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Boilerplate error? (The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (February 13, 2014) is 3018 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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: draft-ietf-dane-registry-acronyms has been published as RFC 7218 == Outdated reference: draft-ietf-dane-smtp-with-dane has been published as RFC 7672 == Outdated reference: draft-ietf-xmpp-dna has been published as RFC 7712 Summary: 0 errors (**), 0 flaws (~~), 5 warnings (==), 1 comment (--).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: August 17, 2014 Cisco Systems, Inc. 6 P. Saint-Andre 7 &yet 8 February 13, 2014 10 Using DNS-Based Authentication of Named Entities (DANE) TLSA records 11 with SRV and MX records. 12 draft-ietf-dane-srv-05 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. The association is secured with DNSSEC. Some 19 application protocols use SRV records (RFC 2782) to indirectly name 20 the server hosts for a service domain (SMTP uses MX records for the 21 same purpose). This specification gives generic instructions for how 22 these application protocols locate and use TLSA records when 23 technologies such as SRV records are used. Separate documents give 24 the details that are specific to particular application protocols. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on August 17, 2014. 43 Copyright Notice 45 Copyright (c) 2014 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 3. Relation between SRV and MX records . . . . . . . . . . . . . 3 63 4. DNS Checks for TLSA and SRV Records . . . . . . . . . . . . . 4 64 4.1. SRV Query . . . . . . . . . . . . . . . . . . . . . . . . 4 65 4.2. TLSA Queries . . . . . . . . . . . . . . . . . . . . . . 5 66 5. TLS Checks for TLSA and SRV Records . . . . . . . . . . . . . 6 67 6. Guidance for Application Protocols . . . . . . . . . . . . . 7 68 7. Guidance for Server Operators . . . . . . . . . . . . . . . . 7 69 8. Internationalization Considerations . . . . . . . . . . . . . 8 70 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 71 10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 72 10.1. Mixed Security Status . . . . . . . . . . . . . . . . . 8 73 10.2. A Service Domain Trusts its Servers . . . . . . . . . . 8 74 10.3. Certificate Subject Name Matching . . . . . . . . . . . 9 75 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 76 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 77 12.1. Normative References . . . . . . . . . . . . . . . . . . 9 78 12.2. Informative References . . . . . . . . . . . . . . . . . 10 79 Appendix A. Mail Example . . . . . . . . . . . . . . . . . . . . 11 80 Appendix B. XMPP Example . . . . . . . . . . . . . . . . . . . . 11 81 Appendix C. Rationale . . . . . . . . . . . . . . . . . . . . . 12 82 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 84 1. Introduction 86 The base DANE specification [RFC6698] describes how to use TLSA 87 resource records in the DNS to associate a server's host name with 88 its TLS certificate. The association is secured using DNSSEC. That 89 document "only relates to securely associating certificates for TLS 90 and DTLS with host names" (see the last paragraph of section 1.2 of 91 [RFC6698]). 93 Some application protocols do not use host names directly; instead, 94 they use a service domain and the relevant host names are located 95 indirectly via SRV records [RFC2782], or MX records in the case of 96 SMTP [RFC5321] (Note: in the "CertID" specification [RFC6125], the 97 source domain and host name are referred to as the "source domain" 98 and the "derived domain"). Because of this intermediate resolution 99 step, the normal DANE rules specified in [RFC6698] do not directly 100 apply to protocols that use SRV or MX records. 102 This document describes how to use DANE TLSA records with SRV and MX 103 records. To summarize: 105 o We rely on DNSSEC to secure the association between the service 106 domain and the target server host names (i.e., the host names that 107 are discovered by the SRV or MX query). 109 o The TLSA records are located using the port, protocol, and target 110 host name fields (not the service domain). 112 o Clients always use TLS when connecting to servers with TLSA 113 records. 115 o Assuming that the association is secure, the server's certificate 116 is expected to authenticate the target server host name, rather 117 than the service domain. 119 Separate documents give the details that are specific to particular 120 application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane] 121 and XMPP [I-D.ietf-xmpp-dna]. 123 2. Terminology 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 127 "OPTIONAL" in this memo are to be interpreted as described in 128 [RFC2119]. 130 This draft uses the definitions for "secure", "insecure", "bogus", 131 and "indeterminate" from [RFC4035]. This draft uses the acronyms 132 from [I-D.ietf-dane-registry-acronyms] for the values of TLSA fields 133 where appropriate. 135 3. Relation between SRV and MX records 137 For the purpose of this specification (to avoid cluttering the 138 description with special cases) we treat each MX record ([RFC5321] 139 section 5) as being equivalent to an SRV record [RFC2782] with 140 corresponding fields copied from the MX record and the remaining 141 fields having fixed values as follows: 143 Table 1: SRV Fields and MX Equivalents 145 +---------------+-----------------------------+ 146 | DNS SRV Field | Equivalent MX Value | 147 +---------------+-----------------------------+ 148 | Service | smtp | 149 +---------------+-----------------------------+ 150 | Proto | tcp | 151 +---------------+-----------------------------+ 152 | Name | MX owner name (mail domain) | 153 +---------------+-----------------------------+ 154 | TTL | MX TTL | 155 +---------------+-----------------------------+ 156 | Class | MX Class | 157 +---------------+-----------------------------+ 158 | Priority | MX Priority | 159 +---------------+-----------------------------+ 160 | Weight | 0 | 161 +---------------+-----------------------------+ 162 | Port | 25 | 163 +---------------+-----------------------------+ 164 | Target | MX Target | 165 +---------------+-----------------------------+ 167 Thus we can treat the following MX record as if it were the SRV 168 record shown below: 170 example.com. 86400 IN MX 10 mx.example.net. 172 _smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net. 174 Other details that are specific to SMTP are described in 175 [I-D.ietf-dane-smtp-with-dane]. 177 4. DNS Checks for TLSA and SRV Records 179 4.1. SRV Query 181 When the client makes an SRV query, a successful result will be a 182 list of one or more SRV records (or possibly a chain of CNAME / DNAME 183 aliases referring to such a list). 185 For this specification to apply, all of these DNS RRsets MUST be 186 "secure" according to DNSSSEC validation ([RFC4033] section 5). In 187 the case of aliases, the whole chain of CNAME and DNAME RRsets MUST 188 be secure as well. This corresponds to the AD bit being set in the 189 response(s); see [RFC4035] section 3.2.3. 191 If they are not all secure, this protocol has not been correctly 192 deployed. The client SHOULD fall back to its non-DNSSEC non-DANE 193 behavior (this corresponds to the AD bit being unset). 195 If any of the responses are "bogus" or "indeterminate" according to 196 DNSSEC validation, the client MUST abort (This usually corresponds to 197 a "server failure" response). 199 In the successful case, the client now has an authentic list of 200 server host names with weight and priority values. It performs 201 server ordering and selection using the weight and priority values 202 without regard to the presence or absence of DNSSEC or TLSA records. 203 It takes note of the DNSSEC validation status of the SRV response for 204 use when checking certificate names (see Section 5). 206 4.2. TLSA Queries 208 If the SRV response was insecure, the client MUST NOT perform any 209 TLSA queries. If the SRV response is "secure" according to DNSSEC 210 validation, the client performs a TLSA query for each SRV target as 211 described in this section. 213 For each SRV target host name, the client performs DNSSEC validation 214 on the address (A, AAAA) response and continues based on the results: 216 o if the response is "insecure", the client MUST NOT perform a TLSA 217 query for that target; the TLSA query will most likely fail. 219 o If the response is "bogus" or "indeterminate", the client MUST NOT 220 connect to this host name; instead it uses the next most 221 appropriate SRV target. 223 The client SHALL construct the TLSA query name as described in 224 [RFC6698] section 3, based on fields from the SRV record: the port 225 from the SRV RDATA, the protocol from the SRV query name, and the 226 TLSA base domain set to the SRV target host name. 228 For example, the following SRV record leads to the TLSA query shown 229 below: 231 _imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net. 233 _143._tcp.imap.example.net. IN TLSA ? 235 The client SHALL determine if the TLSA record(s) are usable according 236 to section 4.1 of [RFC6698]. This affects SRV handling as follows: 238 If the TLSA response is "secure", the client MUST use TLS when 239 connecting to the server. The TLSA records are used when validating 240 the server's certificate as described under Section 5. 242 If the TLSA response is "insecure", the client SHALL proceed as if 243 this server has no TLSA records. It MAY connect to the server with 244 or without TLS. 246 If the TLSA response is "bogus" or "indeterminate", then the client 247 MUST NOT connect to this server (the client can still use other SRV 248 targets). 250 5. TLS Checks for TLSA and SRV Records 252 When connecting to a server, the client MUST use TLS if the responses 253 to the SRV and TLSA queries were "secure" as described above. If the 254 client received zero usable TLSA certificate associations, it SHALL 255 validate the server's TLS certificate using the normal PKIX rules 256 [RFC5280] or protocol-specific rules (e.g., following [RFC6125]) 257 without further input from the TLSA records. If the client received 258 one or more usable TLSA certificate associations, it SHALL process 259 them as described in [RFC6698] section 2.1. 261 If the TLS server's certificate -- or the public key of the server's 262 certificate -- matches a usable TLSA record with Certificate Usage 263 "DANE-EE", the client MUST consider the server to be authenticated. 264 Because the information in such a TLSA record supersedes the non-key 265 information in the certificate, all other [RFC5280] and [RFC6125] 266 authentication checks (e.g., reference identifier, key usage, 267 expiration, issuance, etc.) MUST be ignored or omitted. 269 Otherwise, the client uses the information in the server certificate 270 and DNSSEC validation status of the SRV query in its authentication 271 checks. It SHOULD use the Server Name Indication extension (TLS SNI) 272 [RFC6066] or its functional equivalent in the relevant application 273 protocol (e.g., in XMPP [RFC6120] this is the 'to' address of the 274 initial stream header). The preferred name SHALL be chosen as 275 follows, and the client SHALL verify the identity asserted by the 276 server's certificate according to [RFC6125] section 6, using a list 277 of reference identifiers constructed as follows (note again that in 278 RFC 6125 the terms "source domain" and "derived domain" refer to the 279 same things as "service domain" and "target host name" in this 280 document). 282 SRV is insecure: The reference identifiers SHALL include the service 283 domain and MUST NOT include the SRV target host name. The service 284 domain is the preferred name for TLS SNI or its equivalent. 286 SRV is secure: The reference identifiers SHALL include both the 287 service domain and the SRV target host name. The target host name 288 is the preferred name for TLS SNI or its equivalent. 290 In the latter case, the client will accept either identity so that it 291 is compatible with servers that do and do not support this 292 specification. 294 6. Guidance for Application Protocols 296 Separate documents describe how to apply this specification to 297 particular application protocols. Such documents ought to cover the 298 following points: 300 o Fallback logic in the event of bogus replies and the like. 302 o The use of TLS SNI or its functional equivalent. 304 o Appropriate mappings for non-SRV technologies such as MX. 306 o Compatibility with clients that do not support SRV lookups. 308 7. Guidance for Server Operators 310 To conform to this specification, the published SRV records and 311 subsequent address (A, AAAA) records MUST be secured with DNSSEC. 312 There SHOULD also be at least one TLSA record published that 313 authenticates the server's certificate. 315 When using TLSA records with Certificate Usage "DANE-EE", the 316 deployed certificate does not need to contain any of the possible 317 reference identifiers discussed below. Indeed, none of the 318 certificate's information is necessary for such certificates. 319 However, servers that rely solely on validation using Certificate 320 Usage "DANE-EE" TLSA records might prevent clients that do not 321 support this specification from successfully connecting with TLS. 323 For TLSA records with Certificate Usage types other than "DANE-EE", 324 the certificate(s) MUST contain a reference identifier that matches: 326 o the service domain name (the "source domain" in [RFC6125] terms, 327 which is the SRV query domain); and/or 329 o the server host name (the "derived domain" in [RFC6125] terms, 330 which is the SRV target). 332 Servers that support multiple service domains (i.e., multi-tenant) 333 can implement Server Name Indicator (TLS SNI) [RFC6066] or its 334 functional equivalent to determine which certificate to offer. 335 Clients that do not support this specification will indicate a 336 preference for the service domain name, while clients that support 337 this specification will indicate the server host name. However, the 338 server determines what certificate to present in the TLS handshake; 339 e.g., the presented certificate might only authenticate the server 340 host name. 342 8. Internationalization Considerations 344 If any of the DNS queries are for an internationalized domain name, 345 then they need to use the A-label form [RFC5890]. 347 9. IANA Considerations 349 No IANA action is required. 351 10. Security Considerations 353 10.1. Mixed Security Status 355 We do not specify that clients checking all of a service domain's 356 server host names are consistent in whether they have or do not have 357 TLSA records. This is so that partial or incremental deployment does 358 not break the service. Different levels of deployment are likely if 359 a service domain has a third-party fallback server, for example. 361 The SRV and MX sorting rules are unchanged; in particular they have 362 not been altered in order to prioritize secure servers over insecure 363 servers. If a site wants to be secure it needs to deploy this 364 protocol completely; a partial deployment is not secure and we make 365 no special effort to support it. 367 10.2. A Service Domain Trusts its Servers 369 By signing their zone with DNSSEC, service domain operators 370 implicitly instruct their clients to check their server TLSA records. 371 This implies another point in the trust relationship between service 372 domain holders and their server operators. Most of the setup 373 requirements for this protocol fall on the server operator: 374 installing a TLS certificate with the correct name (where necessary), 375 and publishing a TLSA record for that certificate. If these are not 376 correct then connections from TLSA-aware clients might fail. 378 10.3. Certificate Subject Name Matching 380 Section 4 of the TLSA specification [RFC6698] leaves the details of 381 checking names in certificates to higher level application protocols, 382 though it suggests the use of [RFC6125]. 384 Name checks are not necessary if the matching TLSA record is of 385 Certificate Usage "DANE-EE". Because such a record identifies the 386 specific certificate (or public key of the certificate), additional 387 checks are superfluous and potentially conflicting. 389 Otherwise, while DNSSEC provides a secure binding between the server 390 name and the TLSA record, and the TLSA record provides a binding to a 391 certificate, this latter step can be indirect via a chain of 392 certificates. For example, a Certificate Usage "PKIX-TA" TLSA record 393 only authenticates the CA that issued the certificate, and third 394 parties can obtain certificates from the same CA. Therefore, clients 395 need to check whether the server's certificate matches one of the 396 expected reference identifiers to ensure the certificate was issued 397 by the CA to the server the client expects. 399 11. Acknowledgements 401 Thanks to Mark Andrews for arguing that authenticating the server 402 host name is the right thing, and that we ought to rely on DNSSEC to 403 secure the SRV / MX lookup. Thanks to James Cloos, Viktor Dukhovni, 404 Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, Hector 405 Santos, Jonas Schneider, and Alessandro Vesely for helpful 406 suggestions. 408 12. References 410 12.1. Normative References 412 [I-D.ietf-dane-registry-acronyms] 413 Gudmundsson, O., "Adding acronyms to simplify DANE 414 conversations", draft-ietf-dane-registry-acronyms-03 (work 415 in progress), January 2014. 417 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 418 Requirement Levels", BCP 14, RFC 2119, March 1997. 420 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 421 specifying the location of services (DNS SRV)", RFC 2782, 422 February 2000. 424 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 425 Rose, "DNS Security Introduction and Requirements", RFC 426 4033, March 2005. 428 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 429 Rose, "Protocol Modifications for the DNS Security 430 Extensions", RFC 4035, March 2005. 432 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 433 Housley, R., and W. Polk, "Internet X.509 Public Key 434 Infrastructure Certificate and Certificate Revocation List 435 (CRL) Profile", RFC 5280, May 2008. 437 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 438 October 2008. 440 [RFC5890] Klensin, J., "Internationalized Domain Names for 441 Applications (IDNA): Definitions and Document Framework", 442 RFC 5890, August 2010. 444 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 445 Extension Definitions", RFC 6066, January 2011. 447 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence 448 Protocol (XMPP): Core", RFC 6120, March 2011. 450 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 451 Verification of Domain-Based Application Service Identity 452 within Internet Public Key Infrastructure Using X.509 453 (PKIX) Certificates in the Context of Transport Layer 454 Security (TLS)", RFC 6125, March 2011. 456 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 457 of Named Entities (DANE) Transport Layer Security (TLS) 458 Protocol: TLSA", RFC 6698, August 2012. 460 12.2. Informative References 462 [I-D.ietf-dane-smtp-with-dane] 463 Dukhovni, V. and W. Hardaker, "SMTP security via 464 opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05 465 (work in progress), February 2014. 467 [I-D.ietf-xmpp-dna] 468 Saint-Andre, P. and M. Miller, "Domain Name Associations 469 (DNA) in the Extensible Messaging and Presence Protocol 470 (XMPP)", draft-ietf-xmpp-dna-05 (work in progress), 471 February 2014. 473 Appendix A. Mail Example 475 In the following, most of the DNS resource data is elided for 476 simplicity. 478 ; mail domain 479 example.com. MX 1 mx.example.net. 480 example.com. RRSIG MX ... 482 ; SMTP server host name 483 mx.example.net. A 192.0.2.1 484 mx.example.net. RRSIG A ... 486 mx.example.net. AAAA 2001:db8:212:8::e:1 487 mx.example.net. RRSIG ... 489 ; TLSA resource record 490 _25._tcp.mx.example.net. TLSA ... 491 _25._tcp.mx.example.net. RRSIG TLSA ... 493 Mail for addresses at example.com is delivered by SMTP to 494 mx.example.net. Connections to mx.example.net port 25 that use 495 STARTTLS will get a server certificate that authenticates the name 496 mx.example.net. 498 Appendix B. XMPP Example 500 In the following, most of the DNS resource data is elided for 501 simplicity. 503 ; XMPP domain 504 _xmpp-client.example.com. SRV 1 0 5222 im.example.net. 505 _xmpp-client.example.com. RRSIG SRV ... 507 ; XMPP 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 C. Rationale 525 The long-term goal of this specification is to settle on TLS 526 certificates that verify the server host name rather than the service 527 domain, since this is more convenient for servers hosting multiple 528 domains (so-called "multi-tenanted environments") and scales up more 529 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. 555 o It is common for SMTP servers to act in multiple roles, for 556 example as outgoing relays or as incoming MX servers, depending on 557 the client identity. It is simpler if the server can present the 558 same certificate regardless of the role in which it is to act. 559 Sometimes the server does not know its role until the client has 560 authenticated, which usually occurs after TLS has been 561 established. 563 This specification does not provide an option to put TLSA records 564 under the service domain because that would add complexity without 565 providing any benefit, and security protocols are best kept simple. 566 As described above, there are real-world cases where authenticating 567 the service domain cannot be made to work, so there would be 568 complicated criteria for when service domain TLSA records might be 569 used and when they cannot. This is all avoided by putting the TLSA 570 records under the server host name. 572 The disadvantage is that clients which do not do DNSSEC validation 573 must, according to [RFC6125] rules, check the server certificate 574 against the service domain, since they have no other way to 575 authenticate the server. This means that SNI support or its 576 functional equivalent is necessary for backward compatibility. 578 Authors' Addresses 580 Tony Finch 581 University of Cambridge Computing Service 582 New Museums Site 583 Pembroke Street 584 Cambridge CB2 3QH 585 ENGLAND 587 Phone: +44 797 040 1426 588 Email: dot@dotat.at 589 URI: http://dotat.at/ 591 Matthew Miller 592 Cisco Systems, Inc. 593 1899 Wynkoop Street, Suite 600 594 Denver, CO 80202 595 USA 597 Email: mamille2@cisco.com 598 Peter Saint-Andre 599 &yet 601 Email: ietf@stpeter.im