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Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNS-Based Authentication of Named T. Finch 3 Entities (DANE) University of Cambridge 4 Internet-Draft February 25, 2013 5 Intended status: Standards Track 6 Expires: August 29, 2013 8 Using DNS-Based Authentication of Named Entities (DANE) TLSA records 9 with SRV and MX records. 10 draft-ietf-dane-srv-01 12 Abstract 14 The DANE specification [RFC6698] describes how to use TLSA resource 15 records in the DNS to associate a server's host name with its TLS 16 certificate. The association is secured with DNSSEC. Some 17 application protocols can use SRV records [RFC2782] to indirectly 18 name the server hosts for a service domain. (SMTP uses MX records 19 for the same purpose.) This specification gives generic instructions 20 for how these application protocols locate and use TLSA records. 21 Separate documents give the details that are specific to particular 22 application protocols. 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 August 29, 2013. 41 Copyright Notice 43 Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. DNS checks for TLSA and SRV / MX records . . . . . . . . . . . 3 60 2.1. MX records . . . . . . . . . . . . . . . . . . . . . . . . 3 61 2.2. SRV query . . . . . . . . . . . . . . . . . . . . . . . . 4 62 2.3. TLSA queries . . . . . . . . . . . . . . . . . . . . . . . 5 63 3. TLS checks for TLSA and SRV / MX records . . . . . . . . . . . 5 64 4. Guidance for application protocols . . . . . . . . . . . . . . 6 65 5. Guidance for server operators . . . . . . . . . . . . . . . . 6 66 6. Security considerations . . . . . . . . . . . . . . . . . . . 7 67 6.1. Mixed security status . . . . . . . . . . . . . . . . . . 7 68 6.2. A service domain trusts its servers . . . . . . . . . . . 7 69 6.3. Certificate subject name matching . . . . . . . . . . . . 7 70 6.4. Deliberate omissions . . . . . . . . . . . . . . . . . . . 8 71 7. Internationalization Considerations . . . . . . . . . . . . . 8 72 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 73 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 74 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 10.1. Normative References . . . . . . . . . . . . . . . . . . . 9 76 10.2. Informative References . . . . . . . . . . . . . . . . . . 9 77 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 10 78 Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . . 10 79 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 81 1. Introduction 83 The base DANE specification [RFC6698] describes how to use TLSA 84 resource records in the DNS to associate a server's host name with 85 its TLS certificate. The association is secured using DNSSEC. That 86 document "only relates to securely associating certificates for TLS 87 and DTLS with host names" (see the last paragraph of section 1.2 of 88 [RFC6698]). 90 Some application protocols do not use host names directly, but 91 instead use a service domain. The domain's servers are located 92 indirectly via SRV records [RFC2782] (or MX records in the case of 93 SMTP [RFC5321]). When they do not use host names [RFC6698] does not 94 direcly apply to these protocols. 96 This document describes how to use DANE TLSA records with SRV and MX 97 records. To summarize: 99 o We rely on DNSSEC to secure the association between the service 100 domain and the target server host names, i.e. the result of the 101 SRV or MX query. 103 o The TLSA records are located using the SRV port, protocol, and 104 target host name fields. 106 o Clients always use TLS when connecting to servers with TLSA 107 records. 109 o The server's certificate is expected to authenticate the server 110 host name, rather than the service domain. 112 Separate documents give the details that are specific to particular 113 application protocols. For examples, see [I-D.ietf-dane-smtp] and 114 [I-D.ietf-dane-mua]. 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 118 memo are to be interpreted as described in [RFC2119]. 120 2. DNS checks for TLSA and SRV / MX records 122 2.1. MX records 124 For the purpose of this specification (to avoid cluttering the 125 description with special cases) each MX record ([RFC5321] section 5) 126 is treated as being equivalent to a SRV record [RFC2782] with 127 corresponding fields copied from the MX record and the remaining 128 fields having fixed values as follows: 130 Service - smtp 132 Proto - tcp 134 Name - MX owner name (mail domain) 136 TTL - MX TTL 138 Class - MX Class 140 Priority - MX Priority 142 Weight - 0 144 Port - 25 146 Target - MX Target 148 For example this MX record is treated as if it were the following SRV 149 record: 151 example.com. 86400 IN MX 10 mx.example.net. 153 _smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net. 155 Other details that are specific to SMTP are described in 156 [I-D.ietf-dane-smtp]. 158 2.2. SRV query 160 When the client makes a SRV query, a successful result will be (a 161 possible chain of CNAME / DNAME aliases referring to) a list of one 162 or more SRV records. 164 For this specification to take effect, all of these DNS RRsets MUST 165 be "secure" according to DNSSSEC validation ([RFC4033] section 5). 166 In the case of aliases, the whole chain MUST be secure as well as the 167 ultimate target. (This corresponds to the AD bit being set in the 168 response(s) - see [RFC4035] section 3.2.3.) 170 If they are not all secure, this protocol has not been fully 171 deployed. The client SHOULD fall back to its non-DNSSEC non-DANE 172 behaviour. (This corresponds to the AD bit being unset.) 174 If any of the responses is "bogus" according to DNSSEC validation the 175 client MUST abort. (This usually corresponds to a "server failure" 176 response.) 178 In the successful case, the client now has an authentic list of 179 server host names with weight and priority values. It performs 180 server ordering and selection using the weight and priority values 181 without regard to the presence or absence of DNSSEC or TLSA records. 182 It takes note of the DNSSEC validation status of the SRV response for 183 use when checking certificate names (see section Section 3). 185 2.3. TLSA queries 187 This sub-section applies to each server host name individually, 188 provided the SRV response was secure according to DNSSEC validation. 190 The client SHALL construct the TLSA query name as described in 191 [RFC6698] section 3, based on fields from the SRV record: port from 192 the SRV RDATA, protocol from the SRV query name, and the TLSA base 193 domain is the SRV target host name. 195 For example this SRV record leads to the following TLSA query: 197 _imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net. 199 _143._tcp.imap.example.net. IN TLSA ? 201 The client SHALL determine if the TLSA record(s) are usable according 202 to section 4.1 of [RFC6698]. This affects SRV handling as follows: 204 If the TLSA response is "secure" the client MUST use TLS when 205 connecting to the server. If the client receives zero usable 206 certificate associations, it processes TLS in the normal fashion 207 without any input from the TLSA records. If the client receives one 208 or more usable certificate associations, it processes them as 209 described in [RFC6698]. 211 If the TLSA response is "insecure" or "indeterminate" the client 212 SHALL proceed as if this server has no TLSA records. It MAY connect 213 to the server with or without TLS. 215 If the TLSA response is "bogus" then the client MUST NOT connect to 216 the corresponding server. (The client can still use other SRV 217 targets.) 219 3. TLS checks for TLSA and SRV / MX records 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 client SHALL ensure the server's certificate passes the [RFC6698] 224 checks if there are usable TLSA records. 226 The client uses the DNSSEC validation status of the SRV query in its 227 server certificate identity checks. (The TLSA validation status does 228 not affect the server certificate identity checks.) It MUST use the 229 Server Name Indication extension (TLS SNI) [RFC6066] with the 230 preferred name chosen as follows. It SHALL verify the identity 231 asserted by the server's certificate according to [RFC6125] section 232 6, using a list of reference identifiers constructed as follows. 234 SRV is insecure or indeterminate: The reference identifiers SHALL 235 include the service domain and MUST NOT include the SRV target 236 host name. The service domain is the preferred name for TLS SNI. 238 SRV is secure: The reference identifiers SHALL include both the 239 service domain and the SRV target host name. The target host name 240 is the preferred name for TLS SNI. 242 (In the latter case, the client will accept either identity so it is 243 compatible with servers that do and do not support this 244 specification.) 246 4. Guidance for application protocols 248 Separate documents describe how to apply this specification to 249 particular application protocols. If you are writing such as 250 document the following points ought to be covered: (This section is 251 currently sketchy.) 253 o SRV fallback logic? In the event of bogus replies etc. 255 o Compatibility with non-SRV clients. 257 5. Guidance for server operators 259 In order to support this specification, server software MUST 260 implement the TLS Server Name Indication extension (TLS SNI) 261 [RFC6066] for selecting the appropriate certificate. 263 A server that supports TLS and is the target of a SRV record MUST 264 have a TLS certificate that authenticates the SRV query domain (i.e. 265 the service domain, or "source domain" in [RFC6125] terms). This is 266 necessary for clients that cannot perform DNSSEC validation. This 267 certificate MUST be the default that is presented if the client does 268 not use TLS SNI. 270 In order to support this specification, the server SHOULD also have a 271 certificate that authenticates the SRV target domain (the mail server 272 hostname). This can be done using a multi-name certificate or by 273 using the client's TLS SNI to select the appropriate certificate. 274 The server's TLSA record SHOULD correspond to this certificate. 276 Note: In some application protocols, there are old non-SRV clients 277 that expect a server's TLS certificate to authenticate its host name; 278 they are also unlikely to support SNI. This means that servers for 279 old clients need a different default certificate from servers that 280 are the targets of SRV records. If the server does not have a 281 certificate that authenticates all relevant names, it is necessary to 282 segregate old and new clients. This can be done by using different 283 target hosts or non-standard ports in the SRV targets. (The latter 284 avoids the need for additional certificates.) 286 6. Security considerations 288 6.1. Mixed security status 290 We do not specify that clients check that all of a service domain's 291 server host names are consistent in whether they have or do not have 292 TLSA records. This is so that partial or incremental deployment does 293 not break the service. Different levels of deployment are likely if 294 a service domain has a third-party fall-back server, for example. 296 The SRV and MX sorting rules are unchanged; in particular they have 297 not been altered in order to prioritize secure servers over insecure 298 servers. If a site wants to be secure it needs to deploy this 299 protocol completely; a partial deployment is not secure and we make 300 no special effort to support it. 302 6.2. A service domain trusts its servers 304 By signing their zone with DNSSEC, service domain operators 305 implicitly instruct their clients to check their server TLSA records. 306 This implies another point in the trust relationship between service 307 domain holders and their server operators. Most of the setup 308 requirements for this protocol fall on the server operator: 309 installing a TLS certificate with the correct name, and publishing a 310 TLSA record under that name. If these are not correct then 311 connections from TLSA-aware clients might fail. 313 6.3. Certificate subject name matching 315 Section 4 of the TLSA specification [RFC6698] leaves the details of 316 checking names in certificates to higher level application protocols, 317 though it suggests the use of [RFC6125]. 319 Name checking might appear to be unnecessary, since DNSSEC provides a 320 secure binding between the server name and the TLSA record, which in 321 turn authenticates the certificate. However this latter step can be 322 indirect, via a chain of certificates. A usage=0 TLSA record only 323 authenticates the CA that issued the certificate, and third parties 324 can obtain certificates from the same CA. 326 So this specification says that clients check that the server's 327 certificate matches the server host name, to ensure that the 328 certificate was issued by the CA to the server that the client is 329 connecting to. The client always performs this check regardless of 330 the TLSA usage, to simplify implementation and so that this 331 specification is less likely to need updating when new TLSA usages 332 are added. 334 6.4. Deliberate omissions 336 We do not specify that clients check the DNSSEC state of the server 337 address records. This is not necessary since the certificate checks 338 ensure that the client has connected to the correct server. (The 339 address records will normally have the same security state as the 340 TLSA records, but they can differ if there are CNAME or DNAME 341 indirections.) 343 7. Internationalization Considerations 345 If any of the DNS queries are for an internationalized domain name, 346 then they need to use the A-label form [RFC5890]. 348 8. IANA Considerations 350 No IANA action is required. 352 9. Acknowledgements 354 Thanks to Mark Andrews for arguing that authenticating the server 355 host name is the right thing, and that we ought to rely on DNSSEC to 356 secure the SRV / MX lookup. Thanks to James Cloos, Ned Freed, Olafur 357 Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas 358 Schneider, and Alessandro Vesely for helpful suggestions. 360 10. References 361 10.1. Normative References 363 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 364 Requirement Levels", BCP 14, RFC 2119, March 1997. 366 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 367 specifying the location of services (DNS SRV)", RFC 2782, 368 February 2000. 370 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 371 Rose, "DNS Security Introduction and Requirements", 372 RFC 4033, March 2005. 374 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 375 Rose, "Protocol Modifications for the DNS Security 376 Extensions", RFC 4035, March 2005. 378 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 379 October 2008. 381 [RFC5890] Klensin, J., "Internationalized Domain Names for 382 Applications (IDNA): Definitions and Document Framework", 383 RFC 5890, August 2010. 385 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 386 Extension Definitions", RFC 6066, January 2011. 388 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 389 Verification of Domain-Based Application Service Identity 390 within Internet Public Key Infrastructure Using X.509 391 (PKIX) Certificates in the Context of Transport Layer 392 Security (TLS)", RFC 6125, March 2011. 394 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 395 of Named Entities (DANE) Transport Layer Security (TLS) 396 Protocol: TLSA", RFC 6698, August 2012. 398 10.2. Informative References 400 [I-D.ietf-dane-smtp] 401 Finch, T., "DNS-Based Authentication of Named Entities 402 (DANE) for secure SMTP", draft-ietf-dane-smtp (work in 403 progress), March 2013. 405 [I-D.ietf-dane-mua] 406 Finch, T., "DNS-Based Authentication of Named Entities 407 (DANE) for POP, IMAP, and message submission", 408 draft-ietf-dane-mua (work in progress), March 2013. 410 Appendix A. Example 412 In the following, most of the DNS resource data is elided for 413 simplicity. 415 ; mail domain 416 example.com. MX 1 mx.example.net. 417 example.com. RRSIG MX ... 419 ; SMTP server host name 420 mx.example.net. A 192.0.2.1 421 mx.example.net. AAAA 2001:db8:212:8::e:1 423 ; TLSA resource record 424 _25._tcp.mx.example.net. TLSA ... 425 _25._tcp.mx.example.net. RRSIG TLSA ... 427 Mail for addresses at example.com is delivered by SMTP to 428 mx.example.net. Connections to mx.example.net port 25 that use 429 STARTTLS will get a server certificate that authenticates the name 430 mx.example.net. 432 Appendix B. Rationale 434 The long-term goal of this specification is to settle on TLS 435 certificates that verify the server host name rather than the service 436 domain, since this is more convenient for servers hosting multiple 437 domains and scales up more easily to larger numbers of service 438 domains. 440 There are a number of other reasons for doing it this way: 442 o The certificate is part of the server configuration, so it makes 443 sense to associate it with the server name rather than the service 444 domain. 446 o In the absence of TLS SNI, if the certificate identifies the host 447 name then it does not need to list all the possible service 448 domains. 450 o When the server certificate is replaced it is much easier if there 451 is one part of the DNS that needs updating to match, instead of an 452 unbounded number of hosted service domains. 454 o The same TLSA records work with this specification, and with 455 direct connections to the host name in the style of [RFC6698]. 457 o Some application protocols, such as SMTP, allow a client to 458 perform transactions with multiple service domains in the same 459 connection. It is not in general feasible for the client to 460 specify the service domain using TLS SNI when the connection is 461 established, and the server might not be able to present a 462 certificate that authenticates all possible service domains. 464 o It is common for SMTP servers to act in multiple roles, as 465 outgoing relays or as incoming MX servers, depending on the client 466 identity. It is simpler if the server can present the same 467 certificate regardless of the role in which it is to act. 468 Sometimes the server does not know its role until the client has 469 authenticated, which usually occurs after TLS has been 470 established. 472 This specification does not provide an option to put TLSA records 473 under the service domain because that would add complexity without 474 providing any benefit, and security protocols are best kept simple. 475 As described above, there are real-world cases where authenticating 476 the service domain cannot be made to work, so there would be 477 complicated criteria for when service domain TLSA records might be 478 used and when they cannot. This is all avoided by putting the TLSA 479 records under the server host name. 481 The disadvantage is that clients which do not do DNSSEC validation 482 must, according to [RFC6125] rules, check the server certificate 483 against the service domain, since they have no other way to 484 authenticate the server. This means that Server Name Indication 485 support is necessary for backwards compatibility. 487 Author's Address 489 Tony Finch 490 University of Cambridge Computing Service 491 New Museums Site 492 Pembroke Street 493 Cambridge CB2 3QH 494 ENGLAND 496 Phone: +44 797 040 1426 497 Email: dot@dotat.at 498 URI: http://dotat.at/