-
Notifications
You must be signed in to change notification settings - Fork 19
/
mta-sts.xml
1127 lines (1025 loc) · 50.7 KB
/
mta-sts.xml
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
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE rfc SYSTEM 'rfc2629.dtd' []>
<rfc ipr="trust200902" category="std" docName="draft-ietf-uta-mta-sts-21">
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc private=""?>
<?rfc topblock="yes"?>
<?rfc comments="no"?>
<front>
<title abbrev="MTA-STS">SMTP MTA Strict Transport Security (MTA-STS)</title>
<author initials="D." surname="Margolis" fullname="Daniel Margolis">
<organization>Google, Inc</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
<email>dmargolis@google.com</email>
<uri></uri>
</address>
</author>
<author initials="M." surname="Risher" fullname="Mark Risher">
<organization>Google, Inc</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
<email>risher@google.com</email>
<uri></uri>
</address>
</author>
<author initials="B." surname="Ramakrishnan" fullname="Binu Ramakrishnan">
<organization>Yahoo!, Inc</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
<email>rbinu@yahoo-inc.com</email>
<uri></uri>
</address>
</author>
<author initials="A." surname="Brotman" fullname="Alexander Brotman">
<organization>Comcast, Inc</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
<email>alex_brotman@comcast.com</email>
<uri></uri>
</address>
</author>
<author initials="J." surname="Jones" fullname="Janet Jones">
<organization>Microsoft, Inc</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
<email>janet.jones@microsoft.com</email>
<uri></uri>
</address>
</author>
<date year="2018" month="June" day="16"/>
<area>Applications</area>
<workgroup>Using TLS in Applications</workgroup>
<keyword></keyword>
<abstract>
<t>SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a mechanism
enabling mail service providers to declare their ability to receive Transport
Layer Security (TLS) secure SMTP connections, and to specify whether sending
SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a
trusted server certificate.
</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>The STARTTLS extension to SMTP <xref target="RFC3207"/> allows SMTP clients and hosts to
negotiate the use of a TLS channel for encrypted mail transmission.
</t>
<t>While this opportunistic encryption protocol by itself provides a high barrier
against passive man-in-the-middle traffic interception, any attacker who can
delete parts of the SMTP session (such as the "250 STARTTLS" response) or who
can redirect the entire SMTP session (perhaps by overwriting the resolved MX
record of the delivery domain) can perform downgrade or interception attacks.
</t>
<t>This document defines a mechanism for recipient domains to publish policies, via
a combination of DNS and HTTPS, specifying:
</t>
<t>
<list style="symbols">
<t>whether MTAs sending mail to this domain can expect PKIX-authenticated TLS
support</t>
<t>what a conforming client should do with messages when TLS cannot be
successfully negotiated</t>
</list>
</t>
<section anchor="terminology" title="Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [BCP 14] <xref target="RFC2119"/>
<xref target="RFC8174"/> when, and only when, they appear in all capitals, as shown here.
</t>
<t>We also define the following terms for further use in this document:
</t>
<t>
<list style="symbols">
<t>MTA-STS Policy: A commitment by the Policy Domain to support PKIX <xref target="RFC5280"/>
authenticated TLS for the specified MX hosts.</t>
<t>Policy Domain: The domain for which an MTA-STS Policy is defined. This is the
next-hop domain; when sending mail to "alice@example.com" this would
ordinarily be "example.com", but this may be overridden by explicit routing
rules (as described in <xref target="policy-selection-for-smart-hosts-and-subdomains"/>,
"Policy Selection for Smart Hosts and Subdomains").</t>
<t>Policy Host: The HTTPS host which serves the MTA-STS Policy for a Policy
Domain. Rules for constructing the hostname are described in
<xref target="mtasts-policies"/>, "MTA-STS Policies".</t>
<t>Sender: The SMTP Mail Transfer Agent sending an email message.</t>
<t>ABNF: Augmented Backus-Naur Form, a syntax for formally specifying syntax,
defined in <xref target="RFC5234"/> and <xref target="RFC7405"/>.</t>
</list>
</t>
</section>
</section>
<section anchor="related-technologies" title="Related Technologies">
<t>The DANE TLSA record <xref target="RFC7672"/> is similar, in that DANE is also designed to
upgrade unauthenticated encryption or plaintext transmission into authenticated,
downgrade-resistant encrypted transmission. DANE requires DNSSEC <xref target="RFC4033"/>
for authentication; the mechanism described here instead relies on certificate
authorities (CAs) and does not require DNSSEC, at a cost of risking malicious
downgrades. For a thorough discussion of this trade-off, see
<xref target="security-considerations"/>, "Security Considerations".
</t>
<t>In addition, MTA-STS provides an optional testing-only mode, enabling soft
deployments to detect policy failures; partial deployments can be achieved in
DANE by deploying TLSA records only for some of a domain's MXs, but such a
mechanism is not possible for the per-domain policies used by MTA-STS.
</t>
<t>The primary motivation of MTA-STS is to provide a mechanism for domains to
ensure transport security even when deploying DNSSEC is undesirable or
impractical. However, MTA-STS is designed not to interfere with DANE
deployments when the two overlap; in particular, senders who implement MTA-STS
validation MUST NOT allow a "valid" or "testing"-only MTA-STS validation to
override a failing DANE validation.
</t>
</section>
<section anchor="policy-discovery" title="Policy Discovery">
<t>MTA-STS policies are distributed via HTTPS from a "well-known" <xref target="RFC5785"/> path
served within the Policy Domain, and their presence and current version are
indicated by a TXT record at the Policy Domain. These TXT records additionally
contain a policy <spanx style="verb">id</spanx> field, allowing sending MTAs to check the currency of a
cached policy without performing an HTTPS request.
</t>
<t>To discover if a recipient domain implements MTA-STS, a sender need only resolve
a single TXT record. To see if an updated policy is available for a domain for
which the sender has a previously cached policy, the sender need only check the
TXT record's version <spanx style="verb">id</spanx> against the cached value.
</t>
<section anchor="mtasts-txt-records" title="MTA-STS TXT Records">
<t>The MTA-STS TXT record is a TXT record with the name <spanx style="verb">_mta-sts</spanx> at the Policy
Domain. For the domain <spanx style="verb">example.com</spanx>, this record would be
<spanx style="verb">_mta-sts.example.com</spanx>. MTA-STS TXT records MUST be US-ASCII,
semicolon-separated key/value pairs containing the following fields:
</t>
<t>
<list style="symbols">
<t><spanx style="verb">v</spanx>: (plain-text, required). Currently only "STSv1" is supported.</t>
<t><spanx style="verb">id</spanx>: (plain-text, required). A short string used to track policy updates.
This string MUST uniquely identify a given instance of a policy, such that
senders can determine when the policy has been updated by comparing to the
<spanx style="verb">id</spanx> of a previously seen policy. There is no implied ordering of <spanx style="verb">id</spanx> fields
between revisions.</t>
</list>
</t>
<t>An example TXT record is as below:
</t>
<t>_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
</t>
<t>The formal definition of the <spanx style="verb">_mta-sts</spanx> TXT record, defined using ABNF
(<xref target="RFC7405"/>), is as follows:
</t>
<figure align="center"><artwork align="center">
sts-text-record = sts-version 1*(sts-field-delim sts-field)
[sts-field-delim]
sts-field = sts-id / ; Note that sts-id record
sts-extension ; is required.
sts-field-delim = *WSP ";" *WSP
sts-version = %s"v=STSv1"
sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=...
sts-extension = sts-ext-name "=" sts-ext-value ; name=value
sts-ext-name = (ALPHA / DIGIT)
*31(ALPHA / DIGIT / "_" / "-" / ".")
sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E)
; chars excluding "=", ";", SP, and CTLs
</artwork></figure>
<t>The TXT record MUST begin with sts-version field, and the order of other fields
is not significant. If multiple TXT records for <spanx style="verb">_mta-sts</spanx> are returned by the
resolver, records which do not begin with <spanx style="verb">v=STSv1;</spanx> are discarded. If the
number of resulting records is not one, or if the resulting record is
syntactically invalid, senders MUST assume the recipient domain does not
have an available MTA-STS policy and skip the remaining steps of policy
discovery. (Note that absence of a usable TXT record is not by itself
sufficient to remove a sender's previously cached policy for the Policy Domain,
as discussed in <xref target="policy-application-control-flow"/>, "Policy Application Control
Flow".) If the resulting TXT record contains multiple strings, then the record
MUST be treated as if those strings are concatenated together without adding
spaces.
</t>
</section>
<section anchor="mtasts-policies" title="MTA-STS Policies">
<t>The policy itself is a set of key/value pairs (similar to <xref target="RFC5322"/> header
fields) served via the HTTPS GET method from the fixed <xref target="RFC5785"/> "well-known"
path of <spanx style="verb">.well-known/mta-sts.txt</spanx> served by the Policy Host. The Policy Host
DNS name is constructed by prepending <spanx style="verb">mta-sts</spanx> to the Policy Domain.
</t>
<t>Thus for a Policy Domain of <spanx style="verb">example.com</spanx> the full URL is
<spanx style="verb">https://mta-sts.example.com/.well-known/mta-sts.txt</spanx>.
</t>
<t>When fetching a policy, senders SHOULD validate that the media type is
"text/plain" to guard against cases where webservers allow untrusted users to
host non-text content (typically, HTML or images) at a user-defined path. All
parameters other than charset=utf-8 or charset=us-ascii are ignored. Additional
"Content-Type" parameters are also ignored.
</t>
<t>This resource contains the following CRLF-separated key/value pairs:
</t>
<t>
<list style="symbols">
<t><spanx style="verb">version</spanx>: Currently only "STSv1" is supported.</t>
<t><spanx style="verb">mode</spanx>: One of "enforce", "testing", or "none", indicating the expected
behavior of a sending MTA in the case of a policy validation failure. See
<xref target="policy-application"/>, "Policy Application." for more details about the three
modes.</t>
<t><spanx style="verb">max_age</spanx>: Max lifetime of the policy (plain-text non-negative integer
seconds, maximum value of 31557600). Well-behaved clients SHOULD cache a
policy for up to this value from last policy fetch time. To mitigate the
risks of attacks at policy refresh time, it is expected that this value
typically be in the range of weeks or greater.</t>
<t><spanx style="verb">mx</spanx>: Allowed MX patterns. One or more patterns matching allowed MX hosts for
the Policy Domain. As an example,</t>
</list>
</t>
<figure align="center"><artwork align="center">
mx: mail.example.com <CRLF>
mx: *.example.net
</artwork></figure>
<t>indicates that mail for this domain might be handled by MX <spanx style="verb">mail.example.com</spanx>
or any MX at <spanx style="verb">example.net</spanx>. Valid patterns can be either fully specified
names (<spanx style="verb">example.com</spanx>) or suffixes prefixed by a wildcard (<spanx style="verb">*.example.net</spanx>).
If a policy specifies more than one MX, each MX MUST have its own <spanx style="verb">mx:</spanx> key,
and each MX key/value pair MUST be on its own line in the policy file. In the
case of Internationalized Domain Names (<xref target="RFC5891"/>), the <spanx style="verb">mx</spanx> value MUST
specify the Punycode-encoded A-label <xref target="RFC3492"/> to match against, and not the
Unicode-encoded U-label. The full semantics of certificate validation
(including the use of wildcard patterns) are described in
<xref target="mx-host-validation"/>, "MX Host Validation."
</t>
<t>An example policy is as below:
</t>
<figure align="center"><artwork align="center">
version: STSv1
mode: enforce
mx: mail.example.com
mx: *.example.net
mx: backupmx.example.com
max_age: 604800
</artwork></figure>
<t>The formal definition of the policy resource, defined using <xref target="RFC7405"/>, is as
follows:
</t>
<figure align="center"><artwork align="center">
sts-policy-record = sts-policy-field *WSP
*(sts-policy-term sts-policy-field *WSP)
[sts-policy-term]
sts-policy-field = sts-policy-version / ; required once
sts-policy-mode / ; required once
sts-policy-max-age / ; required once
sts-policy-term /
; required at least once, except when
; mode is "none"
sts-policy-extension ; other fields
sts-policy-field-delim = ":" *WSP
sts-policy-version = sts-policy-version-field sts-policy-field-delim
sts-policy-version-value
sts-policy-version-field = %s"version"
sts-policy-version-value = %s"STSv1"
sts-policy-mode = sts-policy-mode-field sts-policy-field-delim
sts-policy-mode-value
sts-policy-mode-field = %s"mode"
sts-policy-mode-value = %s"testing" / %s"enforce" / %s"none"
sts-policy-mx = sts-policy-mx-field sts-policy-field-delim
sts-policy-mx-value
sts-policy-mx-field = %s"mx"
sts-policy-mx-value = ["*."] Domain
sts-policy-max-age = sts-policy-max-age-field sts-policy-field-delim
sts-policy-max-age-value
sts-policy-max-age-field = %s"max_age"
sts-policy-max-age-value = 1*10(DIGIT)
sts-policy-extension = sts-policy-ext-name ; additional
sts-policy-field-delim ; extension
sts-policy-ext-value ; fields
sts-policy-ext-name = (sts-policy-alphanum)
*31(sta-policy-alphanum / "_" / "-" / ".")
sts-policy-term = LF / CRLF
sts-policy-ext-value = sts-policy-vchar
[*(%x20 / sts-policy-vchar)
sts-policy-vchar]
; chars, including UTF-8 [@!RFC3629],
; excluding CTLs and no
; leading/trailing spaces
sts-policy-alphanum = ALPHA / DIGIT
sts-policy-vchar = %x21-7E / UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = <Defined in Section 4 of [@!RFC3629]>
UTF8-3 = <Defined in Section 4 of [@!RFC3629]>
UTF8-4 = <Defined in Section 4 of [@!RFC3629]>
Domain = <see RFC 5321 4.1.2>
</artwork></figure>
<t>Parsers MUST accept TXT records and policy files which are syntactically valid
(i.e., valid key/value pairs separated by semi-colons for TXT records), possibly
containing additional key/value pairs not specified in this document, in which
case unknown fields SHALL be ignored. If any non-repeated field--i.e., all
fields excepting <spanx style="verb">mx</spanx>--is duplicated, all entries except for the first SHALL be
ignored.
</t>
</section>
<section anchor="https-policy-fetching" title="HTTPS Policy Fetching">
<t>Policy bodies are, as described above, retrieved by sending MTAs via HTTPS
<xref target="RFC2818"/>. During the TLS handshake initiated to fetch a new or updated
policy from the Policy Host, the Policy Host HTTPS server MUST present a X.509
certificate which is valid for the <spanx style="verb">mta-sts</spanx> DNS-ID (<xref target="RFC6125"/>) (e.g.,
<spanx style="verb">mta-sts.example.com</spanx>) as described below, chain to a root CA that is trusted by
the sending MTA, and be non-expired. It is expected that sending MTAs use a set
of trusted CAs similar to those in widely deployed Web browsers and operating
systems. See <xref target="RFC5280"/> for more details about certificate verification.
</t>
<t>The certificate is valid for the Policy Host (i.e., <spanx style="verb">mta-sts</spanx> prepended to the
Policy Domain) with respect to the rules described in <xref target="RFC6125"/>, with the
following application-specific considerations:
</t>
<t>
<list style="symbols">
<t>Matching is performed only against the DNS-ID identifiers.</t>
<t>DNS domain names in server certificates MAY contain the wildcard character
'*' as the complete left-most label within the identifier.</t>
</list>
</t>
<t>The certificate MAY be checked for revocation via the Online Certificate Status
Protocol (OCSP) <xref target="RFC6960"/>, certificate revocation lists (CRLs), or some other
mechanism.
</t>
<t>Policies fetched via HTTPS are only valid if the HTTP response code is 200 (OK).
HTTP 3xx redirects MUST NOT be followed, and HTTP caching (as specified in
<xref target="RFC7234"/>) MUST NOT be used.
</t>
<t>Senders may wish to rate-limit the frequency of attempts to fetch the HTTPS
endpoint even if a valid TXT record for the recipient domain exists. In the
case that the HTTPS GET fails, implementers SHOULD limit further attempts to
a period of five minutes or longer per version ID, to avoid overwhelming
resource-constrained recipients with cascading failures.
</t>
<t>Senders MAY impose a timeout on the HTTPS GET and/or a limit on the maximum size
of the response body to avoid long delays or resource exhaustion during
attempted policy updates. A suggested timeout is one minute, and a suggested
maximum policy size 64 kilobytes; policy hosts SHOULD respond to requests with a
complete policy body within that timeout and size limit.
</t>
<t>If a valid TXT record is found but no policy can be fetched via HTTPS (for any
reason), and there is no valid (non-expired) previously-cached policy, senders
MUST continue with delivery as though the domain has not implemented MTA-STS.
</t>
<t>Conversely, if no "live" policy can be discovered via DNS or fetched via HTTPS,
but a valid (non-expired) policy exists in the sender's cache, the sender MUST
apply that cached policy.
</t>
<t>Finally, to mitigate the risk of persistent interference with policy refresh, as
discussed in-depth in <xref target="security-considerations"/>, MTAs SHOULD proactively
refresh cached policies before they expire; a suggested refresh frequency is
once per day. To enable administrators to discover problems with policy
refresh, MTAs SHOULD alert administrators (through the use of logs or similar)
when such attempts fail, unless the cached policy mode is <spanx style="verb">none</spanx>.
</t>
</section>
<section anchor="policy-selection-for-smart-hosts-and-subdomains" title="Policy Selection for Smart Hosts and Subdomains">
<t>When sending mail via a "smart host"--an administratively configured
intermediate SMTP relay, which is different from the message recipient's server
as determined from DNS --compliant senders MUST treat the smart host domain as
the policy domain for the purposes of policy discovery and application. This
specification does not provide a means of associating policies with email
addresses that employ Address Literals <xref target="RFC5321"/>.
</t>
<t>When sending mail to a mailbox at a subdomain, compliant senders MUST NOT
attempt to fetch a policy from the parent zone. Thus for mail sent to
"user@mail.example.com", the policy can be fetched only from "mail.example.com",
not "example.com".
</t>
</section>
</section>
<section anchor="policy-validation" title="Policy Validation">
<t>When sending to an MX at a domain for which the sender has a valid and
non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST check whether:
</t>
<t>
<list style="numbers">
<t>At least one of the policy's "mx" patterns matches the selected MX host,
as described in <xref target="mx-host-validation"/>, "MX Host Validation".</t>
<t>The recipient mail server supports STARTTLS and offers a PKIX-based TLS
certificate, during TLS handshake, which is valid for that host, as described
in <xref target="recipient-mta-certificate-validation"/>, "Recipient MTA Certificate
Validation".</t>
</list>
</t>
<t>When these conditions are not met, a policy is said to fail to validate. This
section does not dictate the behavior of sending MTAs when the above conditions
are not met; see <xref target="policy-application"/>, "Policy Application" for a description
of sending MTA behavior when policy validation fails.
</t>
<section anchor="mx-host-validation" title="MX Host Validation">
<t>A receiving candidate MX host is valid according to an applied MTA-STS policy if
the MX record name matches one or more of the <spanx style="verb">mx</spanx> fields in the applied policy.
Matching is identical to the rules given in <xref target="RFC6125"/>, with restriction that
the wildcard character <spanx style="verb">*</spanx> may only be used to match the entire left-most label
in the presented identifier. Thus the mx pattern <spanx style="verb">*.example.com</spanx> matches
<spanx style="verb">mail.example.com</spanx> but not <spanx style="verb">example.com</spanx> or <spanx style="verb">foo.bar.example.com</spanx>.
</t>
</section>
<section anchor="recipient-mta-certificate-validation" title="Recipient MTA Certificate Validation">
<t>The certificate presented by the receiving MTA MUST not be expired, and MUST
chain to a root CA that is trusted by the sending MTA. The certificate MUST
have a subject alternative name (SAN, <xref target="RFC5280"/>) with a DNS-ID (<xref target="RFC6125"/>)
matching the host name, per the rules given in <xref target="RFC6125"/>. The MX's
certificate MAY also be checked for revocation via OCSP <xref target="RFC6960"/>, CRLs
<xref target="RFC6818"/>, or some other mechanism.
</t>
</section>
</section>
<section anchor="policy-application" title="Policy Application">
<t>When sending to an MX at a domain for which the sender has a valid, non-expired
MTA-STS policy, a sending MTA honoring MTA-STS applies the result of a policy
validation failure one of two ways, depending on the value of the policy <spanx style="verb">mode</spanx>
field:
</t>
<t>
<list style="numbers">
<t><spanx style="verb">enforce</spanx>: In this mode, sending MTAs MUST NOT deliver the message to hosts
which fail MX matching or certificate validation, or do not support STARTTLS.</t>
<t><spanx style="verb">testing</spanx>: In this mode, sending MTAs which also implement the TLSRPT
specification <xref target="I-D.ietf-uta-smtp-tlsrpt"/> merely send a report indicating
policy application failures (so long as TLSRPT is also implemented by the
recipient domain).</t>
<t><spanx style="verb">none</spanx>: In this mode, sending MTAs should treat the policy domain as though
it does not have any active policy; see <xref target="removing-mtasts"/>, "Removing
MTA-STS", for use of this mode value.</t>
</list>
</t>
<t>When a message fails to deliver due to an <spanx style="verb">enforce</spanx> policy, a compliant MTA MUST
NOT permanently fail to deliver messages before checking, via DNS, for the
presence of an updated policy at the Policy Domain. (In all cases, MTAs SHOULD
treat such failures as transient errors and retry delivery later.) This allows
implementing domains to update long-lived policies on the fly.
</t>
<section anchor="policy-application-control-flow" title="Policy Application Control Flow">
<t>An example control flow for a compliant sender consists of the following steps:
</t>
<t>
<list style="numbers">
<t>Check for a cached policy whose time-since-fetch has not exceeded its
<spanx style="verb">max_age</spanx>. If none exists, attempt to fetch a new policy (perhaps
asynchronously, so as not to block message delivery). Optionally, sending
MTAs may unconditionally check for a new policy at this step.</t>
<t>For each candidate MX, in order of MX priority, attempt to deliver the
message. If a policy is present with an <spanx style="verb">enforce</spanx> mode, when attempting to
deliver to each candidate MX, ensure STARTTLS support and host identity
validity as described in <xref target="policy-validation"/>, "Policy Validation". If a
candidate fails validation, continue to the next candidate (if there is one).</t>
<t>A message delivery MUST NOT be permanently failed until the sender has first
checked for the presence of a new policy (as indicated by the <spanx style="verb">id</spanx> field in
the <spanx style="verb">_mta-sts</spanx> TXT record). If a new policy is not found, existing rules for
the case of temporary message delivery failures apply (as discussed in
<xref target="RFC5321"/> section 4.5.4.1).</t>
</list>
</t>
</section>
</section>
<section anchor="reporting-failures" title="Reporting Failures">
<t>MTA-STS is intended to be used along with TLSRPT <xref target="I-D.ietf-uta-smtp-tlsrpt"/> in
order to ensure implementing domains can detect cases of both benign and
malicious failures, and to ensure that failures that indicate an active attack
are discoverable. As such, senders who also implement TLSRPT SHOULD treat the
following events as reportable failures:
</t>
<t>
<list style="symbols">
<t>HTTPS policy fetch failures when a valid TXT record is present.</t>
<t>Policy fetch failures of any kind when a valid policy exists in the policy
cache, except if that policy's mode is <spanx style="verb">none</spanx>.</t>
<t>Delivery attempts in which a contacted MX does not support STARTTLS or does
not present a certificate which validates according to the applied policy,
except if that policy's mode is <spanx style="verb">none</spanx>.</t>
</list>
</t>
</section>
<section anchor="interoperability-considerations" title="Interoperability Considerations">
<section anchor="sni-support" title="SNI Support">
<t>To ensure that the server sends the right certificate chain, the SMTP client
MUST have support for the TLS SNI extension <xref target="RFC6066"/>. When connecting to a
HTTP server to retrieve the MTA-STS policy, the SNI extension MUST contain the
name of the policy host (e.g., <spanx style="verb">mta-sts.example.com</spanx>). When connecting to an
SMTP server, the SNI extension MUST contain the MX hostname.
</t>
<t>HTTP servers used to deliver MTA-STS policies MAY rely on SNI to determine which
certificate chain to present to the client. HTTP servers MUST respond with a
certificate chain that matches the policy hostname or abort the TLS handshake if
unable to do so. Clients that do not send SNI information may not see the
expected certificate chain.
</t>
<t>SMTP servers MAY rely on SNI to determine which certificate chain to present to
the client. However servers that have one identity and a single matching
certificate do not require SNI support. Servers MUST NOT enforce the use of SNI
by clients, as the client may be using unauthenticated opportunistic TLS and may
not expect any particular certificate from the server. If the client sends no
SNI extension or sends an SNI extension for an unsupported server name, the
server MUST simply send a fallback certificate chain of its choice. The reason
for not enforcing strict matching of the requested SNI hostname is that MTA-STS
TLS clients may be typically willing to accept multiple server names but can
only send one name in the SNI extension. The server's fallback certificate may
match a different name that is acceptable to the client, e.g., the original
next-hop domain.
</t>
</section>
<section anchor="minimum-tls-version-support" title="Minimum TLS Version Support">
<t>MTAs supporting MTA-STS MUST have support for TLS version 1.2 <xref target="RFC5246"/> or
higher. The general TLS usage guidance in <xref target="RFC7525"/> SHOULD be followed.
</t>
</section>
</section>
<section anchor="operational-considerations" title="Operational Considerations">
<section anchor="policy-updates" title="Policy Updates">
<t>Updating the policy requires that the owner make changes in two places: the
<spanx style="verb">_mta-sts</spanx> TXT record in the Policy Domain's DNS zone and at the corresponding
HTTPS endpoint. As a result, recipients should expect a policy will continue to
be used by senders until both the HTTPS and TXT endpoints are updated and the
TXT record's TTL has passed.
</t>
<t>In other words, a sender who is unable to successfully deliver a message while
applying a cache of the recipient's now-outdated policy may be unable to
discover that a new policy exists until the DNS TTL has passed. Recipients
SHOULD therefore ensure that old policies continue to work for message delivery
during this period of time, or risk message delays.
</t>
<t>Recipients SHOULD also update the HTTPS policy body before updating the TXT
record; this ordering avoids the risk that senders, seeing a new TXT record,
mistakenly cache the old policy from HTTPS.
</t>
</section>
<section anchor="policy-delegation" title="Policy Delegation">
<t>Domain owners commonly delegate SMTP hosting to a different organization, such
as an ISP or a Web host. In such a case, they may wish to also delegate the
MTA-STS policy to the same organization which can be accomplished with two
changes.
</t>
<t>First, the Policy Domain must point the <spanx style="verb">_mta-sts</spanx> record, via CNAME, to the
<spanx style="verb">_mta-sts</spanx> record maintained by the hosting organization. This allows the
hosting organization to control update signaling.
</t>
<t>Second, the Policy Domain must point the "well-known" policy location to the
hosting organization. This can be done either by setting the <spanx style="verb">mta-sts</spanx> record
to an IP address or CNAME specified by the hosting organization and by giving
the hosting organization a TLS certificate which is valid for that host, or by
setting up a "reverse proxy" (also known as a "gateway") server that serves as
the Policy Domain's policy the policy currently served by the hosting
organization.
</t>
<t>For example, given a user domain <spanx style="verb">user.example</spanx> hosted by a mail provider
<spanx style="verb">provider.example</spanx>, the following configuration would allow policy delegation:
</t>
<t>DNS:
</t>
<figure align="center"><artwork align="center">
_mta-sts.user.example. IN CNAME _mta-sts.provider.example.
</artwork></figure>
<t>Policy:
</t>
<figure align="center"><artwork align="center">
> GET /.well-known/mta-sts.txt Host: mta-sts.user.example
< HTTP/1.1 200 OK # Response proxies content from
# https://mta-sts.provider.example
</artwork></figure>
<t>Note that in all such cases, the policy endpoint
(<spanx style="verb">https://mta-sts.user.example/.well-known/mta-sts.txt</spanx> in this example) must
still present a certificate valid for the Policy Host (<spanx style="verb">mta-sts.user.example</spanx>),
and not for that host at the provider's domain (<spanx style="verb">mta-sts.provider.example</spanx>).
</t>
<t>Note that while sending MTAs MUST NOT use HTTP caching when fetching policies
via HTTPS, such caching may nonetheless be useful to a reverse proxy configured
as described in this section. An HTTPS policy endpoint expecting to be proxied
for multiple hosted domains--as with a large mail hosting provider or
similar--may wish to indicate an HTTP Cache-Control <spanx style="verb">max-age</spanx> response directive
(as specified in <xref target="RFC7234"/>) of 60 seconds as a reasonable value to save
reverse proxies an unnecessarily high-rate of proxied policy fetching.
</t>
</section>
<section anchor="removing-mtasts" title="Removing MTA-STS">
<t>In order to facilitate clean opt-out of MTA-STS by implementing policy domains,
and to distinguish clearly between failures which indicate attacks and those
which indicate such opt-outs, MTA-STS implements the <spanx style="verb">none</spanx> mode, which allows
validated policies to indicate authoritatively that the policy domain wishes to
no longer implement MTA-STS and may, in the future, remove the MTA-STS TXT and
policy endpoints entirely.
</t>
<t>A suggested workflow to implement such an opt out is as follows:
</t>
<t>
<list style="numbers">
<t>Publish a new policy with <spanx style="verb">mode</spanx> equal to <spanx style="verb">none</spanx> and a small <spanx style="verb">max_age</spanx> (e.g.,
one day).</t>
<t>Publish a new TXT record to trigger fetching of the new policy.</t>
<t>When all previously served policies have expired--normally this is the time
the previously published policy was last served plus that policy's <spanx style="verb">max_age</spanx>,
but note that older policies may have been served with a greater <spanx style="verb">max_age</spanx>,
allowing overlapping policy caches--safely remove the TXT record and HTTPS
endpoint.</t>
</list>
</t>
</section>
<section anchor="preserving-mx-candidate-traversal" title="Preserving MX Candidate Traversal">
<t>Implementors of send-time MTA-STS validation in mail transfer agents should take
note of the risks of modifying the logic of traversing MX candidate lists.
Because an MTA-STS policy can be used to prefilter invalid MX candidates from
the MX candidate list, it is tempting to implement a "two-pass" model, where MX
candidates are first filtered for possible validity according to the MTA-STS
policy, and then the remaining candidates attempted in order as without an
MTA-STS policy. This may lead to incorrect implementations, such a message
loops; implementors are instead recommended to traverse the MX candidate list as
usual, and treat invalid candidates as though they were unreachable (i.e., as
though there were some transient error when trying to deliver to that
candidate).
</t>
<t>One consequence of validating MX hosts in order of ordinary candidate traversal
is that, in the event that a higher-priority MX is MTA-STS valid and a
lower-priority MX is not, senders may never encounter the lower-priority MX,
leading to a risk that policy misconfigurations that apply only to "backup" MXes
may only be discovered in the case of primary MX failure.
</t>
</section>
</section>
<section anchor="iana-considerations" title="IANA Considerations">
<section anchor="wellknown-uris-registry" title="Well-Known URIs Registry">
<t>A new "well-known" URI as described in <xref target="policy-discovery"/> will be registered in
the Well-Known URIs registry as described below:
</t>
<t>URI Suffix: mta-sts.txt Change Controller: IETF
</t>
</section>
<section anchor="mtasts-txt-record-fields" title="MTA-STS TXT Record Fields">
<t>IANA is requested to create a new registry titled "MTA-STS TXT Record Fields".
The initial entries in the registry are:
</t>
<texttable>
<ttcol align="center">Field Name</ttcol>
<ttcol align="center">Description</ttcol>
<ttcol align="center">Reference</ttcol>
<c>v</c><c>Record version</c><c><xref target="mtasts-txt-records"/> of RFC XXX</c>
<c>id</c><c>Policy instance ID</c><c><xref target="mtasts-txt-records"/> of RFC XXX</c>
</texttable>
<t>New fields are added to this registry using IANA's "Expert Review" policy.
</t>
</section>
<section anchor="mtasts-policy-fields" title="MTA-STS Policy Fields">
<t>IANA is requested to create a new registry titled "MTA-STS Policy Fields". The
initial entries in the registry are:
</t>
<texttable>
<ttcol align="center">Field Name</ttcol>
<ttcol align="center">Description</ttcol>
<ttcol align="center">Reference</ttcol>
<c>version</c><c>Policy version</c><c><xref target="mtasts-policies"/> of RFC XXX</c>
<c>mode</c><c>Enforcement behavior</c><c><xref target="mtasts-policies"/> of RFC XXX</c>
<c>max_age</c><c>Policy lifetime</c><c><xref target="mtasts-policies"/> of RFC XXX</c>
<c>mx</c><c>MX identities</c><c><xref target="mtasts-policies"/> of RFC XXX</c>
</texttable>
<t>New fields are added to this registry using IANA's "Expert Review"
policy.
</t>
</section>
</section>
<section anchor="security-considerations" title="Security Considerations">
<t>SMTP MTA Strict Transport Security attempts to protect against an active
attacker trying to intercept or tamper with mail between hosts that support
STARTTLS. There are two classes of attacks considered:
</t>
<t>
<list style="symbols">
<t>Foiling TLS negotiation, for example by deleting the "250 STARTTLS" response
from a server or altering TLS session negotiation. This would result in the
SMTP session occurring over plaintext, despite both parties supporting TLS.</t>
<t>Impersonating the destination mail server, whereby the sender might deliver
the message to an impostor, who could then monitor and/or modify messages
despite opportunistic TLS. This impersonation could be accomplished by
spoofing the DNS MX record for the recipient domain, or by redirecting client
connections intended for the legitimate recipient server (for example, by
altering BGP routing tables).</t>
</list>
</t>
<t>MTA-STS can thwart such attacks only if the sender is able to previously obtain
and cache a policy for the recipient domain, and only if the attacker is unable
to obtain a valid certificate that complies with that policy. Below, we
consider specific attacks on this model.
</t>
<section anchor="obtaining-a-signed-certificate" title="Obtaining a Signed Certificate">
<t>SMTP MTA-STS relies on certificate validation via PKIX based TLS identity
checking <xref target="RFC6125"/>. Attackers who are able to obtain a valid certificate for
the targeted recipient mail service (e.g., by compromising a certificate
authority) are thus able to circumvent STS authentication.
</t>
</section>
<section anchor="preventing-policy-discovery" title="Preventing Policy Discovery">
<t>Since MTA-STS uses DNS TXT records for policy discovery, an attacker who is able
to block DNS responses can suppress the discovery of an MTA-STS Policy, making
the Policy Domain appear not to have an MTA-STS Policy. The sender policy cache
is designed to resist this attack by decreasing the frequency of policy
discovery and thus reducing the window of vulnerability; it is nonetheless a
risk that attackers who can predict or induce policy discovery--for example, by
inducing a sending domain to send mail to a never-before-contacted recipient
while carrying out a man-in-the-middle attack--may be able to foil policy
discovery and effectively downgrade the security of the message delivery.
</t>
<t>Since this attack depends upon intercepting initial policy discovery,
implementers SHOULD prefer policy <spanx style="verb">max_age</spanx> values to be as long as is
practical.
</t>
<t>Because this attack is also possible upon refresh of a cached policy,
implementors SHOULD NOT wait until a cached policy has expired before checking
for an update; if senders attempt to refresh the cache regularly (for example,
by fetching currently live policy in a background task that runs daily or
weekly, regardless of the state of the <spanx style="verb">_mta-sts</spanx> TXT record, and updating their
cache's "max age" accordingly), an attacker would have to foil policy discovery
consistently over the lifetime of a cached policy to prevent a successful
refresh.
</t>
<t>Additionally, MTAs SHOULD alert administrators to repeated policy refresh
failures long before cached policies expire (through warning logs or similar
applicable mechanisms), allowing administrators to detect such a persistent
attack on policy refresh. (However, they should not implement such alerts if
the cached policy has a <spanx style="verb">none</spanx> mode, to allow clean MTA-STS removal, as
described in <xref target="removing-mtasts"/>.)
</t>
<t>Resistance to downgrade attacks of this nature--due to the ability to
authoritatively determine "lack of a record" even for non-participating
recipients--is a feature of DANE, due to its use of DNSSEC for policy discovery.
</t>
</section>
<section anchor="denial-of-service" title="Denial of Service">
<t>We additionally consider the Denial of Service risk posed by an attacker who can
modify the DNS records for a recipient domain. Absent MTA-STS, such an attacker
can cause a sending MTA to cache invalid MX records, but only for however long
the sending resolver caches those records. With MTA-STS, the attacker can
additionally advertise a new, long-<spanx style="verb">max_age</spanx> MTA-STS policy with <spanx style="verb">mx</spanx>
constraints that validate the malicious MX record, causing senders to cache the
policy and refuse to deliver messages once the victim has resecured the MX
records.
</t>
<t>This attack is mitigated in part by the ability of a victim domain to (at any
time) publish a new policy updating the cached, malicious policy, though this
does require the victim domain to both obtain a valid CA-signed certificate and
to understand and properly configure MTA-STS.
</t>
<t>Similarly, we consider the possibility of domains that deliberately allow
untrusted users to serve untrusted content on user-specified subdomains. In
some cases (e.g., the service Tumblr.com) this takes the form of providing HTTPS
hosting of user-registered subdomains; in other cases (e.g. dynamic DNS
providers) this takes the form of allowing untrusted users to register custom
DNS records at the provider's domain.
</t>
<t>In these cases, there is a risk that untrusted users would be able to serve
custom content at the <spanx style="verb">mta-sts</spanx> host, including serving an illegitimate MTA-STS
policy. We believe this attack is rendered more difficult by the need for the
attacker to also serve the <spanx style="verb">_mta-sts</spanx> TXT record on the same domain--something
not, to our knowledge, widely provided to untrusted users. This attack is
additionally mitigated by the aforementioned ability for a victim domain to
update an invalid policy at any future date.
</t>
</section>
<section anchor="weak-policy-constraints" title="Weak Policy Constraints">
<t>Even if an attacker cannot modify a served policy, the potential exists for
configurations that allow attackers on the same domain to receive mail for that
domain. For example, an easy configuration option when authoring an MTA-STS
Policy for <spanx style="verb">example.com</spanx> is to set the <spanx style="verb">mx</spanx> equal to <spanx style="verb">*.example.com</spanx>; recipient
domains must consider in this case the risk that any user possessing a valid
hostname and CA-signed certificate (for example, <spanx style="verb">dhcp-123.example.com</spanx>) will,
from the perspective of MTA-STS Policy validation, be a valid MX host for that
domain.
</t>
</section>
<section anchor="compromise-of-the-web-pki-system" title="Compromise of the Web PKI System">
<t>A host of risks apply to the PKI system used for certificate authentication,
both of the <spanx style="verb">mta-sts</spanx> HTTPS host's certificate and the SMTP servers'
certificates. These risks are broadly applicable within the Web PKI ecosystem
and are not specific to MTA-STS; nonetheless, they deserve some consideration in
this context.
</t>
<t>Broadly speaking, attackers may compromise the system by obtaining certificates
under fraudulent circumstances (i.e., by impersonating the legitimate owner of
the victim domain), by compromising a Certificate Authority or Delegate
Authority's private keys, by obtaining a legitimate certificate issued to the
victim domain, and similar.
</t>
<t>One approach commonly employed by Web browsers to help mitigate against some of
these attacks is to allow for revocation of compromised or fraudulent
certificates via OCSP <xref target="RFC6960"/> or CRLs <xref target="RFC6818"/>. Such mechanisms
themselves represent tradeoffs and are not universally implemented; we
nonetheless recommend implementors of MTA-STS to implement revocation mechanisms
which are most applicable to their implementations.
</t>
</section>
</section>
<section anchor="contributors" title="Contributors">
<t>Wei Chuang
Google, Inc
weihaw@google.com
</t>
<t>Viktor Dukhovni
ietf-dane@dukhovni.de
</t>
<t>Markus Laber
1&1 Mail & Media Development & Technology GmbH
markus.laber@1und1.de
</t>
<t>Nicolas Lidzborski
Google, Inc
nlidz@google.com
</t>
<t>Brandon Long
Google, Inc
blong@google.com
</t>
<t>Franck Martin
LinkedIn, Inc
fmartin@linkedin.com
</t>
<t>Klaus Umbach
1&1 Mail & Media Development & Technology GmbH
klaus.umbach@1und1.de
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-uta-smtp-tlsrpt.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2818.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3207.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3492.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5246.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5280.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5321.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5785.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6066.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6125.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7405.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7525.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"?>
</references>
<references title="Informative References">
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4033.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5322.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5891.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6818.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6960.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7234.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7672.xml"?>
</references>
<section anchor="mtasts-example-record--policy" title="MTA-STS example record & policy">
<t>The owner of <spanx style="verb">example.com</spanx> wishes to begin using MTA-STS with a policy that will
solicit reports from senders without affecting how the messages are processed,
in order to verify the identity of MXs that handle mail for <spanx style="verb">example.com</spanx>,
confirm that TLS is correctly used, and ensure that certificates presented by
the recipient MX validate.
</t>
<t>MTA-STS policy indicator TXT RR:
</t>
<figure align="center"><artwork align="center">
_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
</artwork></figure>
<t>MTA-STS Policy file served as the response body at
<spanx style="verb">https://mta-sts.example.com/.well-known/mta-sts.txt</spanx>:
</t>
<figure align="center"><artwork align="center">
version: STSv1