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(The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (October 21, 2014) is 3474 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 6125 (Obsoleted by RFC 9525) == Outdated reference: A later version (-19) exists of draft-ietf-dane-smtp-with-dane-05 == Outdated reference: A later version (-11) exists of draft-ietf-xmpp-dna-05 Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNS-Based Authentication of Named Entities (DANE) T. Finch 3 Internet-Draft University of Cambridge 4 Intended status: Standards Track M. Miller 5 Expires: April 24, 2015 Cisco Systems, Inc. 6 P. Saint-Andre 7 &yet 8 October 21, 2014 10 Using DNS-Based Authentication of Named Entities (DANE) TLSA Records 11 with SRV Records 12 draft-ietf-dane-srv-08 14 Abstract 16 The DANE specification (RFC 6698) describes how to use TLSA resource 17 records in the DNS to associate a server's host name with its TLS 18 certificate, where the association is secured with DNSSEC. However, 19 application protocols that use SRV records (RFC 2782) to indirectly 20 name the target server host names for a service domain cannot apply 21 the rules from RFC 6698. Therefore this document provides guidelines 22 that enable such protocols to locate and use TLSA records. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on April 24, 2015. 41 Copyright Notice 43 Copyright (c) 2014 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 3. DNS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 3.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 3 62 3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4 63 3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 4 64 3.4. Impact on TLS Usage . . . . . . . . . . . . . . . . . . . 5 65 4. TLS Checks . . . . . . . . . . . . . . . . . . . . . . . . . 5 66 4.1. SRV Records Only . . . . . . . . . . . . . . . . . . . . 5 67 4.2. TLSA Records . . . . . . . . . . . . . . . . . . . . . . 6 68 5. Guidance for Protocol Authors . . . . . . . . . . . . . . . . 6 69 6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7 70 7. Guidance for Application Developers . . . . . . . . . . . . . 8 71 8. Internationalization Considerations . . . . . . . . . . . . . 8 72 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 73 10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 74 10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 8 75 10.2. A Service Domain Trusts its Servers . . . . . . . . . . 8 76 10.3. Certificate Subject Name Matching . . . . . . . . . . . 9 77 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 78 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 79 12.1. Normative References . . . . . . . . . . . . . . . . . . 9 80 12.2. Informative References . . . . . . . . . . . . . . . . . 10 81 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 82 A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 84 Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 12 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 87 1. Introduction 89 The base DANE specification [RFC6698] describes how to use TLSA 90 resource records in the DNS to associate a server's host name with 91 its TLS certificate, where the association is secured using DNSSEC. 92 That document "only relates to securely associating certificates for 93 TLS and DTLS with host names" (see the last paragraph of section 1.2 94 of [RFC6698]). 96 Some application protocols do not use host names directly; instead, 97 they use a service domain, and the relevant target server host names 98 are located indirectly via SRV records [RFC2782]. Because of this 99 intermediate resolution step, the normal DANE rules specified in 100 [RFC6698] cannot be applied to protocols that use SRV records. 101 (Rules for SMTP [RFC5321], which uses MX records instead of SRV 102 records, are described in [I-D.ietf-dane-smtp-with-dane].) 104 This document describes how to use DANE TLSA records with SRV 105 records. To summarize: 107 o We rely on DNSSEC to secure the association between the service 108 domain and the target server host names (i.e., the host names that 109 are discovered by the SRV query). 111 o The TLSA records are located using the port, protocol, and target 112 server host name fields (not the service domain). 114 o Clients always use TLS when connecting to servers with TLSA 115 records. 117 o Assuming that the association is secure, the server's certificate 118 is expected to authenticate the target server host name, rather 119 than the service domain. 121 Note: The "CertID" specification [RFC6125] does not use the terms 122 "service domain" and "target server host name", but refers to the 123 same entities with the terms "source domain" and "derived domain". 125 2. Terminology 127 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 128 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 129 "OPTIONAL" in this memo are to be interpreted as described in 130 [RFC2119]. 132 This draft uses the definitions for "secure", "insecure", "bogus", 133 and "indeterminate" from [RFC4035]. This draft uses the acronyms 134 from [RFC7218] for the values of TLSA fields where appropriate. 136 3. DNS Checks 138 3.1. SRV Query 140 When the client makes an SRV query, a successful result will 141 typically be a list of one or more SRV records (or possibly a chain 142 of CNAME / DNAME aliases leading to such a list). Implementers need 143 to be aware that unsuccessful results can occur because of various 144 DNS-related errors; a helpful summary can be found in section 2.1 of 145 [I-D.ietf-dane-smtp-with-dane]. 147 For this specification to apply, the entire DNS RRset that is 148 returned MUST be "secure" according to DNSSSEC validation ([RFC4033] 149 section 5). In the case of aliases, the whole chain of CNAME and 150 DNAME RRsets MUST be secure as well. This corresponds to the AD bit 151 being set in the response(s); see [RFC4035] section 3.2.3. 153 If the the entire RRset is "insecure", this protocol has not been 154 correctly deployed. The client SHOULD fall back to its non-DNSSEC, 155 non-DANE behavior (this corresponds to the AD bit being unset). If 156 the entire RRset is "bogus", the client MUST abort the attempt. 158 In the successful case, the client now has an authentic list of 159 target server host names with weight and priority values. It 160 performs server ordering and selection using the weight and priority 161 values without regard to the presence or absence of DNSSEC or TLSA 162 records. It also takes note of the DNSSEC validation status of the 163 SRV response for use when checking certificate names (see Section 4). 164 The client can now proceed to making address queries on the target 165 server host names as described in the next section. 167 3.2. Address Queries 169 For each SRV target server host name, the client makes A and AAAA 170 queries, performs DNSSEC validation on the address (A or AAAA) 171 response, and continues as follows based on the results: 173 o If the response is "secure" and usable, the client MUST perform a 174 TLSA query for that target server host name as described in the 175 next section. 177 o If the response is "insecure", the client MUST NOT perform a TLSA 178 query for that target server host name; the TLSA query will most 179 likely fail. 181 o If the response is "bogus" or "indeterminate", the client MUST NOT 182 connect to this target server; instead it uses the next most 183 appropriate SRV target. 185 3.3. TLSA Queries 187 The client SHALL construct the TLSA query name as described in 188 [RFC6698] section 3, based on fields from the SRV record: the port 189 from the SRV RDATA, the protocol from the SRV query name, and the 190 TLSA base domain set to the SRV target server host name. 192 For example, the following SRV record for IMAP (see [RFC6186]) leads 193 to the TLSA query shown below: 195 _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net. 197 _9143._tcp.imap.example.net. IN TLSA ? 199 3.4. Impact on TLS Usage 201 The client SHALL determine if the TLSA record(s) returned in the 202 previous step are usable according to section 4.1 of [RFC6698]. This 203 affects the use TLS as follows: 205 o If the TLSA response is "secure" and usable, then the client MUST 206 use TLS when connecting to the target server. The TLSA records 207 are used when validating the server's certificate as described 208 under Section 4. 210 o If the TLSA response is "insecure", then the client SHALL proceed 211 as if the target server had no TLSA records. It MAY connect to 212 the target server with or without TLS, subject to the policies of 213 the application protocol or client implementation. 215 o If the TLSA response is "bogus" or "indeterminate", then the 216 client MUST NOT connect to the target server (the client can still 217 use other SRV targets). 219 4. TLS Checks 221 When connecting to a server, the client MUST use TLS if the responses 222 to the SRV and TLSA queries were "secure" as described above. The 223 rules described in the next two sections apply. 225 4.1. SRV Records Only 227 If the client received zero usable TLSA certificate associations, it 228 SHALL validate the server's TLS certificate using the normal PKIX 229 rules [RFC5280] or protocol-specific rules (e.g., following 230 [RFC6125]) without further input from the TLSA records. 232 In this case, the client uses the information in the server 233 certificate and the DNSSEC validation status of the SRV query in its 234 authentication checks. It SHOULD use the Server Name Indication 235 extension (TLS SNI) [RFC6066] or its functional equivalent in the 236 relevant application protocol (e.g., in XMPP [RFC6120] this is the 237 'to' address of the initial stream header). The preferred name SHALL 238 be chosen as follows, and the client SHALL verify the identity 239 asserted by the server's certificate according to section 6 of 241 [RFC6125], using a list of reference identifiers constructed as 242 follows (note again that in RFC 6125 the terms "source domain" and 243 "derived domain" refer to the same things as "service domain" and 244 "target server host name" in this document). The examples below 245 assume a service domain of "im.example.com" and a target server host 246 name of "xmpp23.hosting.example.net". 248 SRV is insecure: The reference identifiers SHALL include the service 249 domain and MUST NOT include the SRV target server host name (e.g., 250 include "im.example.com" but not "xmpp23.hosting.example.net"). 251 The service domain is the preferred name for TLS SNI or its 252 equivalent. 254 SRV is secure: The reference identifiers SHALL include both the 255 service domain and the SRV target server host name (e.g., include 256 both "im.example.com" and "xmpp23.hosting.example.net"). The 257 target server host name is the preferred name for TLS SNI or its 258 equivalent. 260 In the latter case, the client will accept either identity to ensure 261 compatibility with servers that support this specification as well as 262 servers that do not support this specification. 264 4.2. TLSA Records 266 If the client received one or more usable TLSA certificate 267 associations, it SHALL process them as described in section 2.1 of 268 [RFC6698]. 270 If the TLS server's certificate -- or the public key of the server's 271 certificate -- matches a usable TLSA record with Certificate Usage 272 "DANE-EE", the client MUST consider the server to be authenticated. 273 Because the information in such a TLSA record supersedes the non-key 274 information in the certificate, all other [RFC5280] and [RFC6125] 275 authentication checks (e.g., reference identifier, key usage, 276 expiration, issuance) MUST be ignored or omitted. 278 5. Guidance for Protocol Authors 280 This document describes how to use DANE with application protocols in 281 which target servers are discovered via SRV records. Although this 282 document attempts to provide generic guidance applying to all such 283 protocols, additional documents for particular application protocols 284 could cover related topics, such as: 286 o Fallback logic in the event that a client is unable to connect 287 securely to a target server by following the procedures defined in 288 this document. 290 o How clients ought to behave if they do not support SRV lookups, or 291 if clients that support SRV lookups encounter service domains that 292 do not offer SRV records. 294 o Whether the application protocol has a functional equivalent for 295 TLS SNI that is preferred within that protocol. 297 o Use of SRV records with additional discovery technologies, such as 298 the use of both SRV records and NAPTR records [RFC3403] for 299 transport selection in the Session Initiation Protocol (SIP). 301 For example, [I-D.ietf-xmpp-dna] covers such topics for the 302 Extensible Messaging and Presence Protocol (XMPP). 304 6. Guidance for Server Operators 306 To conform to this specification, the published SRV records and 307 subsequent address (A and AAAA) records MUST be secured with DNSSEC. 308 There SHOULD also be at least one TLSA record published that 309 authenticates the server's certificate. 311 When using TLSA records with Certificate Usage "DANE-EE", it is not 312 necessary for the deployed certificate to contain an identifier for 313 either the source domain or target server host name. However, 314 operators need to be aware that servers relying solely on validation 315 using Certificate Usage "DANE-EE" TLSA records might prevent clients 316 that do not support this specification from successfully connecting 317 with TLS. 319 For TLSA records with Certificate Usage types other than "DANE-EE", 320 the certificate(s) MUST contain an identifier that matches: 322 o the service domain name (the "source domain" in [RFC6125] terms, 323 which is the SRV query domain); and/or 325 o the target server host name (the "derived domain" in [RFC6125] 326 terms, which is the SRV target). 328 Servers that support multiple service domains (i.e., so-called 329 "multi-tenanted environments") can implement the Transport Layer 330 Security Server Name Indication (TLS SNI) [RFC6066] or its functional 331 equivalent to determine which certificate to offer. Clients that do 332 not support this specification will indicate a preference for the 333 service domain name, while clients that support this specification 334 will indicate the target server host name. However, the server 335 determines what certificate to present in the TLS handshake; e.g., 336 the presented certificate might only authenticate the target server 337 host name. 339 7. Guidance for Application Developers 341 Developers of application clients that depend on DANE-SRV often would 342 like to prepare as quickly as possible for making a connection to the 343 intended service, thus reducing the wait time for end users. To make 344 this optimization possible, a DNS library might perform the SRV 345 queries, address queries, and TLSA queries in parallel (because a 346 TLSA record can be ignored if it turns out that the address record on 347 which it depends is not secure, performing the TLSA queries in 348 parallel with the SRV queries and address queries is not harmful from 349 a security perspective and can yield some operational benefits). 351 8. Internationalization Considerations 353 If any of the DNS queries are for an internationalized domain name, 354 then they need to use the A-label form [RFC5890]. 356 9. IANA Considerations 358 No IANA action is required. 360 10. Security Considerations 362 10.1. Mixed Security Status 364 We do not specify that clients checking all of a service domain's 365 target server host names are consistent in whether they have or do 366 not have TLSA records. This is so that partial or incremental 367 deployment does not break the service. Different levels of 368 deployment are likely if a service domain has a third-party fallback 369 server, for example. 371 The SRV sorting rules are unchanged; in particular they have not been 372 altered in order to prioritize secure servers over insecure servers. 373 If a site wants to be secure it needs to deploy this protocol 374 completely; a partial deployment is not secure and we make no special 375 effort to support it. 377 10.2. A Service Domain Trusts its Servers 379 By signing their zone with DNSSEC, service domain operators 380 implicitly instruct their clients to check their server TLSA records. 381 This implies another point in the trust relationship between service 382 domain holders and their server operators. Most of the setup 383 requirements for this protocol fall on the server operator: 384 installing a TLS certificate with the correct name (where necessary), 385 and publishing a TLSA record for that certificate. If these are not 386 correct then connections from TLSA-aware clients might fail. 388 10.3. Certificate Subject Name Matching 390 Section 4 of the TLSA specification [RFC6698] leaves the details of 391 checking names in certificates to higher level application protocols, 392 though it suggests the use of [RFC6125]. 394 Name checks are not necessary if the matching TLSA record is of 395 Certificate Usage "DANE-EE". Because such a record identifies the 396 specific certificate (or public key of the certificate), additional 397 checks are superfluous and potentially conflicting. 399 Otherwise, while DNSSEC provides a secure binding between the server 400 name and the TLSA record, and the TLSA record provides a binding to a 401 certificate, this latter step can be indirect via a chain of 402 certificates. For example, a Certificate Usage "PKIX-TA" TLSA record 403 only authenticates the CA that issued the certificate, and third 404 parties can obtain certificates from the same CA. Therefore, clients 405 need to check whether the server's certificate matches one of the 406 expected reference identifiers to ensure that the certificate was 407 issued by the CA to the server the client expects. 409 11. Acknowledgements 411 Thanks to Mark Andrews for arguing that authenticating the target 412 server host name is the right thing, and that we ought to rely on 413 DNSSEC to secure the SRV lookup. Thanks to James Cloos, Viktor 414 Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, 415 Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful 416 suggestions. 418 12. References 420 12.1. Normative References 422 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 423 Requirement Levels", BCP 14, RFC 2119, March 1997. 425 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 426 specifying the location of services (DNS SRV)", RFC 2782, 427 February 2000. 429 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 430 Rose, "DNS Security Introduction and Requirements", RFC 431 4033, March 2005. 433 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 434 Rose, "Protocol Modifications for the DNS Security 435 Extensions", RFC 4035, March 2005. 437 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 438 Housley, R., and W. Polk, "Internet X.509 Public Key 439 Infrastructure Certificate and Certificate Revocation List 440 (CRL) Profile", RFC 5280, May 2008. 442 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 443 October 2008. 445 [RFC5890] Klensin, J., "Internationalized Domain Names for 446 Applications (IDNA): Definitions and Document Framework", 447 RFC 5890, August 2010. 449 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 450 Extension Definitions", RFC 6066, January 2011. 452 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence 453 Protocol (XMPP): Core", RFC 6120, March 2011. 455 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 456 Verification of Domain-Based Application Service Identity 457 within Internet Public Key Infrastructure Using X.509 458 (PKIX) Certificates in the Context of Transport Layer 459 Security (TLS)", RFC 6125, March 2011. 461 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 462 Submission/Access Services", RFC 6186, March 2011. 464 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 465 of Named Entities (DANE) Transport Layer Security (TLS) 466 Protocol: TLSA", RFC 6698, August 2012. 468 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify 469 Conversations about DNS-Based Authentication of Named 470 Entities (DANE)", RFC 7218, April 2014. 472 12.2. Informative References 474 [I-D.ietf-dane-smtp-with-dane] 475 Dukhovni, V. and W. Hardaker, "SMTP security via 476 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05 477 (work in progress), February 2014. 479 [I-D.ietf-xmpp-dna] 480 Saint-Andre, P. and M. Miller, "Domain Name Associations 481 (DNA) in the Extensible Messaging and Presence Protocol 482 (XMPP)", draft-ietf-xmpp-dna-05 (work in progress), 483 February 2014. 485 [RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS) 486 Part Three: The Domain Name System (DNS) Database", RFC 487 3403, October 2002. 489 Appendix A. Examples 491 In the following, most of the DNS resource data is elided for 492 simplicity. 494 A.1. IMAP 496 ; mail domain 497 _imap._tcp.example.com. SRV 10 0 9143 imap.example.net. 498 example.com. RRSIG SRV ... 500 ; target server host name 501 imap.example.net. A 192.0.2.1 502 imap.example.net. RRSIG A ... 504 imap.example.net. AAAA 2001:db8:212:8::e:1 505 imap.example.net. RRSIG ... 507 ; TLSA resource record 508 _9143._tcp.imap.example.net. TLSA ... 509 _9143._tcp.imap.example.net. RRSIG TLSA ... 511 Mail messages submitted for addresses at example.com are sent via 512 IMAP to imap.example.net. Connections to imap.example.net port 9143 513 that use STARTTLS will get a server certificate that authenticates 514 the name imap.example.net. 516 A.2. XMPP 518 ; XMPP domain 519 _xmpp-client.example.com. SRV 1 0 5222 im.example.net. 520 _xmpp-client.example.com. RRSIG SRV ... 522 ; target server host name 523 im.example.net. A 192.0.2.3 524 im.example.net. RRSIG A ... 526 im.example.net. AAAA 2001:db8:212:8::e:4 527 im.example.net. RRSIG AAAA ... 529 ; TLSA resource record 530 _5222._tcp.im.example.net. TLSA ... 531 _5222._tcp.im.example.net. RRSIG TLSA ... 533 XMPP sessions for addresses at example.com are established at 534 im.example.net. Connections to im.example.net port 5222 that use 535 STARTTLS will get a server certificate that authenticates the name 536 im.example.net. 538 Appendix B. Rationale 540 The long-term goal of this specification is to settle on TLS 541 certificates that verify the target server host name rather than the 542 service domain, since this is more convenient for servers hosting 543 multiple domains (so-called "multi-tenanted environments") and scales 544 up more easily to larger numbers of service domains. 546 There are a number of other reasons for doing it this way: 548 o The certificate is part of the server configuration, so it makes 549 sense to associate it with the server host name rather than the 550 service domain. 552 o In the absence of TLS SNI, if the certificate identifies the host 553 name then it does not need to list all the possible service 554 domains. 556 o When the server certificate is replaced it is much easier if there 557 is one part of the DNS that needs updating to match, instead of an 558 unbounded number of hosted service domains. 560 o The same TLSA records work with this specification, and with 561 direct connections to the host name in the style of [RFC6698]. 563 o Some application protocols, such as SMTP, allow a client to 564 perform transactions with multiple service domains in the same 565 connection. It is not in general feasible for the client to 566 specify the service domain using TLS SNI when the connection is 567 established, and the server might not be able to present a 568 certificate that authenticates all possible service domains. See 569 [I-D.ietf-dane-smtp-with-dane] for details. 571 o It is common for SMTP servers to act in multiple roles, for 572 example as outgoing relays or as incoming MX servers, depending on 573 the client identity. It is simpler if the server can present the 574 same certificate regardless of the role in which it is to act. 575 Sometimes the server does not know its role until the client has 576 authenticated, which usually occurs after TLS has been 577 established. See [I-D.ietf-dane-smtp-with-dane] for details. 579 This specification does not provide an option to put TLSA records 580 under the service domain because that would add complexity without 581 providing any benefit, and security protocols are best kept simple. 582 As described above, there are real-world cases where authenticating 583 the service domain cannot be made to work, so there would be 584 complicated criteria for when service domain TLSA records might be 585 used and when they cannot. This is all avoided by putting the TLSA 586 records under the target server host name. 588 The disadvantage is that clients which do not complete DNSSEC 589 validation must, according to [RFC6125] rules, check the server 590 certificate against the service domain, since they have no other way 591 to authenticate the server. This means that SNI support or its 592 functional equivalent is necessary for backward compatibility. 594 Authors' Addresses 596 Tony Finch 597 University of Cambridge Computing Service 598 New Museums Site 599 Pembroke Street 600 Cambridge CB2 3QH 601 ENGLAND 603 Phone: +44 797 040 1426 604 Email: dot@dotat.at 605 URI: http://dotat.at/ 607 Matthew Miller 608 Cisco Systems, Inc. 609 1899 Wynkoop Street, Suite 600 610 Denver, CO 80202 611 USA 613 Email: mamille2@cisco.com 615 Peter Saint-Andre 616 &yet 618 Email: peter@andyet.com 619 URI: https://andyet.com/