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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: January 24, 2015 Cisco Systems, Inc. 6 P. Saint-Andre 7 &yet 8 July 23, 2014 10 Using DNS-Based Authentication of Named Entities (DANE) TLSA Records 11 with SRV Records 12 draft-ietf-dane-srv-07 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 January 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 . . . . . . . . . . . . . . . . . . . . . . . . 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 [RFC4035]. 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 "insecure" or "indeterminate", this 155 protocol has not been correctly deployed. The client SHOULD fall 156 back to its non-DNSSEC, non-DANE behavior (this corresponds to the AD 157 bit being unset). If the entire RRset is "bogus", the client MUST 158 abort the attempt. 160 In the successful case, the client now has an authentic list of 161 target server host names with weight and priority values. It 162 performs server ordering and selection using the weight and priority 163 values without regard to the presence or absence of DNSSEC or TLSA 164 records. It also takes note of the DNSSEC validation status of the 165 SRV response for use when checking certificate names (see Section 4). 166 The client can now proceed to making address queries on the target 167 server host names as described in the next section. 169 3.2. Address Queries 171 For each SRV target server host name, the client makes A / AAAA 172 queries, performs DNSSEC validation on the address (A, AAAA) 173 response, and continues as follows based on the results: 175 o If the response is "secure" and usable, the client MUST perform a 176 TLSA query for that target server host name as described in the 177 next section. 179 o If the response is "insecure", the client MUST NOT perform a TLSA 180 query for that target server host name; the TLSA query will most 181 likely fail. 183 o If the response is "bogus" or "indeterminate", the client MUST NOT 184 connect to this target server; instead it uses the next most 185 appropriate SRV target. 187 3.3. TLSA Queries 189 The client SHALL construct the TLSA query name as described in 190 [RFC6698] section 3, based on fields from the SRV record: the port 191 from the SRV RDATA, the protocol from the SRV query name, and the 192 TLSA base domain set to the SRV target server host name. 194 For example, the following SRV record for IMAP (see [RFC6186]) leads 195 to the TLSA query shown below: 197 _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net. 199 _9143._tcp.imap.example.net. IN TLSA ? 201 3.4. Impact on TLS Usage 203 The client SHALL determine if the TLSA record(s) returned in the 204 previous step are usable according to section 4.1 of [RFC6698]. This 205 affects the use TLS as follows: 207 o If the TLSA response is "secure" and usable, then the client MUST 208 use TLS when connecting to the target server. The TLSA records 209 are used when validating the server's certificate as described 210 under Section 4. 212 o If the TLSA response is "insecure", then the client SHALL proceed 213 as if the target server had no TLSA records. It MAY connect to 214 the target server with or without TLS, subject to the policies of 215 the application protocol or client implementation. 217 o If the TLSA response is "bogus" or "indeterminate", then the 218 client MUST NOT connect to the target server (the client can still 219 use other SRV targets). 221 4. TLS Checks 223 When connecting to a server, the client MUST use TLS if the responses 224 to the SRV and TLSA queries were "secure" as described above. The 225 rules described in the next two sections apply. 227 4.1. SRV Records Only 229 If the client received zero usable TLSA certificate associations, it 230 SHALL validate the server's TLS certificate using the normal PKIX 231 rules [RFC5280] or protocol-specific rules (e.g., following 232 [RFC6125]) without further input from the TLSA records. 234 In this case, the client uses the information in the server 235 certificate and the DNSSEC validation status of the SRV query in its 236 authentication checks. It SHOULD use the Server Name Indication 237 extension (TLS SNI) [RFC6066] or its functional equivalent in the 238 relevant application protocol (e.g., in XMPP [RFC6120] this is the 239 'to' address of the initial stream header). The preferred name SHALL 240 be chosen as follows, and the client SHALL verify the identity 241 asserted by the server's certificate according to section 6 of 242 [RFC6125], using a list of reference identifiers constructed as 243 follows (note again that in RFC 6125 the terms "source domain" and 244 "derived domain" refer to the same things as "service domain" and 245 "target server host name" in this document). The examples below 246 assume a service domain of "im.example.com" and a target server host 247 name of "xmpp23.hosting.example.net". 249 SRV is insecure: The reference identifiers SHALL include the service 250 domain and MUST NOT include the SRV target server host name (e.g., 251 include "im.example.com" but not "xmpp23.hosting.example.net"). 252 The service domain is the preferred name for TLS SNI or its 253 equivalent. 255 SRV is secure: The reference identifiers SHALL include both the 256 service domain and the SRV target server host name (e.g., include 257 both "im.example.com" and "xmpp23.hosting.example.net"). The 258 target server host name is the preferred name for TLS SNI or its 259 equivalent. 261 In the latter case, the client will accept either identity to ensure 262 compatibility with servers that support this specification as well as 263 servers that do not support this specification. 265 4.2. TLSA Records 267 If the client received one or more usable TLSA certificate 268 associations, it SHALL process them as described in section 2.1 of 269 [RFC6698]. 271 If the TLS server's certificate -- or the public key of the server's 272 certificate -- matches a usable TLSA record with Certificate Usage 273 "DANE-EE", the client MUST consider the server to be authenticated. 274 Because the information in such a TLSA record supersedes the non-key 275 information in the certificate, all other [RFC5280] and [RFC6125] 276 authentication checks (e.g., reference identifier, key usage, 277 expiration, issuance) MUST be ignored or omitted. 279 5. Guidance for Application Protocols 281 This document describes how to use DANE with application protocols in 282 which target servers are discovered via SRV records. Although this 283 document attempts to provide generic guidance applying to all such 284 protocols, additional documents for particular application protocols 285 could cover related topics, such as: 287 o Fallback logic in the event that a client is unable to connect 288 securely to a target server by following the procedures defined in 289 this document. 291 o How clients ought to behave if they do not support SRV lookups, or 292 if clients that support SRV lookups encounter service domains that 293 do not offer SRV records. 295 o Whether the application protocol has a functional equivalent for 296 TLS SNI that is preferred within that protocol. 298 For example, [I-D.ietf-xmpp-dna] covers such topics for the 299 Extensible Messaging and Presence Protocol (XMPP). 301 6. Guidance for Server Operators 303 To conform to this specification, the published SRV records and 304 subsequent address (A, AAAA) records MUST be secured with DNSSEC. 305 There SHOULD also be at least one TLSA record published that 306 authenticates the server's certificate. 308 When using TLSA records with Certificate Usage "DANE-EE", it is not 309 necessary for the deployed certificate to contain an identifier for 310 either the source domain or target server host name. However, 311 servers that rely solely on validation using Certificate Usage "DANE- 312 EE" TLSA records might prevent clients that do not support this 313 specification from successfully connecting with TLS. 315 For TLSA records with Certificate Usage types other than "DANE-EE", 316 the certificate(s) MUST contain an identifier that matches: 318 o the service domain name (the "source domain" in [RFC6125] terms, 319 which is the SRV query domain); and/or 321 o the target server host name (the "derived domain" in [RFC6125] 322 terms, which is the SRV target). 324 Servers that support multiple service domains (i.e., so-called 325 "multi-tenanted environments") can implement the Transport Layer 326 Security Server Name Indication (TLS SNI) [RFC6066] or its functional 327 equivalent to determine which certificate to offer. Clients that do 328 not support this specification will indicate a preference for the 329 service domain name, while clients that support this specification 330 will indicate the target server host name. However, the server 331 determines what certificate to present in the TLS handshake; e.g., 332 the presented certificate might only authenticate the target server 333 host name. 335 7. Internationalization Considerations 337 If any of the DNS queries are for an internationalized domain name, 338 then they need to use the A-label form [RFC5890]. 340 8. IANA Considerations 342 No IANA action is required. 344 9. Security Considerations 346 9.1. Mixed Security Status 348 We do not specify that clients checking all of a service domain's 349 target server host names are consistent in whether they have or do 350 not have TLSA records. This is so that partial or incremental 351 deployment does not break the service. Different levels of 352 deployment are likely if a service domain has a third-party fallback 353 server, for example. 355 The SRV sorting rules are unchanged; in particular they have not been 356 altered in order to prioritize secure servers over insecure servers. 357 If a site wants to be secure it needs to deploy this protocol 358 completely; a partial deployment is not secure and we make no special 359 effort to support it. 361 9.2. A Service Domain Trusts its Servers 363 By signing their zone with DNSSEC, service domain operators 364 implicitly instruct their clients to check their server TLSA records. 365 This implies another point in the trust relationship between service 366 domain holders and their server operators. Most of the setup 367 requirements for this protocol fall on the server operator: 368 installing a TLS certificate with the correct name (where necessary), 369 and publishing a TLSA record for that certificate. If these are not 370 correct then connections from TLSA-aware clients might fail. 372 9.3. Certificate Subject Name Matching 374 Section 4 of the TLSA specification [RFC6698] leaves the details of 375 checking names in certificates to higher level application protocols, 376 though it suggests the use of [RFC6125]. 378 Name checks are not necessary if the matching TLSA record is of 379 Certificate Usage "DANE-EE". Because such a record identifies the 380 specific certificate (or public key of the certificate), additional 381 checks are superfluous and potentially conflicting. 383 Otherwise, while DNSSEC provides a secure binding between the server 384 name and the TLSA record, and the TLSA record provides a binding to a 385 certificate, this latter step can be indirect via a chain of 386 certificates. For example, a Certificate Usage "PKIX-TA" TLSA record 387 only authenticates the CA that issued the certificate, and third 388 parties can obtain certificates from the same CA. Therefore, clients 389 need to check whether the server's certificate matches one of the 390 expected reference identifiers to ensure that the certificate was 391 issued by the CA to the server the client expects. 393 10. Acknowledgements 395 Thanks to Mark Andrews for arguing that authenticating the target 396 server host name is the right thing, and that we ought to rely on 397 DNSSEC to secure the SRV lookup. Thanks to James Cloos, Viktor 398 Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, 399 Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful 400 suggestions. 402 11. References 404 11.1. Normative References 406 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 407 Requirement Levels", BCP 14, RFC 2119, March 1997. 409 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 410 specifying the location of services (DNS SRV)", RFC 2782, 411 February 2000. 413 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 414 Rose, "DNS Security Introduction and Requirements", RFC 415 4033, March 2005. 417 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 418 Rose, "Protocol Modifications for the DNS Security 419 Extensions", RFC 4035, March 2005. 421 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 422 Housley, R., and W. Polk, "Internet X.509 Public Key 423 Infrastructure Certificate and Certificate Revocation List 424 (CRL) Profile", RFC 5280, May 2008. 426 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 427 October 2008. 429 [RFC5890] Klensin, J., "Internationalized Domain Names for 430 Applications (IDNA): Definitions and Document Framework", 431 RFC 5890, August 2010. 433 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 434 Extension Definitions", RFC 6066, January 2011. 436 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence 437 Protocol (XMPP): Core", RFC 6120, March 2011. 439 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 440 Verification of Domain-Based Application Service Identity 441 within Internet Public Key Infrastructure Using X.509 442 (PKIX) Certificates in the Context of Transport Layer 443 Security (TLS)", RFC 6125, March 2011. 445 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 446 Submission/Access Services", RFC 6186, March 2011. 448 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 449 of Named Entities (DANE) Transport Layer Security (TLS) 450 Protocol: TLSA", RFC 6698, August 2012. 452 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify 453 Conversations about DNS-Based Authentication of Named 454 Entities (DANE)", RFC 7218, April 2014. 456 11.2. Informative References 458 [I-D.ietf-dane-smtp-with-dane] 459 Dukhovni, V. and W. Hardaker, "SMTP security via 460 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05 461 (work in progress), February 2014. 463 [I-D.ietf-xmpp-dna] 464 Saint-Andre, P. and M. Miller, "Domain Name Associations 465 (DNA) in the Extensible Messaging and Presence Protocol 466 (XMPP)", draft-ietf-xmpp-dna-05 (work in progress), 467 February 2014. 469 [RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with 470 Dual-Stack Hosts", RFC 6555, April 2012. 472 Appendix A. Examples 474 In the following, most of the DNS resource data is elided for 475 simplicity. 477 A.1. IMAP 479 ; mail domain 480 _imap._tcp.example.com. SRV 10 0 9143 imap.example.net. 481 example.com. RRSIG SRV ... 483 ; target server host name 484 imap.example.net. A 192.0.2.1 485 imap.example.net. RRSIG A ... 487 imap.example.net. AAAA 2001:db8:212:8::e:1 488 imap.example.net. RRSIG ... 490 ; TLSA resource record 491 _9143._tcp.imap.example.net. TLSA ... 492 _9143._tcp.imap.example.net. RRSIG TLSA ... 494 Mail messages submitted for addresses at example.com are sent via 495 IMAP to imap.example.net. Connections to imap.example.net port 9143 496 that use STARTTLS will get a server certificate that authenticates 497 the name imap.example.net. 499 A.2. XMPP 501 ; XMPP domain 502 _xmpp-client.example.com. SRV 1 0 5222 im.example.net. 503 _xmpp-client.example.com. RRSIG SRV ... 505 ; target server host name 506 im.example.net. A 192.0.2.3 507 im.example.net. RRSIG A ... 509 im.example.net. AAAA 2001:db8:212:8::e:4 510 im.example.net. RRSIG AAAA ... 512 ; TLSA resource record 513 _5222._tcp.im.example.net. TLSA ... 514 _5222._tcp.im.example.net. RRSIG TLSA ... 516 XMPP sessions for addresses at example.com are established at 517 im.example.net. Connections to im.example.net port 5222 that use 518 STARTTLS will get a server certificate that authenticates the name 519 im.example.net. 521 Appendix B. Rationale 523 The long-term goal of this specification is to settle on TLS 524 certificates that verify the target server host name rather than the 525 service domain, since this is more convenient for servers hosting 526 multiple domains (so-called "multi-tenanted environments") and scales 527 up more easily to larger numbers of service domains. 529 There are a number of other reasons for doing it this way: 531 o The certificate is part of the server configuration, so it makes 532 sense to associate it with the server host name rather than the 533 service domain. 535 o In the absence of TLS SNI, if the certificate identifies the host 536 name then it does not need to list all the possible service 537 domains. 539 o When the server certificate is replaced it is much easier if there 540 is one part of the DNS that needs updating to match, instead of an 541 unbounded number of hosted service domains. 543 o The same TLSA records work with this specification, and with 544 direct connections to the host name in the style of [RFC6698]. 546 o Some application protocols, such as SMTP, allow a client to 547 perform transactions with multiple service domains in the same 548 connection. It is not in general feasible for the client to 549 specify the service domain using TLS SNI when the connection is 550 established, and the server might not be able to present a 551 certificate that authenticates all possible service domains. See 552 [I-D.ietf-dane-smtp-with-dane] for details. 554 o It is common for SMTP servers to act in multiple roles, for 555 example as outgoing relays or as incoming MX servers, depending on 556 the client identity. It is simpler if the server can present the 557 same certificate regardless of the role in which it is to act. 558 Sometimes the server does not know its role until the client has 559 authenticated, which usually occurs after TLS has been 560 established. See [I-D.ietf-dane-smtp-with-dane] for details. 562 This specification does not provide an option to put TLSA records 563 under the service domain because that would add complexity without 564 providing any benefit, and security protocols are best kept simple. 565 As described above, there are real-world cases where authenticating 566 the service domain cannot be made to work, so there would be 567 complicated criteria for when service domain TLSA records might be 568 used and when they cannot. This is all avoided by putting the TLSA 569 records under the target server host name. 571 The disadvantage is that clients which do not complete DNSSEC 572 validation must, according to [RFC6125] rules, check the server 573 certificate against the service domain, since they have no other way 574 to authenticate the server. This means that SNI support or its 575 functional equivalent is necessary for backward compatibility. 577 Authors' Addresses 579 Tony Finch 580 University of Cambridge Computing Service 581 New Museums Site 582 Pembroke Street 583 Cambridge CB2 3QH 584 ENGLAND 586 Phone: +44 797 040 1426 587 Email: dot@dotat.at 588 URI: http://dotat.at/ 590 Matthew Miller 591 Cisco Systems, Inc. 592 1899 Wynkoop Street, Suite 600 593 Denver, CO 80202 594 USA 596 Email: mamille2@cisco.com 598 Peter Saint-Andre 599 &yet 600 P.O. Box 787 601 Parker, CO 80134 602 USA 604 Email: peter@andyet.com