idnits 2.17.1 draft-ietf-dane-srv-06.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to lack the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords -- however, there's a paragraph with a matching beginning. Boilerplate error? (The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (June 10, 2014) is 3607 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references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 6125 (Obsoleted by RFC 9525) == Outdated reference: A later version (-19) exists of draft-ietf-dane-smtp-with-dane-05 == Outdated reference: A later version (-11) exists of draft-ietf-xmpp-dna-05 -- Obsolete informational reference (is this intentional?): RFC 6555 (Obsoleted by RFC 8305) Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 2 comments (--). 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: December 12, 2014 Cisco Systems, Inc. 6 P. Saint-Andre 7 &yet 8 June 10, 2014 10 Using DNS-Based Authentication of Named Entities (DANE) TLSA Records 11 with SRV Records 12 draft-ietf-dane-srv-06 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 December 12, 2014. 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 . . . . . . . . . . . . . . . . . . . . . . . . 4 62 3.2. Address Queries . . . . . . . . . . . . . . . . . . . . . 4 63 3.3. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5 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 Application Protocols . . . . . . . . . . . . . 7 69 6. Guidance for Server Operators . . . . . . . . . . . . . . . . 7 70 7. Internationalization Considerations . . . . . . . . . . . . . 8 71 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 72 9. Security Considerations . . . . . . . . . . . . . . . . . . . 8 73 9.1. Mixed Security Status . . . . . . . . . . . . . . . . . . 8 74 9.2. A Service Domain Trusts its Servers . . . . . . . . . . . 8 75 9.3. Certificate Subject Name Matching . . . . . . . . . . . . 9 76 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 77 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 78 11.1. Normative References . . . . . . . . . . . . . . . . . . 9 79 11.2. Informative References . . . . . . . . . . . . . . . . . 10 80 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 81 A.1. IMAP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 82 A.2. XMPP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . 12 84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 86 1. Introduction 88 The base DANE specification [RFC6698] describes how to use TLSA 89 resource records in the DNS to associate a server's host name with 90 its TLS certificate, where the association is secured using DNSSEC. 91 That document "only relates to securely associating certificates for 92 TLS and DTLS with host names" (see the last paragraph of section 1.2 93 of [RFC6698]). 95 Some application protocols do not use host names directly; instead, 96 they use a service domain, and the relevant target server host names 97 are located indirectly via SRV records [RFC2782]. Because of this 98 intermediate resolution step, the normal DANE rules specified in 99 [RFC6698] cannot be applied to protocols that use SRV records. 100 (Rules for SMTP [RFC5321], which uses MX records instead of SRV 101 records, are described in [I-D.ietf-dane-smtp-with-dane].) 103 This document describes how to use DANE TLSA records with SRV 104 records. To summarize: 106 o We rely on DNSSEC to secure the association between the service 107 domain and the target server host names (i.e., the host names that 108 are discovered by the SRV query). 110 o The TLSA records are located using the port, protocol, and target 111 server host name fields (not the service domain). 113 o Clients always use TLS when connecting to servers with TLSA 114 records. 116 o Assuming that the association is secure, the server's certificate 117 is expected to authenticate the target server host name, rather 118 than the service domain. 120 Note: The "CertID" specification [RFC6125] does not use the terms 121 "service domain" and "target server host name", but refers to the 122 same entities with the terms "source domain" and "derived domain". 124 2. Terminology 126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 128 "OPTIONAL" in this memo are to be interpreted as described in 129 [RFC2119]. 131 This draft uses the definitions for "secure", "insecure", "bogus", 132 and "indeterminate" from [RFC4033]. This draft uses the acronyms 133 from [RFC7218] for the values of TLSA fields where appropriate. 135 3. DNS Checks 137 To expedite connection to the intended service, where possible the 138 queries described in the following sections SHOULD be performed in 139 parallel (this is similar to the "happy eyeballs" approach for IPv4 140 and IPv6 connections described in [RFC6555]). 142 3.1. SRV Query 144 When the client makes an SRV query, a successful result will 145 typically be a list of one or more SRV records (or possibly a chain 146 of CNAME / DNAME aliases leading to such a list). 148 For this specification to apply, the entire DNS RRset that is 149 returned MUST be "secure" according to DNSSSEC validation ([RFC4033] 150 section 5). In the case of aliases, the whole chain of CNAME and 151 DNAME RRsets MUST be secure as well. This corresponds to the AD bit 152 being set in the response(s); see [RFC4035] section 3.2.3. 154 If the the entire RRset is not secure, this protocol has not been 155 correctly deployed. The client SHOULD fall back to its non-DNSSEC, 156 non-DANE behavior (this corresponds to the AD bit being unset). 158 If a particular response is "bogus" or "indeterminate" according to 159 DNSSEC validation, the client MUST ignore that target server host 160 name. 162 In the successful case, the client now has an authentic list of 163 target server host names with weight and priority values. It 164 performs server ordering and selection using the weight and priority 165 values without regard to the presence or absence of DNSSEC or TLSA 166 records. It also takes note of the DNSSEC validation status of the 167 SRV response for use when checking certificate names (see Section 4). 168 The client can now proceed to making address queries on the target 169 server host names as described in the next section. 171 3.2. Address Queries 173 For each SRV target server host name, the client makes A / AAAA 174 queries, performs DNSSEC validation on the address (A, AAAA) 175 response, and continues as follows based on the results: 177 o If the response is "secure" and usable, the client MUST perform a 178 TLSA query for that target server host name as described in the 179 next section. 181 o If the response is "insecure", the client MUST NOT perform a TLSA 182 query for that target server host name; the TLSA query will most 183 likely fail. 185 o If the response is "bogus" or "indeterminate", the client MUST NOT 186 connect to this target server; instead it uses the next most 187 appropriate SRV target. 189 3.3. TLSA Queries 191 The client SHALL construct the TLSA query name as described in 192 [RFC6698] section 3, based on fields from the SRV record: the port 193 from the SRV RDATA, the protocol from the SRV query name, and the 194 TLSA base domain set to the SRV target server host name. 196 For example, the following SRV record for IMAP (see [RFC6186]) leads 197 to the TLSA query shown below: 199 _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net. 201 _9143._tcp.imap.example.net. IN TLSA ? 203 3.4. Impact on TLS Usage 205 The client SHALL determine if the TLSA record(s) returned in the 206 previous step are usable according to section 4.1 of [RFC6698]. This 207 affects the use TLS as follows: 209 o If the TLSA response is "secure" and usable, then the client MUST 210 use TLS when connecting to the target server. The TLSA records 211 are used when validating the server's certificate as described 212 under Section 4. 214 o If the TLSA response is "insecure", then the client SHALL proceed 215 as if the target server had no TLSA records. It MAY connect to 216 the target server with or without TLS, subject to the policies of 217 the application protocol or client implementation. 219 o If the TLSA response is "bogus" or "indeterminate", then the 220 client MUST NOT connect to the target server (the client can still 221 use other SRV targets). 223 4. TLS Checks 225 When connecting to a server, the client MUST use TLS if the responses 226 to the SRV and TLSA queries were "secure" as described above. The 227 rules described in the next two sections apply. 229 4.1. SRV Records Only 231 If the client received zero usable TLSA certificate associations, it 232 SHALL validate the server's TLS certificate using the normal PKIX 233 rules [RFC5280] or protocol-specific rules (e.g., following 234 [RFC6125]) without further input from the TLSA records. 236 In this case, the client uses the information in the server 237 certificate and the DNSSEC validation status of the SRV query in its 238 authentication checks. It SHOULD use the Server Name Indication 239 extension (TLS SNI) [RFC6066] or its functional equivalent in the 240 relevant application protocol (e.g., in XMPP [RFC6120] this is the 241 'to' address of the initial stream header). The preferred name SHALL 242 be chosen as follows, and the client SHALL verify the identity 243 asserted by the server's certificate according to section 6 of 244 [RFC6125], using a list of reference identifiers constructed as 245 follows (note again that in RFC 6125 the terms "source domain" and 246 "derived domain" refer to the same things as "service domain" and 247 "target server host name" in this document). The examples below 248 assume a service domain of "im.example.com" and a target server host 249 name of "xmpp23.hosting.example.net". 251 SRV is insecure: The reference identifiers SHALL include the service 252 domain and MUST NOT include the SRV target server host name (e.g., 253 include "im.example.com" but not "xmpp23.hosting.example.net"). 254 The service domain is the preferred name for TLS SNI or its 255 equivalent. 257 SRV is secure: The reference identifiers SHALL include both the 258 service domain and the SRV target server host name (e.g., include 259 both "im.example.com" and "xmpp23.hosting.example.net"). The 260 target server host name is the preferred name for TLS SNI or its 261 equivalent. 263 In the latter case, the client will accept either identity to ensure 264 compatibility with servers that support this specification as well as 265 servers that do not support this specification. 267 4.2. TLSA Records 269 If the client received one or more usable TLSA certificate 270 associations, it SHALL process them as described in section 2.1 of 271 [RFC6698]. 273 If the TLS server's certificate -- or the public key of the server's 274 certificate -- matches a usable TLSA record with Certificate Usage 275 "DANE-EE", the client MUST consider the server to be authenticated. 276 Because the information in such a TLSA record supersedes the non-key 277 information in the certificate, all other [RFC5280] and [RFC6125] 278 authentication checks (e.g., reference identifier, key usage, 279 expiration, issuance) MUST be ignored or omitted. 281 5. Guidance for Application Protocols 283 This document describes how to use DANE with application protocols in 284 which target servers are discovered via SRV records. Although this 285 document attempts to provide generic guidance applying to all such 286 protocols, additional documents for particular application protocols 287 could cover related topics, such as: 289 o Fallback logic in the event that a client is unable to connect 290 securely to a target server by following the procedures defined in 291 this document. 293 o How clients ought to behave if they do not support SRV lookups, or 294 if clients that support SRV lookups encounter service domains that 295 do not offer SRV records. 297 o Whether the application protocol has a functional equivalent for 298 TLS SNI that is preferred within that protocol. 300 For example, [I-D.ietf-xmpp-dna] covers such topics for the 301 Extensible Messaging and Presence Protocol (XMPP). 303 6. Guidance for Server Operators 305 To conform to this specification, the published SRV records and 306 subsequent address (A, AAAA) records MUST be secured with DNSSEC. 307 There SHOULD also be at least one TLSA record published that 308 authenticates the server's certificate. 310 When using TLSA records with Certificate Usage "DANE-EE", it is not 311 necessary for the deployed certificate to contain an identifier for 312 either the source domain or target server host name. However, 313 servers that rely solely on validation using Certificate Usage "DANE- 314 EE" TLSA records might prevent clients that do not support this 315 specification from successfully connecting with TLS. 317 For TLSA records with Certificate Usage types other than "DANE-EE", 318 the certificate(s) MUST contain an identifier that matches: 320 o the service domain name (the "source domain" in [RFC6125] terms, 321 which is the SRV query domain); and/or 323 o the target server host name (the "derived domain" in [RFC6125] 324 terms, which is the SRV target). 326 Servers that support multiple service domains (i.e., so-called 327 "multi-tenanted environments") can implement the Transport Layer 328 Security Server Name Indication (TLS SNI) [RFC6066] or its functional 329 equivalent to determine which certificate to offer. Clients that do 330 not support this specification will indicate a preference for the 331 service domain name, while clients that support this specification 332 will indicate the target server host name. However, the server 333 determines what certificate to present in the TLS handshake; e.g., 334 the presented certificate might only authenticate the target server 335 host name. 337 7. Internationalization Considerations 339 If any of the DNS queries are for an internationalized domain name, 340 then they need to use the A-label form [RFC5890]. 342 8. IANA Considerations 344 No IANA action is required. 346 9. Security Considerations 348 9.1. Mixed Security Status 350 We do not specify that clients checking all of a service domain's 351 target server host names are consistent in whether they have or do 352 not have TLSA records. This is so that partial or incremental 353 deployment does not break the service. Different levels of 354 deployment are likely if a service domain has a third-party fallback 355 server, for example. 357 The SRV sorting rules are unchanged; in particular they have not been 358 altered in order to prioritize secure servers over insecure servers. 359 If a site wants to be secure it needs to deploy this protocol 360 completely; a partial deployment is not secure and we make no special 361 effort to support it. 363 9.2. A Service Domain Trusts its Servers 365 By signing their zone with DNSSEC, service domain operators 366 implicitly instruct their clients to check their server TLSA records. 367 This implies another point in the trust relationship between service 368 domain holders and their server operators. Most of the setup 369 requirements for this protocol fall on the server operator: 370 installing a TLS certificate with the correct name (where necessary), 371 and publishing a TLSA record for that certificate. If these are not 372 correct then connections from TLSA-aware clients might fail. 374 9.3. Certificate Subject Name Matching 376 Section 4 of the TLSA specification [RFC6698] leaves the details of 377 checking names in certificates to higher level application protocols, 378 though it suggests the use of [RFC6125]. 380 Name checks are not necessary if the matching TLSA record is of 381 Certificate Usage "DANE-EE". Because such a record identifies the 382 specific certificate (or public key of the certificate), additional 383 checks are superfluous and potentially conflicting. 385 Otherwise, while DNSSEC provides a secure binding between the server 386 name and the TLSA record, and the TLSA record provides a binding to a 387 certificate, this latter step can be indirect via a chain of 388 certificates. For example, a Certificate Usage "PKIX-TA" TLSA record 389 only authenticates the CA that issued the certificate, and third 390 parties can obtain certificates from the same CA. Therefore, clients 391 need to check whether the server's certificate matches one of the 392 expected reference identifiers to ensure that the certificate was 393 issued by the CA to the server the client expects. 395 10. Acknowledgements 397 Thanks to Mark Andrews for arguing that authenticating the target 398 server host name is the right thing, and that we ought to rely on 399 DNSSEC to secure the SRV lookup. Thanks to James Cloos, Viktor 400 Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, 401 Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful 402 suggestions. 404 11. References 406 11.1. Normative References 408 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 409 Requirement Levels", BCP 14, RFC 2119, March 1997. 411 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 412 specifying the location of services (DNS SRV)", RFC 2782, 413 February 2000. 415 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 416 Rose, "DNS Security Introduction and Requirements", RFC 417 4033, March 2005. 419 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 420 Rose, "Protocol Modifications for the DNS Security 421 Extensions", RFC 4035, March 2005. 423 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 424 Housley, R., and W. Polk, "Internet X.509 Public Key 425 Infrastructure Certificate and Certificate Revocation List 426 (CRL) Profile", RFC 5280, May 2008. 428 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 429 October 2008. 431 [RFC5890] Klensin, J., "Internationalized Domain Names for 432 Applications (IDNA): Definitions and Document Framework", 433 RFC 5890, August 2010. 435 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 436 Extension Definitions", RFC 6066, January 2011. 438 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence 439 Protocol (XMPP): Core", RFC 6120, March 2011. 441 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 442 Verification of Domain-Based Application Service Identity 443 within Internet Public Key Infrastructure Using X.509 444 (PKIX) Certificates in the Context of Transport Layer 445 Security (TLS)", RFC 6125, March 2011. 447 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 448 Submission/Access Services", RFC 6186, March 2011. 450 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 451 of Named Entities (DANE) Transport Layer Security (TLS) 452 Protocol: TLSA", RFC 6698, August 2012. 454 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify 455 Conversations about DNS-Based Authentication of Named 456 Entities (DANE)", RFC 7218, April 2014. 458 11.2. Informative References 460 [I-D.ietf-dane-smtp-with-dane] 461 Dukhovni, V. and W. Hardaker, "SMTP security via 462 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05 463 (work in progress), February 2014. 465 [I-D.ietf-xmpp-dna] 466 Saint-Andre, P. and M. Miller, "Domain Name Associations 467 (DNA) in the Extensible Messaging and Presence Protocol 468 (XMPP)", draft-ietf-xmpp-dna-05 (work in progress), 469 February 2014. 471 [RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with 472 Dual-Stack Hosts", RFC 6555, April 2012. 474 Appendix A. Examples 476 In the following, most of the DNS resource data is elided for 477 simplicity. 479 A.1. IMAP 481 ; mail domain 482 _imap._tcp.example.com. SRV 10 0 9143 imap.example.net. 483 example.com. RRSIG SRV ... 485 ; target server host name 486 imap.example.net. A 192.0.2.1 487 imap.example.net. RRSIG A ... 489 imap.example.net. AAAA 2001:db8:212:8::e:1 490 imap.example.net. RRSIG ... 492 ; TLSA resource record 493 _9143._tcp.imap.example.net. TLSA ... 494 _9143._tcp.imap.example.net. RRSIG TLSA ... 496 Mail messages submitted for addresses at example.com are sent via 497 IMAP to imap.example.net. Connections to imap.example.net port 9143 498 that use STARTTLS will get a server certificate that authenticates 499 the name imap.example.net. 501 A.2. XMPP 503 ; XMPP domain 504 _xmpp-client.example.com. SRV 1 0 5222 im.example.net. 505 _xmpp-client.example.com. RRSIG SRV ... 507 ; target server host name 508 im.example.net. A 192.0.2.3 509 im.example.net. RRSIG A ... 511 im.example.net. AAAA 2001:db8:212:8::e:4 512 im.example.net. RRSIG AAAA ... 514 ; TLSA resource record 515 _5222._tcp.im.example.net. TLSA ... 516 _5222._tcp.im.example.net. RRSIG TLSA ... 518 XMPP sessions for addresses at example.com are established at 519 im.example.net. Connections to im.example.net port 5222 that use 520 STARTTLS will get a server certificate that authenticates the name 521 im.example.net. 523 Appendix B. Rationale 525 The long-term goal of this specification is to settle on TLS 526 certificates that verify the target server host name rather than the 527 service domain, since this is more convenient for servers hosting 528 multiple domains (so-called "multi-tenanted environments") and scales 529 up more easily to larger numbers of service domains. 531 There are a number of other reasons for doing it this way: 533 o The certificate is part of the server configuration, so it makes 534 sense to associate it with the server host name rather than the 535 service domain. 537 o In the absence of TLS SNI, if the certificate identifies the host 538 name then it does not need to list all the possible service 539 domains. 541 o When the server certificate is replaced it is much easier if there 542 is one part of the DNS that needs updating to match, instead of an 543 unbounded number of hosted service domains. 545 o The same TLSA records work with this specification, and with 546 direct connections to the host name in the style of [RFC6698]. 548 o Some application protocols, such as SMTP, allow a client to 549 perform transactions with multiple service domains in the same 550 connection. It is not in general feasible for the client to 551 specify the service domain using TLS SNI when the connection is 552 established, and the server might not be able to present a 553 certificate that authenticates all possible service domains. See 554 [I-D.ietf-dane-smtp-with-dane] for details. 556 o It is common for SMTP servers to act in multiple roles, for 557 example as outgoing relays or as incoming MX servers, depending on 558 the client identity. It is simpler if the server can present the 559 same certificate regardless of the role in which it is to act. 560 Sometimes the server does not know its role until the client has 561 authenticated, which usually occurs after TLS has been 562 established. See [I-D.ietf-dane-smtp-with-dane] for details. 564 This specification does not provide an option to put TLSA records 565 under the service domain because that would add complexity without 566 providing any benefit, and security protocols are best kept simple. 567 As described above, there are real-world cases where authenticating 568 the service domain cannot be made to work, so there would be 569 complicated criteria for when service domain TLSA records might be 570 used and when they cannot. This is all avoided by putting the TLSA 571 records under the target server host name. 573 The disadvantage is that clients which do not complete DNSSEC 574 validation must, according to [RFC6125] rules, check the server 575 certificate against the service domain, since they have no other way 576 to authenticate the server. This means that SNI support or its 577 functional equivalent is necessary for backward compatibility. 579 Authors' Addresses 581 Tony Finch 582 University of Cambridge Computing Service 583 New Museums Site 584 Pembroke Street 585 Cambridge CB2 3QH 586 ENGLAND 588 Phone: +44 797 040 1426 589 Email: dot@dotat.at 590 URI: http://dotat.at/ 592 Matthew Miller 593 Cisco Systems, Inc. 594 1899 Wynkoop Street, Suite 600 595 Denver, CO 80202 596 USA 598 Email: mamille2@cisco.com 600 Peter Saint-Andre 601 &yet 602 P.O. Box 787 603 Parker, CO 80134 604 USA 606 Email: peter@andyet.com