doc/doc-txt/experimental-spec.txt

   1 From time to time, experimental features may be added to Exim.
   2 While a feature  is experimental, there  will be a  build-time
   3 option whose name starts  "EXPERIMENTAL_" that must be  set in
   4 order to include the  feature. This file contains  information
   5 about experimenatal  features, all  of which  are unstable and
   6 liable to incompatibile change.
   7
   8
   9 Brightmail AntiSpam (BMI) suppport
  10 --------------------------------------------------------------
  11
  12 Brightmail  AntiSpam  is  a  commercial  package.  Please  see
  13 http://www.brightmail.com    for    more    information     on
  14 the product. For  the sake of  clarity, we'll refer  to it  as
  15 "BMI" from now on.
  16
  17
  18 0) BMI concept and implementation overview
  19
  20 In  contrast  to   how  spam-scanning  with   SpamAssassin  is
  21 implemented  in  exiscan-acl,  BMI  is  more  suited  for  per
  22 -recipient  scanning of  messages. However,  each messages  is
  23 scanned  only  once,  but  multiple  "verdicts"  for  multiple
  24 recipients can be  returned from the  BMI server. The  exiscan
  25 implementation  passes  the  message to  the  BMI  server just
  26 before accepting it.  It then adds  the retrieved verdicts  to
  27 the messages header file in the spool. These verdicts can then
  28 be  queried  in  routers,  where  operation  is  per-recipient
  29 instead  of per-message.  To use  BMI, you  need to  take the
  30 following steps:
  31
  32   1) Compile Exim with BMI support
  33   2) Set up main BMI options (top section of Exim config file)
  34   3) Set up ACL control statement (ACL section of the config
  35      file)
  36   4) Set up your routers to use BMI verdicts (routers section
  37      of the config file).
  38   5) (Optional) Set up per-recipient opt-in information.
  39
  40 These four steps are explained in more details below.
  41
  42 1) Adding support for BMI at compile time
  43
  44   To compile with BMI support,  you need to link Exim  against
  45   the   Brighmail  client   SDK,  consisting   of  a   library
  46   (libbmiclient_single.so)  and  a  header  file  (bmi_api.h).
  47   You'll also need to explicitly set a flag in the Makefile to
  48   include BMI support in the Exim binary. Both can be achieved
  49   with  these lines in Local/Makefile:
  50
  51   EXPERIMENTAL_BRIGHTMAIL=yes
  52   CFLAGS=-I/path/to/the/dir/with/the/includefile
  53   EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
  54
  55   If  you use  other CFLAGS  or EXTRALIBS_EXIM  settings then
  56   merge the content of these lines with them.
  57
  58   Note for BMI6.x users: You'll also have to add -lxml2_single
  59   to the EXTRALIBS_EXIM line. Users of 5.5x do not need to  do
  60   this.
  61
  62   You    should     also    include     the    location     of
  63   libbmiclient_single.so in your dynamic linker  configuration
  64   file   (usually   /etc/ld.so.conf)   and   run    "ldconfig"
  65   afterwards, or  else the  produced Exim  binary will  not be
  66   able to find the library file.
  67
  68
  69 2) Setting up BMI support in the Exim main configuration
  70
  71   To enable BMI  support in the  main Exim configuration,  you
  72   should set the path to the main BMI configuration file  with
  73   the "bmi_config_file" option, like this:
  74
  75   bmi_config_file = /opt/brightmail/etc/brightmail.cfg
  76
  77   This must go into section 1 of Exim's configuration file (You
  78   can  put it  right on  top). If  you omit  this option,  it
  79   defaults to /opt/brightmail/etc/brightmail.cfg.
  80
  81   Note for BMI6.x users: This  file is in XML format  in V6.xx
  82   and its  name is  /opt/brightmail/etc/bmiconfig.xml. So  BMI
  83   6.x users MUST set the bmi_config_file option.
  84
  85
  86 3) Set up ACL control statement
  87
  88   To  optimize performance,  it makes  sense only  to process
  89   messages coming from remote, untrusted sources with the  BMI
  90   server.  To set  up a  messages for  processing by  the BMI
  91   server, you MUST set the "bmi_run" control statement in  any
  92   ACL for an incoming message.  You will typically do this  in
  93   an "accept"  block in  the "acl_check_rcpt"  ACL. You should
  94   use the "accept" block(s)  that accept messages from  remote
  95   servers for your own domain(s). Here is an example that uses
  96   the "accept" blocks from Exim's default configuration file:
  97
  98
  99   accept  domains       = +local_domains
 100           endpass
 101           verify        = recipient
 102           control       = bmi_run
 103
 104   accept  domains       = +relay_to_domains
 105           endpass
 106           verify        = recipient
 107           control       = bmi_run
 108
 109   If bmi_run  is not  set in  any ACL  during reception of the
 110   message, it will NOT be passed to the BMI server.
 111
 112
 113 4) Setting up routers to use BMI verdicts
 114
 115   When a message has been  run through the BMI server,  one or
 116   more "verdicts" are  present. Different recipients  can have
 117   different verdicts. Each  recipient is treated  individually
 118   during routing, so you  can query the verdicts  by recipient
 119   at  that stage.  From Exim's  view, a  verdict can  have the
 120   following outcomes:
 121
 122   o deliver the message normally
 123   o deliver the message to an alternate location
 124   o do not deliver the message
 125
 126   To query  the verdict  for a  recipient, the  implementation
 127   offers the following tools:
 128
 129
 130   - Boolean router  preconditions. These  can be  used in  any
 131     router. For a simple  implementation of BMI, these  may be
 132     all  that  you  need.  The  following  preconditions   are
 133     available:
 134
 135     o bmi_deliver_default
 136
 137       This  precondition  is  TRUE  if  the  verdict  for  the
 138       recipient is  to deliver  the message  normally. If  the
 139       message has not been  processed by the BMI  server, this
 140       variable defaults to TRUE.
 141
 142     o bmi_deliver_alternate
 143
 144       This  precondition  is  TRUE  if  the  verdict  for  the
 145       recipient  is to  deliver the  message to  an alternate
 146       location.  You  can  get the  location  string  from the
 147       $bmi_alt_location expansion variable if you need it. See
 148       further below. If the message has not been processed  by
 149       the BMI server, this variable defaults to FALSE.
 150
 151     o bmi_dont_deliver
 152
 153       This  precondition  is  TRUE  if  the  verdict  for  the
 154       recipient  is  NOT  to   deliver  the  message  to   the
 155       recipient. You will typically use this precondition in a
 156       top-level blackhole router, like this:
 157
 158         # don't deliver messages handled by the BMI server
 159         bmi_blackhole:
 160           driver = redirect
 161           bmi_dont_deliver
 162           data = :blackhole:
 163
 164       This router should be on top of all others, so  messages
 165       that should not be delivered do not reach other  routers
 166       at all. If   the  message  has  not  been  processed  by
 167       the  BMI server, this variable defaults to FALSE.
 168
 169
 170   - A list router  precondition to query  if rules "fired"  on
 171     the message for the recipient. Its name is "bmi_rule". You
 172     use  it  by  passing it  a  colon-separated  list of  rule
 173     numbers. You can use this condition to route messages that
 174     matched specific rules. Here is an example:
 175
 176       # special router for BMI rule #5, #8 and #11
 177       bmi_rule_redirect:
 178         driver = redirect
 179         bmi_rule = 5:8:11
 180         data = postmaster@mydomain.com
 181
 182
 183   - Expansion variables. Several  expansion variables are  set
 184     during  routing.  You  can  use  them  in  custom   router
 185     conditions,  for  example.  The  following  variables  are
 186     available:
 187
 188     o $bmi_base64_verdict
 189
 190       This variable  will contain  the BASE64  encoded verdict
 191       for the recipient being routed. You can use it to add  a
 192       header to messages for tracking purposes, for example:
 193
 194       localuser:
 195         driver = accept
 196         check_local_user
 197         headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
 198         transport = local_delivery
 199
 200       If there is no verdict available for the recipient being
 201       routed, this variable contains the empty string.
 202
 203     o $bmi_base64_tracker_verdict
 204
 205       This variable  will contain  a BASE64  encoded subset of
 206       the  verdict  information  concerning  the  "rules" that
 207       fired  on the  message. You  can add  this string  to a
 208       header, commonly named "X-Brightmail-Tracker". Example:
 209
 210       localuser:
 211         driver = accept
 212         check_local_user
 213         headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
 214         transport = local_delivery
 215
 216       If there is no verdict available for the recipient being
 217       routed, this variable contains the empty string.
 218
 219     o $bmi_alt_location
 220
 221       If  the  verdict  is  to  redirect  the  message  to  an
 222       alternate  location,  this  variable  will  contain  the
 223       alternate location string returned by the BMI server. In
 224       its default configuration, this is a header-like  string
 225       that can be added to the message with "headers_add".  If
 226       there is  no verdict  available for  the recipient being
 227       routed, or if the  message is to be  delivered normally,
 228       this variable contains the empty string.
 229
 230     o $bmi_deliver
 231
 232       This is an additional integer variable that can be  used
 233       to query if the message should be delivered at all.  You
 234       should use router preconditions instead if possible.
 235
 236       $bmi_deliver is '0': the message should NOT be delivered.
 237       $bmi_deliver is '1': the message should be delivered.
 238
 239
 240   IMPORTANT NOTE: Verdict inheritance.
 241   The  message  is passed  to  the BMI  server  during message
 242   reception,  using the  target addresses  from the  RCPT TO:
 243   commands in the SMTP transaction. If recipients get expanded
 244   or re-written (for example by aliasing), the new address(es)
 245   inherit the  verdict from  the original  address. This means
 246   that verdicts also apply to all "child" addresses  generated
 247   from top-level addresses that were sent to the BMI server.
 248
 249
 250 5) Using per-recipient opt-in information (Optional)
 251
 252   The  BMI server  features multiple  scanning "profiles"  for
 253   individual recipients.  These are  usually stored  in a LDAP
 254   server and are  queried by the  BMI server itself.  However,
 255   you can also  pass opt-in data  for each recipient  from the
 256   MTA to the  BMI server. This  is particularly useful  if you
 257   already look  up recipient  data in  Exim anyway  (which can
 258   also be  stored in  a SQL  database or  other source).  This
 259   implementation enables you  to pass opt-in  data to the  BMI
 260   server  in  the  RCPT   ACL.  This  works  by   setting  the
 261   'bmi_optin' modifier in  a block of  that ACL. If  should be
 262   set to a list  of comma-separated strings that  identify the
 263   features which the BMI server should use for that particular
 264   recipient. Ideally, you  would use the  'bmi_optin' modifier
 265   in the same  ACL block where  you set the  'bmi_run' control
 266   flag. Here is an example that will pull opt-in data for each
 267   recipient      from       a      flat       file      called
 268   '/etc/exim/bmi_optin_data'.
 269
 270   The file format:
 271
 272     user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
 273     user2@thatdomain.com: <OPTIN STRING3>
 274
 275
 276   The example:
 277
 278     accept  domains       = +relay_to_domains
 279             endpass
 280             verify        = recipient
 281             bmi_optin     = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
 282             control       = bmi_run
 283
 284   Of course,  you can  also use  any other  lookup method that
 285   Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
 286   as long as  the result is  a list of  colon-separated opt-in
 287   strings.
 288
 289   For a list of available opt-in strings, please contact  your
 290   Brightmail representative.
 291
 292
 293
 294
 295 Sender Policy Framework (SPF) support
 296 --------------------------------------------------------------
 297
 298 To learn  more  about  SPF, visit   http://www.openspf.org. This
 299 document does   not explain  the SPF  fundamentals, you should
 300 read and understand the implications of deploying SPF on  your
 301 system before doing so.
 302
 303 SPF support is added via the libspf2 library. Visit
 304
 305   http://www.libspf2.org/
 306
 307 to obtain  a copy,  then compile  and install  it. By default,
 308 this will  put headers  in /usr/local/include  and the  static
 309 library in /usr/local/lib.
 310
 311 To compile Exim with SPF support, set these additional flags in
 312 Local/Makefile:
 313
 314 EXPERIMENTAL_SPF=yes
 315 CFLAGS=-DSPF -I/usr/local/include
 316 EXTRALIBS_EXIM=-L/usr/local/lib -lspf2
 317
 318 This assumes   that the   libspf2 files   are installed  in
 319 their default locations.
 320
 321 You can now run SPF checks in incoming SMTP by using the "spf"
 322 ACL condition  in either  the MAIL,  RCPT or  DATA ACLs.  When
 323 using it in the RCPT ACL, you can make the checks dependend on
 324 the RCPT  address (or  domain), so  you can  check SPF records
 325 only  for   certain  target   domains.  This   gives  you  the
 326 possibility  to opt-out  certain customers  that do  not want
 327 their mail to be subject to SPF checking.
 328
 329 The spf condition  takes a list  of strings on  its right-hand
 330 side. These strings describe the outcome of the SPF check  for
 331 which the spf condition should succeed. Valid strings are:
 332
 333   o pass      The SPF check passed, the sending host
 334               is positively verified by SPF.
 335   o fail      The SPF check failed, the sending host
 336               is NOT allowed to send mail for the domain
 337               in the envelope-from address.
 338   o softfail  The SPF check failed, but the queried
 339               domain can't absolutely confirm that this
 340               is a forgery.
 341   o none      The queried domain does not publish SPF
 342               records.
 343   o neutral   The SPF check returned a "neutral" state.
 344               This means the queried domain has published
 345               a SPF record, but wants to allow outside
 346               servers to send mail under its domain as well.
 347   o err_perm  This indicates a syntax error in the SPF
 348               record of the queried domain. This should be
 349               treated like "none".
 350   o err_temp  This indicates a temporary error during all
 351               processing, including Exim's SPF processing.
 352               You may defer messages when this occurs.
 353
 354 You can prefix each string with an exclamation mark to  invert
 355 is meaning,  for example  "!fail" will  match all  results but
 356 "fail".  The  string  list is  evaluated  left-to-right,  in a
 357 short-circuit fashion.  When a  string matches  the outcome of
 358 the SPF check, the condition  succeeds. If none of the  listed
 359 strings matches the  outcome of the  SPF check, the  condition
 360 fails.
 361
 362 Here is an example to fail forgery attempts from domains that
 363 publish SPF records:
 364
 365 /* -----------------
 366 deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}.  \
 367               Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address
 368      spf = fail
 369 --------------------- */
 370
 371 You can also give special treatment to specific domains:
 372
 373 /* -----------------
 374 deny message = AOL sender, but not from AOL-approved relay.
 375      sender_domains = aol.com
 376      spf = fail:neutral
 377 --------------------- */
 378
 379 Explanation: AOL  publishes SPF  records, but  is liberal  and
 380 still allows  non-approved relays  to send  mail from aol.com.
 381 This will result in a "neutral" state, while mail from genuine
 382 AOL servers  will result  in "pass".  The example  above takes
 383 this into account and  treats "neutral" like "fail",  but only
 384 for aol.com. Please note that this violates the SPF draft.
 385
 386 When the spf condition has run, it sets up several expansion
 387 variables.
 388
 389   $spf_header_comment
 390   This contains a human-readable string describing the outcome
 391   of the SPF check. You can add it to a custom header or use
 392   it for logging purposes.
 393
 394   $spf_received
 395   This contains a complete Received-SPF: header that can be
 396   added to the message. Please note that according to the SPF
 397   draft, this header must be added at the top of the header
 398   list. Please see section 10 on how you can do this.
 399
 400   Note: in case of "Best-guess" (see below), the convention is
 401   to put this string in a header called X-SPF-Guess: instead.
 402
 403   $spf_result
 404   This contains the outcome of the SPF check in string form,
 405   one of pass, fail, softfail, none, neutral, err_perm or
 406   err_temp.
 407
 408   $spf_smtp_comment
 409   This contains a string that can be used in a SMTP response
 410   to the calling party. Useful for "fail".
 411
 412 In addition to SPF, you can also perform checks for so-called
 413 "Best-guess".  Strictly speaking, "Best-guess" is not standard
 414 SPF, but it is supported by the same framework that enables SPF
 415 capability.  Refer to http://www.openspf.org/FAQ/Best_guess_record
 416 for a description of what it means.
 417
 418 To access this feature, simply use the spf_guess condition in place
 419 of the spf one.  For example:
 420
 421 /* -----------------
 422 deny message = $sender_host_address doesn't look trustworthy to me
 423      spf_guess = fail
 424 --------------------- */
 425
 426 In case you decide to reject messages based on this check, you
 427 should note that although it uses the same framework, "Best-guess"
 428 is NOT SPF, and therefore you should not mention SPF at all in your
 429 reject message.
 430
 431 When the spf_guess condition has run, it sets up the same expansion
 432 variables as when spf condition is run, described above.
 433
 434 Additionally, since Best-guess is not standarized, you may redefine
 435 what "Best-guess" means to you by redefining spf_guess variable in
 436 global config.  For example, the following:
 437
 438 /* -----------------
 439 spf_guess = v=spf1 a/16 mx/16 ptr ?all
 440 --------------------- */
 441
 442 would relax host matching rules to a broader network range.
 443
 444
 445 SRS (Sender Rewriting Scheme) Support
 446 --------------------------------------------------------------
 447
 448 Exiscan  currently  includes SRS  support  via Miles  Wilton's
 449 libsrs_alt library. The current version of the supported
 450 library is 0.5.
 451
 452 In order to  use SRS, you  must get a  copy of libsrs_alt from
 453
 454 http://srs.mirtol.com/
 455
 456 Unpack the tarball, then refer to MTAs/README.EXIM
 457 to proceed. You need to set
 458
 459 EXPERIMENTAL_SRS=yes
 460
 461 in your Local/Makefile.
 462
 463
 464 --------------------------------------------------------------
 465 End of file
 466 --------------------------------------------------------------