SRS: add basic documentation
[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
4c04137d 9Brightmail AntiSpam (BMI) support
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10--------------------------------------------------------------
11
12Brightmail AntiSpam is a commercial package. Please see
13http://www.brightmail.com for more information on
14the product. For the sake of clarity, we'll refer to it as
15"BMI" from now on.
16
17
180) BMI concept and implementation overview
19
20In contrast to how spam-scanning with SpamAssassin is
21implemented in exiscan-acl, BMI is more suited for per
22-recipient scanning of messages. However, each messages is
23scanned only once, but multiple "verdicts" for multiple
24recipients can be returned from the BMI server. The exiscan
25implementation passes the message to the BMI server just
26before accepting it. It then adds the retrieved verdicts to
27the messages header file in the spool. These verdicts can then
28be queried in routers, where operation is per-recipient
29instead of per-message. To use BMI, you need to take the
30following steps:
31
32 1) Compile Exim with BMI support
3ec3e3bb 33 2) Set up main BMI options (top section of Exim config file)
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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
8ff3788c 40These four steps are explained in more details below.
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41
421) Adding support for BMI at compile time
43
44 To compile with BMI support, you need to link Exim against
4c04137d 45 the Brightmail client SDK, consisting of a library
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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
47bbda99 52 CFLAGS=-I/path/to/the/dir/with/the/includefile
ee161e8f 53 EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
8ff3788c 54
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55 If you use other CFLAGS or EXTRALIBS_EXIM settings then
56 merge the content of these lines with them.
57
7c0c8547 58 Note for BMI6.x users: You'll also have to add -lxml2_single
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59 to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do
60 this.
8ff3788c 61
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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
3ec3e3bb 692) Setting up BMI support in the Exim main configuration
ee161e8f 70
3ec3e3bb 71 To enable BMI support in the main Exim configuration, you
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72 should set the path to the main BMI configuration file with
73 the "bmi_config_file" option, like this:
8ff3788c 74
ee161e8f 75 bmi_config_file = /opt/brightmail/etc/brightmail.cfg
8ff3788c 76
3ec3e3bb 77 This must go into section 1 of Exim's configuration file (You
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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.
8ff3788c 84
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85
863) 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
3ec3e3bb 96 the "accept" blocks from Exim's default configuration file:
8ff3788c 97
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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
8ff3788c 108
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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
1134) 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
3ec3e3bb 119 at that stage. From Exim's view, a verdict can have the
ee161e8f 120 following outcomes:
8ff3788c 121
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122 o deliver the message normally
123 o deliver the message to an alternate location
124 o do not deliver the message
8ff3788c 125
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126 To query the verdict for a recipient, the implementation
127 offers the following tools:
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128
129
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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:
8ff3788c 134
ee161e8f 135 o bmi_deliver_default
8ff3788c 136
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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.
8ff3788c 141
ee161e8f 142 o bmi_deliver_alternate
8ff3788c 143
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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.
8ff3788c 150
ee161e8f 151 o bmi_dont_deliver
8ff3788c 152
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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:
8ff3788c 157
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158 # don't deliver messages handled by the BMI server
159 bmi_blackhole:
160 driver = redirect
161 bmi_dont_deliver
162 data = :blackhole:
8ff3788c 163
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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.
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168
169
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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:
8ff3788c 175
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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
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181
182
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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:
8ff3788c 187
ee161e8f 188 o $bmi_base64_verdict
8ff3788c 189
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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:
8ff3788c 193
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194 localuser:
195 driver = accept
196 check_local_user
197 headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
198 transport = local_delivery
8ff3788c 199
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200 If there is no verdict available for the recipient being
201 routed, this variable contains the empty string.
8ff3788c 202
ee161e8f 203 o $bmi_base64_tracker_verdict
8ff3788c 204
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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:
8ff3788c 209
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210 localuser:
211 driver = accept
212 check_local_user
213 headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
214 transport = local_delivery
8ff3788c 215
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216 If there is no verdict available for the recipient being
217 routed, this variable contains the empty string.
8ff3788c 218
ee161e8f 219 o $bmi_alt_location
8ff3788c 220
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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.
8ff3788c 229
ee161e8f 230 o $bmi_deliver
8ff3788c 231
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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.
8ff3788c 235
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236 $bmi_deliver is '0': the message should NOT be delivered.
237 $bmi_deliver is '1': the message should be delivered.
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238
239
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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.
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248
249
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2505) 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
3ec3e3bb 257 already look up recipient data in Exim anyway (which can
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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'.
8ff3788c 269
ee161e8f 270 The file format:
8ff3788c 271
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272 user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
273 user2@thatdomain.com: <OPTIN STRING3>
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274
275
ee161e8f 276 The example:
8ff3788c 277
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278 accept domains = +relay_to_domains
279 endpass
280 verify = recipient
281 bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
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282 control = bmi_run
283
ee161e8f 284 Of course, you can also use any other lookup method that
3ec3e3bb 285 Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
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286 as long as the result is a list of colon-separated opt-in
287 strings.
8ff3788c 288
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289 For a list of available opt-in strings, please contact your
290 Brightmail representative.
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292
293
294
0b23848a 295SRS (Sender Rewriting Scheme) Support
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296--------------------------------------------------------------
297
298Exiscan currently includes SRS support via Miles Wilton's
8ff3788c 299libsrs_alt library. The current version of the supported
f413521a 300library is 0.5, there are reports of 1.0 working.
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301
302In order to use SRS, you must get a copy of libsrs_alt from
303
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304https://opsec.eu/src/srs/
305
306(not the original source, which has disappeared.)
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307
308Unpack the tarball, then refer to MTAs/README.EXIM
309to proceed. You need to set
310
311EXPERIMENTAL_SRS=yes
312
313in your Local/Makefile.
314
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315The following main-section options become available:
316 srs_config string
317 srs_hashlength int
318 srs_hashmin int
319 srs_maxage int
320 srs_secrets string
321 srs_usehash bool
322 srs_usetimestamp bool
323
324The redirect router gains these options (all of type string, unset by default):
325 srs
326 srs_alias
327 srs_condition
328 srs_dbinsert
329 srs_dbselect
330
331The following variables become available:
332 $srs_db_address
333 $srs_db_key
334 $srs_orig_recipient
335 $srs_orig_sender
336 $srs_recipient
337 $srs_status
338
339The predefined feature-macro _HAVE_SRS will be present.
340Additional delivery log line elements, tagged with "SRS=" will show the srs sender.
341For configuration information see https://github.com/Exim/exim/wiki/SRS .
342
343
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f413521a 345
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346DCC Support
347--------------------------------------------------------------
d36254f2 348Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
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349
350*) Building exim
351
352In order to build exim with DCC support add
353
354EXPERIMENTAL_DCC=yes
355
356to your Makefile. (Re-)build/install exim. exim -d should show
357EXPERIMENTAL_DCC under "Support for".
358
359
360*) Configuration
361
362In the main section of exim.cf add at least
363 dccifd_address = /usr/local/dcc/var/dccifd
364or
365 dccifd_address = <ip> <port>
366
367In the DATA ACL you can use the new condition
368 dcc = *
369
370After that "$dcc_header" contains the X-DCC-Header.
371
d36a0501 372Return values are:
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373 fail for overall "R", "G" from dccifd
374 defer for overall "T" from dccifd
375 accept for overall "A", "S" from dccifd
376
377dcc = */defer_ok works as for spamd.
378
379The "$dcc_result" variable contains the overall result from DCC
380answer. There will an X-DCC: header added to the mail.
381
382Usually you'll use
383 defer !dcc = *
384to greylist with DCC.
385
386If you set, in the main section,
387 dcc_direct_add_header = true
388then the dcc header will be added "in deep" and if the spool
389file was already written it gets removed. This forces Exim to
390write it again if needed. This helps to get the DCC Header
391through to eg. SpamAssassin.
392
393If you want to pass even more headers in the middle of the
394DATA stage you can set
395 $acl_m_dcc_add_header
05c39afa 396to tell the DCC routines to add more information; eg, you might set
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397this to some results from ClamAV. Be careful. Header syntax is
398not checked and is added "as is".
399
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400In case you've troubles with sites sending the same queue items from several
401hosts and fail to get through greylisting you can use
402$acl_m_dcc_override_client_ip
403
404Setting $acl_m_dcc_override_client_ip to an IP address overrides the default
405of $sender_host_address. eg. use the following ACL in DATA stage:
406
407 warn set acl_m_dcc_override_client_ip = \
408 ${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}}
409 condition = ${if def:acl_m_dcc_override_client_ip}
410 log_message = dbg: acl_m_dcc_override_client_ip set to \
411 $acl_m_dcc_override_client_ip
412
413Then set something like
414# cat /etc/mail/multipleip_sites
415mout-xforward.gmx.net 82.165.159.12
416mout.gmx.net 212.227.15.16
417
4c04137d 418Use a reasonable IP. eg. one the sending cluster actually uses.
0e1ccf44 419
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420
421
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422DSN extra information
423---------------------
424If compiled with EXPERIMENTAL_DSN_INFO extra information will be added
425to DSN fail messages ("bounces"), when available. The intent is to aid
426tracing of specific failing messages, when presented with a "bounce"
427complaint and needing to search logs.
428
429
430The remote MTA IP address, with port number if nonstandard.
431Example:
432 Remote-MTA: X-ip; [127.0.0.1]:587
433Rationale:
434 Several addresses may correspond to the (already available)
435 dns name for the remote MTA.
436
437The remote MTA connect-time greeting.
438Example:
439 X-Remote-MTA-smtp-greeting: X-str; 220 the.local.host.name ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000
440Rationale:
441 This string sometimes presents the remote MTA's idea of its
442 own name, and sometimes identifies the MTA software.
443
444The remote MTA response to HELO or EHLO.
445Example:
446 X-Remote-MTA-helo-response: X-str; 250-the.local.host.name Hello localhost [127.0.0.1]
447Limitations:
448 Only the first line of a multiline response is recorded.
449Rationale:
450 This string sometimes presents the remote MTA's view of
451 the peer IP connecting to it.
452
453The reporting MTA detailed diagnostic.
454Example:
455 X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:<d3@myhost.test.ex>: 550 hard error
456Rationale:
4c04137d 457 This string sometimes give extra information over the
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458 existing (already available) Diagnostic-Code field.
459
460
461Note that non-RFC-documented field names and data types are used.
462
463
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464LMDB Lookup support
465-------------------
466LMDB is an ultra-fast, ultra-compact, crash-proof key-value embedded data store.
4c04137d 467It is modeled loosely on the BerkeleyDB API. You should read about the feature
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468set as well as operation modes at https://symas.com/products/lightning-memory-mapped-database/
469
470LMDB single key lookup support is provided by linking to the LMDB C library.
471The current implementation does not support writing to the LMDB database.
472
473Visit https://github.com/LMDB/lmdb to download the library or find it in your
474operating systems package repository.
475
476If building from source, this description assumes that headers will be in
477/usr/local/include, and that the libraries are in /usr/local/lib.
478
4791. In order to build exim with LMDB lookup support add or uncomment
480
481EXPERIMENTAL_LMDB=yes
482
483to your Local/Makefile. (Re-)build/install exim. exim -d should show
484Experimental_LMDB in the line "Support for:".
485
486EXPERIMENTAL_LMDB=yes
487LDFLAGS += -llmdb
488# CFLAGS += -I/usr/local/include
489# LDFLAGS += -L/usr/local/lib
490
491The first line sets the feature to include the correct code, and
492the second line says to link the LMDB libraries into the
493exim binary. The commented out lines should be uncommented if you
494built LMDB from source and installed in the default location.
495Adjust the paths if you installed them elsewhere, but you do not
496need to uncomment them if an rpm (or you) installed them in the
497package controlled locations (/usr/include and /usr/lib).
498
4992. Create your LMDB files, you can use the mdb_load utility which is
500part of the LMDB distribution our your favourite language bindings.
501
5023. Add the single key lookups to your exim.conf file, example lookups
503are below.
504
505${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}}
506${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}fail}
507${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}}
895fbaf2 508
ed0512a1 509
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510Queuefile transport
511-------------------
512Queuefile is a pseudo transport which does not perform final delivery.
513It simply copies the exim spool files out of the spool directory into
514an external directory retaining the exim spool format.
515
516The spool files can then be processed by external processes and then
517requeued into exim spool directories for final delivery.
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518However, note carefully the warnings in the main documentation on
519qpool file formats.
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520
521The motivation/inspiration for the transport is to allow external
522processes to access email queued by exim and have access to all the
523information which would not be available if the messages were delivered
524to the process in the standard email formats.
525
526The mailscanner package is one of the processes that can take advantage
527of this transport to filter email.
528
529The transport can be used in the same way as the other existing transports,
530i.e by configuring a router to route mail to a transport configured with
531the queuefile driver.
532
533The transport only takes one option:
534
535* directory - This is used to specify the directory messages should be
9604413b 536copied to. Expanded.
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537
538The generic transport options (body_only, current_directory, disable_logging,
539debug_print, delivery_date_add, envelope_to_add, event_action, group,
540headers_add, headers_only, headers_remove, headers_rewrite, home_directory,
541initgroups, max_parallel, message_size_limit, rcpt_include_affixes,
542retry_use_local_part, return_path, return_path_add, shadow_condition,
543shadow_transport, transport_filter, transport_filter_timeout, user) are
544ignored.
545
546Sample configuration:
547
548(Router)
549
550scan:
551 driver = accept
552 transport = scan
553
554(Transport)
555
556scan:
557 driver = queuefile
558 directory = /var/spool/baruwa-scanner/input
559
560
561In order to build exim with Queuefile transport support add or uncomment
562
563EXPERIMENTAL_QUEUEFILE=yes
564
565to your Local/Makefile. (Re-)build/install exim. exim -d should show
566Experimental_QUEUEFILE in the line "Support for:".
567
568
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569ARC support
570-----------
571Specification: https://tools.ietf.org/html/draft-ietf-dmarc-arc-protocol-11
572Note that this is not an RFC yet, so may change.
573
574ARC is intended to support the utility of SPF and DKIM in the presence of
575intermediaries in the transmission path - forwarders and mailinglists -
576by establishing a cryptographically-signed chain in headers.
577
578Normally one would only bother doing ARC-signing when functioning as
579an intermediary. One might do verify for local destinations.
580
581ARC uses the notion of a "ADministrative Management Domain" (ADMD).
582Described in RFC 5598 (section 2.3), this is essentially the set of
583mail-handling systems that the mail transits. A label should be chosen to
584identify the ADMD. Messages should be ARC-verified on entry to the ADMD,
585and ARC-signed on exit from it.
586
587
588Verification
589--
590An ACL condition is provided to perform the "verifier actions" detailed
591in section 6 of the above specification. It may be called from the DATA ACL
592and succeeds if the result matches any of a given list.
593It also records the highest ARC instance number (the chain size)
594and verification result for later use in creating an Authentication-Results:
595standard header.
596
597 verify = arc/<acceptable_list> none:fail:pass
598
599 add_header = :at_start:${authresults {<admd-identifier>}}
600
601 Note that it would be wise to strip incoming messages of A-R headers
602 that claim to be from our own <admd-identifier>.
603
a1d4300b 604There are four new variables:
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605
606 $arc_state One of pass, fail, none
607 $arc_state_reason (if fail, why)
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608 $arc_domains colon-sep list of ARC chain domains, in chain order.
609 problematic elements may have empty list elements
ea7b1f16 610 $arc_oldest_pass lowest passing instance number of chain
93c931f8 611
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612Example:
613 logwrite = oldest-p-ams: <${reduce {$lh_ARC-Authentication-Results:} \
614 {} \
615 {${if = {$arc_oldest_pass} \
616 {${extract {i}{${extract {1}{;}{$item}}}}} \
617 {$item} {$value}}} \
618 }>
619
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620Receive log lines for an ARC pass will be tagged "ARC".
621
622
623Signing
624--
a93dbf44 625arc_sign = <admd-identifier> : <selector> : <privkey> [ : <options> ]
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626An option on the smtp transport, which constructs and prepends to the message
627an ARC set of headers. The textually-first Authentication-Results: header
628is used as a basis (you must have added one on entry to the ADMD).
9f7d9fa1 629Expanded as a whole; if unset, empty or forced-failure then no signing is done.
5054c4fd 630If it is set, all of the first three elements must be non-empty.
617d3932 631
a93dbf44 632The fourth element is optional, and if present consists of a comma-separated list
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633of options. The options implemented are
634
635 timestamps Add a t= tag to the generated AMS and AS headers, with the
636 current time.
637 expire[=<val>] Add an x= tag to the generated AMS header, with an expiry time.
638 If the value <val> is an plain number it is used unchanged.
639 If it starts with a '+' then the following number is added
640 to the current time, as an offset in seconds.
641 If a value is not given it defaults to a one month offset.
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642
643[As of writing, gmail insist that a t= tag on the AS is mandatory]
644
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645Caveats:
646 * There must be an Authentication-Results header, presumably added by an ACL
647 while receiving the message, for the same ADMD, for arc_sign to succeed.
648 This requires careful coordination between inbound and outbound logic.
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649
650 Only one A-R header is taken account of. This is a limitation versus
651 the ARC spec (which says that all A-R headers from within the ADMD must
652 be used).
653
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654 * If passing a message to another system, such as a mailing-list manager
655 (MLM), between receipt and sending, be wary of manipulations to headers made
656 by the MLM.
657 + For instance, Mailman with REMOVE_DKIM_HEADERS==3 might improve
658 deliverability in a pre-ARC world, but that option also renames the
659 Authentication-Results header, which breaks signing.
5054c4fd 660
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661 * Even if you use multiple DKIM keys for different domains, the ARC concept
662 should try to stick to one ADMD, so pick a primary domain and use that for
663 AR headers and outbound signing.
664
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665Signing is not compatible with cutthrough delivery; any (before expansion)
666value set for the option will result in cutthrough delivery not being
667used via the transport in question.
668
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669
670
8ac90765 671
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672TLS Session Resumption
673----------------------
43e2db44 674TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
b10c87b3 675in RFC 5077 for 1.2). The support for this can be included by building with
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676EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1
677(or later).
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678
679Session resumption (this is the "stateless" variant) involves the server sending
680a "session ticket" to the client on one connection, which can be stored by the
681client and used for a later session. The ticket contains sufficient state for
682the server to reconstruct the TLS session, avoiding some expensive crypto
683calculation and one full packet roundtrip time.
684
685Operational cost/benefit:
686 The extra data being transmitted costs a minor amount, and the client has
68c62739 687 extra costs in storing and retrieving the data.
b10c87b3 688
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689 In the Exim/Gnutls implementation the extra cost on an initial connection
690 which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
691 The saved cost on a subsequent connection is about 4ms; three or more
692 connections become a net win. On longer network paths, two or more
693 connections will have an average lower startup time thanks to the one
694 saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
695 packet roundtrips.
696
697 Since a new hints DB is used, the hints DB maintenance should be updated
698 to additionally handle "tls".
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699
700Security aspects:
701 The session ticket is encrypted, but is obviously an additional security
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702 vulnarability surface. An attacker able to decrypt it would have access
703 all connections using the resumed session.
704 The session ticket encryption key is not committed to storage by the server
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705 and is rotated regularly (OpenSSL: 1hr, and one previous key is used for
706 overlap; GnuTLS 6hr but does not specify any overlap).
707 Tickets have limited lifetime (2hr, and new ones issued after 1hr under
708 OpenSSL. GnuTLS 2hr, appears to not do overlap).
b10c87b3 709
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710 There is a question-mark over the security of the Diffie-Helman parameters
711 used for session negotiation. TBD. q-value; cf bug 1895
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712
713Observability:
714 New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X="
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715 element.
716
4a1bd6b9 717 Variables $tls_{in,out}_resumption have bits 0-4 indicating respectively
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718 support built, client requested ticket, client offered session,
719 server issued ticket, resume used. A suitable decode list is provided
720 in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
721
722Issues:
723 In a resumed session:
11c4a22b 724 $tls_{in,out}_cipher will have values different to the original (under GnuTLS)
4f1d23a1 725 $tls_{in,out}_ocsp will be "not requested" or "no response", and
f4e62a87 726 hosts_require_ocsp will fail
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727
728
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729--------------------------------------------------------------
730End of file
731--------------------------------------------------------------