1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
|
From time to time, experimental features may be added to Exim.
While a feature is experimental, there will be a build-time
option whose name starts "EXPERIMENTAL_" that must be set in
order to include the feature. This file contains information
about experimental features, all of which are unstable and
liable to incompatible change.
Brightmail AntiSpam (BMI) support
--------------------------------------------------------------
Brightmail AntiSpam is a commercial package. Please see
http://www.brightmail.com for more information on
the product. For the sake of clarity, we'll refer to it as
"BMI" from now on.
0) BMI concept and implementation overview
In contrast to how spam-scanning with SpamAssassin is
implemented in exiscan-acl, BMI is more suited for per
-recipient scanning of messages. However, each messages is
scanned only once, but multiple "verdicts" for multiple
recipients can be returned from the BMI server. The exiscan
implementation passes the message to the BMI server just
before accepting it. It then adds the retrieved verdicts to
the messages header file in the spool. These verdicts can then
be queried in routers, where operation is per-recipient
instead of per-message. To use BMI, you need to take the
following steps:
1) Compile Exim with BMI support
2) Set up main BMI options (top section of Exim config file)
3) Set up ACL control statement (ACL section of the config
file)
4) Set up your routers to use BMI verdicts (routers section
of the config file).
5) (Optional) Set up per-recipient opt-in information.
These four steps are explained in more details below.
1) Adding support for BMI at compile time
To compile with BMI support, you need to link Exim against
the Brightmail client SDK, consisting of a library
(libbmiclient_single.so) and a header file (bmi_api.h).
You'll also need to explicitly set a flag in the Makefile to
include BMI support in the Exim binary. Both can be achieved
with these lines in Local/Makefile:
EXPERIMENTAL_BRIGHTMAIL=yes
CFLAGS=-I/path/to/the/dir/with/the/includefile
EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
If you use other CFLAGS or EXTRALIBS_EXIM settings then
merge the content of these lines with them.
Note for BMI6.x users: You'll also have to add -lxml2_single
to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do
this.
You should also include the location of
libbmiclient_single.so in your dynamic linker configuration
file (usually /etc/ld.so.conf) and run "ldconfig"
afterwards, or else the produced Exim binary will not be
able to find the library file.
2) Setting up BMI support in the Exim main configuration
To enable BMI support in the main Exim configuration, you
should set the path to the main BMI configuration file with
the "bmi_config_file" option, like this:
bmi_config_file = /opt/brightmail/etc/brightmail.cfg
This must go into section 1 of Exim's configuration file (You
can put it right on top). If you omit this option, it
defaults to /opt/brightmail/etc/brightmail.cfg.
Note for BMI6.x users: This file is in XML format in V6.xx
and its name is /opt/brightmail/etc/bmiconfig.xml. So BMI
6.x users MUST set the bmi_config_file option.
3) Set up ACL control statement
To optimize performance, it makes sense only to process
messages coming from remote, untrusted sources with the BMI
server. To set up a messages for processing by the BMI
server, you MUST set the "bmi_run" control statement in any
ACL for an incoming message. You will typically do this in
an "accept" block in the "acl_check_rcpt" ACL. You should
use the "accept" block(s) that accept messages from remote
servers for your own domain(s). Here is an example that uses
the "accept" blocks from Exim's default configuration file:
accept domains = +local_domains
endpass
verify = recipient
control = bmi_run
accept domains = +relay_to_domains
endpass
verify = recipient
control = bmi_run
If bmi_run is not set in any ACL during reception of the
message, it will NOT be passed to the BMI server.
4) Setting up routers to use BMI verdicts
When a message has been run through the BMI server, one or
more "verdicts" are present. Different recipients can have
different verdicts. Each recipient is treated individually
during routing, so you can query the verdicts by recipient
at that stage. From Exim's view, a verdict can have the
following outcomes:
o deliver the message normally
o deliver the message to an alternate location
o do not deliver the message
To query the verdict for a recipient, the implementation
offers the following tools:
- Boolean router preconditions. These can be used in any
router. For a simple implementation of BMI, these may be
all that you need. The following preconditions are
available:
o bmi_deliver_default
This precondition is TRUE if the verdict for the
recipient is to deliver the message normally. If the
message has not been processed by the BMI server, this
variable defaults to TRUE.
o bmi_deliver_alternate
This precondition is TRUE if the verdict for the
recipient is to deliver the message to an alternate
location. You can get the location string from the
$bmi_alt_location expansion variable if you need it. See
further below. If the message has not been processed by
the BMI server, this variable defaults to FALSE.
o bmi_dont_deliver
This precondition is TRUE if the verdict for the
recipient is NOT to deliver the message to the
recipient. You will typically use this precondition in a
top-level blackhole router, like this:
# don't deliver messages handled by the BMI server
bmi_blackhole:
driver = redirect
bmi_dont_deliver
data = :blackhole:
This router should be on top of all others, so messages
that should not be delivered do not reach other routers
at all. If the message has not been processed by
the BMI server, this variable defaults to FALSE.
- A list router precondition to query if rules "fired" on
the message for the recipient. Its name is "bmi_rule". You
use it by passing it a colon-separated list of rule
numbers. You can use this condition to route messages that
matched specific rules. Here is an example:
# special router for BMI rule #5, #8 and #11
bmi_rule_redirect:
driver = redirect
bmi_rule = 5:8:11
data = postmaster@mydomain.com
- Expansion variables. Several expansion variables are set
during routing. You can use them in custom router
conditions, for example. The following variables are
available:
o $bmi_base64_verdict
This variable will contain the BASE64 encoded verdict
for the recipient being routed. You can use it to add a
header to messages for tracking purposes, for example:
localuser:
driver = accept
check_local_user
headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
transport = local_delivery
If there is no verdict available for the recipient being
routed, this variable contains the empty string.
o $bmi_base64_tracker_verdict
This variable will contain a BASE64 encoded subset of
the verdict information concerning the "rules" that
fired on the message. You can add this string to a
header, commonly named "X-Brightmail-Tracker". Example:
localuser:
driver = accept
check_local_user
headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
transport = local_delivery
If there is no verdict available for the recipient being
routed, this variable contains the empty string.
o $bmi_alt_location
If the verdict is to redirect the message to an
alternate location, this variable will contain the
alternate location string returned by the BMI server. In
its default configuration, this is a header-like string
that can be added to the message with "headers_add". If
there is no verdict available for the recipient being
routed, or if the message is to be delivered normally,
this variable contains the empty string.
o $bmi_deliver
This is an additional integer variable that can be used
to query if the message should be delivered at all. You
should use router preconditions instead if possible.
$bmi_deliver is '0': the message should NOT be delivered.
$bmi_deliver is '1': the message should be delivered.
IMPORTANT NOTE: Verdict inheritance.
The message is passed to the BMI server during message
reception, using the target addresses from the RCPT TO:
commands in the SMTP transaction. If recipients get expanded
or re-written (for example by aliasing), the new address(es)
inherit the verdict from the original address. This means
that verdicts also apply to all "child" addresses generated
from top-level addresses that were sent to the BMI server.
5) Using per-recipient opt-in information (Optional)
The BMI server features multiple scanning "profiles" for
individual recipients. These are usually stored in a LDAP
server and are queried by the BMI server itself. However,
you can also pass opt-in data for each recipient from the
MTA to the BMI server. This is particularly useful if you
already look up recipient data in Exim anyway (which can
also be stored in a SQL database or other source). This
implementation enables you to pass opt-in data to the BMI
server in the RCPT ACL. This works by setting the
'bmi_optin' modifier in a block of that ACL. If should be
set to a list of comma-separated strings that identify the
features which the BMI server should use for that particular
recipient. Ideally, you would use the 'bmi_optin' modifier
in the same ACL block where you set the 'bmi_run' control
flag. Here is an example that will pull opt-in data for each
recipient from a flat file called
'/etc/exim/bmi_optin_data'.
The file format:
user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
user2@thatdomain.com: <OPTIN STRING3>
The example:
accept domains = +relay_to_domains
endpass
verify = recipient
bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
control = bmi_run
Of course, you can also use any other lookup method that
Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
as long as the result is a list of colon-separated opt-in
strings.
For a list of available opt-in strings, please contact your
Brightmail representative.
Sender Policy Framework (SPF) support
--------------------------------------------------------------
To learn more about SPF, visit http://www.openspf.org. This
document does not explain the SPF fundamentals, you should
read and understand the implications of deploying SPF on your
system before doing so.
SPF support is added via the libspf2 library. Visit
http://www.libspf2.org/
to obtain a copy, then compile and install it. By default,
this will put headers in /usr/local/include and the static
library in /usr/local/lib.
To compile Exim with SPF support, set these additional flags in
Local/Makefile:
EXPERIMENTAL_SPF=yes
CFLAGS=-DSPF -I/usr/local/include
EXTRALIBS_EXIM=-L/usr/local/lib -lspf2
This assumes that the libspf2 files are installed in
their default locations.
You can now run SPF checks in incoming SMTP by using the "spf"
ACL condition in either the MAIL, RCPT or DATA ACLs. When
using it in the RCPT ACL, you can make the checks dependent on
the RCPT address (or domain), so you can check SPF records
only for certain target domains. This gives you the
possibility to opt-out certain customers that do not want
their mail to be subject to SPF checking.
The spf condition takes a list of strings on its right-hand
side. These strings describe the outcome of the SPF check for
which the spf condition should succeed. Valid strings are:
o pass The SPF check passed, the sending host
is positively verified by SPF.
o fail The SPF check failed, the sending host
is NOT allowed to send mail for the domain
in the envelope-from address.
o softfail The SPF check failed, but the queried
domain can't absolutely confirm that this
is a forgery.
o none The queried domain does not publish SPF
records.
o neutral The SPF check returned a "neutral" state.
This means the queried domain has published
a SPF record, but wants to allow outside
servers to send mail under its domain as well.
This should be treated like "none".
o permerror This indicates a syntax error in the SPF
record of the queried domain. You may deny
messages when this occurs. (Changed in 4.83)
o temperror This indicates a temporary error during all
processing, including Exim's SPF processing.
You may defer messages when this occurs.
(Changed in 4.83)
o err_temp Same as permerror, deprecated in 4.83, will be
removed in a future release.
o err_perm Same as temperror, deprecated in 4.83, will be
removed in a future release.
You can prefix each string with an exclamation mark to invert
its meaning, for example "!fail" will match all results but
"fail". The string list is evaluated left-to-right, in a
short-circuit fashion. When a string matches the outcome of
the SPF check, the condition succeeds. If none of the listed
strings matches the outcome of the SPF check, the condition
fails.
Here is an example to fail forgery attempts from domains that
publish SPF records:
/* -----------------
deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}. \
Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address
spf = fail
--------------------- */
You can also give special treatment to specific domains:
/* -----------------
deny message = AOL sender, but not from AOL-approved relay.
sender_domains = aol.com
spf = fail:neutral
--------------------- */
Explanation: AOL publishes SPF records, but is liberal and
still allows non-approved relays to send mail from aol.com.
This will result in a "neutral" state, while mail from genuine
AOL servers will result in "pass". The example above takes
this into account and treats "neutral" like "fail", but only
for aol.com. Please note that this violates the SPF draft.
When the spf condition has run, it sets up several expansion
variables.
$spf_header_comment
This contains a human-readable string describing the outcome
of the SPF check. You can add it to a custom header or use
it for logging purposes.
$spf_received
This contains a complete Received-SPF: header that can be
added to the message. Please note that according to the SPF
draft, this header must be added at the top of the header
list. Please see section 10 on how you can do this.
Note: in case of "Best-guess" (see below), the convention is
to put this string in a header called X-SPF-Guess: instead.
$spf_result
This contains the outcome of the SPF check in string form,
one of pass, fail, softfail, none, neutral, permerror or
temperror.
$spf_smtp_comment
This contains a string that can be used in a SMTP response
to the calling party. Useful for "fail".
In addition to SPF, you can also perform checks for so-called
"Best-guess". Strictly speaking, "Best-guess" is not standard
SPF, but it is supported by the same framework that enables SPF
capability. Refer to http://www.openspf.org/FAQ/Best_guess_record
for a description of what it means.
To access this feature, simply use the spf_guess condition in place
of the spf one. For example:
/* -----------------
deny message = $sender_host_address doesn't look trustworthy to me
spf_guess = fail
--------------------- */
In case you decide to reject messages based on this check, you
should note that although it uses the same framework, "Best-guess"
is NOT SPF, and therefore you should not mention SPF at all in your
reject message.
When the spf_guess condition has run, it sets up the same expansion
variables as when spf condition is run, described above.
Additionally, since Best-guess is not standardized, you may redefine
what "Best-guess" means to you by redefining spf_guess variable in
global config. For example, the following:
/* -----------------
spf_guess = v=spf1 a/16 mx/16 ptr ?all
--------------------- */
would relax host matching rules to a broader network range.
A lookup expansion is also available. It takes an email
address as the key and an IP address as the database:
${lookup {username@domain} spf {ip.ip.ip.ip}}
The lookup will return the same result strings as they can appear in
$spf_result (pass,fail,softfail,neutral,none,err_perm,err_temp).
Currently, only IPv4 addresses are supported.
SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------
Exiscan currently includes SRS support via Miles Wilton's
libsrs_alt library. The current version of the supported
library is 0.5, there are reports of 1.0 working.
In order to use SRS, you must get a copy of libsrs_alt from
https://opsec.eu/src/srs/
(not the original source, which has disappeared.)
Unpack the tarball, then refer to MTAs/README.EXIM
to proceed. You need to set
EXPERIMENTAL_SRS=yes
in your Local/Makefile.
DCC Support
--------------------------------------------------------------
Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
*) Building exim
In order to build exim with DCC support add
EXPERIMENTAL_DCC=yes
to your Makefile. (Re-)build/install exim. exim -d should show
EXPERIMENTAL_DCC under "Support for".
*) Configuration
In the main section of exim.cf add at least
dccifd_address = /usr/local/dcc/var/dccifd
or
dccifd_address = <ip> <port>
In the DATA ACL you can use the new condition
dcc = *
After that "$dcc_header" contains the X-DCC-Header.
Return values are:
fail for overall "R", "G" from dccifd
defer for overall "T" from dccifd
accept for overall "A", "S" from dccifd
dcc = */defer_ok works as for spamd.
The "$dcc_result" variable contains the overall result from DCC
answer. There will an X-DCC: header added to the mail.
Usually you'll use
defer !dcc = *
to greylist with DCC.
If you set, in the main section,
dcc_direct_add_header = true
then the dcc header will be added "in deep" and if the spool
file was already written it gets removed. This forces Exim to
write it again if needed. This helps to get the DCC Header
through to eg. SpamAssassin.
If you want to pass even more headers in the middle of the
DATA stage you can set
$acl_m_dcc_add_header
to tell the DCC routines to add more information; eg, you might set
this to some results from ClamAV. Be careful. Header syntax is
not checked and is added "as is".
In case you've troubles with sites sending the same queue items from several
hosts and fail to get through greylisting you can use
$acl_m_dcc_override_client_ip
Setting $acl_m_dcc_override_client_ip to an IP address overrides the default
of $sender_host_address. eg. use the following ACL in DATA stage:
warn set acl_m_dcc_override_client_ip = \
${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}}
condition = ${if def:acl_m_dcc_override_client_ip}
log_message = dbg: acl_m_dcc_override_client_ip set to \
$acl_m_dcc_override_client_ip
Then set something like
# cat /etc/mail/multipleip_sites
mout-xforward.gmx.net 82.165.159.12
mout.gmx.net 212.227.15.16
Use a reasonable IP. eg. one the sending cluster actually uses.
DMARC Support
--------------------------------------------------------------
DMARC combines feedback from SPF, DKIM, and header From: in order
to attempt to provide better indicators of the authenticity of an
email. This document does not explain the fundamentals, you
should read and understand how it works by visiting the website at
http://www.dmarc.org/.
DMARC support is added via the libopendmarc library. Visit:
http://sourceforge.net/projects/opendmarc/
to obtain a copy, or find it in your favorite rpm package
repository. If building from source, this description assumes
that headers will be in /usr/local/include, and that the libraries
are in /usr/local/lib.
1. To compile Exim with DMARC support, you must first enable SPF.
Please read the above section on enabling the EXPERIMENTAL_SPF
feature. You must also have DKIM support, so you cannot set the
DISABLE_DKIM feature. Once both of those conditions have been met
you can enable DMARC in Local/Makefile:
EXPERIMENTAL_DMARC=yes
LDFLAGS += -lopendmarc
# CFLAGS += -I/usr/local/include
# LDFLAGS += -L/usr/local/lib
The first line sets the feature to include the correct code, and
the second line says to link the libopendmarc libraries into the
exim binary. The commented out lines should be uncommented if you
built opendmarc from source and installed in the default location.
Adjust the paths if you installed them elsewhere, but you do not
need to uncomment them if an rpm (or you) installed them in the
package controlled locations (/usr/include and /usr/lib).
2. Use the following global settings to configure DMARC:
Required:
dmarc_tld_file Defines the location of a text file of valid
top level domains the opendmarc library uses
during domain parsing. Maintained by Mozilla,
the most current version can be downloaded
from a link at http://publicsuffix.org/list/.
Optional:
dmarc_history_file Defines the location of a file to log results
of dmarc verification on inbound emails. The
contents are importable by the opendmarc tools
which will manage the data, send out DMARC
reports, and expire the data. Make sure the
directory of this file is writable by the user
exim runs as.
dmarc_forensic_sender The email address to use when sending a
forensic report detailing alignment failures
if a sender domain's dmarc record specifies it
and you have configured Exim to send them.
Default: do-not-reply@$default_hostname
3. By default, the DMARC processing will run for any remote,
non-authenticated user. It makes sense to only verify DMARC
status of messages coming from remote, untrusted sources. You can
use standard conditions such as hosts, senders, etc, to decide that
DMARC verification should *not* be performed for them and disable
DMARC with a control setting:
control = dmarc_disable_verify
A DMARC record can also specify a "forensic address", which gives
exim an email address to submit reports about failed alignment.
Exim does not do this by default because in certain conditions it
results in unintended information leakage (what lists a user might
be subscribed to, etc). You must configure exim to submit forensic
reports to the owner of the domain. If the DMARC record contains a
forensic address and you specify the control statement below, then
exim will send these forensic emails. It's also advised that you
configure a dmarc_forensic_sender because the default sender address
construction might be inadequate.
control = dmarc_enable_forensic
(AGAIN: You can choose not to send these forensic reports by simply
not putting the dmarc_enable_forensic control line at any point in
your exim config. If you don't tell it to send them, it will not
send them.)
There are no options to either control. Both must appear before
the DATA acl.
4. You can now run DMARC checks in incoming SMTP by using the
"dmarc_status" ACL condition in the DATA ACL. You are required to
call the spf condition first in the ACLs, then the "dmarc_status"
condition. Putting this condition in the ACLs is required in order
for a DMARC check to actually occur. All of the variables are set
up before the DATA ACL, but there is no actual DMARC check that
occurs until a "dmarc_status" condition is encountered in the ACLs.
The dmarc_status condition takes a list of strings on its
right-hand side. These strings describe recommended action based
on the DMARC check. To understand what the policy recommendations
mean, refer to the DMARC website above. Valid strings are:
o accept The DMARC check passed and the library recommends
accepting the email.
o reject The DMARC check failed and the library recommends
rejecting the email.
o quarantine The DMARC check failed and the library recommends
keeping it for further inspection.
o none The DMARC check passed and the library recommends
no specific action, neutral.
o norecord No policy section in the DMARC record for this
sender domain.
o nofrom Unable to determine the domain of the sender.
o temperror Library error or dns error.
o off The DMARC check was disabled for this email.
You can prefix each string with an exclamation mark to invert its
meaning, for example "!accept" will match all results but
"accept". The string list is evaluated left-to-right in a
short-circuit fashion. When a string matches the outcome of the
DMARC check, the condition succeeds. If none of the listed
strings matches the outcome of the DMARC check, the condition
fails.
Of course, you can also use any other lookup method that Exim
supports, including LDAP, Postgres, MySQL, etc, as long as the
result is a list of colon-separated strings.
Several expansion variables are set before the DATA ACL is
processed, and you can use them in this ACL. The following
expansion variables are available:
o $dmarc_status
This is a one word status indicating what the DMARC library
thinks of the email. It is a combination of the results of
DMARC record lookup and the SPF/DKIM/DMARC processing results
(if a DMARC record was found). The actual policy declared
in the DMARC record is in a separate expansion variable.
o $dmarc_status_text
This is a slightly longer, human readable status.
o $dmarc_used_domain
This is the domain which DMARC used to look up the DMARC
policy record.
o $dmarc_domain_policy
This is the policy declared in the DMARC record. Valid values
are "none", "reject" and "quarantine". It is blank when there
is any error, including no DMARC record.
o $dmarc_ar_header
This is the entire Authentication-Results header which you can
add using an add_header modifier.
5. How to enable DMARC advanced operation:
By default, Exim's DMARC configuration is intended to be
non-intrusive and conservative. To facilitate this, Exim will not
create any type of logging files without explicit configuration by
you, the admin. Nor will Exim send out any emails/reports about
DMARC issues without explicit configuration by you, the admin (other
than typical bounce messages that may come about due to ACL
processing or failure delivery issues).
In order to log statistics suitable to be imported by the opendmarc
tools, you need to:
a. Configure the global setting dmarc_history_file.
b. Configure cron jobs to call the appropriate opendmarc history
import scripts and truncating the dmarc_history_file.
In order to send forensic reports, you need to:
a. Configure the global setting dmarc_forensic_sender.
b. Configure, somewhere before the DATA ACL, the control option to
enable sending DMARC forensic reports.
6. Example usage:
(RCPT ACL)
warn domains = +local_domains
hosts = +local_hosts
control = dmarc_disable_verify
warn !domains = +screwed_up_dmarc_records
control = dmarc_enable_forensic
warn condition = (lookup if destined to mailing list)
set acl_m_mailing_list = 1
(DATA ACL)
warn dmarc_status = accept : none : off
!authenticated = *
log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain
add_header = $dmarc_ar_header
warn dmarc_status = !accept
!authenticated = *
log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain
warn dmarc_status = quarantine
!authenticated = *
set $acl_m_quarantine = 1
# Do something in a transport with this flag variable
deny condition = ${if eq{$dmarc_domain_policy}{reject}}
condition = ${if eq{$acl_m_mailing_list}{1}}
message = Messages from $dmarc_used_domain break mailing lists
deny dmarc_status = reject
!authenticated = *
message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT
DANE
------------------------------------------------------------
DNS-based Authentication of Named Entities, as applied
to SMTP over TLS, provides assurance to a client that
it is actually talking to the server it wants to rather
than some attacker operating a Man In The Middle (MITM)
operation. The latter can terminate the TLS connection
you make, and make another one to the server (so both
you and the server still think you have an encrypted
connection) and, if one of the "well known" set of
Certificate Authorities has been suborned - something
which *has* been seen already (2014), a verifiable
certificate (if you're using normal root CAs, eg. the
Mozilla set, as your trust anchors).
What DANE does is replace the CAs with the DNS as the
trust anchor. The assurance is limited to a) the possibility
that the DNS has been suborned, b) mistakes made by the
admins of the target server. The attack surface presented
by (a) is thought to be smaller than that of the set
of root CAs.
It also allows the server to declare (implicitly) that
connections to it should use TLS. An MITM could simply
fail to pass on a server's STARTTLS.
DANE scales better than having to maintain (and
side-channel communicate) copies of server certificates
for every possible target server. It also scales
(slightly) better than having to maintain on an SMTP
client a copy of the standard CAs bundle. It also
means not having to pay a CA for certificates.
DANE requires a server operator to do three things:
1) run DNSSEC. This provides assurance to clients
that DNS lookups they do for the server have not
been tampered with. The domain MX record applying
to this server, its A record, its TLSA record and
any associated CNAME records must all be covered by
DNSSEC.
2) add TLSA DNS records. These say what the server
certificate for a TLS connection should be.
3) offer a server certificate, or certificate chain,
in TLS connections which is traceable to the one
defined by (one of?) the TSLA records
There are no changes to Exim specific to server-side
operation of DANE.
The TLSA record for the server may have "certificate
usage" of DANE-TA(2) or DANE-EE(3). The latter specifies
the End Entity directly, i.e. the certificate involved
is that of the server (and should be the sole one transmitted
during the TLS handshake); this is appropriate for a
single system, using a self-signed certificate.
DANE-TA usage is effectively declaring a specific CA
to be used; this might be a private CA or a public,
well-known one. A private CA at simplest is just
a self-signed certificate which is used to sign
cerver certificates, but running one securely does
require careful arrangement. If a private CA is used
then either all clients must be primed with it, or
(probably simpler) the server TLS handshake must transmit
the entire certificate chain from CA to server-certificate.
If a public CA is used then all clients must be primed with it
(losing one advantage of DANE) - but the attack surface is
reduced from all public CAs to that single CA.
DANE-TA is commonly used for several services and/or
servers, each having a TLSA query-domain CNAME record,
all of which point to a single TLSA record.
The TLSA record should have a Selector field of SPKI(1)
and a Matching Type field of SHA2-512(2).
At the time of writing, https://www.huque.com/bin/gen_tlsa
is useful for quickly generating TLSA records; and commands like
openssl x509 -in -pubkey -noout <certificate.pem \
| openssl rsa -outform der -pubin 2>/dev/null \
| openssl sha512 \
| awk '{print $2}'
are workable for 4th-field hashes.
For use with the DANE-TA model, server certificates
must have a correct name (SubjectName or SubjectAltName).
The use of OCSP-stapling should be considered, allowing
for fast revocation of certificates (which would otherwise
be limited by the DNS TTL on the TLSA records). However,
this is likely to only be usable with DANE-TA. NOTE: the
default of requesting OCSP for all hosts is modified iff
DANE is in use, to:
hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \
{= {4}{$tls_out_tlsa_usage}} } \
{*}{}}
The (new) variable $tls_out_tlsa_usage is a bitfield with
numbered bits set for TLSA record usage codes.
The zero above means DANE was not in use,
the four means that only DANE-TA usage TLSA records were
found. If the definition of hosts_request_ocsp includes the
string "tls_out_tlsa_usage", they are re-expanded in time to
control the OCSP request.
This modification of hosts_request_ocsp is only done if
it has the default value of "*". Admins who change it, and
those who use hosts_require_ocsp, should consider the interaction
with DANE in their OCSP settings.
For client-side DANE there are two new smtp transport options,
hosts_try_dane and hosts_require_dane.
[ should they be domain-based rather than host-based? ]
Hosts_require_dane will result in failure if the target host
is not DNSSEC-secured.
DANE will only be usable if the target host has DNSSEC-secured
MX, A and TLSA records.
A TLSA lookup will be done if either of the above options match
and the host-lookup succeeded using dnssec.
If a TLSA lookup is done and succeeds, a DANE-verified TLS connection
will be required for the host. If it does not, the host will not
be used; there is no fallback to non-DANE or non-TLS.
If DANE is requested and useable (see above) the following transport
options are ignored:
hosts_require_tls
tls_verify_hosts
tls_try_verify_hosts
tls_verify_certificates
tls_crl
tls_verify_cert_hostnames
If DANE is not usable, whether requested or not, and CA-anchored
verification evaluation is wanted, the above variables should be set
appropriately.
Currently dnssec_request_domains must be active (need to think about that)
and dnssec_require_domains is ignored.
If verification was successful using DANE then the "CV" item
in the delivery log line will show as "CV=dane".
There is a new variable $tls_out_dane which will have "yes" if
verification succeeded using DANE and "no" otherwise (only useful
in combination with EXPERIMENTAL_EVENT), and a new variable
$tls_out_tlsa_usage (detailed above).
DSN extra information
---------------------
If compiled with EXPERIMENTAL_DSN_INFO extra information will be added
to DSN fail messages ("bounces"), when available. The intent is to aid
tracing of specific failing messages, when presented with a "bounce"
complaint and needing to search logs.
The remote MTA IP address, with port number if nonstandard.
Example:
Remote-MTA: X-ip; [127.0.0.1]:587
Rationale:
Several addresses may correspond to the (already available)
dns name for the remote MTA.
The remote MTA connect-time greeting.
Example:
X-Remote-MTA-smtp-greeting: X-str; 220 the.local.host.name ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000
Rationale:
This string sometimes presents the remote MTA's idea of its
own name, and sometimes identifies the MTA software.
The remote MTA response to HELO or EHLO.
Example:
X-Remote-MTA-helo-response: X-str; 250-the.local.host.name Hello localhost [127.0.0.1]
Limitations:
Only the first line of a multiline response is recorded.
Rationale:
This string sometimes presents the remote MTA's view of
the peer IP connecting to it.
The reporting MTA detailed diagnostic.
Example:
X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:<d3@myhost.test.ex>: 550 hard error
Rationale:
This string sometimes give extra information over the
existing (already available) Diagnostic-Code field.
Note that non-RFC-documented field names and data types are used.
LMDB Lookup support
-------------------
LMDB is an ultra-fast, ultra-compact, crash-proof key-value embedded data store.
It is modeled loosely on the BerkeleyDB API. You should read about the feature
set as well as operation modes at https://symas.com/products/lightning-memory-mapped-database/
LMDB single key lookup support is provided by linking to the LMDB C library.
The current implementation does not support writing to the LMDB database.
Visit https://github.com/LMDB/lmdb to download the library or find it in your
operating systems package repository.
If building from source, this description assumes that headers will be in
/usr/local/include, and that the libraries are in /usr/local/lib.
1. In order to build exim with LMDB lookup support add or uncomment
EXPERIMENTAL_LMDB=yes
to your Local/Makefile. (Re-)build/install exim. exim -d should show
Experimental_LMDB in the line "Support for:".
EXPERIMENTAL_LMDB=yes
LDFLAGS += -llmdb
# CFLAGS += -I/usr/local/include
# LDFLAGS += -L/usr/local/lib
The first line sets the feature to include the correct code, and
the second line says to link the LMDB libraries into the
exim binary. The commented out lines should be uncommented if you
built LMDB from source and installed in the default location.
Adjust the paths if you installed them elsewhere, but you do not
need to uncomment them if an rpm (or you) installed them in the
package controlled locations (/usr/include and /usr/lib).
2. Create your LMDB files, you can use the mdb_load utility which is
part of the LMDB distribution our your favourite language bindings.
3. Add the single key lookups to your exim.conf file, example lookups
are below.
${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}}
${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}fail}
${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}}
Queuefile transport
-------------------
Queuefile is a pseudo transport which does not perform final delivery.
It simply copies the exim spool files out of the spool directory into
an external directory retaining the exim spool format.
The spool files can then be processed by external processes and then
requeued into exim spool directories for final delivery.
The motivation/inspiration for the transport is to allow external
processes to access email queued by exim and have access to all the
information which would not be available if the messages were delivered
to the process in the standard email formats.
The mailscanner package is one of the processes that can take advantage
of this transport to filter email.
The transport can be used in the same way as the other existing transports,
i.e by configuring a router to route mail to a transport configured with
the queuefile driver.
The transport only takes one option:
* directory - This is used to specify the directory messages should be
copied to
The generic transport options (body_only, current_directory, disable_logging,
debug_print, delivery_date_add, envelope_to_add, event_action, group,
headers_add, headers_only, headers_remove, headers_rewrite, home_directory,
initgroups, max_parallel, message_size_limit, rcpt_include_affixes,
retry_use_local_part, return_path, return_path_add, shadow_condition,
shadow_transport, transport_filter, transport_filter_timeout, user) are
ignored.
Sample configuration:
(Router)
scan:
driver = accept
transport = scan
(Transport)
scan:
driver = queuefile
directory = /var/spool/baruwa-scanner/input
In order to build exim with Queuefile transport support add or uncomment
EXPERIMENTAL_QUEUEFILE=yes
to your Local/Makefile. (Re-)build/install exim. exim -d should show
Experimental_QUEUEFILE in the line "Support for:".
--------------------------------------------------------------
End of file
--------------------------------------------------------------
|