LDAP: Check for errors of TLS initialisation
[exim.git] / 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.
9 Brightmail AntiSpam (BMI) suppport
10 --------------------------------------------------------------
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.
18 0) BMI concept and implementation overview
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:
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.
40 These four steps are explained in more details below.
42 1) Adding support for BMI at compile time
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:
52 CFLAGS=-I/path/to/the/dir/with/the/includefile
53 EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
55 If you use other CFLAGS or EXTRALIBS_EXIM settings then
56 merge the content of these lines with them.
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.
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.
69 2) Setting up BMI support in the Exim main configuration
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:
75 bmi_config_file = /opt/brightmail/etc/brightmail.cfg
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.
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.
86 3) Set up ACL control statement
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:
99 accept domains = +local_domains
100 endpass
101 verify = recipient
102 control = bmi_run
104 accept domains = +relay_to_domains
105 endpass
106 verify = recipient
107 control = bmi_run
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.
113 4) Setting up routers to use BMI verdicts
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:
122 o deliver the message normally
123 o deliver the message to an alternate location
124 o do not deliver the message
126 To query the verdict for a recipient, the implementation
127 offers the following tools:
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:
135 o bmi_deliver_default
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.
142 o bmi_deliver_alternate
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.
151 o bmi_dont_deliver
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:
158 # don't deliver messages handled by the BMI server
159 bmi_blackhole:
160 driver = redirect
161 bmi_dont_deliver
162 data = :blackhole:
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.
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:
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
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:
188 o $bmi_base64_verdict
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:
194 localuser:
195 driver = accept
196 check_local_user
197 headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
198 transport = local_delivery
200 If there is no verdict available for the recipient being
201 routed, this variable contains the empty string.
203 o $bmi_base64_tracker_verdict
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:
210 localuser:
211 driver = accept
212 check_local_user
213 headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
214 transport = local_delivery
216 If there is no verdict available for the recipient being
217 routed, this variable contains the empty string.
219 o $bmi_alt_location
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.
230 o $bmi_deliver
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.
236 $bmi_deliver is '0': the message should NOT be delivered.
237 $bmi_deliver is '1': the message should be delivered.
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.
250 5) Using per-recipient opt-in information (Optional)
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'.
270 The file format:
272 user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
273 user2@thatdomain.com: <OPTIN STRING3>
276 The example:
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
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.
289 For a list of available opt-in strings, please contact your
290 Brightmail representative.
295 Sender Policy Framework (SPF) support
296 --------------------------------------------------------------
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.
303 SPF support is added via the libspf2 library. Visit
305 http://www.libspf2.org/
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.
311 To compile Exim with SPF support, set these additional flags in
312 Local/Makefile:
315 CFLAGS=-DSPF -I/usr/local/include
316 EXTRALIBS_EXIM=-L/usr/local/lib -lspf2
318 This assumes that the libspf2 files are installed in
319 their default locations.
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.
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:
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.
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.
362 Here is an example to fail forgery attempts from domains that
363 publish SPF records:
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 --------------------- */
371 You can also give special treatment to specific domains:
373 /* -----------------
374 deny message = AOL sender, but not from AOL-approved relay.
375 sender_domains = aol.com
376 spf = fail:neutral
377 --------------------- */
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.
386 When the spf condition has run, it sets up several expansion
387 variables.
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.
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.
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.
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.
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".
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.
418 To access this feature, simply use the spf_guess condition in place
419 of the spf one. For example:
421 /* -----------------
422 deny message = $sender_host_address doesn't look trustworthy to me
423 spf_guess = fail
424 --------------------- */
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.
431 When the spf_guess condition has run, it sets up the same expansion
432 variables as when spf condition is run, described above.
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:
438 /* -----------------
439 spf_guess = v=spf1 a/16 mx/16 ptr ?all
440 --------------------- */
442 would relax host matching rules to a broader network range.
445 SRS (Sender Rewriting Scheme) Support
446 --------------------------------------------------------------
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.
452 In order to use SRS, you must get a copy of libsrs_alt from
454 http://srs.mirtol.com/
456 Unpack the tarball, then refer to MTAs/README.EXIM
457 to proceed. You need to set
461 in your Local/Makefile.
464 --------------------------------------------------------------
465 End of file
466 --------------------------------------------------------------