Fix Proxy Protocol v2 handling
[exim.git] / doc / doc-txt / experimental-spec.txt
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1From time to time, experimental features may be added to Exim.
2While a feature is experimental, there will be a build-time
3option whose name starts "EXPERIMENTAL_" that must be set in
4order to include the feature. This file contains information
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5about experimental features, all of which are unstable and
6liable to incompatible change.
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7
8
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9PRDR support
10--------------------------------------------------------------
11
12Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall
13in a (now-expired) IETF draft from 2007. It's not hit mainstream
14use, but has apparently been implemented in the META1 MTA.
15
16There is mention at http://mail.aegee.org/intern/sendmail.html
17of a patch to sendmail "to make it PRDR capable".
18
19 ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt
20
21If Exim is built with EXPERIMENTAL_PRDR there is a new config
22boolean "prdr_enable" which controls whether PRDR is advertised
23as part of an EHLO response, a new "acl_data_smtp_prdr" ACL
24(called for each recipient, after data arrives but before the
25data ACL), and a new smtp transport option "hosts_try_prdr".
26
27PRDR may be used to support per-user content filtering. Without it
28one must defer any recipient after the first that has a different
29content-filter configuration. With PRDR, the RCPT-time check
30for this can be disabled when the MAIL-time $smtp_command included
31"PRDR". Any required difference in behaviour of the main DATA-time
32ACL should however depend on the PRDR-time ACL having run, as Exim
33will avoid doing so in some situations (eg. single-recipient mails).
34
35
36
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37OCSP Stapling support
38--------------------------------------------------------------
39
d36a0501 40X.509 PKI certificates expire and can be revoked; to handle this, the
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41clients need some way to determine if a particular certificate, from a
42particular Certificate Authority (CA), is still valid. There are three
43main ways to do so.
44
45The simplest way is to serve up a Certificate Revocation List (CRL) with
46an ordinary web-server, regenerating the CRL before it expires. The
47downside is that clients have to periodically re-download a potentially
48huge file from every certificate authority it knows of.
49
50The way with most moving parts at query time is Online Certificate
51Status Protocol (OCSP), where the client verifies the certificate
52against an OCSP server run by the CA. This lets the CA track all
53usage of the certs. This requires running software with access to the
54private key of the CA, to sign the responses to the OCSP queries. OCSP
55is based on HTTP and can be proxied accordingly.
56
57The only widespread OCSP server implementation (known to this writer)
58comes as part of OpenSSL and aborts on an invalid request, such as
59connecting to the port and then disconnecting. This requires
60re-entering the passphrase each time some random client does this.
61
62The third way is OCSP Stapling; in this, the server using a certificate
63issued by the CA periodically requests an OCSP proof of validity from
64the OCSP server, then serves it up inline as part of the TLS
65negotiation. This approach adds no extra round trips, does not let the
66CA track users, scales well with number of certs issued by the CA and is
67resilient to temporary OCSP server failures, as long as the server
68starts retrying to fetch an OCSP proof some time before its current
69proof expires. The downside is that it requires server support.
70
71If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
f5d78688 72then it gains a new global option: "tls_ocsp_file".
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73
74The file specified therein is expected to be in DER format, and contain
75an OCSP proof. Exim will serve it as part of the TLS handshake. This
76option will be re-expanded for SNI, if the tls_certificate option
77contains $tls_sni, as per other TLS options.
78
79Exim does not at this time implement any support for fetching a new OCSP
80proof. The burden is on the administrator to handle this, outside of
81Exim. The file specified should be replaced atomically, so that the
82contents are always valid. Exim will expand the "tls_ocsp_file" option
83on each connection, so a new file will be handled transparently on the
84next connection.
85
98a3b527 86Exim will check for a valid next update timestamp in the OCSP proof;
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87if not present, or if the proof has expired, it will be ignored.
88
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89Also, given EXPERIMENTAL_OCSP and OpenSSL, the smtp transport gains
90a "hosts_require_ocsp" option; a host-list for which an OCSP Stapling
91is requested and required for the connection to proceed. The host(s)
92should also be in "hosts_require_tls", and "tls_verify_certificates"
93configured for the transport.
94
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95For the client to be able to verify the stapled OCSP the server must
96also supply, in its stapled information, any intermediate
97certificates for the chain leading to the OCSP proof from the signer
98of the server certificate. There may be zero or one such. These
99intermediate certificates should be added to the server OCSP stapling
100file (named by tls_ocsp_file).
101
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102At this point in time, we're gathering feedback on use, to determine if
103it's worth adding complexity to the Exim daemon to periodically re-fetch
f5d78688 104OCSP files and somehow handling multiple files.
3f7eeb86 105
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106 A helper script "ocsp_fetch.pl" for fetching a proof from a CA
107 OCSP server is supplied. The server URL may be included in the
108 server certificate, if the CA is helpful.
109
110 One fail mode seen was the OCSP Signer cert expiring before the end
111 of vailidity of the OCSP proof. The checking done by Exim/OpenSSL
112 noted this as invalid overall, but the re-fetch script did not.
113
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114
115
116
0b23848a 117Brightmail AntiSpam (BMI) suppport
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118--------------------------------------------------------------
119
120Brightmail AntiSpam is a commercial package. Please see
121http://www.brightmail.com for more information on
122the product. For the sake of clarity, we'll refer to it as
123"BMI" from now on.
124
125
1260) BMI concept and implementation overview
127
128In contrast to how spam-scanning with SpamAssassin is
129implemented in exiscan-acl, BMI is more suited for per
130-recipient scanning of messages. However, each messages is
131scanned only once, but multiple "verdicts" for multiple
132recipients can be returned from the BMI server. The exiscan
133implementation passes the message to the BMI server just
134before accepting it. It then adds the retrieved verdicts to
135the messages header file in the spool. These verdicts can then
136be queried in routers, where operation is per-recipient
137instead of per-message. To use BMI, you need to take the
138following steps:
139
140 1) Compile Exim with BMI support
3ec3e3bb 141 2) Set up main BMI options (top section of Exim config file)
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142 3) Set up ACL control statement (ACL section of the config
143 file)
144 4) Set up your routers to use BMI verdicts (routers section
145 of the config file).
146 5) (Optional) Set up per-recipient opt-in information.
147
8ff3788c 148These four steps are explained in more details below.
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149
1501) Adding support for BMI at compile time
151
152 To compile with BMI support, you need to link Exim against
153 the Brighmail client SDK, consisting of a library
154 (libbmiclient_single.so) and a header file (bmi_api.h).
155 You'll also need to explicitly set a flag in the Makefile to
156 include BMI support in the Exim binary. Both can be achieved
157 with these lines in Local/Makefile:
158
159 EXPERIMENTAL_BRIGHTMAIL=yes
47bbda99 160 CFLAGS=-I/path/to/the/dir/with/the/includefile
ee161e8f 161 EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
8ff3788c 162
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163 If you use other CFLAGS or EXTRALIBS_EXIM settings then
164 merge the content of these lines with them.
165
7c0c8547 166 Note for BMI6.x users: You'll also have to add -lxml2_single
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167 to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do
168 this.
8ff3788c 169
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170 You should also include the location of
171 libbmiclient_single.so in your dynamic linker configuration
172 file (usually /etc/ld.so.conf) and run "ldconfig"
173 afterwards, or else the produced Exim binary will not be
174 able to find the library file.
175
176
3ec3e3bb 1772) Setting up BMI support in the Exim main configuration
ee161e8f 178
3ec3e3bb 179 To enable BMI support in the main Exim configuration, you
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180 should set the path to the main BMI configuration file with
181 the "bmi_config_file" option, like this:
8ff3788c 182
ee161e8f 183 bmi_config_file = /opt/brightmail/etc/brightmail.cfg
8ff3788c 184
3ec3e3bb 185 This must go into section 1 of Exim's configuration file (You
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186 can put it right on top). If you omit this option, it
187 defaults to /opt/brightmail/etc/brightmail.cfg.
188
189 Note for BMI6.x users: This file is in XML format in V6.xx
190 and its name is /opt/brightmail/etc/bmiconfig.xml. So BMI
191 6.x users MUST set the bmi_config_file option.
8ff3788c 192
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193
1943) Set up ACL control statement
195
196 To optimize performance, it makes sense only to process
197 messages coming from remote, untrusted sources with the BMI
198 server. To set up a messages for processing by the BMI
199 server, you MUST set the "bmi_run" control statement in any
200 ACL for an incoming message. You will typically do this in
201 an "accept" block in the "acl_check_rcpt" ACL. You should
202 use the "accept" block(s) that accept messages from remote
203 servers for your own domain(s). Here is an example that uses
3ec3e3bb 204 the "accept" blocks from Exim's default configuration file:
8ff3788c 205
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206
207 accept domains = +local_domains
208 endpass
209 verify = recipient
210 control = bmi_run
211
212 accept domains = +relay_to_domains
213 endpass
214 verify = recipient
215 control = bmi_run
8ff3788c 216
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217 If bmi_run is not set in any ACL during reception of the
218 message, it will NOT be passed to the BMI server.
219
220
2214) Setting up routers to use BMI verdicts
222
223 When a message has been run through the BMI server, one or
224 more "verdicts" are present. Different recipients can have
225 different verdicts. Each recipient is treated individually
226 during routing, so you can query the verdicts by recipient
3ec3e3bb 227 at that stage. From Exim's view, a verdict can have the
ee161e8f 228 following outcomes:
8ff3788c 229
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230 o deliver the message normally
231 o deliver the message to an alternate location
232 o do not deliver the message
8ff3788c 233
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234 To query the verdict for a recipient, the implementation
235 offers the following tools:
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236
237
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238 - Boolean router preconditions. These can be used in any
239 router. For a simple implementation of BMI, these may be
240 all that you need. The following preconditions are
241 available:
8ff3788c 242
ee161e8f 243 o bmi_deliver_default
8ff3788c 244
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245 This precondition is TRUE if the verdict for the
246 recipient is to deliver the message normally. If the
247 message has not been processed by the BMI server, this
248 variable defaults to TRUE.
8ff3788c 249
ee161e8f 250 o bmi_deliver_alternate
8ff3788c 251
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252 This precondition is TRUE if the verdict for the
253 recipient is to deliver the message to an alternate
254 location. You can get the location string from the
255 $bmi_alt_location expansion variable if you need it. See
256 further below. If the message has not been processed by
257 the BMI server, this variable defaults to FALSE.
8ff3788c 258
ee161e8f 259 o bmi_dont_deliver
8ff3788c 260
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261 This precondition is TRUE if the verdict for the
262 recipient is NOT to deliver the message to the
263 recipient. You will typically use this precondition in a
264 top-level blackhole router, like this:
8ff3788c 265
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266 # don't deliver messages handled by the BMI server
267 bmi_blackhole:
268 driver = redirect
269 bmi_dont_deliver
270 data = :blackhole:
8ff3788c 271
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272 This router should be on top of all others, so messages
273 that should not be delivered do not reach other routers
274 at all. If the message has not been processed by
275 the BMI server, this variable defaults to FALSE.
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276
277
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278 - A list router precondition to query if rules "fired" on
279 the message for the recipient. Its name is "bmi_rule". You
280 use it by passing it a colon-separated list of rule
281 numbers. You can use this condition to route messages that
282 matched specific rules. Here is an example:
8ff3788c 283
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284 # special router for BMI rule #5, #8 and #11
285 bmi_rule_redirect:
286 driver = redirect
287 bmi_rule = 5:8:11
288 data = postmaster@mydomain.com
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289
290
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291 - Expansion variables. Several expansion variables are set
292 during routing. You can use them in custom router
293 conditions, for example. The following variables are
294 available:
8ff3788c 295
ee161e8f 296 o $bmi_base64_verdict
8ff3788c 297
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298 This variable will contain the BASE64 encoded verdict
299 for the recipient being routed. You can use it to add a
300 header to messages for tracking purposes, for example:
8ff3788c 301
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302 localuser:
303 driver = accept
304 check_local_user
305 headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
306 transport = local_delivery
8ff3788c 307
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308 If there is no verdict available for the recipient being
309 routed, this variable contains the empty string.
8ff3788c 310
ee161e8f 311 o $bmi_base64_tracker_verdict
8ff3788c 312
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313 This variable will contain a BASE64 encoded subset of
314 the verdict information concerning the "rules" that
315 fired on the message. You can add this string to a
316 header, commonly named "X-Brightmail-Tracker". Example:
8ff3788c 317
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318 localuser:
319 driver = accept
320 check_local_user
321 headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
322 transport = local_delivery
8ff3788c 323
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324 If there is no verdict available for the recipient being
325 routed, this variable contains the empty string.
8ff3788c 326
ee161e8f 327 o $bmi_alt_location
8ff3788c 328
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329 If the verdict is to redirect the message to an
330 alternate location, this variable will contain the
331 alternate location string returned by the BMI server. In
332 its default configuration, this is a header-like string
333 that can be added to the message with "headers_add". If
334 there is no verdict available for the recipient being
335 routed, or if the message is to be delivered normally,
336 this variable contains the empty string.
8ff3788c 337
ee161e8f 338 o $bmi_deliver
8ff3788c 339
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340 This is an additional integer variable that can be used
341 to query if the message should be delivered at all. You
342 should use router preconditions instead if possible.
8ff3788c 343
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344 $bmi_deliver is '0': the message should NOT be delivered.
345 $bmi_deliver is '1': the message should be delivered.
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346
347
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348 IMPORTANT NOTE: Verdict inheritance.
349 The message is passed to the BMI server during message
350 reception, using the target addresses from the RCPT TO:
351 commands in the SMTP transaction. If recipients get expanded
352 or re-written (for example by aliasing), the new address(es)
353 inherit the verdict from the original address. This means
354 that verdicts also apply to all "child" addresses generated
355 from top-level addresses that were sent to the BMI server.
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356
357
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3585) Using per-recipient opt-in information (Optional)
359
360 The BMI server features multiple scanning "profiles" for
361 individual recipients. These are usually stored in a LDAP
362 server and are queried by the BMI server itself. However,
363 you can also pass opt-in data for each recipient from the
364 MTA to the BMI server. This is particularly useful if you
3ec3e3bb 365 already look up recipient data in Exim anyway (which can
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366 also be stored in a SQL database or other source). This
367 implementation enables you to pass opt-in data to the BMI
368 server in the RCPT ACL. This works by setting the
369 'bmi_optin' modifier in a block of that ACL. If should be
370 set to a list of comma-separated strings that identify the
371 features which the BMI server should use for that particular
372 recipient. Ideally, you would use the 'bmi_optin' modifier
373 in the same ACL block where you set the 'bmi_run' control
374 flag. Here is an example that will pull opt-in data for each
375 recipient from a flat file called
376 '/etc/exim/bmi_optin_data'.
8ff3788c 377
ee161e8f 378 The file format:
8ff3788c 379
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380 user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
381 user2@thatdomain.com: <OPTIN STRING3>
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382
383
ee161e8f 384 The example:
8ff3788c 385
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386 accept domains = +relay_to_domains
387 endpass
388 verify = recipient
389 bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
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390 control = bmi_run
391
ee161e8f 392 Of course, you can also use any other lookup method that
3ec3e3bb 393 Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
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394 as long as the result is a list of colon-separated opt-in
395 strings.
8ff3788c 396
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397 For a list of available opt-in strings, please contact your
398 Brightmail representative.
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400
401
402
0b23848a 403Sender Policy Framework (SPF) support
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404--------------------------------------------------------------
405
f413481d 406To learn more about SPF, visit http://www.openspf.org. This
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407document does not explain the SPF fundamentals, you should
408read and understand the implications of deploying SPF on your
409system before doing so.
410
8ff3788c 411SPF support is added via the libspf2 library. Visit
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412
413 http://www.libspf2.org/
8ff3788c 414
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415to obtain a copy, then compile and install it. By default,
416this will put headers in /usr/local/include and the static
417library in /usr/local/lib.
418
3ec3e3bb 419To compile Exim with SPF support, set these additional flags in
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420Local/Makefile:
421
422EXPERIMENTAL_SPF=yes
423CFLAGS=-DSPF -I/usr/local/include
424EXTRALIBS_EXIM=-L/usr/local/lib -lspf2
425
426This assumes that the libspf2 files are installed in
427their default locations.
428
429You can now run SPF checks in incoming SMTP by using the "spf"
430ACL condition in either the MAIL, RCPT or DATA ACLs. When
d36a0501 431using it in the RCPT ACL, you can make the checks dependent on
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432the RCPT address (or domain), so you can check SPF records
433only for certain target domains. This gives you the
434possibility to opt-out certain customers that do not want
435their mail to be subject to SPF checking.
436
437The spf condition takes a list of strings on its right-hand
438side. These strings describe the outcome of the SPF check for
439which the spf condition should succeed. Valid strings are:
440
441 o pass The SPF check passed, the sending host
442 is positively verified by SPF.
443 o fail The SPF check failed, the sending host
444 is NOT allowed to send mail for the domain
445 in the envelope-from address.
446 o softfail The SPF check failed, but the queried
447 domain can't absolutely confirm that this
448 is a forgery.
449 o none The queried domain does not publish SPF
450 records.
451 o neutral The SPF check returned a "neutral" state.
452 This means the queried domain has published
453 a SPF record, but wants to allow outside
454 servers to send mail under its domain as well.
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455 This should be treated like "none".
456 o permerror This indicates a syntax error in the SPF
457 record of the queried domain. You may deny
458 messages when this occurs. (Changed in 4.83)
459 o temperror This indicates a temporary error during all
3ec3e3bb 460 processing, including Exim's SPF processing.
ee161e8f 461 You may defer messages when this occurs.
8ddef691 462 (Changed in 4.83)
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463 o err_temp Same as permerror, deprecated in 4.83, will be
464 removed in a future release.
465 o err_perm Same as temperror, deprecated in 4.83, will be
466 removed in a future release.
8ff3788c 467
ee161e8f 468You can prefix each string with an exclamation mark to invert
982650ec 469its meaning, for example "!fail" will match all results but
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470"fail". The string list is evaluated left-to-right, in a
471short-circuit fashion. When a string matches the outcome of
472the SPF check, the condition succeeds. If none of the listed
473strings matches the outcome of the SPF check, the condition
474fails.
475
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476Here is an example to fail forgery attempts from domains that
477publish SPF records:
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478
479/* -----------------
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480deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}. \
481 Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address
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482 spf = fail
483--------------------- */
484
485You can also give special treatment to specific domains:
486
487/* -----------------
488deny message = AOL sender, but not from AOL-approved relay.
489 sender_domains = aol.com
490 spf = fail:neutral
491--------------------- */
492
493Explanation: AOL publishes SPF records, but is liberal and
494still allows non-approved relays to send mail from aol.com.
495This will result in a "neutral" state, while mail from genuine
496AOL servers will result in "pass". The example above takes
497this into account and treats "neutral" like "fail", but only
498for aol.com. Please note that this violates the SPF draft.
499
500When the spf condition has run, it sets up several expansion
501variables.
502
503 $spf_header_comment
504 This contains a human-readable string describing the outcome
505 of the SPF check. You can add it to a custom header or use
506 it for logging purposes.
8ff3788c 507
ee161e8f 508 $spf_received
8fe685ad 509 This contains a complete Received-SPF: header that can be
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510 added to the message. Please note that according to the SPF
511 draft, this header must be added at the top of the header
512 list. Please see section 10 on how you can do this.
8ff3788c 513
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514 Note: in case of "Best-guess" (see below), the convention is
515 to put this string in a header called X-SPF-Guess: instead.
516
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517 $spf_result
518 This contains the outcome of the SPF check in string form,
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519 one of pass, fail, softfail, none, neutral, permerror or
520 temperror.
8ff3788c 521
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522 $spf_smtp_comment
523 This contains a string that can be used in a SMTP response
524 to the calling party. Useful for "fail".
8ff3788c 525
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526In addition to SPF, you can also perform checks for so-called
527"Best-guess". Strictly speaking, "Best-guess" is not standard
528SPF, but it is supported by the same framework that enables SPF
529capability. Refer to http://www.openspf.org/FAQ/Best_guess_record
530for a description of what it means.
531
532To access this feature, simply use the spf_guess condition in place
533of the spf one. For example:
534
535/* -----------------
536deny message = $sender_host_address doesn't look trustworthy to me
537 spf_guess = fail
538--------------------- */
539
540In case you decide to reject messages based on this check, you
541should note that although it uses the same framework, "Best-guess"
542is NOT SPF, and therefore you should not mention SPF at all in your
543reject message.
544
545When the spf_guess condition has run, it sets up the same expansion
546variables as when spf condition is run, described above.
547
d36a0501 548Additionally, since Best-guess is not standardized, you may redefine
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549what "Best-guess" means to you by redefining spf_guess variable in
550global config. For example, the following:
551
552/* -----------------
553spf_guess = v=spf1 a/16 mx/16 ptr ?all
554--------------------- */
555
556would relax host matching rules to a broader network range.
8ff3788c 557
ee161e8f 558
0b23848a 559SRS (Sender Rewriting Scheme) Support
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560--------------------------------------------------------------
561
562Exiscan currently includes SRS support via Miles Wilton's
8ff3788c 563libsrs_alt library. The current version of the supported
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564library is 0.5.
565
566In order to use SRS, you must get a copy of libsrs_alt from
567
568http://srs.mirtol.com/
569
570Unpack the tarball, then refer to MTAs/README.EXIM
571to proceed. You need to set
572
573EXPERIMENTAL_SRS=yes
574
575in your Local/Makefile.
576
577
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578DCC Support
579--------------------------------------------------------------
580
581*) Building exim
582
583In order to build exim with DCC support add
584
585EXPERIMENTAL_DCC=yes
586
587to your Makefile. (Re-)build/install exim. exim -d should show
588EXPERIMENTAL_DCC under "Support for".
589
590
591*) Configuration
592
593In the main section of exim.cf add at least
594 dccifd_address = /usr/local/dcc/var/dccifd
595or
596 dccifd_address = <ip> <port>
597
598In the DATA ACL you can use the new condition
599 dcc = *
600
601After that "$dcc_header" contains the X-DCC-Header.
602
d36a0501 603Return values are:
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604 fail for overall "R", "G" from dccifd
605 defer for overall "T" from dccifd
606 accept for overall "A", "S" from dccifd
607
608dcc = */defer_ok works as for spamd.
609
610The "$dcc_result" variable contains the overall result from DCC
611answer. There will an X-DCC: header added to the mail.
612
613Usually you'll use
614 defer !dcc = *
615to greylist with DCC.
616
617If you set, in the main section,
618 dcc_direct_add_header = true
619then the dcc header will be added "in deep" and if the spool
620file was already written it gets removed. This forces Exim to
621write it again if needed. This helps to get the DCC Header
622through to eg. SpamAssassin.
623
624If you want to pass even more headers in the middle of the
625DATA stage you can set
626 $acl_m_dcc_add_header
05c39afa 627to tell the DCC routines to add more information; eg, you might set
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628this to some results from ClamAV. Be careful. Header syntax is
629not checked and is added "as is".
630
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631In case you've troubles with sites sending the same queue items from several
632hosts and fail to get through greylisting you can use
633$acl_m_dcc_override_client_ip
634
635Setting $acl_m_dcc_override_client_ip to an IP address overrides the default
636of $sender_host_address. eg. use the following ACL in DATA stage:
637
638 warn set acl_m_dcc_override_client_ip = \
639 ${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}}
640 condition = ${if def:acl_m_dcc_override_client_ip}
641 log_message = dbg: acl_m_dcc_override_client_ip set to \
642 $acl_m_dcc_override_client_ip
643
644Then set something like
645# cat /etc/mail/multipleip_sites
646mout-xforward.gmx.net 82.165.159.12
647mout.gmx.net 212.227.15.16
648
649Use a reasonable IP. eg. one the sending cluster acutally uses.
0e1ccf44 650
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651DMARC Support
652--------------------------------------------------------------
653
654DMARC combines feedback from SPF, DKIM, and header From: in order
655to attempt to provide better indicators of the authenticity of an
656email. This document does not explain the fundamentals, you
657should read and understand how it works by visiting the website at
658http://www.dmarc.org/.
659
660DMARC support is added via the libopendmarc library. Visit:
661
662 http://sourceforge.net/projects/opendmarc/
663
664to obtain a copy, or find it in your favorite rpm package
665repository. If building from source, this description assumes
666that headers will be in /usr/local/include, and that the libraries
667are in /usr/local/lib.
668
6691. To compile Exim with DMARC support, you must first enable SPF.
670Please read the above section on enabling the EXPERIMENTAL_SPF
671feature. You must also have DKIM support, so you cannot set the
672DISABLE_DKIM feature. Once both of those conditions have been met
673you can enable DMARC in Local/Makefile:
674
675EXPERIMENTAL_DMARC=yes
676LDFLAGS += -lopendmarc
677# CFLAGS += -I/usr/local/include
678# LDFLAGS += -L/usr/local/lib
679
680The first line sets the feature to include the correct code, and
681the second line says to link the libopendmarc libraries into the
682exim binary. The commented out lines should be uncommented if you
683built opendmarc from source and installed in the default location.
684Adjust the paths if you installed them elsewhere, but you do not
685need to uncomment them if an rpm (or you) installed them in the
686package controlled locations (/usr/include and /usr/lib).
687
688
6892. Use the following global settings to configure DMARC:
690
691Required:
692dmarc_tld_file Defines the location of a text file of valid
693 top level domains the opendmarc library uses
694 during domain parsing. Maintained by Mozilla,
695 the most current version can be downloaded
696 from a link at http://publicsuffix.org/list/.
697
698Optional:
699dmarc_history_file Defines the location of a file to log results
700 of dmarc verification on inbound emails. The
701 contents are importable by the opendmarc tools
702 which will manage the data, send out DMARC
703 reports, and expire the data. Make sure the
704 directory of this file is writable by the user
705 exim runs as.
706
707dmarc_forensic_sender The email address to use when sending a
708 forensic report detailing alignment failures
709 if a sender domain's dmarc record specifies it
710 and you have configured Exim to send them.
711 Default: do-not-reply@$default_hostname
712
713
7143. By default, the DMARC processing will run for any remote,
715non-authenticated user. It makes sense to only verify DMARC
716status of messages coming from remote, untrusted sources. You can
717use standard conditions such as hosts, senders, etc, to decide that
718DMARC verification should *not* be performed for them and disable
719DMARC with a control setting:
720
12d0043d 721 control = dmarc_disable_verify
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722
723A DMARC record can also specify a "forensic address", which gives
724exim an email address to submit reports about failed alignment.
725Exim does not do this by default because in certain conditions it
726results in unintended information leakage (what lists a user might
727be subscribed to, etc). You must configure exim to submit forensic
728reports to the owner of the domain. If the DMARC record contains a
729forensic address and you specify the control statement below, then
730exim will send these forensic emails. It's also advised that you
731configure a dmarc_forensic_sender because the default sender address
732construction might be inadequate.
733
734 control = dmarc_forensic_enable
735
736(AGAIN: You can choose not to send these forensic reports by simply
737not putting the dmarc_forensic_enable control line at any point in
738your exim config. If you don't tell it to send them, it will not
739send them.)
740
741There are no options to either control. Both must appear before
742the DATA acl.
743
744
7454. You can now run DMARC checks in incoming SMTP by using the
746"dmarc_status" ACL condition in the DATA ACL. You are required to
747call the spf condition first in the ACLs, then the "dmarc_status"
748condition. Putting this condition in the ACLs is required in order
749for a DMARC check to actually occur. All of the variables are set
750up before the DATA ACL, but there is no actual DMARC check that
751occurs until a "dmarc_status" condition is encountered in the ACLs.
752
753The dmarc_status condition takes a list of strings on its
754right-hand side. These strings describe recommended action based
755on the DMARC check. To understand what the policy recommendations
756mean, refer to the DMARC website above. Valid strings are:
757
758 o accept The DMARC check passed and the library recommends
759 accepting the email.
760 o reject The DMARC check failed and the library recommends
761 rejecting the email.
762 o quarantine The DMARC check failed and the library recommends
763 keeping it for further inspection.
7a8678e6 764 o none The DMARC check passed and the library recommends
765 no specific action, neutral.
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766 o norecord No policy section in the DMARC record for this
767 sender domain.
768 o nofrom Unable to determine the domain of the sender.
7a8678e6 769 o temperror Library error or dns error.
05070e30 770 o off The DMARC check was disabled for this email.
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771
772You can prefix each string with an exclamation mark to invert its
773meaning, for example "!accept" will match all results but
774"accept". The string list is evaluated left-to-right in a
775short-circuit fashion. When a string matches the outcome of the
776DMARC check, the condition succeeds. If none of the listed
777strings matches the outcome of the DMARC check, the condition
778fails.
779
780Of course, you can also use any other lookup method that Exim
781supports, including LDAP, Postgres, MySQL, etc, as long as the
8c8b8274 782result is a list of colon-separated strings.
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783
784Several expansion variables are set before the DATA ACL is
785processed, and you can use them in this ACL. The following
786expansion variables are available:
787
788 o $dmarc_status
789 This is a one word status indicating what the DMARC library
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790 thinks of the email. It is a combination of the results of
791 DMARC record lookup and the SPF/DKIM/DMARC processing results
792 (if a DMARC record was found). The actual policy declared
793 in the DMARC record is in a separate expansion variable.
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794
795 o $dmarc_status_text
796 This is a slightly longer, human readable status.
797
798 o $dmarc_used_domain
799 This is the domain which DMARC used to look up the DMARC
800 policy record.
801
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802 o $dmarc_domain_policy
803 This is the policy declared in the DMARC record. Valid values
804 are "none", "reject" and "quarantine". It is blank when there
805 is any error, including no DMARC record.
806
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807 o $dmarc_ar_header
808 This is the entire Authentication-Results header which you can
809 add using an add_header modifier.
810
811
8125. How to enable DMARC advanced operation:
813By default, Exim's DMARC configuration is intended to be
814non-intrusive and conservative. To facilitate this, Exim will not
815create any type of logging files without explicit configuration by
816you, the admin. Nor will Exim send out any emails/reports about
817DMARC issues without explicit configuration by you, the admin (other
818than typical bounce messages that may come about due to ACL
819processing or failure delivery issues).
820
821In order to log statistics suitable to be imported by the opendmarc
822tools, you need to:
823a. Configure the global setting dmarc_history_file.
824b. Configure cron jobs to call the appropriate opendmarc history
825 import scripts and truncating the dmarc_history_file.
826
827In order to send forensic reports, you need to:
828a. Configure the global setting dmarc_forensic_sender.
829b. Configure, somewhere before the DATA ACL, the control option to
830 enable sending DMARC forensic reports.
831
832
8336. Example usage:
834(RCPT ACL)
835 warn domains = +local_domains
836 hosts = +local_hosts
12d0043d 837 control = dmarc_disable_verify
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838
839 warn !domains = +screwed_up_dmarc_records
840 control = dmarc_enable_forensic
841
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842 warn condition = (lookup if destined to mailing list)
843 set acl_m_mailing_list = 1
844
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845(DATA ACL)
846 warn dmarc_status = accept : none : off
847 !authenticated = *
848 log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain
849 add_header = $dmarc_ar_header
850
851 warn dmarc_status = !accept
852 !authenticated = *
853 log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain
854
855 warn dmarc_status = quarantine
856 !authenticated = *
857 set $acl_m_quarantine = 1
858 # Do something in a transport with this flag variable
859
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860 deny condition = ${if eq{$dmarc_domain_policy}{reject}}
861 condition = ${if eq{$acl_m_mailing_list}{1}}
862 message = Messages from $dmarc_used_domain break mailing lists
863
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864 deny dmarc_status = reject
865 !authenticated = *
866 message = Message from $domain_used_domain failed sender's DMARC policy, REJECT
867
868
869
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JH
870Transport post-delivery actions
871--------------------------------------------------------------
872
873An arbitrary per-transport string can be expanded on successful delivery,
874and (for SMTP transports) a second string on deferrals caused by a host error.
875This feature may be used, for example, to write exim internal log information
876(not available otherwise) into a database.
877
878In order to use the feature, you must set
879
880EXPERIMENTAL_TPDA=yes
881
882in your Local/Makefile
883
884and define the expandable strings in the runtime config file, to
885be executed at end of delivery.
886
887Additionally, there are 6 more variables, available at end of
888delivery:
889
890tpda_delivery_ip IP of host, which has accepted delivery
891tpda_delivery_port Port of remote host which has accepted delivery
892tpda_delivery_fqdn FQDN of host, which has accepted delivery
893tpda_delivery_local_part local part of address being delivered
894tpda_delivery_domain domain part of address being delivered
895tpda_delivery_confirmation SMTP confirmation message
896
897In case of a deferral caused by a host-error:
898tpda_defer_errno Error number
899tpda_defer_errstr Error string possibly containing more details
900
901The $router_name and $transport_name variables are also usable.
902
903
904To take action after successful deliveries, set the following option
905on any transport of interest.
906
907tpda_delivery_action
908
909An example might look like:
910
911tpda_delivery_action = \
912${lookup pgsql {SELECT * FROM record_Delivery( \
913 '${quote_pgsql:$sender_address_domain}',\
914 '${quote_pgsql:${lc:$sender_address_local_part}}', \
915 '${quote_pgsql:$tpda_delivery_domain}', \
916 '${quote_pgsql:${lc:$tpda_delivery_local_part}}', \
917 '${quote_pgsql:$tpda_delivery_ip}', \
918 '${quote_pgsql:${lc:$tpda_delivery_fqdn}}', \
919 '${quote_pgsql:$message_exim_id}')}}
920
921The string is expanded after the delivery completes and any
922side-effects will happen. The result is then discarded.
923Note that for complex operations an ACL expansion can be used.
924
925
926In order to log host deferrals, add the following option to an SMTP
927transport:
928
929tpda_host_defer_action
930
931This is a private option of the SMTP transport. It is intended to
932log failures of remote hosts. It is executed only when exim has
933attempted to deliver a message to a remote host and failed due to
934an error which doesn't seem to be related to the individual
935message, sender, or recipient address.
7c261cc6 936See section 47.2 of the exim documentation for more details on how
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JH
937this is determined.
938
939Example:
940
941tpda_host_defer_action = \
942${lookup mysql {insert into delivlog set \
943 msgid = '${quote_mysql:$message_exim_id}', \
944 senderlp = '${quote_mysql:${lc:$sender_address_local_part}}', \
945 senderdom = '${quote_mysql:$sender_address_domain}', \
946 delivlp = '${quote_mysql:${lc:$tpda_delivery_local_part}}', \
947 delivdom = '${quote_mysql:$tpda_delivery_domain}', \
948 delivip = '${quote_mysql:$tpda_delivery_ip}', \
949 delivport = '${quote_mysql:$tpda_delivery_port}', \
950 delivfqdn = '${quote_mysql:$tpda_delivery_fqdn}', \
951 deliverrno = '${quote_mysql:$tpda_defer_errno}', \
952 deliverrstr = '${quote_mysql:$tpda_defer_errstr}' \
953 }}
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955
956Redis Lookup
957--------------------------------------------------------------
958
959Redis is open source advanced key-value data store. This document
960does not explain the fundamentals, you should read and understand how
961it works by visiting the website at http://www.redis.io/.
962
963Redis lookup support is added via the hiredis library. Visit:
964
965 https://github.com/redis/hiredis
966
967to obtain a copy, or find it in your operating systems package repository.
968If building from source, this description assumes that headers will be in
969/usr/local/include, and that the libraries are in /usr/local/lib.
970
9711. In order to build exim with Redis lookup support add
972
973EXPERIMENTAL_REDIS=yes
974
975to your Local/Makefile. (Re-)build/install exim. exim -d should show
976Experimental_Redis in the line "Support for:".
977
978EXPERIMENTAL_REDIS=yes
979LDFLAGS += -lhiredis
980# CFLAGS += -I/usr/local/include
981# LDFLAGS += -L/usr/local/lib
982
983The first line sets the feature to include the correct code, and
984the second line says to link the hiredis libraries into the
985exim binary. The commented out lines should be uncommented if you
986built hiredis from source and installed in the default location.
987Adjust the paths if you installed them elsewhere, but you do not
988need to uncomment them if an rpm (or you) installed them in the
989package controlled locations (/usr/include and /usr/lib).
990
991
9922. Use the following global settings to configure Redis lookup support:
993
994Required:
995redis_servers This option provides a list of Redis servers
996 and associated connection data, to be used in
997 conjunction with redis lookups. The option is
998 only available if Exim is configured with Redis
999 support.
1000
1001For example:
1002
1003redis_servers = 127.0.0.1/10/ - using database 10 with no password
1004redis_servers = 127.0.0.1//password - to make use of the default database of 0 with a password
1005redis_servers = 127.0.0.1// - for default database of 0 with no password
1006
10073. Once you have the Redis servers defined you can then make use of the
1008experimental Redis lookup by specifying ${lookup redis{}} in a lookup query.
1009
10104. Example usage:
1011
1012(Host List)
1013hostlist relay_from_ips = <\n ${lookup redis{SMEMBERS relay_from_ips}}
1014
1015Where relay_from_ips is a Redis set which contains entries such as "192.168.0.0/24" "10.0.0.0/8" and so on.
1016The result set is returned as
1017192.168.0.0/24
101810.0.0.0/8
1019..
1020.
1021
1022(Domain list)
1023domainlist virtual_domains = ${lookup redis {HGET $domain domain}}
1024
1025Where $domain is a hash which includes the key 'domain' and the value '$domain'.
1026
1027(Adding or updating an existing key)
1028set acl_c_spammer = ${if eq{${lookup redis{SPAMMER_SET}}}{OK}}
1029
1030Where SPAMMER_SET is a macro and it is defined as
1031
1032"SET SPAMMER <some_value>"
1033
1034(Getting a value from Redis)
1035
1036set acl_c_spam_host = ${lookup redis{GET...}}
1037
1038
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1039Proxy Protocol Support
1040--------------------------------------------------------------
1041
1042Exim now has Experimental "Proxy Protocol" support. It was built on
1043specifications from:
1044http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt
1045
1046The purpose of this function is so that an application load balancer,
1047such as HAProxy, can sit in front of several Exim servers and Exim
1048will log the IP that is connecting to the proxy server instead of
1049the IP of the proxy server when it connects to Exim. It resets the
1050$sender_address_host and $sender_address_port to the IP:port of the
1051connection to the proxy. It also re-queries the DNS information for
1052this new IP address so that the original sender's hostname and IP
1053get logged in the Exim logfile. There is no logging if a host passes or
1054fails Proxy Protocol negotiation, but it can easily be determined and
1055recorded in an ACL (example is below).
1056
10571. To compile Exim with Proxy Protocol support, put this in
1058Local/Makefile:
1059
1060EXPERIMENTAL_PROXY=yes
1061
10622. Global configuration settings:
1063
1064proxy_required_hosts = HOSTLIST
1065
1066The proxy_required_hosts option will require any IP in that hostlist
1067to use Proxy Protocol. The specification of Proxy Protocol is very
1068strict, and if proxy negotiation fails, Exim will not allow any SMTP
1069command other than QUIT. (See end of this section for an example.)
1070The option is expanded when used, so it can be a hostlist as well as
1071string of IP addresses. Since it is expanded, specifying an alternate
1072separator is supported for ease of use with IPv6 addresses.
1073
1074To log the IP of the proxy in the incoming logline, add:
1075 log_selector = +proxy
1076
1077A default incoming logline (wrapped for appearance) will look like this:
1078
1079 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
1080 H=mail.example.net [1.2.3.4] P=esmtp S=433
1081
1082With the log selector enabled, an email that was proxied through a
1083Proxy Protocol server at 192.168.1.2 will look like this:
1084
1085 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
1086 H=mail.example.net [1.2.3.4] P=esmtp PRX=192.168.1.2 S=433
1087
10883. In the ACL's the following expansion variables are available.
1089
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1090proxy_host_address The (internal) src IP of the proxy server
1091 making the connection to the Exim server.
1092proxy_host_port The (internal) src port the proxy server is
1093 using to connect to the Exim server.
1094proxy_target_address The dest (public) IP of the remote host to
1095 the proxy server.
1096proxy_target_port The dest port the remote host is using to
1097 connect to the proxy server.
1098proxy_session Boolean, yes/no, the connected host is required
1099 to use Proxy Protocol.
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1100
1101There is no expansion for a failed proxy session, however you can detect
1102it by checking if $proxy_session is true but $proxy_host is empty. As
1103an example, in my connect ACL, I have:
1104
1105 warn condition = ${if and{ {bool{$proxy_session}} \
a3bddaa8 1106 {eq{$proxy_host_address}{}} } }
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1107 log_message = Failed required proxy protocol negotiation \
1108 from $sender_host_name [$sender_host_address]
1109
1110 warn condition = ${if and{ {bool{$proxy_session}} \
a3bddaa8 1111 {!eq{$proxy_host_address}{}} } }
a3c86431 1112 # But don't log health probes from the proxy itself
a3bddaa8 1113 condition = ${if eq{$proxy_host_address}{$sender_host_address} \
a3c86431
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1114 {false}{true}}
1115 log_message = Successfully proxied from $sender_host_name \
1116 [$sender_host_address] through proxy protocol \
a3bddaa8 1117 host $proxy_host_address
a3c86431 1118
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1119 # Possibly more clear
1120 warn logwrite = Remote Source Address: $sender_host_address:$sender_host_port
1121 logwrite = Proxy Target Address: $proxy_target_address:$proxy_target_port
1122 logwrite = Proxy Internal Address: $proxy_host_address:$proxy_host_port
1123 logwrite = Internal Server Address: $received_ip_address:$received_port
1124
1125
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11264. Runtime issues to be aware of:
1127 - Since the real connections are all coming from your proxy, and the
1128 per host connection tracking is done before Proxy Protocol is
1129 evaluated, smtp_accept_max_per_host must be set high enough to
1130 handle all of the parallel volume you expect per inbound proxy.
1131 - The proxy has 3 seconds (hard-coded in the source code) to send the
1132 required Proxy Protocol header after it connects. If it does not,
1133 the response to any commands will be:
1134 "503 Command refused, required Proxy negotiation failed"
1135 - If the incoming connection is configured in Exim to be a Proxy
1136 Protocol host, but the proxy is not sending the header, the banner
1137 does not get sent until the timeout occurs. If the sending host
1138 sent any input (before the banner), this causes a standard Exim
1139 synchronization error (i.e. trying to pipeline before PIPELINING
1140 was advertised).
1141 - This is not advised, but is mentioned for completeness if you have
1142 a specific internal configuration that you want this: If the Exim
1143 server only has an internal IP address and no other machines in your
1144 organization will connect to it to try to send email, you may
1145 simply set the hostlist to "*", however, this will prevent local
1146 mail programs from working because that would require mail from
1147 localhost to use Proxy Protocol. Again, not advised!
1148
11495. Example of a refused connection because the Proxy Protocol header was
1150not sent from a host configured to use Proxy Protocol. In the example,
1151the 3 second timeout occurred (when a Proxy Protocol banner should have
1152been sent), the banner was displayed to the user, but all commands are
1153rejected except for QUIT:
1154
1155# nc mail.example.net 25
1156220-mail.example.net, ESMTP Exim 4.82+proxy, Mon, 04 Nov 2013 10:45:59
1157220 -0800 RFC's enforced
1158EHLO localhost
1159503 Command refused, required Proxy negotiation failed
1160QUIT
1161221 mail.example.net closing connection
1162
1163
9bdd29ad 1164
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PH
1165--------------------------------------------------------------
1166End of file
1167--------------------------------------------------------------