external/ibm-public/postfix/dist/html/header_checks.5.html

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   7 HEADER_CHECKS(5)                                              HEADER_CHECKS(5)
   8
   9 <b>NAME</b>
  10        <a href="postconf.5.html#header_checks">header_checks</a> - Postfix built-in content inspection
  11
  12 <b>SYNOPSIS</b>
  13        <b><a href="postconf.5.html#header_checks">header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/header_checks</b>
  14        <b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/mime_header_checks</b>
  15        <b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/nested_header_checks</b>
  16        <b><a href="postconf.5.html#body_checks">body_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/body_checks</b>
  17
  18        <b>postmap -q "</b><i>string</i><b>" <a href="pcre_table.5.html">pcre</a>:/etc/postfix/</b><i>filename</i>
  19        <b>postmap -q - <a href="pcre_table.5.html">pcre</a>:/etc/postfix/</b><i>filename</i> &lt;<i>inputfile</i>
  20
  21 <b>DESCRIPTION</b>
  22        This  document  describes access control on the content of
  23        message headers and message body lines; it is  implemented
  24        by  the  Postfix  <a href="cleanup.8.html"><b>cleanup</b>(8)</a> server before mail is queued.
  25        See <a href="access.5.html"><b>access</b>(5)</a> for access control  on  remote  SMTP  client
  26        information.
  27
  28        Each  message  header  or  message  body  line is compared
  29        against a list of patterns.  When a  match  is  found  the
  30        corresponding action is executed, and the matching process
  31        is repeated for the next message header  or  message  body
  32        line.
  33
  34        For  examples, see the EXAMPLES section at the end of this
  35        manual page.
  36
  37        Postfix header or <a href="postconf.5.html#body_checks">body_checks</a> are designed to stop a flood
  38        of  mail from worms or viruses; they do not decode attach-
  39        ments, and they do not unzip archives. See  the  documents
  40        referenced  below  in the README FILES section if you need
  41        more sophisticated content analysis.
  42
  43        Postfix supports four built-in content inspection classes:
  44
  45        <b><a href="postconf.5.html#header_checks">header_checks</a></b>
  46               These   are  applied  to  initial  message  headers
  47               (except for the headers  that  are  processed  with
  48               <b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b>).
  49
  50        <b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
  51               These  are  applied to MIME related message headers
  52               only.
  53
  54               This feature is available in Postfix 2.0 and later.
  55
  56        <b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
  57               These  are  applied  to message headers of attached
  58               email messages (except for  the  headers  that  are
  59               processed with <b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b>).
  60
  61               This feature is available in Postfix 2.0 and later.
  62
  63        <b><a href="postconf.5.html#body_checks">body_checks</a></b>
  64               These are applied to all other  content,  including
  65               multi-part message boundaries.
  66
  67               With Postfix versions before 2.0, all content after
  68               the initial message headers is treated as body con-
  69               tent.
  70
  71        Note: message headers are examined one logical header at a
  72        time, even when a message  header  spans  multiple  lines.
  73        Body lines are always examined one line at a time.
  74
  75 <b>COMPATIBILITY</b>
  76        With Postfix version 2.2 and earlier specify "<b>postmap -fq</b>"
  77        to query a table that contains case sensitive patterns. By
  78        default,  <a href="regexp_table.5.html">regexp</a>: and <a href="pcre_table.5.html">pcre</a>: patterns are case insensitive.
  79
  80 <b>TABLE FORMAT</b>
  81        This document assumes that header  and  <a href="postconf.5.html#body_checks">body_checks</a>  rules
  82        are  specified  in  the form of Postfix regular expression
  83        lookup tables. Usually the best  performance  is  obtained
  84        with <b>pcre</b> (Perl Compatible Regular Expression) tables, but
  85        the slower <b>regexp</b> (POSIX regular expressions)  support  is
  86        more  widely  available.  Use the command "<b>postconf -m</b>" to
  87        find out what lookup table types your Postfix system  sup-
  88        ports.
  89
  90        The general format of Postfix regular expression tables is
  91        given below.  For a  discussion  of  specific  pattern  or
  92        flags   syntax,   see  <a href="pcre_table.5.html"><b>pcre_table</b>(5)</a>  or  <a href="regexp_table.5.html"><b>regexp_table</b>(5)</a>,
  93        respectively.
  94
  95        <b>/</b><i>pattern</i><b>/</b><i>flags action</i>
  96               When /<i>pattern</i>/ matches the  input  string,  execute
  97               the  corresponding  <i>action</i>. See below for a list of
  98               possible actions.
  99
 100        <b>!/</b><i>pattern</i><b>/</b><i>flags action</i>
 101               When /<i>pattern</i>/ does <b>not</b>  match  the  input  string,
 102               execute the corresponding <i>action</i>.
 103
 104        <b>if /</b><i>pattern</i><b>/</b><i>flags</i>
 105
 106        <b>endif</b>  Match the input string against the patterns between
 107               <b>if</b> and <b>endif</b>, if and only if the same input  string
 108               also matches /<i>pattern</i>/. The <b>if</b>..<b>endif</b> can nest.
 109
 110               Note:  do not prepend whitespace to patterns inside
 111               <b>if</b>..<b>endif</b>.
 112
 113        <b>if !/</b><i>pattern</i><b>/</b><i>flags</i>
 114
 115        <b>endif</b>  Match the input string against the patterns between
 116               <b>if</b>  and <b>endif</b>, if and only if the same input string
 117               does <b>not</b> match /<i>pattern</i>/. The <b>if</b>..<b>endif</b> can nest.
 118
 119        blank lines and comments
 120               Empty lines and whitespace-only lines are  ignored,
 121               as  are  lines whose first non-whitespace character
 122               is a `#'.
 123
 124        multi-line text
 125               A pattern/action line  starts  with  non-whitespace
 126               text.  A line that starts with whitespace continues
 127               a logical line.
 128
 129 <b>TABLE SEARCH ORDER</b>
 130        For each line of message input, the patterns  are  applied
 131        in  the order as specified in the table. When a pattern is
 132        found that  matches  the  input  line,  the  corresponding
 133        action  is  executed  and  then  the  next  input  line is
 134        inspected.
 135
 136 <b>TEXT SUBSTITUTION</b>
 137        Substitution of substrings  from  the  matched  expression
 138        into  the <i>action</i> string is possible using the conventional
 139        Perl syntax (<b>$1</b>, <b>$2</b>, etc.).   The  macros  in  the  result
 140        string  may  need  to  be  written as <b>${n}</b> or <b>$(n)</b> if they
 141        aren't followed by whitespace.
 142
 143        Note: since negated patterns (those preceded by <b>!</b>)  return
 144        a result when the expression does not match, substitutions
 145        are not available for negated patterns.
 146
 147 <b>ACTIONS</b>
 148        Action names are case insensitive. They are shown in upper
 149        case for consistency with other Postfix documentation.
 150
 151        <b>DISCARD</b> <i>optional text...</i>
 152               Claim  successful delivery and silently discard the
 153               message.  Log the optional text if specified,  oth-
 154               erwise log a generic message.
 155
 156               Note:   this  action  disables  further  header  or
 157               <a href="postconf.5.html#body_checks">body_checks</a> inspection of the current  message  and
 158               affects all recipients.  To discard only one recip-
 159               ient without discarding the entire message, use the
 160               <a href="transport.5.html">transport(5)</a> table to direct mail to the <a href="discard.8.html">discard(8)</a>
 161               service.
 162
 163               This feature is available in Postfix 2.0 and later.
 164
 165        <b>DUNNO</b>  Pretend  that the input line did not match any pat-
 166               tern, and inspect the next input line. This  action
 167               can be used to shorten the table search.
 168
 169               For  backwards  compatibility reasons, Postfix also
 170               accepts <b>OK</b> but it is (and always has been)  treated
 171               as <b>DUNNO</b>.
 172
 173               This feature is available in Postfix 2.1 and later.
 174
 175        <b>FILTER</b> <i>transport:destination</i>
 176               Write a content filter request to the  queue  file,
 177               and  inspect  the  next input line.  After the com-
 178               plete message is received it will be  sent  through
 179               the specified external content filter.  More infor-
 180               mation about external content  filters  is  in  the
 181               Postfix <a href="FILTER_README.html">FILTER_README</a> file.
 182
 183               Note: this action overrides the <b><a href="postconf.5.html#content_filter">content_filter</a></b> set-
 184               ting, and affects all recipients of the message. In
 185               the  case  that  multiple <b>FILTER</b> actions fire, only
 186               the last one is executed.
 187
 188               This feature is available in Postfix 2.0 and later.
 189
 190        <b>HOLD</b> <i>optional text...</i>
 191               Arrange  for  the  message to be placed on the <b>hold</b>
 192               queue, and inspect the next input line.   The  mes-
 193               sage  remains  on <b>hold</b> until someone either deletes
 194               it or releases it for delivery.  Log  the  optional
 195               text if specified, otherwise log a generic message.
 196
 197               Mail that is placed on hold can  be  examined  with
 198               the  <a href="postcat.1.html"><b>postcat</b>(1)</a>  command,  and  can be destroyed or
 199               released with the <a href="postsuper.1.html"><b>postsuper</b>(1)</a> command.
 200
 201               Note: use "<b>postsuper -r</b>" to release mail  that  was
 202               kept  on  hold for a significant fraction of <b>$<a href="postconf.5.html#maximal_queue_lifetime">maxi</a>-</b>
 203               <b><a href="postconf.5.html#maximal_queue_lifetime">mal_queue_lifetime</a></b>  or  <b>$<a href="postconf.5.html#bounce_queue_lifetime">bounce_queue_lifetime</a></b>,  or
 204               longer.  Use "<b>postsuper -H</b>" only for mail that will
 205               not expire within a few delivery attempts.
 206
 207               Note: this action affects  all  recipients  of  the
 208               message.
 209
 210               This feature is available in Postfix 2.0 and later.
 211
 212        <b>IGNORE</b> Delete the current line from the input, and inspect
 213               the next input line.
 214
 215        <b>PREPEND</b> <i>text...</i>
 216               Prepend  one  line  with  the  specified  text, and
 217               inspect the next input line.
 218
 219               Notes:
 220
 221               <b>o</b>      The prepended text is output on  a  separate
 222                      line,  immediately  before  the  input  that
 223                      triggered the <b>PREPEND</b> action.
 224
 225               <b>o</b>      The prepended text is not considered part of
 226                      the  input  stream:  it  is  not  subject to
 227                      header/body checks or address rewriting, and
 228                      it does not affect the way that Postfix adds
 229                      missing message headers.
 230
 231               <b>o</b>      When prepending text before a message header
 232                      line,  the  prepended text must begin with a
 233                      valid message header label.
 234
 235               <b>o</b>      This action cannot be used to prepend multi-
 236                      line text.
 237
 238               This feature is available in Postfix 2.1 and later.
 239
 240        <b>REDIRECT</b> <i>user@domain</i>
 241               Write a message redirection request  to  the  queue
 242               file,  and  inspect  the next input line. After the
 243               message is queued, it will be sent to the specified
 244               address instead of the intended recipient(s).
 245
 246               Note:  this action overrides the <b>FILTER</b> action, and
 247               affects all recipients of the message. If  multiple
 248               <b>REDIRECT</b>  actions  fire,  only the last one is exe-
 249               cuted.
 250
 251               This feature is available in Postfix 2.1 and later.
 252
 253        <b>REPLACE</b> <i>text...</i>
 254               Replace  the  current line with the specified text,
 255               and inspect the next input line.
 256
 257               This feature is available in Postfix 2.2 and later.
 258               The  description below applies to Postfix 2.2.2 and
 259               later.
 260
 261               Notes:
 262
 263               <b>o</b>      When replacing a message  header  line,  the
 264                      replacement  text  must  begin  with a valid
 265                      header label.
 266
 267               <b>o</b>      The replaced text remains part of the  input
 268                      stream.  Unlike  the result from the <b>PREPEND</b>
 269                      action, a replaced  message  header  may  be
 270                      subject  to address rewriting and may affect
 271                      the way that Postfix  adds  missing  message
 272                      headers.
 273
 274        <b>REJECT</b> <i>optional text...</i>
 275               Reject  the  entire  message.  Reply  with <i>optional</i>
 276               <i>text...</i> when the optional text is specified, other-
 277               wise reply with a generic error message.
 278
 279               Note:   this  action  disables  further  header  or
 280               <a href="postconf.5.html#body_checks">body_checks</a> inspection of the current  message  and
 281               affects all recipients.
 282
 283               Postfix version 2.3 and later support enhanced sta-
 284               tus codes.  When no code is specified at the begin-
 285               ning of <i>optional text...</i>, Postfix inserts a default
 286               enhanced status code of "5.7.1".
 287
 288        <b>WARN</b> <i>optional text...</i>
 289               Log a warning with the <i>optional text...</i> (or  log  a
 290               generic  message), and inspect the next input line.
 291               This action is useful for debugging and for testing
 292               a pattern before applying more drastic actions.
 293
 294 <b>BUGS</b>
 295        Empty lines never match, because some map types mis-behave
 296        when given a zero-length search string.   This  limitation
 297        may  be  removed for regular expression tables in a future
 298        release.
 299
 300        Many people overlook the main limitations  of  header  and
 301        <a href="postconf.5.html#body_checks">body_checks</a> rules.
 302
 303        <b>o</b>      These  rules  operate on one logical message header
 304               or one body line at a time. A decision made for one
 305               line is not carried over to the next line.
 306
 307        <b>o</b>      If  text  in the message body is encoded (<a href="http://tools.ietf.org/html/rfc2045">RFC 2045</a>)
 308               then the rules need to be specified for the encoded
 309               form.
 310
 311        <b>o</b>      Likewise,  when  message  headers  are encoded (<a href="http://tools.ietf.org/html/rfc2047">RFC</a>
 312               <a href="http://tools.ietf.org/html/rfc2047">2047</a>) then the rules need to be specified  for  the
 313               encoded form.
 314
 315        Message  headers added by the <a href="cleanup.8.html"><b>cleanup</b>(8)</a> daemon itself are
 316        excluded from inspection. Examples of such message headers
 317        are <b>From:</b>, <b>To:</b>, <b>Message-ID:</b>, <b>Date:</b>.
 318
 319        Message  headers  deleted by the <a href="cleanup.8.html"><b>cleanup</b>(8)</a> daemon will be
 320        examined before they are deleted. Examples are: <b>Bcc:, Con-</b>
 321        <b>tent-Length:</b>, <b>Return-Path:</b>.
 322
 323 <b>CONFIGURATION PARAMETERS</b>
 324        <b><a href="postconf.5.html#body_checks">body_checks</a></b>
 325               Lookup tables with content filter rules for message
 326               body lines.  These filters see one physical line at
 327               a  time,  in  chunks  of at most <b>$<a href="postconf.5.html#line_length_limit">line_length_limit</a></b>
 328               bytes.
 329
 330        <b><a href="postconf.5.html#body_checks_size_limit">body_checks_size_limit</a></b>
 331               The amount of  content  per  message  body  segment
 332               (attachment) that is subjected to <b>$<a href="postconf.5.html#body_checks">body_checks</a></b> fil-
 333               tering.
 334
 335        <b><a href="postconf.5.html#header_checks">header_checks</a></b>
 336
 337        <b><a href="postconf.5.html#mime_header_checks">mime_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
 338
 339        <b><a href="postconf.5.html#nested_header_checks">nested_header_checks</a></b> (default: <b>$<a href="postconf.5.html#header_checks">header_checks</a></b>)
 340               Lookup tables with content filter rules for message
 341               header  lines:  respectively,  these are applied to
 342               the initial message  headers  (not  including  MIME
 343               headers),  to the MIME headers anywhere in the mes-
 344               sage, and to the initial headers of  attached  mes-
 345               sages.
 346
 347               Note:  these filters see one logical message header
 348               at a time, even when a message header spans  multi-
 349               ple  lines.  Message  headers  that are longer than
 350               <b>$<a href="postconf.5.html#header_size_limit">header_size_limit</a></b> characters are truncated.
 351
 352        <b><a href="postconf.5.html#disable_mime_input_processing">disable_mime_input_processing</a></b>
 353               While receiving mail, give no special treatment  to
 354               MIME  related  message  headers; all text after the
 355               initial message headers is considered to be part of
 356               the  message body. This means that <b><a href="postconf.5.html#header_checks">header_checks</a></b> is
 357               applied to all the  initial  message  headers,  and
 358               that <b><a href="postconf.5.html#body_checks">body_checks</a></b> is applied to the remainder of the
 359               message.
 360
 361               Note: when used in this  manner,  <b><a href="postconf.5.html#body_checks">body_checks</a></b>  will
 362               process  a  multi-line message header one line at a
 363               time.
 364
 365 <b>EXAMPLES</b>
 366        Header pattern to block attachments  with  bad  file  name
 367        extensions.   For  convenience, the PCRE /x flag is speci-
 368        fied, so that there is no need  to  collapse  the  pattern
 369        into   a   single  line  of  text.   The  purpose  of  the
 370        [[:xdigit:]] sub-expressions is to recognize Windows CLSID
 371        strings.
 372
 373        /etc/postfix/<a href="postconf.5.html">main.cf</a>:
 374            <a href="postconf.5.html#header_checks">header_checks</a> = <a href="pcre_table.5.html">pcre</a>:/etc/postfix/header_checks.pcre
 375
 376        /etc/postfix/header_checks.<a href="pcre_table.5.html">pcre</a>:
 377            /^Content-(Disposition|Type).*name\s*=\s*"?(.*(\.|=2E)(
 378              ade|adp|asp|bas|bat|chm|cmd|com|cpl|crt|dll|exe|
 379              hlp|ht[at]|
 380              inf|ins|isp|jse?|lnk|md[betw]|ms[cipt]|nws|
 381              \{[[:xdigit:]]{8}(?:-[[:xdigit:]]{4}){3}-[[:xdigit:]]{12}\}|
 382              ops|pcd|pif|prf|reg|sc[frt]|sh[bsm]|swf|
 383              vb[esx]?|vxd|ws[cfh]))(\?=)?"?\s*(;|$)/x
 384                REJECT Attachment name "$2" may not end with ".$4"
 385
 386        Body pattern to stop a specific HTML browser vulnerability
 387        exploit.
 388
 389        /etc/postfix/<a href="postconf.5.html">main.cf</a>:
 390            <a href="postconf.5.html#body_checks">body_checks</a> = <a href="regexp_table.5.html">regexp</a>:/etc/postfix/body_checks
 391
 392        /etc/postfix/body_checks:
 393            /^&lt;iframe src=(3D)?cid:.* height=(3D)?0 width=(3D)?0&gt;$/
 394                REJECT IFRAME vulnerability exploit
 395
 396 <b>SEE ALSO</b>
 397        <a href="cleanup.8.html">cleanup(8)</a>, canonicalize and enqueue Postfix message
 398        <a href="pcre_table.5.html">pcre_table(5)</a>, format of PCRE lookup tables
 399        <a href="regexp_table.5.html">regexp_table(5)</a>, format of POSIX regular expression tables
 400        <a href="postconf.1.html">postconf(1)</a>, Postfix configuration utility
 401        <a href="postmap.1.html">postmap(1)</a>, Postfix lookup table management
 402        <a href="postsuper.1.html">postsuper(1)</a>, Postfix janitor
 403        <a href="postcat.1.html">postcat(1)</a>, show Postfix queue file contents
 404        <a href="http://tools.ietf.org/html/rfc2045">RFC 2045</a>, base64 and quoted-printable encoding rules
 405        <a href="http://tools.ietf.org/html/rfc2047">RFC 2047</a>, message header encoding for non-ASCII text
 406
 407 <b>README FILES</b>
 408        <a href="DATABASE_README.html">DATABASE_README</a>, Postfix lookup table overview
 409        <a href="CONTENT_INSPECTION_README.html">CONTENT_INSPECTION_README</a>, Postfix content inspection overview
 410        <a href="BUILTIN_FILTER_README.html">BUILTIN_FILTER_README</a>, Postfix built-in content inspection
 411        <a href="BACKSCATTER_README.html">BACKSCATTER_README</a>, blocking returned forged mail
 412
 413 <b>LICENSE</b>
 414        The  Secure  Mailer  license must be distributed with this
 415        software.
 416
 417 <b>AUTHOR(S)</b>
 418        Wietse Venema
 419        IBM T.J. Watson Research
 420        P.O. Box 704
 421        Yorktown Heights, NY 10598, USA
 422
 423                                                               HEADER_CHECKS(5)
 424 </pre> </body> </html>