summaryrefslogtreecommitdiff
path: root/doc/doc-txt/draft-ietf-dane-smtp-with-dane-11.txt
blob: 26bed33a5a048f055bc0f447c0735e0ec55236dc (plain)
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
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
DANE                                                         V. Dukhovni
Internet-Draft                                                 Two Sigma
Intended status: Standards Track                             W. Hardaker
Expires: February 3, 2015                                        Parsons
                                                          August 2, 2014


                SMTP security via opportunistic DANE TLS
                   draft-ietf-dane-smtp-with-dane-11

Abstract

   This memo describes a downgrade-resistant protocol for SMTP transport
   security between Mail Transfer Agents (MTAs) based on the DNS-Based
   Authentication of Named Entities (DANE) TLSA DNS record.  Adoption of
   this protocol enables an incremental transition of the Internet email
   backbone to one using encrypted and authenticated Transport Layer
   Security (TLS).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 3, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



Dukhovni & Hardaker     Expires February 3, 2015                [Page 1]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  SMTP channel security . . . . . . . . . . . . . . . . . .   6
       1.3.1.  STARTTLS downgrade attack . . . . . . . . . . . . . .   6
       1.3.2.  Insecure server name without DNSSEC . . . . . . . . .   7
       1.3.3.  Sender policy does not scale  . . . . . . . . . . . .   8
       1.3.4.  Too many certification authorities  . . . . . . . . .   8
   2.  Identifying applicable TLSA records . . . . . . . . . . . . .   9
     2.1.  DNS considerations  . . . . . . . . . . . . . . . . . . .   9
       2.1.1.  DNS errors, bogus and indeterminate responses . . . .   9
       2.1.2.  DNS error handling  . . . . . . . . . . . . . . . . .  11
       2.1.3.  Stub resolver considerations  . . . . . . . . . . . .  12
     2.2.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .  13
       2.2.1.  MX resolution . . . . . . . . . . . . . . . . . . . .  14
       2.2.2.  Non-MX destinations . . . . . . . . . . . . . . . . .  15
       2.2.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .  17
   3.  DANE authentication . . . . . . . . . . . . . . . . . . . . .  19
     3.1.  TLSA certificate usages . . . . . . . . . . . . . . . . .  19
       3.1.1.  Certificate usage DANE-EE(3)  . . . . . . . . . . . .  21
       3.1.2.  Certificate usage DANE-TA(2)  . . . . . . . . . . . .  22
       3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)  . . . .  23
     3.2.  Certificate matching  . . . . . . . . . . . . . . . . . .  24
       3.2.1.  DANE-EE(3) name checks  . . . . . . . . . . . . . . .  24
       3.2.2.  DANE-TA(2) name checks  . . . . . . . . . . . . . . .  24
       3.2.3.  Reference identifier matching . . . . . . . . . . . .  25
   4.  Server key management . . . . . . . . . . . . . . . . . . . .  26
   5.  Digest algorithm agility  . . . . . . . . . . . . . . . . . .  26
   6.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  28
   7.  Note on DANE for Message User Agents  . . . . . . . . . . . .  29
   8.  Interoperability considerations . . . . . . . . . . . . . . .  29
     8.1.  SNI support . . . . . . . . . . . . . . . . . . . . . . .  29
     8.2.  Anonymous TLS cipher suites . . . . . . . . . . . . . . .  30
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  30
     9.1.  Client Operational Considerations . . . . . . . . . . . .  30
     9.2.  Publisher Operational Considerations  . . . . . . . . . .  31
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  31
   11. IANA considerations . . . . . . . . . . . . . . . . . . . . .  32
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  33
     13.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34



Dukhovni & Hardaker     Expires February 3, 2015                [Page 2]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


1.  Introduction

   This memo specifies a new connection security model for Message
   Transfer Agents (MTAs).  This model is motivated by key features of
   inter-domain SMTP delivery, in particular the fact that the
   destination server is selected indirectly via DNS Mail Exchange (MX)
   records and that neither email addresses nor MX hostnames signal a
   requirement for either secure or cleartext transport.  Therefore,
   aside from a few manually configured exceptions, SMTP transport
   security is of necessity opportunistic.

   This specification uses the presence of DANE TLSA records to securely
   signal TLS support and to publish the means by which SMTP clients can
   successfully authenticate legitimate SMTP servers.  This becomes
   "opportunistic DANE TLS" and is resistant to downgrade and man-in-
   the-middle (MITM) attacks.  It enables an incremental transition of
   the email backbone to authenticated TLS delivery, with increased
   global protection as adoption increases.

   With opportunistic DANE TLS, traffic from SMTP clients to domains
   that publish "usable" DANE TLSA records in accordance with this memo
   is authenticated and encrypted.  Traffic from legacy clients or to
   domains that do not publish TLSA records will continue to be sent in
   the same manner as before, via manually configured security, (pre-
   DANE) opportunistic TLS or just cleartext SMTP.

   Problems with existing use of TLS in MTA to MTA SMTP that motivate
   this specification are described in Section 1.3.  The specification
   itself follows in Section 2 and Section 3 which describe respectively
   how to locate and use DANE TLSA records with SMTP.  In Section 6, we
   discuss application of DANE TLS to destinations for which channel
   integrity and confidentiality are mandatory.  In Section 7 we briefly
   comment on potential applicability of this specification to Message
   User Agents.

1.1.  Terminology

   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
   [RFC2119].

   The following terms or concepts are used through the document:

   Man-in-the-middle or MITM attack:  Active modification of network
      traffic by an adversary able to thereby compromise the
      confidentiality or integrity of the data.




Dukhovni & Hardaker     Expires February 3, 2015                [Page 3]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   secure, bogus, insecure, indeterminate:  DNSSEC validation results,
      as defined in Section 4.3 of [RFC4035].

   Validating Security-Aware Stub Resolver and     Non-Validating
   Security-Aware Stub Resolver:
      Capabilities of the stub resolver in use as defined in [RFC4033];
      note that this specification requires the use of a Security-Aware
      Stub Resolver.

   (pre-DANE) opportunistic TLS:  Best-effort use of TLS that is
      generally vulnerable to DNS forgery and STARTTLS downgrade
      attacks.  When a TLS-encrypted communication channel is not
      available, message transmission takes place in the clear.  MX
      record indirection generally precludes authentication even when
      TLS is available.

   opportunistic DANE TLS:  Best-effort use of TLS, resistant to
      downgrade attacks for destinations with DNSSEC-validated TLSA
      records.  When opportunistic DANE TLS is determined to be
      unavailable, clients should fall back to opportunistic TLS.
      Opportunistic DANE TLS requires support for DNSSEC, DANE and
      STARTTLS on the client side and STARTTLS plus a DNSSEC published
      TLSA record on the server side.

   reference identifier:  (Special case of [RFC6125] definition).  One
      of the domain names associated by the SMTP client with the
      destination SMTP server for performing name checks on the server
      certificate.  When name checks are applicable, at least one of the
      reference identifiers MUST match an [RFC6125] DNS-ID (or if none
      are present the [RFC6125] CN-ID) of the server certificate (see
      Section 3.2.3).

   MX hostname:  The RRDATA of an MX record consists of a 16 bit
      preference followed by a Mail Exchange domain name (see [RFC1035],
      Section 3.3.9).  We will use the term "MX hostname" to refer to
      the latter, that is, the DNS domain name found after the
      preference value in an MX record.  Thus an "MX hostname" is
      specifically a reference to a DNS domain name, rather than any
      host that bears that name.

   delayed delivery:  Email delivery is a multi-hop store & forward
      process.  When an MTA is unable forward a message that may become
      deliverable later the message is queued and delivery is retried
      periodically.  Some MTAs may be configured with a fallback next-
      hop destination that handles messages that the MTA would otherwise
      queue and retry.  When a fallback next-hop is configured, messages
      that would otherwise have to be delayed may be sent to the
      fallback next-hop destination instead.  The fallback destination



Dukhovni & Hardaker     Expires February 3, 2015                [Page 4]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


      may itself be subject to opportunistic or mandatory DANE TLS as
      though it were the original message destination.

   original next hop destination:   The logical destination for mail
      delivery.  By default this is the domain portion of the recipient
      address, but MTAs may be configured to forward mail for some or
      all recipients via designated relays.  The original next hop
      destination is, respectively, either the recipient domain or the
      associated configured relay.

   MTA:   Message Transfer Agent ([RFC5598], Section 4.3.2).

   MSA:   Message Submission Agent ([RFC5598], Section 4.3.1).

   MUA:   Message User Agent ([RFC5598], Section 4.2.1).

   RR:   A DNS Resource Record

   RRset:   A set of DNS Resource Records for a particular class, domain
      and record type.

1.2.  Background

   The Domain Name System Security Extensions (DNSSEC) add data origin
   authentication, data integrity and data non-existence proofs to the
   Domain Name System (DNS).  DNSSEC is defined in [RFC4033], [RFC4034]
   and [RFC4035].

   As described in the introduction of [RFC6698], TLS authentication via
   the existing public Certification Authority (CA) PKI suffers from an
   over-abundance of trusted parties capable of issuing certificates for
   any domain of their choice.  DANE leverages the DNSSEC infrastructure
   to publish trusted public keys and certificates for use with the
   Transport Layer Security (TLS) [RFC5246] protocol via a new "TLSA"
   DNS record type.  With DNSSEC each domain can only vouch for the keys
   of its directly delegated sub-domains.

   The TLS protocol enables secure TCP communication.  In the context of
   this memo, channel security is assumed to be provided by TLS.  Used
   without authentication, TLS provides only privacy protection against
   eavesdropping attacks.  With authentication, TLS also provides data
   integrity protection to guard against MITM attacks.









Dukhovni & Hardaker     Expires February 3, 2015                [Page 5]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


1.3.  SMTP channel security

   With HTTPS, Transport Layer Security (TLS) employs X.509 certificates
   [RFC5280] issued by one of the many Certificate Authorities (CAs)
   bundled with popular web browsers to allow users to authenticate
   their "secure" websites.  Before we specify a new DANE TLS security
   model for SMTP, we will explain why a new security model is needed.
   In the process, we will explain why the familiar HTTPS security model
   is inadequate to protect inter-domain SMTP traffic.

   The subsections below outline four key problems with applying
   traditional PKI to SMTP that are addressed by this specification.
   Since SMTP channel security policy is not explicitly specified in
   either the recipient address or the MX record, a new signaling
   mechanism is required to indicate when channel security is possible
   and should be used.  The publication of TLSA records allows server
   operators to securely signal to SMTP clients that TLS is available
   and should be used.  DANE TLSA makes it possible to simultaneously
   discover which destination domains support secure delivery via TLS
   and how to verify the authenticity of the associated SMTP services,
   providing a path forward to ubiquitous SMTP channel security.

1.3.1.  STARTTLS downgrade attack

   The Simple Mail Transfer Protocol (SMTP) [RFC5321] is a single-hop
   protocol in a multi-hop store & forward email delivery process.  An
   SMTP envelope recipient address does not correspond to a specific
   transport-layer endpoint address, rather at each relay hop the
   transport-layer endpoint is the next-hop relay, while the envelope
   recipient address typically remains the same.  Unlike the Hypertext
   Transfer Protocol (HTTP) and its corresponding secured version,
   HTTPS, where the use of TLS is signaled via the URI scheme, email
   recipient addresses do not directly signal transport security policy.
   Indeed, no such signaling could work well with SMTP since TLS
   encryption of SMTP protects email traffic on a hop-by-hop basis while
   email addresses could only express end-to-end policy.















Dukhovni & Hardaker     Expires February 3, 2015                [Page 6]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   With no mechanism available to signal transport security policy, SMTP
   relays employ a best-effort "opportunistic" security model for TLS.
   A single SMTP server TCP listening endpoint can serve both TLS and
   non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS
   command ([RFC3207]).  The server signals TLS support to the client
   over a cleartext SMTP connection, and, if the client also supports
   TLS, it may negotiate a TLS encrypted channel to use for email
   transmission.  The server's indication of TLS support can be easily
   suppressed by an MITM attacker.  Thus pre-DANE SMTP TLS security can
   be subverted by simply downgrading a connection to cleartext.  No TLS
   security feature, such as the use of PKIX, can prevent this.  The
   attacker can simply disable TLS.

1.3.2.  Insecure server name without DNSSEC

   With SMTP, DNS Mail Exchange (MX) records abstract the next-hop
   transport endpoint and allow administrators to specify a set of
   target servers to which SMTP traffic should be directed for a given
   domain.

   A PKIX TLS client is vulnerable to MITM attacks unless it verifies
   that the server's certificate binds the public key to a name that
   matches one of the client's reference identifiers.  A natural choice
   of reference identifier is the server's domain name.  However, with
   SMTP, server names are not directly encoded in the recipient address,
   instead they are obtained indirectly via MX records.  Without DNSSEC,
   the MX lookup is vulnerable to MITM and DNS cache poisoning attacks.
   Active attackers can forge DNS replies with fake MX records and can
   redirect email to servers with names of their choice.  Therefore,
   secure verification of SMTP TLS certificates matching the server name
   is not possible without DNSSEC.

   One might try to harden TLS for SMTP against DNS attacks by using the
   envelope recipient domain as a reference identifier and requiring
   each SMTP server to possess a trusted certificate for the envelope
   recipient domain rather than the MX hostname.  Unfortunately, this is
   impractical as email for many domains is handled by third parties
   that are not in a position to obtain certificates for all the domains
   they serve.  Deployment of the Server Name Indication (SNI) extension
   to TLS (see [RFC6066] Section 3) is no panacea, since SNI key
   management is operationally challenging except when the email service
   provider is also the domain's registrar and its certificate issuer;
   this is rarely the case for email.

   Since the recipient domain name cannot be used as the SMTP server
   reference identifier, and neither can the MX hostname without DNSSEC,
   large-scale deployment of authenticated TLS for SMTP requires that
   the DNS be secure.



Dukhovni & Hardaker     Expires February 3, 2015                [Page 7]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   Since SMTP security depends critically on DNSSEC, it is important to
   point out that consequently SMTP with DANE is the most conservative
   possible trust model.  It trusts only what must be trusted and no
   more.  Adding any other trusted actors to the mix can only reduce
   SMTP security.  A sender may choose to further harden DNSSEC for
   selected high-value receiving domains by configuring explicit trust
   anchors for those domains instead of relying on the chain of trust
   from the root domain.  However, detailed discussion of DNSSEC
   security practices is out of scope for this document.

1.3.3.  Sender policy does not scale

   Sending systems are in some cases explicitly configured to use TLS
   for mail sent to selected peer domains.  This requires sending MTAs
   to be configured with appropriate subject names or certificate
   content digests to expect in the presented server certificates.
   Because of the heavy administrative burden, such statically
   configured SMTP secure channels are used rarely (generally only
   between domains that make bilateral arrangements with their business
   partners).  Internet email, on the other hand, requires regularly
   contacting new domains for which security configurations cannot be
   established in advance.

   The abstraction of the SMTP transport endpoint via DNS MX records,
   often across organization boundaries, limits the use of public CA PKI
   with SMTP to a small set of sender-configured peer domains.  With
   little opportunity to use TLS authentication, sending MTAs are rarely
   configured with a comprehensive list of trusted CAs.  SMTP services
   that support STARTTLS often deploy X.509 certificates that are self-
   signed or issued by a private CA.

1.3.4.  Too many certification authorities

   Even if it were generally possible to determine a secure server name,
   the SMTP client would still need to verify that the server's
   certificate chain is issued by a trusted Certification Authority (a
   trust anchor).  MTAs are not interactive applications where a human
   operator can make a decision (wisely or otherwise) to selectively
   disable TLS security policy when certificate chain verification
   fails.  With no user to "click OK", the MTA's list of public CA trust
   anchors would need to be comprehensive in order to avoid bouncing
   mail addressed to sites that employ unknown Certification
   Authorities.








Dukhovni & Hardaker     Expires February 3, 2015                [Page 8]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   On the other hand, each trusted CA can issue certificates for any
   domain.  If even one of the configured CAs is compromised or operated
   by an adversary, it can subvert TLS security for all destinations.
   Any set of CAs is simultaneously both overly inclusive and not
   inclusive enough.

2.  Identifying applicable TLSA records

2.1.  DNS considerations

2.1.1.  DNS errors, bogus and indeterminate responses

   An SMTP client that implements opportunistic DANE TLS per this
   specification depends critically on the integrity of DNSSEC lookups,
   as discussed in Section 1.3.2.  This section lists the DNS resolver
   requirements needed to avoid downgrade attacks when using
   opportunistic DANE TLS.

   A DNS lookup may signal an error or return a definitive answer.  A
   security-aware resolver must be used for this specification.
   Security-aware resolvers will indicate the security status of a DNS
   RRset with one of four possible values defined in Section 4.3 of
   [RFC4035]: "secure", "insecure", "bogus" and "indeterminate".  In
   [RFC4035] the meaning of the "indeterminate" security status is:

     An RRset for which the resolver is not able to determine whether
     the RRset should be signed, as the resolver is not able to obtain
     the necessary DNSSEC RRs.  This can occur when the security-aware
     resolver is not able to contact security-aware name servers for
     the relevant zones.

   Note, the "indeterminate" security status has a conflicting
   definition in section 5 of [RFC4033].

     There is no trust anchor that would indicate that a specific
     portion of the tree is secure.

   To avoid further confusion, the adjective "anchorless" will be used
   below to refer to domains or RRsets that are "indeterminate" in the
   [RFC4033] sense, and the term "indeterminate" will be used
   exclusively in the sense of [RFC4035].

   SMTP clients following this specification SHOULD NOT distinguish
   between "insecure" and "anchorless" DNS responses.  Both "insecure"
   and "anchorless" RRsets MUST be handled identically: in either case
   unvalidated data for the query domain is all that is and can be
   available, and authentication using the data is impossible.  In what
   follows, the term "insecure" will also includes the case of



Dukhovni & Hardaker     Expires February 3, 2015                [Page 9]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   "anchorless" domains that lie in a portion of the DNS tree for which
   there is no applicable trust anchor.  With the DNS root zone signed,
   we expect that validating resolvers used by Internet-facing MTAs will
   be configured with trust anchor data for the root zone, and that
   therefore "anchorless" domains should be rare in practice.

   As noted in section 4.3 of [RFC4035], a security-aware DNS resolver
   MUST be able to determine whether a given non-error DNS response is
   "secure", "insecure", "bogus" or "indeterminate".  It is expected
   that most security-aware stub resolvers will not signal an
   "indeterminate" security status (in the sense of RFC4035) to the
   application, and will signal a "bogus" or error result instead.  If a
   resolver does signal an RFC4035 "indeterminate" security status, this
   MUST be treated by the SMTP client as though a "bogus" or error
   result had been returned.

   An MTA making use of a non-validating security-aware stub resolver
   MAY use the stub resolver's ability, if available, to signal DNSSEC
   validation status based on information the stub resolver has learned
   from an upstream validating recursive resolver.  Security-Oblivious
   stub-resolvers MUST NOT be used.  In accordance with section 4.9.3 of
   [RFC4035]:

     ... a security-aware stub resolver MUST NOT place any reliance on
     signature validation allegedly performed on its behalf, except
     when the security-aware stub resolver obtained the data in question
     from a trusted security-aware recursive name server via a secure
     channel.

   To avoid much repetition in the text below, we will pause to explain
   the handling of "bogus" or "indeterminate" DNSSEC query responses.
   These are not necessarily the result of a malicious actor; they can,
   for example, occur when network packets are corrupted or lost in
   transit.  Therefore, "bogus" or "indeterminate" replies are equated
   in this memo with lookup failure.

   There is an important non-failure condition we need to highlight in
   addition to the obvious case of the DNS client obtaining a non-empty
   "secure" or "insecure" RRset of the requested type.  Namely, it is
   not an error when either "secure" or "insecure" non-existence is
   determined for the requested data.  When a DNSSEC response with a
   validation status that is either "secure" or "insecure" reports
   either no records of the requested type or non-existence of the query
   domain, the response is not a DNS error condition.  The DNS client
   has not been left without an answer; it has learned that records of
   the requested type do not exist.





Dukhovni & Hardaker     Expires February 3, 2015               [Page 10]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   Security-aware stub resolvers will, of course, also signal DNS lookup
   errors in other cases, for example when processing a "ServFail"
   RCODE, which will not have an associated DNSSEC status.  All lookup
   errors are treated the same way by this specification, regardless of
   whether they are from a "bogus" or "indeterminate" DNSSEC status or
   from a more generic DNS error: the information that was requested
   cannot be obtained by the security-aware resolver at this time.  A
   lookup error is thus a failure to obtain the relevant RRset if it
   exists, or to determine that no such RRset exists when it does not.

   In contrast to a "bogus" or an "indeterminate" response, an
   "insecure" DNSSEC response is not an error, rather it indicates that
   the target DNS zone is either securely opted out of DNSSEC validation
   or is not connected with the DNSSEC trust anchors being used.
   Insecure results will leave the SMTP client with degraded channel
   security, but do not stand in the way of message delivery.  See
   section Section 2.2 for further details.

2.1.2.  DNS error handling

   When a DNS lookup failure (error or "bogus" or "indeterminate" as
   defined above) prevents an SMTP client from determining which SMTP
   server or servers it should connect to, message delivery MUST be
   delayed.  This naturally includes, for example, the case when a
   "bogus" or "indeterminate" response is encountered during MX
   resolution.  When multiple MX hostnames are obtained from a
   successful MX lookup, but a later DNS lookup failure prevents network
   address resolution for a given MX hostname, delivery may proceed via
   any remaining MX hosts.

   When a particular SMTP server is securely identified as the delivery
   destination, a set of DNS lookups (Section 2.2) MUST be performed to
   locate any related TLSA records.  If any DNS queries used to locate
   TLSA records fail (be it due to "bogus" or "indeterminate" records,
   timeouts, malformed replies, ServFails, etc.), then the SMTP client
   MUST treat that server as unreachable and MUST NOT deliver the
   message via that server.  If no servers are reachable, delivery is
   delayed.

   In what follows, we will only describe what happens when all relevant
   DNS queries succeed.  If any DNS failure occurs, the SMTP client MUST
   behave as described in this section, by skipping the problem SMTP
   server, or the problem destination.  Queries for candidate TLSA
   records are explicitly part of "all relevant DNS queries" and SMTP
   clients MUST NOT continue to connect to an SMTP server or destination
   whose TLSA record lookup fails.





Dukhovni & Hardaker     Expires February 3, 2015               [Page 11]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


2.1.3.  Stub resolver considerations

   SMTP clients that employ opportunistic DANE TLS to secure connections
   to SMTP servers MUST NOT use Security-Oblivious stub-resolvers.

   A note about DNAME aliases: a query for a domain name whose ancestor
   domain is a DNAME alias returns the DNAME RR for the ancestor domain
   along with a CNAME that maps the query domain to the corresponding
   sub-domain of the target domain of the DNAME alias [RFC6672].
   Therefore, whenever we speak of CNAME aliases, we implicitly allow
   for the possibility that the alias in question is the result of an
   ancestor domain DNAME record.  Consequently, no explicit support for
   DNAME records is needed in SMTP software; it is sufficient to process
   the resulting CNAME aliases.  DNAME records only require special
   processing in the validating stub-resolver library that checks the
   integrity of the combined DNAME + CNAME reply.  When DNSSEC
   validation is handled by a local caching resolver, rather than the
   MTA itself, even that part of the DNAME support logic is outside the
   MTA.

   When a stub resolver returns a response containing a CNAME alias that
   does not also contain the corresponding query results for the target
   of the alias, the SMTP client will need to repeat the query at the
   target of the alias, and should do so recursively up to some
   configured or implementation-dependent recursion limit.  If at any
   stage of CNAME expansion an error is detected, the lookup of the
   original requested records MUST be considered to have failed.

   Whether a chain of CNAME records was returned in a single stub
   resolver response or via explicit recursion by the SMTP client, if at
   any stage of recursive expansion an "insecure" CNAME record is
   encountered, then it and all subsequent results (in particular, the
   final result) MUST be considered "insecure" regardless of whether any
   earlier CNAME records leading to the "insecure" record were "secure".

   Note that a security-aware non-validating stub resolver may return to
   the SMTP client an "insecure" reply received from a validating
   recursive resolver that contains a CNAME record along with additional
   answers recursively obtained starting at the target of the CNAME.  In
   this case, the only possible conclusion is that some record in the
   set of records returned is "insecure", and it is in fact possible
   that the initial CNAME record and a subset of the subsequent records
   are "secure".

   If the SMTP client needs to determine the security status of the DNS
   zone containing the initial CNAME record, it may need to issue a
   separate query of type "CNAME" that returns only the initial CNAME
   record.  In particular in Section 2.2.2 when insecure A or AAAA



Dukhovni & Hardaker     Expires February 3, 2015               [Page 12]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   records are found for an SMTP server via a CNAME alias, it may be
   necessary to perform an additional CNAME query to determine whether
   the DNS zone in which the alias is published is signed.

2.2.  TLS discovery

   As noted previously (in Section 1.3.1), opportunistic TLS with SMTP
   servers that advertise TLS support via STARTTLS is subject to an MITM
   downgrade attack.  Also some SMTP servers that are not, in fact, TLS
   capable erroneously advertise STARTTLS by default and clients need to
   be prepared to retry cleartext delivery after STARTTLS fails.  In
   contrast, DNSSEC validated TLSA records MUST NOT be published for
   servers that do not support TLS.  Clients can safely interpret their
   presence as a commitment by the server operator to implement TLS and
   STARTTLS.

   This memo defines four actions to be taken after the search for a
   TLSA record returns secure usable results, secure unusable results,
   insecure or no results or an error signal.  The term "usable" in this
   context is in the sense of Section 4.1 of [RFC6698].  Specifically,
   if the DNS lookup for a TLSA record returns:

   A secure TLSA RRset with at least one usable record:  A connection to
      the MTA MUST be made using authenticated and encrypted TLS, using
      the techniques discussed in the rest of this document.  Failure to
      establish an authenticated TLS connection MUST result in falling
      back to the next SMTP server or delayed delivery.

   A secure non-empty TLSA RRset where all the records are unusable:  A
      connection to the MTA MUST be made via TLS, but authentication is
      not required.  Failure to establish an encrypted TLS connection
      MUST result in falling back to the next SMTP server or delayed
      delivery.

   An insecure TLSA RRset or DNSSEC validated proof-of-non-existent TLSA
    records:
      A connection to the MTA SHOULD be made using (pre-DANE)
      opportunistic TLS, this includes using cleartext delivery when the
      remote SMTP server does not appear to support TLS.  The MTA MAY
      retry in cleartext when delivery via TLS fails either during the
      handshake or even during data transfer.

   Any lookup error:  Lookup errors, including "bogus" and
      "indeterminate", as explained in Section 2.1.1 MUST result in
      falling back to the next SMTP server or delayed delivery.

   An SMTP client MAY be configured to require DANE verified delivery
   for some destinations.  We will call such a configuration "mandatory



Dukhovni & Hardaker     Expires February 3, 2015               [Page 13]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   DANE TLS".  With mandatory DANE TLS, delivery proceeds only when
   "secure" TLSA records are used to establish an encrypted and
   authenticated TLS channel with the SMTP server.

   When the original next-hop destination is an address literal, rather
   than a DNS domain, DANE TLS does not apply.  Delivery proceeds using
   any relevant security policy configured by the MTA administrator.
   Similarly, when an MX RRset incorrectly lists a network address in
   lieu of an MX hostname, if an MTA chooses to connect to the network
   address in the non-conformat MX record, DANE TLSA does not apply for
   such a connection.

   In the subsections that follow we explain how to locate the SMTP
   servers and the associated TLSA records for a given next-hop
   destination domain.  We also explain which name or names are to be
   used in identity checks of the SMTP server certificate.

2.2.1.  MX resolution

   In this section we consider next-hop domains that are subject to MX
   resolution and have MX records.  The TLSA records and the associated
   base domain are derived separately for each MX hostname that is used
   to attempt message delivery.  DANE TLS can authenticate message
   delivery to the intended next-hop domain only when the MX records are
   obtained securely via a DNSSEC validated lookup.

   MX records MUST be sorted by preference; an MX hostname with a worse
   (numerically higher) MX preference that has TLSA records MUST NOT
   preempt an MX hostname with a better (numerically lower) preference
   that has no TLSA records.  In other words, prevention of delivery
   loops by obeying MX preferences MUST take precedence over channel
   security considerations.  Even with two equal-preference MX records,
   an MTA is not obligated to choose the MX hostname that offers more
   security.  Domains that want secure inbound mail delivery need to
   ensure that all their SMTP servers and MX records are configured
   accordingly.

   In the language of [RFC5321] Section 5.1, the original next-hop
   domain is the "initial name".  If the MX lookup of the initial name
   results in a CNAME alias, the MTA replaces the initial name with the
   resulting name and performs a new lookup with the new name.  MTAs
   typically support recursion in CNAME expansion, so this replacement
   is performed repeatedly (up to the MTA's recursion limit) until the
   ultimate non-CNAME domain is found.

   If the MX RRset (or any CNAME leading to it) is "insecure" (see
   Section 2.1.1), DANE TLS need not apply, and delivery MAY proceed via
   pre-DANE opportunistic TLS.  That said, the protocol in this memo is



Dukhovni & Hardaker     Expires February 3, 2015               [Page 14]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   an "opportunistic security" protocol, meaning that it strives to
   communicate with each peer as securely as possible, while maintaining
   broad interoperability.  Therefore, the SMTP client MAY proceed to
   use DANE TLS (as described in Section 2.2.2 below) even with MX hosts
   obtained via an "insecure" MX RRset.  For example, when a hosting
   provider has a signed DNS zone and publishes TLSA records for its
   SMTP servers, hosted domains that are not signed may still benefit
   from the provider's TLSA records.  Deliveries via the provider's SMTP
   servers will not be subject to active attacks when sending SMTP
   clients elect to make use of the provider's TLSA records.

   When the MX records are not (DNSSEC) signed, an active attacker can
   redirect SMTP clients to MX hosts of his choice.  Such redirection is
   tamper-evident when SMTP servers found via "insecure" MX records are
   recorded as the next-hop relay in the MTA delivery logs in their
   original (rather than CNAME expanded) form.  Sending MTAs SHOULD log
   unexpanded MX hostnames when these result from insecure MX lookups.
   Any successful authentication via an insecurely determined MX host
   MUST NOT be misrepresented in the mail logs as secure delivery to the
   intended next-hop domain.  When DANE TLS is mandatory (Section 6) for
   a given destination, delivery MUST be delayed when the MX RRset is
   not "secure".

   Otherwise, assuming no DNS errors (Section 2.1.1), the MX RRset is
   "secure", and the SMTP client MUST treat each MX hostname as a
   separate non-MX destination for opportunistic DANE TLS as described
   in Section 2.2.2.  When, for a given MX hostname, no TLSA records are
   found, or only "insecure" TLSA records are found, DANE TLSA is not
   applicable with the SMTP server in question and delivery proceeds to
   that host as with pre-DANE opportunistic TLS.  To avoid downgrade
   attacks, any errors during TLSA lookups MUST, as explained in
   Section 2.1.1, cause the SMTP server in question to be treated as
   unreachable.

2.2.2.  Non-MX destinations

   This section describes the algorithm used to locate the TLSA records
   and associated TLSA base domain for an input domain not subject to MX
   resolution.  Such domains include:

   o  Each MX hostname used in a message delivery attempt for an
      original next-hop destination domain subject to MX resolution.
      Note, MTAs are not obligated to support CNAME expansion of MX
      hostnames.

   o  Any administrator configured relay hostname, not subject to MX
      resolution.  This frequently involves configuration set by the MTA
      administrator to handle some or all mail.



Dukhovni & Hardaker     Expires February 3, 2015               [Page 15]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   o  A next-hop destination domain subject to MX resolution that has no
      MX records.  In this case the domain's name is implicitly also its
      sole SMTP server name.

   Note that DNS queries with type TLSA are mishandled by load balancing
   nameservers that serve the MX hostnames of some large email
   providers.  The DNS zones served by these nameservers are not signed
   and contain no TLSA records, but queries for TLSA records fail,
   rather than returning the non-existence of the requested TLSA
   records.

   To avoid problems delivering mail to domains whose SMTP servers are
   served by the problem nameservers the SMTP client MUST perform any A
   and/or AAAA queries for the destination before attempting to locate
   the associated TLSA records.  This lookup is needed in any case to
   determine whether the destination domain is reachable and the DNSSEC
   validation status of the chain of CNAME queries required to reach the
   ultimate address records.

   If no address records are found, the destination is unreachable.  If
   address records are found, but the DNSSEC validation status of the
   first query response is "insecure" (see Section 2.1.3), the SMTP
   client SHOULD NOT proceed to search for any associated TLSA records.
   With the problem domains, TLSA queries will lead to DNS lookup errors
   and cause messages to be consistently delayed and ultimately returned
   to the sender.  We don't expect to find any "secure" TLSA records
   associated with a TLSA base domain that lies in an unsigned DNS zone.
   Therefore, skipping TLSA lookups in this case will also reduce
   latency with no detrimental impact on security.

   If the A and/or AAAA lookup of the "initial name" yields a CNAME, we
   replace it with the resulting name as if it were the initial name and
   perform a lookup again using the new name.  This replacement is
   performed recursively (up to the MTA's recursion limit).

   We consider the following cases for handling a DNS response for an A
   or AAAA DNS lookup:

   Not found:   When the DNS queries for A and/or AAAA records yield
      neither a list of addresses nor a CNAME (or CNAME expansion is not
      supported) the destination is unreachable.










Dukhovni & Hardaker     Expires February 3, 2015               [Page 16]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   Non-CNAME:   The answer is not a CNAME alias.  If the address RRset
      is "secure", TLSA lookups are performed as described in
      Section 2.2.3 with the initial name as the candidate TLSA base
      domain.  If no "secure" TLSA records are found, DANE TLS is not
      applicable and mail delivery proceeds with pre-DANE opportunistic
      TLS (which, being best-effort, degrades to cleartext delivery when
      STARTTLS is not available or the TLS handshake fails).

   Insecure CNAME:   The input domain is a CNAME alias, but the ultimate
      network address RRset is "insecure" (see Section 2.1.1).  If the
      initial CNAME response is also "insecure", DANE TLS does not
      apply.  Otherwise, this case is treated just like the non-CNAME
      case above, where a search is performed for a TLSA record with the
      original input domain as the candidate TLSA base domain.

   Secure CNAME:   The input domain is a CNAME alias, and the ultimate
      network address RRset is "secure" (see Section 2.1.1).  Two
      candidate TLSA base domains are tried: the fully CNAME-expanded
      initial name and, failing that, then the initial name itself.

   In summary, if it is possible to securely obtain the full, CNAME-
   expanded, DNSSEC-validated address records for the input domain, then
   that name is the preferred TLSA base domain.  Otherwise, the
   unexpanded input-MX domain is the candidate TLSA base domain.  When
   no "secure" TLSA records are found at either the CNAME-expanded or
   unexpanded domain, then DANE TLS does not apply for mail delivery via
   the input domain in question.  And, as always, errors, bogus or
   indeterminate results for any query in the process MUST result in
   delaying or abandoning delivery.

2.2.3.  TLSA record lookup

   Each candidate TLSA base domain (the original or fully CNAME-expanded
   name of a non-MX destination or a particular MX hostname of an MX
   destination) is in turn prefixed with service labels of the form
   "_<port>._tcp".  The resulting domain name is used to issue a DNSSEC
   query with the query type set to TLSA ([RFC6698] Section 7.1).

   For SMTP, the destination TCP port is typically 25, but this may be
   different with custom routes specified by the MTA administrator in
   which case the SMTP client MUST use the appropriate number in the
   "_<port>" prefix in place of "_25".  If, for example, the candidate
   base domain is "mx.example.com", and the SMTP connection is to port
   25, the TLSA RRset is obtained via a DNSSEC query of the form:

   _25._tcp.mx.example.com. IN TLSA ?





Dukhovni & Hardaker     Expires February 3, 2015               [Page 17]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   The query response may be a CNAME, or the actual TLSA RRset.  If the
   response is a CNAME, the SMTP client (through the use of its
   security-aware stub resolver) restarts the TLSA query at the target
   domain, following CNAMEs as appropriate and keeping track of whether
   the entire chain is "secure".  If any "insecure" records are
   encountered, or the TLSA records don't exist, the next candidate TLSA
   base domain is tried instead.

   If the ultimate response is a "secure" TLSA RRset, then the candidate
   TLSA base domain will be the actual TLSA base domain and the TLSA
   RRset will constitute the TLSA records for the destination.  If none
   of the candidate TLSA base domains yield "secure" TLSA records then
   delivery MAY proceed via pre-DANE opportunistic TLS.  SMTP clients
   MAY elect to use "insecure" TLSA records to avoid STARTTLS downgrades
   or even to skip SMTP servers that fail authentication, but MUST NOT
   misrepresent authentication success as either a secure connection to
   the SMTP server or as a secure delivery to the intended next-hop
   domain.

   TLSA record publishers may leverage CNAMEs to reference a single
   authoritative TLSA RRset specifying a common Certification Authority
   or a common end entity certificate to be used with multiple TLS
   services.  Such CNAME expansion does not change the SMTP client's
   notion of the TLSA base domain; thus, when _25._tcp.mx.example.com is
   a CNAME, the base domain remains mx.example.com and this is still the
   reference identifier used together with the next-hop domain in peer
   certificate name checks.

   Note that shared end entity certificate associations expose the
   publishing domain to substitution attacks, where an MITM attacker can
   reroute traffic to a different server that shares the same end entity
   certificate.  Such shared end entity TLSA records SHOULD be avoided
   unless the servers in question are functionally equivalent or employ
   mutually incompatible protocols (an active attacker gains nothing by
   diverting client traffic from one such server to another).

   A better example, employing a shared trust anchor rather than shared
   end-entity certificates, is illustrated by the DNSSEC validated
   records below:

     example.com.                IN MX 0 mx1.example.com.
     example.com.                IN MX 0 mx2.example.com.
     _25._tcp.mx1.example.com.   IN CNAME tlsa201._dane.example.com.
     _25._tcp.mx2.example.com.   IN CNAME tlsa201._dane.example.com.
     tlsa201._dane.example.com.  IN TLSA 2 0 1 e3b0c44298fc1c149a...

   The SMTP servers mx1.example.com and mx2.example.com will be expected
   to have certificates issued under a common trust anchor, but each MX



Dukhovni & Hardaker     Expires February 3, 2015               [Page 18]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   hostname's TLSA base domain remains unchanged despite the above CNAME
   records.  Correspondingly, each SMTP server will be associated with a
   pair of reference identifiers consisting of its hostname plus the
   next-hop domain "example.com".

   If, during TLSA resolution (including possible CNAME indirection), at
   least one "secure" TLSA record is found (even if not usable because
   it is unsupported by the implementation or support is
   administratively disabled), then the corresponding host has signaled
   its commitment to implement TLS.  The SMTP client MUST NOT deliver
   mail via the corresponding host unless a TLS session is negotiated
   via STARTTLS.  This is required to avoid MITM STARTTLS downgrade
   attacks.

   As noted previously (in Section Section 2.2.2), when no "secure" TLSA
   records are found at the fully CNAME-expanded name, the original
   unexpanded name MUST be tried instead.  This supports customers of
   hosting providers where the provider's zone cannot be validated with
   DNSSEC, but the customer has shared appropriate key material with the
   hosting provider to enable TLS via SNI.  Intermediate names that
   arise during CNAME expansion that are neither the original, nor the
   final name, are never candidate TLSA base domains, even if "secure".

3.  DANE authentication

   This section describes which TLSA records are applicable to SMTP
   opportunistic DANE TLS and how to apply such records to authenticate
   the SMTP server.  With opportunistic DANE TLS, both the TLS support
   implied by the presence of DANE TLSA records and the verification
   parameters necessary to authenticate the TLS peer are obtained
   together.  In contrast to protocols where channel security policy is
   set exclusively by the client, authentication via this protocol is
   expected to be less prone to connection failure caused by
   incompatible configuration of the client and server.

3.1.  TLSA certificate usages

   The DANE TLSA specification [RFC6698] defines multiple TLSA RR types
   via combinations of 3 numeric parameters.  The numeric values of
   these parameters were later given symbolic names in [RFC7218].  The
   rest of the TLSA record is the "certificate association data field",
   which specifies the full or digest value of a certificate or public
   key.  The parameters are:








Dukhovni & Hardaker     Expires February 3, 2015               [Page 19]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   The TLSA Certificate Usage field:  Section 2.1.1 of [RFC6698]
      specifies four values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and
      DANE-EE(3).  There is an additional private-use value:
      PrivCert(255).  All other values are reserved for use by future
      specifications.

   The selector field:  Section 2.1.2 of [RFC6698] specifies two values:
      Cert(0) and SPKI(1).  There is an additional private-use value:
      PrivSel(255).  All other values are reserved for use by future
      specifications.

   The matching type field:  Section 2.1.3 of [RFC6698] specifies three
      values: Full(0), SHA2-256(1) and SHA2-512(2).  There is an
      additional private-use value: PrivMatch(255).  All other values
      are reserved for use by future specifications.

   We may think of TLSA Certificate Usage values 0 through 3 as a
   combination of two one-bit flags.  The low bit chooses between trust
   anchor (TA) and end entity (EE) certificates.  The high bit chooses
   between public PKI issued and domain-issued certificates.

   The selector field specifies whether the TLSA RR matches the whole
   certificate: Cert(0), or just its subjectPublicKeyInfo: SPKI(1).  The
   subjectPublicKeyInfo is an ASN.1 DER ([X.690]) encoding of the
   certificate's algorithm id, any parameters and the public key data.

   The matching type field specifies how the TLSA RR Certificate
   Association Data field is to be compared with the certificate or
   public key.  A value of Full(0) means an exact match: the full DER
   encoding of the certificate or public key is given in the TLSA RR.  A
   value of SHA2-256(1) means that the association data matches the
   SHA2-256 digest of the certificate or public key, and likewise
   SHA2-512(2) means a SHA2-512 digest is used.

   Since opportunistic DANE TLS will be used by non-interactive MTAs,
   with no user to "press OK" when authentication fails, reliability of
   peer authentication is paramount.  Server operators are advised to
   publish TLSA records that are least likely to fail authentication due
   to interoperability or operational problems.  Because DANE TLS relies
   on coordinated changes to DNS and SMTP server settings, the best
   choice of records to publish will depend on site-specific practices.










Dukhovni & Hardaker     Expires February 3, 2015               [Page 20]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   The certificate usage element of a TLSA record plays a critical role
   in determining how the corresponding certificate association data
   field is used to authenticate server's certificate chain.  The next
   two subsections explain the process for certificate usages DANE-EE(3)
   and DANE-TA(2).  The third subsection briefly explains why
   certificate usages PKIX-TA(0) and PKIX-EE(1) are not applicable with
   opportunistic DANE TLS.

   In summary, we recommend the use of either "DANE-EE(3) SPKI(1)
   SHA2-256(1)" or "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records
   depending on site needs.  Other combinations of TLSA parameters are
   either explicitly unsupported, or offer little to recommend them over
   these two.

   The mandatory to support digest algorithm in [RFC6698] is
   SHA2-256(1).  When the server's TLSA RRset includes records with a
   matching type indicating a digest record (i.e., a value other than
   Full(0)), a TLSA record with a SHA2-256(1) matching type SHOULD be
   provided along with any other digest published, since some SMTP
   clients may support only SHA2-256(1).  If at some point the SHA2-256
   digest algorithm is tarnished by new cryptanalytic attacks,
   publishers will need to include an appropriate stronger digest in
   their TLSA records, initially along with, and ultimately in place of,
   SHA2-256.

3.1.1.  Certificate usage DANE-EE(3)

   Authentication via certificate usage DANE-EE(3) TLSA records involves
   simply checking that the server's leaf certificate matches the TLSA
   record.  In particular the binding of the server public key to its
   name is based entirely on the TLSA record association.  The server
   MUST be considered authenticated even if none of the names in the
   certificate match the client's reference identity for the server.

   Similarly, the expiration date of the server certificate MUST be
   ignored, the validity period of the TLSA record key binding is
   determined by the validity interval of the TLSA record DNSSEC
   signature.

   With DANE-EE(3) servers need not employ SNI (may ignore the client's
   SNI message) even when the server is known under independent names
   that would otherwise require separate certificates.  It is instead
   sufficient for the TLSA RRsets for all the domains in question to
   match the server's default certificate.  Of course with SMTP servers
   it is simpler still to publish the same MX hostname for all the
   hosted domains.





Dukhovni & Hardaker     Expires February 3, 2015               [Page 21]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   For domains where it is practical to make coordinated changes in DNS
   TLSA records during SMTP server key rotation, it is often best to
   publish end-entity DANE-EE(3) certificate associations.  DANE-EE(3)
   certificates don't suddenly stop working when leaf or intermediate
   certificates expire, and don't fail when the server operator neglects
   to configure all the required issuer certificates in the server
   certificate chain.

   TLSA records published for SMTP servers SHOULD, in most cases, be
   "DANE-EE(3) SPKI(1) SHA2-256(1)" records.  Since all DANE
   implementations are required to support SHA2-256, this record type
   works for all clients and need not change across certificate renewals
   with the same key.

3.1.2.  Certificate usage DANE-TA(2)

   Some domains may prefer to avoid the operational complexity of
   publishing unique TLSA RRs for each TLS service.  If the domain
   employs a common issuing Certification Authority to create
   certificates for multiple TLS services, it may be simpler to publish
   the issuing authority as a trust anchor (TA) for the certificate
   chains of all relevant services.  The TLSA query domain (TLSA base
   domain with port and protocol prefix labels) for each service issued
   by the same TA may then be set to a CNAME alias that points to a
   common TLSA RRset that matches the TA.  For example:

     example.com.                IN MX 0 mx1.example.com.
     example.com.                IN MX 0 mx2.example.com.
     _25._tcp.mx1.example.com.   IN CNAME tlsa201._dane.example.com.
     _25._tcp.mx2.example.com.   IN CNAME tlsa201._dane.example.com.
     tlsa201._dane.example.com.  IN TLSA 2 0 1 e3b0c44298fc1c14....

   With usage DANE-TA(2) the server certificates will need to have names
   that match one of the client's reference identifiers (see [RFC6125]).
   The server MAY employ SNI to select the appropriate certificate to
   present to the client.

   SMTP servers that rely on certificate usage DANE-TA(2) TLSA records
   for TLS authentication MUST include the TA certificate as part of the
   certificate chain presented in the TLS handshake server certificate
   message even when it is a self-signed root certificate.  At this
   time, many SMTP servers are not configured with a comprehensive list
   of trust anchors, nor are they expected to at any point in the
   future.  Some MTAs will ignore all locally trusted certificates when
   processing usage DANE-TA(2) TLSA records.  Thus even when the TA
   happens to be a public Certification Authority known to the SMTP
   client, authentication is likely to fail unless the TA certificate is
   included in the TLS server certificate message.



Dukhovni & Hardaker     Expires February 3, 2015               [Page 22]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   TLSA records with selector Full(0) are discouraged.  While these
   potentially obviate the need to transmit the TA certificate in the
   TLS server certificate message, client implementations may not be
   able to augment the server certificate chain with the data obtained
   from DNS, especially when the TLSA record supplies a bare key
   (selector SPKI(1)).  Since the server will need to transmit the TA
   certificate in any case, server operators SHOULD publish TLSA records
   with a selector other than Full(0) and avoid potential
   interoperability issues with large TLSA records containing full
   certificates or keys.

   TLSA Publishers employing DANE-TA(2) records SHOULD publish records
   with a selector of Cert(0).  Such TLSA records are associated with
   the whole trust anchor certificate, not just with the trust anchor
   public key.  In particular, the SMTP client SHOULD then apply any
   relevant constraints from the trust anchor certificate, such as, for
   example, path length constraints.

   While a selector of SPKI(1) may also be employed, the resulting TLSA
   record will not specify the full trust anchor certificate content,
   and elements of the trust anchor certificate other than the public
   key become mutable.  This may, for example, allow a subsidiary CA to
   issue a chain that violates the trust anchor's path length or name
   constraints.

3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)

   As noted in the introduction, SMTP clients cannot, without relying on
   DNSSEC for secure MX records and DANE for STARTTLS support signaling,
   perform server identity verification or prevent STARTTLS downgrade
   attacks.  The use of PKIX CAs offers no added security since an
   attacker capable of compromising DNSSEC is free to replace any PKIX-
   TA(0) or PKIX-EE(1) TLSA records with records bearing any convenient
   non-PKIX certificate usage.

   SMTP servers SHOULD NOT publish TLSA RRs with certificate usage PKIX-
   TA(0) or PKIX-EE(1).  SMTP clients cannot be expected to be
   configured with a suitably complete set of trusted public CAs.
   Lacking a complete set of public CAs, clients would not be able to
   verify the certificates of SMTP servers whose issuing root CAs are
   not trusted by the client.

   Opportunistic DANE TLS needs to interoperate without bilateral
   coordination of security settings between client and server systems.
   Therefore, parameter choices that are fragile in the absence of
   bilateral coordination are unsupported.  Nothing is lost since the
   PKIX certificate usages cannot aid SMTP TLS security, they can only
   impede SMTP TLS interoperability.



Dukhovni & Hardaker     Expires February 3, 2015               [Page 23]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0)
   or PKIX-EE(1) is undefined.  SMTP clients should generally treat such
   TLSA records as unusable.

3.2.  Certificate matching

   When at least one usable "secure" TLSA record is found, the SMTP
   client MUST use TLSA records to authenticate the SMTP server.
   Messages MUST NOT be delivered via the SMTP server if authentication
   fails, otherwise the SMTP client is vulnerable to MITM attacks.

3.2.1.  DANE-EE(3) name checks

   The SMTP client MUST NOT perform certificate name checks with
   certificate usage DANE-EE(3); see Section 3.1.1 above.

3.2.2.  DANE-TA(2) name checks

   To match a server via a TLSA record with certificate usage DANE-
   TA(2), the client MUST perform name checks to ensure that it has
   reached the correct server.  In all DANE-TA(2) cases the SMTP client
   MUST include the TLSA base domain as one of the valid reference
   identifiers for matching the server certificate.

   TLSA records for MX hostnames:  If the TLSA base domain was obtained
      indirectly via a "secure" MX lookup (including any CNAME-expanded
      name of an MX hostname), then the original next-hop domain used in
      the MX lookup MUST be included as as a second reference
      identifier.  The CNAME-expanded original next-hop domain MUST be
      included as a third reference identifier if different from the
      original next-hop domain.  When the client MTA is employing DANE
      TLS security despite "insecure" MX redirection the MX hostname is
      the only reference identifier.

   TLSA records for Non-MX hostnames:  If MX records were not used
      (e.g., if none exist) and the TLSA base domain is the CNAME-
      expanded original next-hop domain, then the original next-hop
      domain MUST be included as a second reference identifier.

   Accepting certificates with the original next-hop domain in addition
   to the MX hostname allows a domain with multiple MX hostnames to
   field a single certificate bearing a single domain name (i.e., the
   email domain) across all the SMTP servers.  This also aids
   interoperability with pre-DANE SMTP clients that are configured to
   look for the email domain name in server certificates.  For example,
   with "secure" DNS records as below:





Dukhovni & Hardaker     Expires February 3, 2015               [Page 24]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


     exchange.example.org.            IN CNAME mail.example.org.
     mail.example.org.                IN CNAME example.com.
     example.com.                     IN MX    10 mx10.example.com.
     example.com.                     IN MX    15 mx15.example.com.
     example.com.                     IN MX    20 mx20.example.com.
     ;
     mx10.example.com.                IN A     192.0.2.10
     _25._tcp.mx10.example.com.       IN TLSA  2 0 1 ...
     ;
     mx15.example.com.                IN CNAME mxbackup.example.com.
     mxbackup.example.com.            IN A     192.0.2.15
     ; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN)
     _25._tcp.mx15.example.com.       IN TLSA  2 0 1 ...
     ;
     mx20.example.com.                IN CNAME mxbackup.example.net.
     mxbackup.example.net.            IN A     198.51.100.20
     _25._tcp.mxbackup.example.net.   IN TLSA  2 0 1 ...

   Certificate name checks for delivery of mail to exchange.example.org
   via any of the associated SMTP servers MUST accept at least the names
   "exchange.example.org" and "example.com", which are respectively the
   original and fully expanded next-hop domain.  When the SMTP server is
   mx10.example.com, name checks MUST accept the TLSA base domain
   "mx10.example.com".  If, despite the fact that MX hostnames are
   required to not be aliases, the MTA supports delivery via
   "mx15.example.com" or "mx20.example.com" then name checks MUST accept
   the respective TLSA base domains "mx15.example.com" and
   "mxbackup.example.net".

3.2.3.  Reference identifier matching

   When name checks are applicable (certificate usage DANE-TA(2)), if
   the server certificate contains a Subject Alternative Name extension
   ([RFC5280]), with at least one DNS-ID ([RFC6125]) then only the DNS-
   IDs are matched against the client's reference identifiers.  The CN-
   ID ([RFC6125]) is only considered when no DNS-IDs are present.  The
   server certificate is considered matched when one of its presented
   identifiers ([RFC5280]) matches any of the client's reference
   identifiers.

   Wildcards are valid in either DNS-IDs or the CN-ID when applicable.
   The wildcard character must be entire first label of the DNS-ID or
   CN-ID.  Thus, "*.example.com" is valid, while "smtp*.example.com" and
   "*smtp.example.com" are not.  SMTP clients MUST support wildcards
   that match the first label of the reference identifier, with the
   remaining labels matching verbatim.  For example, the DNS-ID
   "*.example.com" matches the reference identifier "mx1.example.com".
   SMTP clients MAY, subject to local policy allow wildcards to match



Dukhovni & Hardaker     Expires February 3, 2015               [Page 25]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   multiple reference identifier labels, but servers cannot expect broad
   support for such a policy.  Therefore any wildcards in server
   certificates SHOULD match exactly one label in either the TLSA base
   domain or the next-hop domain.

4.  Server key management

   Two TLSA records MUST be published before employing a new EE or TA
   public key or certificate, one matching the currently deployed key
   and the other matching the new key scheduled to replace it.  Once
   sufficient time has elapsed for all DNS caches to expire the previous
   TLSA RRset and related signature RRsets, servers may be configured to
   use the new EE private key and associated public key certificate or
   may employ certificates signed by the new trust anchor.

   Once the new public key or certificate is in use, the TLSA RR that
   matches the retired key can be removed from DNS, leaving only RRs
   that match keys or certificates in active use.

   As described in Section 3.1.2, when server certificates are validated
   via a DANE-TA(2) trust anchor, and CNAME records are employed to
   store the TA association data at a single location, the
   responsibility of updating the TLSA RRset shifts to the operator of
   the trust anchor.  Before a new trust anchor is used to sign any new
   server certificates, its certificate (digest) is added to the
   relevant TLSA RRset.  After enough time elapses for the original TLSA
   RRset to age out of DNS caches, the new trust anchor can start
   issuing new server certificates.  Once all certificates issued under
   the previous trust anchor have expired, its associated RRs can be
   removed from the TLSA RRset.

   In the DANE-TA(2) key management model server operators do not
   generally need to update DNS TLSA records after initially creating a
   CNAME record that references the centrally operated DANE-TA(2) RRset.
   If a particular server's key is compromised, its TLSA CNAME SHOULD be
   replaced with a DANE-EE(3) association until the certificate for the
   compromised key expires, at which point it can return to using CNAME
   record.  If the central trust anchor is compromised, all servers need
   to be issued new keys by a new TA, and a shared DANE-TA(2) TLSA RRset
   needs to be published containing just the new TA.  SMTP servers
   cannot expect broad SMTP client CRL or OCSP support.

5.  Digest algorithm agility

   While [RFC6698] specifies multiple digest algorithms, it does not
   specify a protocol by which the SMTP client and TLSA record publisher
   can agree on the strongest shared algorithm.  Such a protocol would
   allow the client and server to avoid exposure to any deprecated



Dukhovni & Hardaker     Expires February 3, 2015               [Page 26]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   weaker algorithms that are published for compatibility with less
   capable clients, but should be ignored when possible.  We specify
   such a protocol below.

   Suppose that a DANE TLS client authenticating a TLS server considers
   digest algorithm "BetterAlg" stronger than digest algorithm
   "WorseAlg".  Suppose further that a server's TLSA RRset contains some
   records with "BetterAlg" as the digest algorithm.  Finally, suppose
   that for every raw public key or certificate object that is included
   in the server's TLSA RRset in digest form, whenever that object
   appears with algorithm "WorseAlg" with some usage and selector it
   also appears with algorithm "BetterAlg" with the same usage and
   selector.  In that case our client can safely ignore TLSA records
   with the weaker algorithm "WorseAlg", because it suffices to check
   the records with the stronger algorithm "BetterAlg".

   Server operators MUST ensure that for any given usage and selector,
   each object (certificate or public key), for which a digest
   association exists in the TLSA RRset, is published with the SAME SET
   of digest algorithms as all other objects that published with that
   usage and selector.  In other words, for each usage and selector, the
   records with non-zero matching types will correspond to on a cross-
   product of a set of underlying objects and a fixed set of digest
   algorithms that apply uniformly to all the objects.

   To achieve digest algorithm agility, all published TLSA RRsets for
   use with opportunistic DANE TLS for SMTP MUST conform to the above
   requirements.  Then, for each combination of usage and selector, SMTP
   clients can simply ignore all digest records except those that employ
   the strongest digest algorithm.  The ordering of digest algorithms by
   strength is not specified in advance, it is entirely up to the SMTP
   client.  SMTP client implementations SHOULD make the digest algorithm
   preference order configurable.  Only the future will tell which
   algorithms might be weakened by new attacks and when.

   Note, TLSA records with a matching type of Full(0), that publish the
   full value of a certificate or public key object, play no role in
   digest algorithm agility.  They neither trump the processing of
   records that employ digests, nor are they ignored in the presence of
   any records with a digest (i.e. non-zero) matching type.











Dukhovni & Hardaker     Expires February 3, 2015               [Page 27]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   SMTP clients SHOULD use digest algorithm agility when processing the
   DANE TLSA records of an SMTP server.  Algorithm agility is to be
   applied after first discarding any unusable or malformed records
   (unsupported digest algorithm, or incorrect digest length).  Thus,
   for each usage and selector, the client SHOULD process only any
   usable records with a matching type of Full(0) and the usable records
   whose digest algorithm is believed to be the strongest among usable
   records with the given usage and selector.

   The main impact of this requirement is on key rotation, when the TLSA
   RRset is pre-populated with digests of new certificates or public
   keys, before these replace or augment their predecessors.  Were the
   newly introduced RRs to include previously unused digest algorithms,
   clients that employ this protocol could potentially ignore all the
   digests corresponding to the current keys or certificates, causing
   connectivity issues until the new keys or certificates are deployed.
   Similarly, publishing new records with fewer digests could cause
   problems for clients using cached TLSA RRsets that list both the old
   and new objects once the new keys are deployed.

   To avoid problems, server operators SHOULD apply the following
   strategy:

   o  When changing the set of objects published via the TLSA RRset
      (e.g. during key rotation), DO NOT change the set of digest
      algorithms used; change just the list of objects.

   o  When changing the set of digest algorithms, change only the set of
      algorithms, and generate a new RRset in which all the current
      objects are re-published with the new set of digest algorithms.

   After either of these two changes are made, the new TLSA RRset should
   be left in place long enough that the older TLSA RRset can be flushed
   from caches before making another change.

6.  Mandatory TLS Security

   An MTA implementing this protocol may require a stronger security
   assurance when sending email to selected destinations.  The sending
   organization may need to send sensitive email and/or may have
   regulatory obligations to protect its content.  This protocol is not
   in conflict with such a requirement, and in fact can often simplify
   authenticated delivery to such destinations.

   Specifically, with domains that publish DANE TLSA records for their
   MX hostnames, a sending MTA can be configured to use the receiving
   domains's DANE TLSA records to authenticate the corresponding SMTP
   server.  Authentication via DANE TLSA records is easier to manage, as



Dukhovni & Hardaker     Expires February 3, 2015               [Page 28]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   changes in the receiver's expected certificate properties are made on
   the receiver end and don't require manually communicated
   configuration changes.  With mandatory DANE TLS, when no usable TLSA
   records are found, message delivery is delayed.  Thus, mail is only
   sent when an authenticated TLS channel is established to the remote
   SMTP server.

   Administrators of mail servers that employ mandatory DANE TLS, need
   to carefully monitor their mail logs and queues.  If a partner domain
   unwittingly misconfigures their TLSA records, disables DNSSEC, or
   misconfigures SMTP server certificate chains, mail will be delayed
   and may bounce if the issue is not resolved in a timely manner.

7.  Note on DANE for Message User Agents

   We note that the SMTP protocol is also used between Message User
   Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409].  In
   [RFC6186] a protocol is specified that enables an MUA to dynamically
   locate the MSA based on the user's email address.  SMTP connection
   security considerations for MUAs implementing [RFC6186] are largely
   analogous to connection security requirements for MTAs, and this
   specification could be applied largely verbatim with DNS MX records
   replaced by corresponding DNS Service (SRV) records
   [I-D.ietf-dane-srv].

   However, until MUAs begin to adopt the dynamic configuration
   mechanisms of [RFC6186] they are adequately served by more
   traditional static TLS security policies.  Specification of DANE TLS
   for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP
   is left to future documents that focus specifically on SMTP security
   between MUAs and MSAs.

8.  Interoperability considerations

8.1.  SNI support

   To ensure that the server sends the right certificate chain, the SMTP
   client MUST send the TLS SNI extension containing the TLSA base
   domain.  This precludes the use of the backward compatible SSL 2.0
   compatible SSL HELLO by the SMTP client.  The minimum SSL/TLS client
   HELLO version for SMTP clients performing DANE authentication is SSL
   3.0, but a client that offers SSL 3.0 MUST also offer at least TLS
   1.0 and MUST include the SNI extension.  Servers that don't make use
   of SNI MAY negotiate SSL 3.0 if offered by the client.

   Each SMTP server MUST present a certificate chain (see [RFC5246]
   Section 7.4.2) that matches at least one of the TLSA records.  The
   server MAY rely on SNI to determine which certificate chain to



Dukhovni & Hardaker     Expires February 3, 2015               [Page 29]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   present to the client.  Clients that don't send SNI information may
   not see the expected certificate chain.

   If the server's TLSA records match the server's default certificate
   chain, the server need not support SNI.  In either case, the server
   need not include the SNI extension in its TLS HELLO as simply
   returning a matching certificate chain is sufficient.  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 domain, the server MUST
   simply send some fallback certificate chain of its choice.  The
   reason for not enforcing strict matching of the requested SNI
   hostname is that DANE TLS clients are 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 acceptable to the client, e.g., the original next-hop domain.

8.2.  Anonymous TLS cipher suites

   Since many SMTP servers either do not support or do not enable any
   anonymous TLS cipher suites, SMTP client TLS HELLO messages SHOULD
   offer to negotiate a typical set of non-anonymous cipher suites
   required for interoperability with such servers.  An SMTP client
   employing pre-DANE opportunistic TLS MAY in addition include one or
   more anonymous TLS cipher suites in its TLS HELLO.  SMTP servers,
   that need to interoperate with opportunistic TLS clients SHOULD be
   prepared to interoperate with such clients by either always selecting
   a mutually supported non-anonymous cipher suite or by correctly
   handling client connections that negotiate anonymous cipher suites.

   Note that while SMTP server operators are under no obligation to
   enable anonymous cipher suites, no security is gained by sending
   certificates to clients that will ignore them.  Indeed support for
   anonymous cipher suites in the server makes audit trails more
   informative.  Log entries that record connections that employed an
   anonymous cipher suite record the fact that the clients did not care
   to authenticate the server.

9.  Operational Considerations

9.1.  Client Operational Considerations

   An operational error on the sending or receiving side that cannot be
   corrected in a timely manner may, at times, lead to consistent
   failure to deliver time-sensitive email.  The sending MTA
   administrator may have to choose between letting email queue until
   the error is resolved and disabling opportunistic or mandatory DANE



Dukhovni & Hardaker     Expires February 3, 2015               [Page 30]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   TLS for one or more destinations.  The choice to disable DANE TLS
   security should not be made lightly.  Every reasonable effort should
   be made to determine that problems with mail delivery are the result
   of an operational error, and not an attack.  A fallback strategy may
   be to configure explicit out-of-band TLS security settings if
   supported by the sending MTA.

   SMTP clients may deploy opportunistic DANE TLS incrementally by
   enabling it only for selected sites, or may occasionally need to
   disable opportunistic DANE TLS for peers that fail to interoperate
   due to misconfiguration or software defects on either end.  Some
   implementations MAY support DANE TLS in an "audit only" mode in which
   failure to achieve the requisite security level is logged as a
   warning and delivery proceeds at a reduced security level.  Unless
   local policy specifies "audit only" or that opportunistic DANE TLS is
   not to be used for a particular destination, an SMTP client MUST NOT
   deliver mail via a server whose certificate chain fails to match at
   least one TLSA record when usable TLSA records are found for that
   server.

9.2.  Publisher Operational Considerations

   SMTP servers that publish certificate usage DANE-TA(2) associations
   MUST include the TA certificate in their TLS server certificate
   chain, even when that TA certificate is a self-signed root
   certificate.

   TLSA Publishers MUST follow the digest agility guidelines in
   Section 5 and MUST make sure that all objects published in digest
   form for a particular usage and selector are published with the same
   set of digest algorithms.

   TLSA Publishers should follow the TLSA publication size guidance
   found in [I-D.ietf-dane-ops] about "DANE DNS Record Size Guidelines".

10.  Security Considerations

   This protocol leverages DANE TLSA records to implement MITM resistant
   opportunistic security ([I-D.dukhovni-opportunistic-security]) for
   SMTP.  For destination domains that sign their MX records and publish
   signed TLSA records for their MX hostnames, this protocol allows
   sending MTAs to securely discover both the availability of TLS and
   how to authenticate the destination.








Dukhovni & Hardaker     Expires February 3, 2015               [Page 31]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   This protocol does not aim to secure all SMTP traffic, as that is not
   practical until DNSSEC and DANE adoption are universal.  The
   incremental deployment provided by following this specification is a
   best possible path for securing SMTP.  This protocol coexists and
   interoperates with the existing insecure Internet email backbone.

   The protocol does not preclude existing non-opportunistic SMTP TLS
   security arrangements, which can continue to be used as before via
   manual configuration with negotiated out-of-band key and TLS
   configuration exchanges.

   Opportunistic SMTP TLS depends critically on DNSSEC for downgrade
   resistance and secure resolution of the destination name.  If DNSSEC
   is compromised, it is not possible to fall back on the public CA PKI
   to prevent MITM attacks.  A successful breach of DNSSEC enables the
   attacker to publish TLSA usage 3 certificate associations, and
   thereby bypass any security benefit the legitimate domain owner might
   hope to gain by publishing usage 0 or 1 TLSA RRs.  Given the lack of
   public CA PKI support in existing MTA deployments, avoiding
   certificate usages 0 and 1 simplifies implementation and deployment
   with no adverse security consequences.

   Implementations must strictly follow the portions of this
   specification that indicate when it is appropriate to initiate a non-
   authenticated connection or cleartext connection to a SMTP server.
   Specifically, in order to prevent downgrade attacks on this protocol,
   implementation must not initiate a connection when this specification
   indicates a particular SMTP server must be considered unreachable.

11.  IANA considerations

   This specification requires no support from IANA.

12.  Acknowledgements

   The authors would like to extend great thanks to Tony Finch, who
   started the original version of a DANE SMTP document.  His work is
   greatly appreciated and has been incorporated into this document.
   The authors would like to additionally thank Phil Pennock for his
   comments and advice on this document.

   Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me
   to begin work on this memo and provided feedback on early drafts.
   Thanks to Patrick Koetter, Perry Metzger and Nico Williams for
   valuable review comments.  Thanks also to Wietse Venema who created
   Postfix, and whose advice and feedback were essential to the
   development of the Postfix DANE implementation.




Dukhovni & Hardaker     Expires February 3, 2015               [Page 32]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


13.  References

13.1.  Normative References

   [I-D.ietf-dane-ops]
              Dukhovni, V. and W. Hardaker, "DANE TLSA implementation
              and operational guidance", draft-ietf-dane-ops-00 (work in
              progress), October 2013.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.








Dukhovni & Hardaker     Expires February 3, 2015               [Page 33]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186, March 2011.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, June 2012.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, April 2014.

   [X.690]    International Telecommunications Union, "Recommendation
              ITU-T X.690 (2002) | ISO/IEC 8825-1:2002, Information
              technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", July 2002.

13.2.  Informative References

   [I-D.dukhovni-opportunistic-security]
              Dukhovni, V., "Opportunistic Security: some protection
              most of the time", draft-dukhovni-opportunistic-
              security-01 (work in progress), July 2014.

   [I-D.ietf-dane-srv]
              Finch, T., "Using DNS-Based Authentication of Named
              Entities (DANE) TLSA records with SRV and MX records.",
              draft-ietf-dane-srv-02 (work in progress), February 2013.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598, July
              2009.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 2011.

Authors' Addresses






Dukhovni & Hardaker     Expires February 3, 2015               [Page 34]

Internet-Draft  SMTP security via opportunistic DANE TLS     August 2014


   Viktor Dukhovni
   Two Sigma

   Email: ietf-dane@dukhovni.org


   Wes Hardaker
   Parsons
   P.O. Box 382
   Davis, CA  95617
   US

   Email: ietf@hardakers.net






































Dukhovni & Hardaker     Expires February 3, 2015               [Page 35]