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Finch 3 Entities (DANE) University of Cambridge 4 Internet-Draft February 18, 2013 5 Intended status: Standards Track 6 Expires: August 22, 2013 8 Using DNS-Based Authentication of Named Entities (DANE) TLSA records 9 with SRV and MX records. 10 draft-ietf-dane-srv-00 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 22, 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. Using TLSA records with SRV and MX . . . . . . . . . . . . . . 3 60 2.1. MX records . . . . . . . . . . . . . . . . . . . . . . . . 3 61 2.2. SRV query . . . . . . . . . . . . . . . . . . . . . . . . 4 62 2.3. TLSA queries . . . . . . . . . . . . . . . . . . . . . . . 5 63 3. Guidelines for application protocols . . . . . . . . . . . . . 5 64 4. Security considerations . . . . . . . . . . . . . . . . . . . 5 65 4.1. Mixed security status . . . . . . . . . . . . . . . . . . 5 66 4.2. A service domain trusts its servers . . . . . . . . . . . 6 67 4.3. Certificate subject name matching . . . . . . . . . . . . 6 68 4.4. Deliberate omissions . . . . . . . . . . . . . . . . . . . 6 69 5. Internationalization Considerations . . . . . . . . . . . . . 7 70 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 71 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 72 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 73 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 74 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 75 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 8 76 Appendix B. Rationale . . . . . . . . . . . . . . . . . . . . . . 8 77 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 79 1. Introduction 81 The base DANE specification [RFC6698] describes how to use TLSA 82 resource records in the DNS to associate a server's host name with 83 its TLS certificate. The association is secured using DNSSEC. That 84 document "only relates to securely associating certificates for TLS 85 and DTLS with host names" (see the last paragraph of section 1.2 of 86 [RFC6698]). 88 Some application protocols do not use host names directly, but 89 instead use a service domain. The domain's servers are located 90 indirectly via SRV records [RFC2782] (or MX records in the case of 91 SMTP [RFC5321]). When they do not use host names [RFC6698] does not 92 direcly apply to these protocols. 94 This document describes how to use DANE TLSA records with SRV and MX 95 records. To summarize: 97 o We rely on DNSSEC to secure the association between the service 98 domain and the target server host names, i.e. the SRV or MX query. 100 o The TLSA records are located alongside the SRV target host names. 102 o Clients always use TLS when connecting to servers with TLSA 103 records. 105 o The server's certificate is expected to match the server host 106 name, rather than the service domain. 108 Separate documents give the details that are specific to particular 109 application protocols. For examples, see [I-D.ietf-dane-smtp] and 110 [I-D.ietf-dane-mua]. 112 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 113 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 114 memo are to be interpreted as described in [RFC2119]. 116 2. Using TLSA records with SRV and MX 118 2.1. MX records 120 For the purpose of this specification (to avoid cluttering the 121 description with special cases) each MX record ([RFC5321] section 5) 122 is treated as being equivalent to a SRV record [RFC2782] with 123 corresponding fields copied from the MX record and the remaining 124 fields having fixed values as follows: 126 Service smtp 128 Proto tcp 130 Name MX owner name 132 TTL MX TTL 134 Class MX Class 136 Priority MX Priority 138 Weight 0 140 Port 25 142 Target MX Target 144 For example this MX record is treated as if it were the following SRV 145 record: 147 example.com. 86400 IN MX 10 mx.example.net. 149 _smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net. 151 2.2. SRV query 153 When the client makes a SRV query, a successful result can be a 154 (possible chain of CNAME / DNAME aliases referring to a) list of one 155 or more SRV records. 157 For this specification to take effect, all of these DNS RRsets MUST 158 be "secure" according to DNSSSEC validation ([RFC4033] section 5). 159 In the case of a (chain of) aliases, the whole chain MUST be secure 160 as well as the ultimate target. (This corresponds to the AD bit 161 being set in the response(s) - see [RFC4035] section 3.2.3.) 163 If they are not all secure, this protocol has not been fully 164 deployed. The client SHOULD fall back to its non-DNSSEC non-DANE 165 behaviour. (This corresponds to the AD bit being unset.) 167 If any of the responses is "bogus" according to DNSSEC validation the 168 client MUST abort. (This usually corresponds to a "server failure" 169 response.) 171 The client now has an authentic list of server host names with weight 172 and priority values. It performs server ordering and selection using 173 the weight and priority values without regard to the presence or 174 absence of DNSSEC or TLSA records. 176 2.3. TLSA queries 178 This sub-section applies to each server host name individually. 180 The client SHALL construct the TLSA query name as described in 181 [RFC6698] section 3 based on fields from the SRV record: port (from 182 the SRV RDATA), protocol (from the SRV query name), and the TLSA base 183 domain is the SRV target host name. 185 For example this SRV record leads to the following TLSA query: 187 _imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net. 189 _143._tcp.imap.example.net. IN TLSA ? 191 o A secure answer containing one or more TLSA records, in which case 192 the client SHALL proceed as descrbed below. 194 o A bogus answer or other failure, which the client MUST treat as a 195 temporary error. 197 o If there is no TLSA record or its DNSSEC validation state is 198 insecure or indeterminate, this protocol has not been fully 199 deployed. The client SHOULD deliver to this server insecurely 200 (which might be over unauthenticated TLS, as described in the 201 introduction). 203 3. Guidelines for application protocols 205 Separate documents describe how to apply this specification to 206 particular application protocols. If you are writing such as 207 document the following points ought to be covered: 209 o How should the client react to a "bogus" DNSSEC status? 211 4. Security considerations 213 4.1. Mixed security status 215 We do not specify that clients check that all of a service domain's 216 server host names are consistent in whether they have or do not have 217 TLSA records. This is so that partial or incremental deployment does 218 not break the service. Different levels of deployment are likely if 219 a service domain has a third-party fall-back server, for example. 221 The SRV and MX sorting rules are unchanged; in particular they have 222 not been altered in order to prioritize secure servers over insecure 223 servers. If a site wants to be secure it needs to deploy this 224 protocol completely; a partial deployment is not secure and we make 225 no special effort to support it. 227 4.2. A service domain trusts its servers 229 By signing their zone with DNSSEC, service domain operators 230 implicitly instruct their clients to check their server TLSA records. 231 This implies another point in the trust relationship between service 232 domain holders and their server operators. Most of the setup 233 requirements for this protocol fall on the server operator: 234 installing a TLS certificate with the correct name, and publishing a 235 TLSA record under that name. If these are not correct then 236 connections from TLSA-aware clients might fail. 238 4.3. Certificate subject name matching 240 Section 4 of the TLSA specification [RFC6698] leaves the details of 241 checking names in certificates to higher level application protocols, 242 though it suggests the use of [RFC6125]. 244 Name checking might appear to be unnecessary, since DNSSEC provides a 245 secure binding between the server name and the TLSA record, which in 246 turn authenticates the certificate. However this latter step can be 247 indirect, via a chain of certificates. A usage=0 TLSA record only 248 authenticates the CA that issued the certificate, and third parties 249 can obtain certificates from the same CA. 251 So this specification says that clients check that the server's 252 certificate matches the server host name, to ensure that the 253 certificate was issued by the CA to the server that the client is 254 connecting to. The client always performs this check regardless of 255 the TLSA usage, to simplify implementation and so that this 256 specification is less likely to need updating when new TLSA usages 257 are added. 259 4.4. Deliberate omissions 261 We do not specify that clients check the DNSSEC state of the server 262 address records. This is not necessary since the certificate checks 263 ensure that the client has connected to the correct server. (The 264 address records will normally have the same security state as the 265 TLSA records, but they can differ if there are CNAME or DNAME 266 indirections.) 268 5. Internationalization Considerations 270 If any of the DNS queries are for an internationalized domain name, 271 then they need to use the A-label form [RFC5890]. 273 6. IANA Considerations 275 No IANA action is required. 277 7. Acknowledgements 279 Thanks to Mark Andrews for arguing that authenticating the server 280 host name is the right thing, and that we ought to rely on DNSSEC to 281 secure the SRV / MX lookup. Thanks to James Cloos, Ned Freed, Olafur 282 Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas 283 Schneider, and Alessandro Vesely for helpful suggestions. 285 8. References 287 8.1. Normative References 289 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 290 Requirement Levels", BCP 14, RFC 2119, March 1997. 292 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 293 specifying the location of services (DNS SRV)", RFC 2782, 294 February 2000. 296 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 297 Rose, "DNS Security Introduction and Requirements", 298 RFC 4033, March 2005. 300 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 301 Rose, "Protocol Modifications for the DNS Security 302 Extensions", RFC 4035, March 2005. 304 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 305 October 2008. 307 [RFC5890] Klensin, J., "Internationalized Domain Names for 308 Applications (IDNA): Definitions and Document Framework", 309 RFC 5890, August 2010. 311 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 312 Verification of Domain-Based Application Service Identity 313 within Internet Public Key Infrastructure Using X.509 314 (PKIX) Certificates in the Context of Transport Layer 315 Security (TLS)", RFC 6125, March 2011. 317 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 318 of Named Entities (DANE) Transport Layer Security (TLS) 319 Protocol: TLSA", RFC 6698, August 2012. 321 8.2. Informative References 323 [I-D.ietf-dane-smtp] 324 Finch, T., "DNS-Based Authentication of Named Entities 325 (DANE) for secure SMTP", draft-ietf-dane-smtp (work in 326 progress), March 2013. 328 [I-D.ietf-dane-mua] 329 Finch, T., "DNS-Based Authentication of Named Entities 330 (DANE) for POP, IMAP, and message submission", 331 draft-ietf-dane-mua (work in progress), March 2013. 333 Appendix A. Example 335 In the following, most of the DNS resource data is elided for 336 simplicity. 338 ; mail domain 339 example.com. MX 1 mx.example.net. 340 example.com. RRSIG MX ... 342 ; SMTP server host name 343 mx.example.net. A 192.0.2.1 344 mx.example.net. AAAA 2001:db8:212:8::e:1 346 ; TLSA resource record 347 _25._tcp.mx.example.net. TLSA ... 348 _25._tcp.mx.example.net. RRSIG TLSA ... 350 Mail for addresses at example.com is delivered by SMTP to 351 mx.example.net. Connections to mx.example.net port 25 that use 352 STARTTLS will get a server certificate that authenticates the name 353 mx.example.net. 355 Appendix B. Rationale 357 The long-term goal of this specification is to settle on TLS 358 certificates that verify the server host name rather than the service 359 domain, since this is more convenient for servers hosting multiple 360 domains and scales up more easily to larger numbers of service 361 domains. 363 There are a number of other reasons for doing it this way: 365 o The certificate is part of the server configuration, so it makes 366 sense to associate it with the server name rather than the service 367 domain. 369 o In the absence of TLS SNI, if the certificate identifies the host 370 name then it does not need to list all the possible service 371 domains. 373 o When the server certificate is replaced it is much easier if there 374 is one part of the DNS that needs updating to match, instead of an 375 unbounded number of hosted service domains. 377 o The same TLSA records work with this specification, and with 378 direct connections to the host name in the style of [RFC6698]. 380 o Some application protocols, such as SMTP, allow a client to 381 perform transactions with multiple service domains in the same 382 connection. It is not in general feasible for the client to 383 specify the service domain using TLS SNI when the connection is 384 established, and the server might not be able to present a 385 certificate that authenticates all possible service domains. 387 o It is common for SMTP servers to act in multiple roles, as 388 outgoing relays or as incoming MX servers, depending on the client 389 identity. It is simpler if the server can present the same 390 certificate regardless of the role in which it is to act. 391 Sometimes the server does not know its role until the client has 392 authenticated, which usually occurs after TLS has been 393 established. 395 This specification does not provide an option to put TLSA records 396 under the service domain because that would add complexity without 397 providing any benefit, and security protocols are best kept simple. 398 As described above, there are real-world cases where authenticating 399 the service domain cannot be made to work, so there would be 400 complicated criteria for when service domain TLSA records might be 401 used and when they cannot. This is all avoided by puttling the TLSA 402 records under the server host name. 404 The disadvantage is that clients which do not do DNSSEC validation 405 must, according to [RFC6125] rules, check the server certificate 406 against the service domain, since they have no other way to 407 authenticate the server. This means that Server Name Indication 408 support is necessary for backwards compatibility. 410 Author's Address 412 Tony Finch 413 University of Cambridge Computing Service 414 New Museums Site 415 Pembroke Street 416 Cambridge CB2 3QH 417 ENGLAND 419 Phone: +44 797 040 1426 420 Email: dot@dotat.at 421 URI: http://dotat.at/