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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-02 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. Relation between SRV and MX records . . . . . . . . . . . . . 3 60 3. DNS checks for TLSA and SRV records . . . . . . . . . . . . . 4 61 3.1. SRV query . . . . . . . . . . . . . . . . . . . . . . . . 4 62 3.2. TLSA queries . . . . . . . . . . . . . . . . . . . . . . . 5 63 4. TLS checks for TLSA and SRV records . . . . . . . . . . . . . 5 64 5. Guidance for application protocols . . . . . . . . . . . . . . 6 65 6. Guidance for server operators . . . . . . . . . . . . . . . . 6 66 7. Security considerations . . . . . . . . . . . . . . . . . . . 7 67 7.1. Mixed security status . . . . . . . . . . . . . . . . . . 7 68 7.2. A service domain trusts its servers . . . . . . . . . . . 7 69 7.3. Certificate subject name matching . . . . . . . . . . . . 8 70 7.4. Deliberate omissions . . . . . . . . . . . . . . . . . . . 8 71 8. Internationalization Considerations . . . . . . . . . . . . . 8 72 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 73 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 74 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 11.1. Normative References . . . . . . . . . . . . . . . . . . . 9 76 11.2. Informative References . . . . . . . . . . . . . . . . . . 10 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. Relation between SRV and MX records 122 For the purpose of this specification (to avoid cluttering the 123 description with special cases) we treat each MX record ([RFC5321] 124 section 5) as being equivalent to a SRV record [RFC2782] with 125 corresponding fields copied from the MX record and the remaining 126 fields having fixed values as follows: 128 Service - smtp 130 Proto - tcp 132 Name - MX owner name (mail domain) 134 TTL - MX TTL 136 Class - MX Class 138 Priority - MX Priority 140 Weight - 0 142 Port - 25 144 Target - MX Target 146 For example this MX record is treated as if it were the following SRV 147 record: 149 example.com. 86400 IN MX 10 mx.example.net. 151 _smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net. 153 Other details that are specific to SMTP are described in 154 [I-D.ietf-dane-smtp]. 156 3. DNS checks for TLSA and SRV records 158 3.1. 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 4). 185 3.2. 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. The TLSA records are used when validating 206 the server's certificate as described in section Section 4. 208 If the TLSA response is "insecure" or "indeterminate" the client 209 SHALL proceed as if this server has no TLSA records. It MAY connect 210 to the server with or without TLS. 212 If the TLSA response is "bogus" then the client MUST NOT connect to 213 the corresponding server. (The client can still use other SRV 214 targets.) 216 4. TLS checks for TLSA and SRV records 218 When connecting to a server, the client MUST use TLS if the responses 219 to the SRV and TLSA queries were "secure" as described above. If the 220 client received zero usable TLSA certificate associations, it SHALL 221 validate the server's TLS certificate using the normal PKIX rules 223 [RFC5280] without further input from the TLSA records. If the client 224 received one or more usable TLSA certificate associations, it SHALL 225 process them as described in [RFC6698] section 2.1. 227 The client uses the DNSSEC validation status of the SRV query in its 228 server certificate identity checks. (The TLSA validation status does 229 not affect the server certificate identity checks.) It SHALL use the 230 Server Name Indication extension (TLS SNI) [RFC6066] with the 231 preferred name chosen as follows. It SHALL verify the identity 232 asserted by the server's certificate according to [RFC6125] section 233 6, using a list of reference identifiers constructed as follows. 235 SRV is insecure or indeterminate: The reference identifiers SHALL 236 include the service domain and MUST NOT include the SRV target 237 host name. The service domain is the preferred name for TLS SNI. 239 SRV is secure: The reference identifiers SHALL include both the 240 service domain and the SRV target host name. The target host name 241 is the preferred name for TLS SNI. 243 (In the latter case, the client will accept either identity so that 244 it is compatible with servers that do and do not support this 245 specification.) 247 5. Guidance for application protocols 249 Separate documents describe how to apply this specification to 250 particular application protocols. If you are writing such as 251 document the following points ought to be covered: (This section is 252 currently sketchy.) 254 o SRV fallback logic? In the event of bogus replies etc. 256 o Compatibility with non-SRV clients. 258 6. Guidance for server operators 260 In order to support this specification, server software MUST 261 implement the TLS Server Name Indication extension (TLS SNI) 262 [RFC6066] for selecting the appropriate certificate. 264 A server that supports TLS and is the target of a SRV record MUST 265 have a TLS certificate that authenticates the SRV query domain (i.e. 266 the service domain, or "source domain" in [RFC6125] terms). This is 267 necessary for clients that cannot perform DNSSEC validation. This 268 certificate MUST be the default that is presented if the client does 269 not use TLS SNI. 271 In order to support this specification, the server SHOULD also have a 272 certificate that authenticates the SRV target domain (the mail server 273 hostname). This can be done using a multi-name certificate or by 274 using the client's TLS SNI to select the appropriate certificate. 275 The server's TLSA record SHOULD correspond to this certificate. 277 Note: In some application protocols, there are old non-SRV clients 278 that expect a server's TLS certificate to authenticate its host name; 279 they are also unlikely to support SNI. This means that servers for 280 old clients need a different default certificate from servers that 281 are the targets of SRV records. If the server does not have a 282 certificate that authenticates all relevant names, it is necessary to 283 segregate old and new clients. This can be done by using different 284 target hosts or non-standard ports in the SRV targets. (The latter 285 avoids the need for additional certificates.) 287 7. Security considerations 289 7.1. Mixed security status 291 We do not specify that clients check that all of a service domain's 292 server host names are consistent in whether they have or do not have 293 TLSA records. This is so that partial or incremental deployment does 294 not break the service. Different levels of deployment are likely if 295 a service domain has a third-party fall-back server, for example. 297 The SRV and MX sorting rules are unchanged; in particular they have 298 not been altered in order to prioritize secure servers over insecure 299 servers. If a site wants to be secure it needs to deploy this 300 protocol completely; a partial deployment is not secure and we make 301 no special effort to support it. 303 7.2. A service domain trusts its servers 305 By signing their zone with DNSSEC, service domain operators 306 implicitly instruct their clients to check their server TLSA records. 307 This implies another point in the trust relationship between service 308 domain holders and their server operators. Most of the setup 309 requirements for this protocol fall on the server operator: 310 installing a TLS certificate with the correct name, and publishing a 311 TLSA record under that name. If these are not correct then 312 connections from TLSA-aware clients might fail. 314 7.3. Certificate subject name matching 316 Section 4 of the TLSA specification [RFC6698] leaves the details of 317 checking names in certificates to higher level application protocols, 318 though it suggests the use of [RFC6125]. 320 Name checking might appear to be unnecessary, since DNSSEC provides a 321 secure binding between the server name and the TLSA record, which in 322 turn authenticates the certificate. However this latter step can be 323 indirect, via a chain of certificates. A usage=0 TLSA record only 324 authenticates the CA that issued the certificate, and third parties 325 can obtain certificates from the same CA. 327 So this specification says that clients check that the server's 328 certificate matches the server host name, to ensure that the 329 certificate was issued by the CA to the server that the client is 330 connecting to. The client always performs this check regardless of 331 the TLSA usage, to simplify implementation and so that this 332 specification is less likely to need updating when new TLSA usages 333 are added. 335 7.4. Deliberate omissions 337 We do not specify that clients check the DNSSEC state of the server 338 address records. This is not necessary since the certificate checks 339 ensure that the client has connected to the correct server. (The 340 address records will normally have the same security state as the 341 TLSA records, but they can differ if there are CNAME or DNAME 342 indirections.) 344 8. Internationalization Considerations 346 If any of the DNS queries are for an internationalized domain name, 347 then they need to use the A-label form [RFC5890]. 349 9. IANA Considerations 351 No IANA action is required. 353 10. Acknowledgements 355 Thanks to Mark Andrews for arguing that authenticating the server 356 host name is the right thing, and that we ought to rely on DNSSEC to 357 secure the SRV / MX lookup. Thanks to James Cloos, Ned Freed, Olafur 358 Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas 359 Schneider, and Alessandro Vesely for helpful suggestions. 361 11. References 363 11.1. Normative References 365 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 366 Requirement Levels", BCP 14, RFC 2119, March 1997. 368 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 369 specifying the location of services (DNS SRV)", RFC 2782, 370 February 2000. 372 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 373 Rose, "DNS Security Introduction and Requirements", 374 RFC 4033, March 2005. 376 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 377 Rose, "Protocol Modifications for the DNS Security 378 Extensions", RFC 4035, March 2005. 380 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 381 Housley, R., and W. Polk, "Internet X.509 Public Key 382 Infrastructure Certificate and Certificate Revocation List 383 (CRL) Profile", RFC 5280, May 2008. 385 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 386 October 2008. 388 [RFC5890] Klensin, J., "Internationalized Domain Names for 389 Applications (IDNA): Definitions and Document Framework", 390 RFC 5890, August 2010. 392 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 393 Extension Definitions", RFC 6066, January 2011. 395 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 396 Verification of Domain-Based Application Service Identity 397 within Internet Public Key Infrastructure Using X.509 398 (PKIX) Certificates in the Context of Transport Layer 399 Security (TLS)", RFC 6125, March 2011. 401 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 402 of Named Entities (DANE) Transport Layer Security (TLS) 403 Protocol: TLSA", RFC 6698, August 2012. 405 11.2. Informative References 407 [I-D.ietf-dane-smtp] 408 Finch, T., "Secure SMTP using DNS-Based Authentication of 409 Named Entities (DANE) TLSA records.", draft-ietf-dane-smtp 410 (work in progress), March 2013. 412 [I-D.ietf-dane-mua] 413 Finch, T., "Using DNS-Based Authentication of Named 414 Entities (DANE) with POP, IMAP, and message submission.", 415 draft-ietf-dane-mua (work in progress), March 2013. 417 Appendix A. Example 419 In the following, most of the DNS resource data is elided for 420 simplicity. 422 ; mail domain 423 example.com. MX 1 mx.example.net. 424 example.com. RRSIG MX ... 426 ; SMTP server host name 427 mx.example.net. A 192.0.2.1 428 mx.example.net. AAAA 2001:db8:212:8::e:1 430 ; TLSA resource record 431 _25._tcp.mx.example.net. TLSA ... 432 _25._tcp.mx.example.net. RRSIG TLSA ... 434 Mail for addresses at example.com is delivered by SMTP to 435 mx.example.net. Connections to mx.example.net port 25 that use 436 STARTTLS will get a server certificate that authenticates the name 437 mx.example.net. 439 Appendix B. Rationale 441 The long-term goal of this specification is to settle on TLS 442 certificates that verify the server host name rather than the service 443 domain, since this is more convenient for servers hosting multiple 444 domains and scales up more easily to larger numbers of service 445 domains. 447 There are a number of other reasons for doing it this way: 449 o The certificate is part of the server configuration, so it makes 450 sense to associate it with the server name rather than the service 451 domain. 453 o In the absence of TLS SNI, if the certificate identifies the host 454 name then it does not need to list all the possible service 455 domains. 457 o When the server certificate is replaced it is much easier if there 458 is one part of the DNS that needs updating to match, instead of an 459 unbounded number of hosted service domains. 461 o The same TLSA records work with this specification, and with 462 direct connections to the host name in the style of [RFC6698]. 464 o Some application protocols, such as SMTP, allow a client to 465 perform transactions with multiple service domains in the same 466 connection. It is not in general feasible for the client to 467 specify the service domain using TLS SNI when the connection is 468 established, and the server might not be able to present a 469 certificate that authenticates all possible service domains. 471 o It is common for SMTP servers to act in multiple roles, as 472 outgoing relays or as incoming MX servers, depending on the client 473 identity. It is simpler if the server can present the same 474 certificate regardless of the role in which it is to act. 475 Sometimes the server does not know its role until the client has 476 authenticated, which usually occurs after TLS has been 477 established. 479 This specification does not provide an option to put TLSA records 480 under the service domain because that would add complexity without 481 providing any benefit, and security protocols are best kept simple. 482 As described above, there are real-world cases where authenticating 483 the service domain cannot be made to work, so there would be 484 complicated criteria for when service domain TLSA records might be 485 used and when they cannot. This is all avoided by putting the TLSA 486 records under the server host name. 488 The disadvantage is that clients which do not do DNSSEC validation 489 must, according to [RFC6125] rules, check the server certificate 490 against the service domain, since they have no other way to 491 authenticate the server. This means that Server Name Indication 492 support is necessary for backwards compatibility. 494 Author's Address 496 Tony Finch 497 University of Cambridge Computing Service 498 New Museums Site 499 Pembroke Street 500 Cambridge CB2 3QH 501 ENGLAND 503 Phone: +44 797 040 1426 504 Email: dot@dotat.at 505 URI: http://dotat.at/