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Jensen 10 Microsoft 11 14 June 2021 13 Discovery of Designated Resolvers 14 draft-ietf-add-ddr-01 16 Abstract 18 This document defines Discovery of Designated Resolvers (DDR), a 19 mechanism for DNS clients to use DNS records to discover a resolver's 20 encrypted DNS configuration. This mechanism can be used to move from 21 unencrypted DNS to encrypted DNS when only the IP address of an 22 encrypted resolver is known. It can also be used to discover support 23 for encrypted DNS protocols when the name of an encrypted resolver is 24 known. This mechanism is designed to be limited to cases where 25 unencrypted resolvers and their designated resolvers are operated by 26 the same entity or cooperating entities. 28 Discussion Venues 30 This note is to be removed before publishing as an RFC. 32 Discussion of this document takes place on the Adaptive DNS Discovery 33 Working Group mailing list (add@ietf.org), which is archived at 34 https://mailarchive.ietf.org/arch/browse/add/. 36 Source for this draft and an issue tracker can be found at 37 https://github.com/ietf-wg-add/draft-ietf-add-ddr. 39 Status of This Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at https://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on 16 December 2021. 56 Copyright Notice 58 Copyright (c) 2021 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 63 license-info) in effect on the date of publication of this document. 64 Please review these documents carefully, as they describe your rights 65 and restrictions with respect to this document. Code Components 66 extracted from this document must include Simplified BSD License text 67 as described in Section 4.e of the Trust Legal Provisions and are 68 provided without warranty as described in the Simplified BSD License. 70 Table of Contents 72 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 73 1.1. Specification of Requirements . . . . . . . . . . . . . . 3 74 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 75 3. DNS Service Binding Records . . . . . . . . . . . . . . . . . 4 76 4. Discovery Using Resolver IP Addresses . . . . . . . . . . . . 5 77 4.1. Authenticated Discovery . . . . . . . . . . . . . . . . . 5 78 4.2. Opportunistic Discovery . . . . . . . . . . . . . . . . . 6 79 5. Discovery Using Resolver Names . . . . . . . . . . . . . . . 6 80 6. Deployment Considerations . . . . . . . . . . . . . . . . . . 7 81 6.1. Caching Forwarders . . . . . . . . . . . . . . . . . . . 7 82 6.2. Certificate Management . . . . . . . . . . . . . . . . . 8 83 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 84 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 85 8.1. Special Use Domain Name "resolver.arpa" . . . . . . . . . 8 86 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 87 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 88 9.2. Informative References . . . . . . . . . . . . . . . . . 9 89 Appendix A. Rationale for using SVCB records . . . . . . . . . . 10 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 92 1. Introduction 94 When DNS clients wish to use encrypted DNS protocols such as DNS- 95 over-TLS (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [RFC8484], they 96 require additional information beyond the IP address of the DNS 97 server, such as the resolver's hostname, non-standard ports, or URL 98 paths. However, common configuration mechanisms only provide the 99 resolver's IP address during configuration. Such mechanisms include 100 network provisioning protocols like DHCP [RFC2132] and IPv6 Router 101 Advertisement (RA) options [RFC8106], as well as manual 102 configuration. 104 This document defines two mechanisms for clients to discover 105 designated resolvers using DNS server Service Binding (SVCB, 106 [I-D.ietf-dnsop-svcb-https]) records: 108 1. When only an IP address of an Unencrypted Resolver is known, the 109 client queries a special use domain name to discover DNS SVCB 110 records associated with the Unencrypted Resolver (Section 4). 112 2. When the hostname of an encrypted DNS server is known, the client 113 requests details by sending a query for a DNS SVCB record. This 114 can be used to discover alternate encrypted DNS protocols 115 supported by a known server, or to provide details if a resolver 116 name is provisioned by a network (Section 5). 118 Both of these approaches allow clients to confirm that a discovered 119 Encrypted Resolver is designated by the originally provisioned 120 resolver. "Designated" in this context means that the resolvers are 121 operated by the same entity or cooperating entities; for example, the 122 resolvers are accessible on the same IP address, or there is a 123 certificate that claims ownership over both resolvers. 125 1.1. Specification of Requirements 127 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 128 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 129 "OPTIONAL" in this document are to be interpreted as described in BCP 130 14 [RFC2119] [RFC8174] when, and only when, they appear in all 131 capitals, as shown here. 133 2. Terminology 135 This document defines the following terms: 137 DDR: Discovery of Designated Resolvers. Refers to the mechanisms 138 defined in this document. 140 Designated Resolver: A resolver, presumably an Encrypted Resolver, 141 designated by another resolver for use in its own place. This 142 designation can be authenticated with TLS certificates. 144 Encrypted Resolver: A DNS resolver using any encrypted DNS 145 transport. This includes current mechanisms such as DoH and DoT 146 as well as future mechanisms. 148 Unencrypted Resolver: A DNS resolver using TCP or UDP port 53. 150 3. DNS Service Binding Records 152 DNS resolvers can advertise one or more Designated Resolvers that may 153 offer support over encrypted channels and are controlled by the same 154 entity. 156 When a client discovers Designated Resolvers, it learns information 157 such as the supported protocols, ports, and server name to use in 158 certificate validation. This information is provided in Service 159 Binding (SVCB) records for DNS Servers, defined by 160 [I-D.schwartz-svcb-dns]. 162 The following is an example of an SVCB record describing a DoH 163 server: 165 _dns.example.net 7200 IN SVCB 1 . ( 166 alpn=h2 dohpath=/dns-query{?dns} ) 168 The following is an example of an SVCB record describing a DoT 169 server: 171 _dns.example.net 7200 IN SVCB 1 dot.example.net ( 172 alpn=dot port=8530 ) 174 If multiple Designated Resolvers are available, using one or more 175 encrypted DNS protocols, the resolver deployment can indicate a 176 preference using the priority fields in each SVCB record 177 [I-D.ietf-dnsop-svcb-https]. 179 This document focuses on discovering DoH and DoT Designated 180 Resolvers. Other protocols can also use the format defined by 181 [I-D.schwartz-svcb-dns]. However, if any protocol does not involve 182 some form of certificate validation, new validation mechanisms will 183 need to be defined to support validating designation as defined in 184 Section 4.1. 186 4. Discovery Using Resolver IP Addresses 188 When a DNS client is configured with an Unencrypted Resolver IP 189 address, it SHOULD query the resolver for SVCB records for 190 "dns://resolver.arpa" before making other queries. Specifically, the 191 client issues a query for "_dns.resolver.arpa" with the SVCB resource 192 record type (64) [I-D.ietf-dnsop-svcb-https]. 194 If the recursive resolver that receives this query has one or more 195 Designated Resolvers, it will return the corresponding SVCB records. 196 When responding to these special queries for "dns://resolver.arpa", 197 the recursive resolver SHOULD include the A and AAAA records for the 198 name of the Designated Resolver in the Additional Answers section. 199 This will allow the DNS client to make queries over an encrypted 200 connection without waiting to resolve the Encrypted Resolver name per 201 [I-D.ietf-dnsop-svcb-https]. If no A/AAAA records or SVCB IP address 202 hints are included, clients will be forced to delay use of the 203 Encrypted Resolver until an additional DNS lookup for the A and AAAA 204 records can be made to the Unencrypted Resolver (or some other 205 resolver the DNS client has been configured to use). 207 If the recursive resolver that receives this query has no Designated 208 Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa" 209 SUDN. 211 4.1. Authenticated Discovery 213 In order to be considered an authenticated Designated Resolver, the 214 TLS certificate presented by the Encrypted Resolver MUST contain both 215 the domain name (from the SVCB answer) and the IP address of the 216 designating Unencrypted Resolver within the SubjectAlternativeName 217 certificate field. The client MUST check the SubjectAlternativeName 218 field for both the Unencrypted Resolver's IP address and the 219 advertised name of the Designated Resolver. If the certificate can 220 be validated, the client SHOULD use the discovered Designated 221 Resolver for any cases in which it would have otherwise used the 222 Unencrypted Resolver. If the Designated Resolver has a different IP 223 address than the Unencrypted Resolver and the TLS certificate does 224 not cover the Unencrypted Resolver address, the client MUST NOT use 225 the discovered Encrypted Resolver. Additionally, the client SHOULD 226 suppress any further queries for Designated Resolvers using this 227 Unencrypted Resolver for the length of time indicated by the SVCB 228 record's Time to Live (TTL). 230 If the Designated Resolver and the Unencrypted Resolver share an IP 231 address, clients MAY choose to opportunistically use the Encrypted 232 Resolver even without this certificate check (Section 4.2). 234 If resolving the name of an Encrypted Resolver from an SVCB record 235 yields an IP address that was not presented in the Additional Answers 236 section or ipv4hint or ipv6hint fields of the original SVCB query, 237 the connection made to that IP address MUST pass the same TLS 238 certificate checks before being allowed to replace a previously known 239 and validated IP address for the same Encrypted Resolver name. 241 4.2. Opportunistic Discovery 243 There are situations where authenticated discovery of encrypted DNS 244 configuration over unencrypted DNS is not possible. This includes 245 Unencrypted Resolvers on non-public IP addresses such as those 246 defined in [RFC1918] whose identity cannot be confirmed using TLS 247 certificates. 249 Opportunistic Privacy is defined for DoT in Section 4.1 of [RFC7858] 250 as a mode in which clients do not validate the name of the resolver 251 presented in the certificate. A client MAY use information from the 252 SVCB record for "dns://resolver.arpa" with this "opportunistic" 253 approach (not validating the names presented in the 254 SubjectAlternativeName field of the certificate) as long as the IP 255 address of the Encrypted Resolver does not differ from the IP address 256 of the Unencrypted Resolver. This approach can be used for any 257 encrypted DNS protocol that uses TLS. 259 5. Discovery Using Resolver Names 261 A DNS client that already knows the name of an Encrypted Resolver can 262 use DDR to discover details about all supported encrypted DNS 263 protocols. This situation can arise if a client has been configured 264 to use a given Encrypted Resolver, or if a network provisioning 265 protocol (such as DHCP or IPv6 Router Advertisements) provides a name 266 for an Encrypted Resolver alongside the resolver IP address. 268 For these cases, the client simply sends a DNS SVCB query using the 269 known name of the resolver. This query can be issued to the named 270 Encrypted Resolver itself or to any other resolver. Unlike the case 271 of bootstrapping from an Unencrypted Resolver (Section 4), these 272 records SHOULD be available in the public DNS. 274 For example, if the client already knows about a DoT server 275 "resolver.example.com", it can issue an SVCB query for 276 "_dns.resolver.example.com" to discover if there are other encrypted 277 DNS protocols available. In the following example, the SVCB answers 278 indicate that "resolver.example.com" supports both DoH and DoT, and 279 that the DoH server indicates a higher priority than the DoT server. 281 _dns.resolver.example.com 7200 IN SVCB 1 . ( 282 alpn=h2 dohpath=/dns-query{?dns} ) 283 _dns.resolver.example.com 7200 IN SVCB 2 . ( 284 alpn=dot ) 286 Often, the various supported encrypted DNS protocols will be 287 accessible using the same hostname. In the example above, both DoH 288 and DoT use the name "resolver.example.com" for their TLS 289 certificates. If a deployment uses a different hostname for one 290 protocol, but still wants clients to treat both DNS servers as 291 designated, the TLS certificates MUST include both names in the 292 SubjectAlternativeName fields. Note that this name verification is 293 not related to the DNS resolver that provided the SVCB answer. 295 For example, being able to discover a Designated Resolver for a known 296 Encrypted Resolver is useful when a client has a DoT configuration 297 for "foo.resolver.example.com" but is on a network that blocks DoT 298 traffic. The client can still send a query to any other accessible 299 resolver (either the local network resolver or an accessible DoH 300 server) to discover if there is a designated DoH server for 301 "foo.resolver.example.com". 303 6. Deployment Considerations 305 Resolver deployments that support DDR are advised to consider the 306 following points. 308 6.1. Caching Forwarders 310 A DNS forwarder SHOULD NOT forward queries for "resolver.arpa" 311 upstream. This prevents a client from receiving an SVCB record that 312 will fail to authenticate because the forwarder's IP address is not 313 in the upstream resolver's Designated Resolver's TLS certificate SAN 314 field. A DNS forwarder which already acts as a completely blind 315 forwarder MAY choose to forward these queries when the operator 316 expects that this does not apply, either because the operator knows 317 the upstream resolver does have the forwarder's IP address in its TLS 318 certificate's SAN field or that the operator expects clients of the 319 unencrypted resolver to use the SVCB information opportunistically. 321 Operators who choose to forward queries for "resolver.arpa" upstream 322 should note that client behavior is never guaranteed and use of DDR 323 by a resolver does not communicate a requirement for clients to use 324 the SVCB record when it cannot be authenticated. 326 6.2. Certificate Management 328 Resolver owners that support authenticated discovery will need to 329 list valid referring IP addresses in their TLS certificates. This 330 may pose challenges for resolvers with a large number of referring IP 331 addresses. 333 7. Security Considerations 335 Since client can receive DNS SVCB answers over unencrypted DNS, on- 336 path attackers can prevent successful discovery by dropping SVCB 337 packets. Clients should be aware that it might not be possible to 338 distinguish between resolvers that do not have any Designated 339 Resolver and such an active attack. 341 While the IP address of the Unencrypted Resolver is often provisioned 342 over insecure mechanisms, it can also be provisioned securely, such 343 as via manual configuration, a VPN, or on a network with protections 344 like RA guard [RFC6105]. An attacker might try to direct Encrypted 345 DNS traffic to itself by causing the client to think that a 346 discovered Designated Resolver uses a different IP address from the 347 Unencrypted Resolver. Such an Encrypted Resolver might have a valid 348 certificate, but be operated by an attacker that is trying to observe 349 or modify user queries without the knowledge of the client or 350 network. 352 If the IP address of a Designated Resolver differs from that of an 353 Unencrypted Resolver, clients MUST validate that the IP address of 354 the Unencrypted Resolver is covered by the SubjectAlternativeName of 355 the Encrypted Resolver's TLS certificate (Section 4.1). 357 Opportunistic use of Encrypted Resolvers MUST be limited to cases 358 where the Unencrypted Resolver and Designated Resolver have the same 359 IP address (Section 4.2). 361 8. IANA Considerations 363 8.1. Special Use Domain Name "resolver.arpa" 365 This document calls for the creation of the "resolver.arpa" SUDN. 366 This will allow resolvers to respond to queries directed at 367 themselves rather than a specific domain name. While this document 368 uses "resolver.arpa" to return SVCB records indicating designated 369 encrypted capability, the name is generic enough to allow future 370 reuse for other purposes where the resolver wishes to provide 371 information about itself to the client. 373 The "resolver.arpa" SUDN is similar to "ipv4only.arpa" in that the 374 querying client is not interested in an answer from the authoritative 375 "arpa" name servers. The intent of the SUDN is to allow clients to 376 communicate with the Unencrypted Resolver much like "ipv4only.arpa" 377 allows for client-to-middlebox communication. For more context, see 378 the rationale behind "ipv4only.arpa" in [RFC8880]. 380 9. References 382 9.1. Normative References 384 [I-D.ietf-dnsop-svcb-https] 385 Schwartz, B., Bishop, M., and E. Nygren, "Service binding 386 and parameter specification via the DNS (DNS SVCB and 387 HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf- 388 dnsop-svcb-https-05, 21 April 2021, 389 . 392 [I-D.ietf-tls-esni] 393 Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS 394 Encrypted Client Hello", Work in Progress, Internet-Draft, 395 draft-ietf-tls-esni-10, 8 March 2021, 396 . 399 [I-D.schwartz-svcb-dns] 400 Schwartz, B., "Service Binding Mapping for DNS Servers", 401 Work in Progress, Internet-Draft, draft-schwartz-svcb-dns- 402 03, 19 April 2021, . 405 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. 406 J., and E. Lear, "Address Allocation for Private 407 Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, 408 February 1996, . 410 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 411 and P. Hoffman, "Specification for DNS over Transport 412 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 413 2016, . 415 [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS 416 (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, 417 . 419 9.2. Informative References 421 [I-D.schinazi-httpbis-doh-preference-hints] 422 Schinazi, D., Sullivan, N., and J. Kipp, "DoH Preference 423 Hints for HTTP", Work in Progress, Internet-Draft, draft- 424 schinazi-httpbis-doh-preference-hints-02, 13 July 2020, 425 . 428 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 429 Requirement Levels", BCP 14, RFC 2119, 430 DOI 10.17487/RFC2119, March 1997, 431 . 433 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 434 Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, 435 . 437 [RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch, 438 Ed., "Design Choices When Expanding the DNS", RFC 5507, 439 DOI 10.17487/RFC5507, April 2009, 440 . 442 [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J. 443 Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, 444 DOI 10.17487/RFC6105, February 2011, 445 . 447 [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 448 "IPv6 Router Advertisement Options for DNS Configuration", 449 RFC 8106, DOI 10.17487/RFC8106, March 2017, 450 . 452 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 453 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 454 May 2017, . 456 [RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name 457 'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August 458 2020, . 460 Appendix A. Rationale for using SVCB records 462 This mechanism uses SVCB/HTTPS resource records 463 [I-D.ietf-dnsop-svcb-https] to communicate that a given domain 464 designates a particular Designated Resolver for clients to use in 465 place of an Unencrypted Resolver (using a SUDN) or another Encrypted 466 Resolver (using its domain name). 468 There are various other proposals for how to provide similar 469 functionality. There are several reasons that this mechanism has 470 chosen SVCB records: 472 * Discovering encrypted resolver using DNS records keeps client 473 logic for DNS self-contained and allows a DNS resolver operator to 474 define which resolver names and IP addresses are related to one 475 another. 477 * Using DNS records also does not rely on bootstrapping with higher- 478 level application operations (such as 479 [I-D.schinazi-httpbis-doh-preference-hints]). 481 * SVCB records are extensible and allow definition of parameter 482 keys. This makes them a superior mechanism for extensibility as 483 compared to approaches such as overloading TXT records. The same 484 keys can be used for discovering Designated Resolvers of different 485 transport types as well as those advertised by Unencrypted 486 Resolvers or another Encrypted Resolver. 488 * Clients and servers that are interested in privacy of names will 489 already need to support SVCB records in order to use Encrypted TLS 490 Client Hello [I-D.ietf-tls-esni]. Without encrypting names in 491 TLS, the value of encrypting DNS is reduced, so pairing the 492 solutions provides the largest benefit. 494 * Clients that support SVCB will generally send out three queries 495 when accessing web content on a dual-stack network: A, AAAA, and 496 HTTPS queries. Discovering a Designated Resolver as part of one 497 of these queries, without having to add yet another query, 498 minimizes the total number of queries clients send. While 499 [RFC5507] recommends adding new RRTypes for new functionality, 500 SVCB provides an extension mechanism that simplifies client 501 behavior. 503 Authors' Addresses 505 Tommy Pauly 506 Apple Inc. 507 One Apple Park Way 508 Cupertino, California 95014, 509 United States of America 511 Email: tpauly@apple.com 512 Eric Kinnear 513 Apple Inc. 514 One Apple Park Way 515 Cupertino, California 95014, 516 United States of America 518 Email: ekinnear@apple.com 520 Christopher A. Wood 521 Cloudflare 522 101 Townsend St 523 San Francisco, 524 United States of America 526 Email: caw@heapingbits.net 528 Patrick McManus 529 Fastly 531 Email: mcmanus@ducksong.com 533 Tommy Jensen 534 Microsoft 536 Email: tojens@microsoft.com