  the queue file, and
306              inspect the next input line. After the  message  is  queued,  it
307              will  be  sent  to the specified address instead of the intended
308              recipient(s).
309
310              Note: this action overrides the FILTER action, and  affects  all
311              recipients  of  the  message. If multiple REDIRECT actions fire,
312              only the last one is executed.
313
314              This feature is available in Postfix 2.1 and later.
315
316              This feature is not supported with smtp header/body checks.
317
318       REPLACE text...
319              Replace the current line with the specified  text,  and  inspect
320              the next input line.
321
322              This feature is available in Postfix 2.2 and later. The descrip‐
323              tion below applies to Postfix 2.2.2 and later.
324
325              Notes:
326
327              ·      When replacing a message  header  line,  the  replacement
328                     text must begin with a valid header label.
329
330              ·      The  replaced  text  remains  part  of  the input stream.
331                     Unlike the result from the  PREPEND  action,  a  replaced
332                     message  header  may  be subject to address rewriting and
333                     may affect the way  that  Postfix  adds  missing  message
334                     headers.
335
336       REJECT optional text...
337              Reject  the  entire message. Do not inspect the remainder of the
338              input message.  Reply with optional text...  when  the  optional
339              text is specified, otherwise reply with a generic error message.
340
341              Note: this action disables further header or body_checks inspec‐
342              tion of the current message and affects all recipients.
343
344              Postfix version 2.3 and later  support  enhanced  status  codes.
345              When  no code is specified at the beginning of optional text...,
346              Postfix inserts a default enhanced status code of "5.7.1".
347
348              This feature is not supported with smtp header/body checks.
349
350       STRIP optional text...
351              Log a "strip:" record  with  the  optional  text...  (or  log  a
352              generic text), delete the input line from the input, and inspect
353              the next input line. See IGNORE for a silent alternative.
354
355              This feature is available in Postfix 3.2 and later.
356
357       WARN optional text...
358              Log a "warning:" record with the  optional  text...  (or  log  a
359              generic  text),  and inspect the next input line. This action is
360              useful for debugging and for testing a pattern  before  applying
361              more drastic actions.
362

BUGS

364       Empty lines never match, because some map types mis-behave when given a
365       zero-length search string.  This limitation may be removed for  regular
366       expression tables in a future release.
367
368       Many  people  overlook  the  main limitations of header and body_checks
369       rules.
370
371       ·      These rules operate on one logical message header  or  one  body
372              line at a time. A decision made for one line is not carried over
373              to the next line.
374
375       ·      If text in the message body is encoded (RFC 2045) then the rules
376              need to be specified for the encoded form.
377
378       ·      Likewise,  when  message headers are encoded (RFC 2047) then the
379              rules need to be specified for the encoded form.
380
381       Message headers added by the cleanup(8) daemon itself are excluded from
382       inspection.  Examples  of  such  message  headers  are From:, To:, Mes‐
383       sage-ID:, Date:.
384
385       Message headers deleted by  the  cleanup(8)  daemon  will  be  examined
386       before   they   are   deleted.  Examples  are:  Bcc:,  Content-Length:,
387       Return-Path:.
388

CONFIGURATION PARAMETERS

390       body_checks
391              Lookup tables with content filter rules for message body  lines.
392              These  filters  see one physical line at a time, in chunks of at
393              most $line_length_limit bytes.
394
395       body_checks_size_limit
396              The amount of content per message body segment (attachment) that
397              is subjected to $body_checks filtering.
398
399       header_checks
400
401       mime_header_checks (default: $header_checks)
402
403       nested_header_checks (default: $header_checks)
404              Lookup  tables  with  content  filter  rules  for message header
405              lines: respectively, these are applied to  the  initial  message
406              headers  (not  including MIME headers), to the MIME headers any‐
407              where in the message, and to the  initial  headers  of  attached
408              messages.
409
410              Note:  these  filters  see one logical message header at a time,
411              even when a message header spans multiple lines. Message headers
412              that  are  longer  than  $header_size_limit characters are trun‐
413              cated.
414
415       disable_mime_input_processing
416              While receiving mail, give no special treatment to MIME  related
417              message  headers;  all text after the initial message headers is
418              considered to be part of  the  message  body.  This  means  that
419              header_checks is applied to all the initial message headers, and
420              that body_checks is applied to the remainder of the message.
421
422              Note: when used in  this  manner,  body_checks  will  process  a
423              multi-line message header one line at a time.
424

EXAMPLES

426       Header pattern to block attachments with bad file name extensions.  For
427       convenience, the PCRE /x flag is specified, so that there is no need to
428       collapse  the  pattern  into a single line of text.  The purpose of the
429       [[:xdigit:]] sub-expressions is to recognize Windows CLSID strings.
430
431       /etc/postfix/main.cf:
432           header_checks = pcre:/etc/postfix/header_checks.pcre
433
434       /etc/postfix/header_checks.pcre:
435           /^Content-(Disposition|Type).*name\s*=\s*"?([^;]*(\.|=2E)(
436             ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
437             hlp|ht[at]|
438             inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
439             \{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
440             ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
441             vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x
442               REJECT Attachment name "$2" may not end with ".$4"
443
444       Body pattern to stop a specific HTML browser vulnerability exploit.
445
446       /etc/postfix/main.cf:
447           body_checks = regexp:/etc/postfix/body_checks
448
449       /etc/postfix/body_checks:
450           /^<iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0>$/
451               REJECT IFRAME vulnerability exploit
452

SEE ALSO

454       cleanup(8), canonicalize and enqueue Postfix message
455       pcre_table(5), format of PCRE lookup tables
456       regexp_table(5), format of POSIX regular expression tables
457       postconf(1), Postfix configuration utility
458       postmap(1), Postfix lookup table management
459       postsuper(1), Postfix janitor
460       postcat(1), show Postfix queue file contents
461       RFC 2045, base64 and quoted-printable encoding rules
462       RFC 2047, message header encoding for non-ASCII text
463

README FILES

465       Use "postconf readme_directory" or "postconf html_directory" to  locate
466       this information.
467       DATABASE_README, Postfix lookup table overview
468       CONTENT_INSPECTION_README, Postfix content inspection overview
469       BUILTIN_FILTER_README, Postfix built-in content inspection
470       BACKSCATTER_README, blocking returned forged mail
471

LICENSE

473       The Secure Mailer license must be distributed with this software.
474

AUTHOR(S)

476       Wietse Venema
477       IBM T.J. Watson Research
478       P.O. Box 704
479       Yorktown Heights, NY 10598, USA
480
481       Wietse Venema
482       Google, Inc.
483       111 8th Avenue
484       New York, NY 10011, USA
485
486
487
488                                                              HEADER_CHECKS(5)
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