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Jensen 10 Microsoft 11 31 January 2022 13 Discovery of Designated Resolvers 14 draft-ietf-add-ddr-05 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 a 22 resolver is known. This mechanism is designed to be limited to cases 23 where unencrypted resolvers and their designated resolvers are 24 operated by the same entity or cooperating entities. It can also be 25 used to discover support for encrypted DNS protocols when the name of 26 an encrypted resolver is known. 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 4 August 2022. 56 Copyright Notice 58 Copyright (c) 2022 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 Revised BSD License text as 67 described in Section 4.e of the Trust Legal Provisions and are 68 provided without warranty as described in the Revised 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. Use of Designated Resolvers . . . . . . . . . . . . . . . 6 78 4.2. Verified Discovery . . . . . . . . . . . . . . . . . . . 7 79 4.3. Opportunistic Discovery . . . . . . . . . . . . . . . . . 8 80 5. Discovery Using Resolver Names . . . . . . . . . . . . . . . 8 81 6. Deployment Considerations . . . . . . . . . . . . . . . . . . 9 82 6.1. Caching Forwarders . . . . . . . . . . . . . . . . . . . 9 83 6.2. Certificate Management . . . . . . . . . . . . . . . . . 10 84 6.3. Server Name Handling . . . . . . . . . . . . . . . . . . 10 85 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 86 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 87 8.1. Special Use Domain Name "resolver.arpa" . . . . . . . . . 11 88 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 89 9.1. Normative References . . . . . . . . . . . . . . . . . . 11 90 9.2. Informative References . . . . . . . . . . . . . . . . . 12 91 Appendix A. Rationale for using SVCB records . . . . . . . . . . 13 92 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 94 1. Introduction 96 When DNS clients wish to use encrypted DNS protocols such as DNS- 97 over-TLS (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [RFC8484], they 98 require additional information beyond the IP address of the DNS 99 server, such as the resolver's hostname, non-standard ports, or URL 100 paths. However, common configuration mechanisms only provide the 101 resolver's IP address during configuration. Such mechanisms include 102 network provisioning protocols like DHCP [RFC2132] and IPv6 Router 103 Advertisement (RA) options [RFC8106], as well as manual 104 configuration. 106 This document defines two mechanisms for clients to discover 107 designated resolvers using DNS server Service Binding (SVCB, 108 [I-D.ietf-dnsop-svcb-https]) records: 110 1. When only an IP address of an Unencrypted Resolver is known, the 111 client queries a special use domain name (SUDN) [RFC6761] to 112 discover DNS SVCB records associated with one or more Encrypted 113 Resolvers the Unencrypted Resolver has designated for use when 114 support for DNS encryption is requested (Section 4). 116 2. When the hostname of an Encrypted Resolver is known, the client 117 requests details by sending a query for a DNS SVCB record. This 118 can be used to discover alternate encrypted DNS protocols 119 supported by a known server, or to provide details if a resolver 120 name is provisioned by a network (Section 5). 122 Both of these approaches allow clients to confirm that a discovered 123 Encrypted Resolver is designated by the originally provisioned 124 resolver. "Designated" in this context means that the resolvers are 125 operated by the same entity or cooperating entities; for example, the 126 resolvers are accessible on the same IP address, or there is a 127 certificate that claims ownership over the IP address for the 128 original designating resolver. 130 1.1. Specification of Requirements 132 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 133 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 134 "OPTIONAL" in this document are to be interpreted as described in BCP 135 14 [RFC2119] [RFC8174] when, and only when, they appear in all 136 capitals, as shown here. 138 2. Terminology 140 This document defines the following terms: 142 DDR: Discovery of Designated Resolvers. Refers to the mechanisms 143 defined in this document. 145 Designated Resolver: A resolver, presumably an Encrypted Resolver, 146 designated by another resolver for use in its own place. This 147 designation can be verified with TLS certificates. 149 Encrypted Resolver: A DNS resolver using any encrypted DNS 150 transport. This includes current mechanisms such as DoH and DoT 151 as well as future mechanisms. 153 Unencrypted Resolver: A DNS resolver using TCP or UDP port 53. 155 3. DNS Service Binding Records 157 DNS resolvers can advertise one or more Designated Resolvers that may 158 offer support over encrypted channels and are controlled by the same 159 entity. 161 When a client discovers Designated Resolvers, it learns information 162 such as the supported protocols and ports. This information is 163 provided in ServiceMode Service Binding (SVCB) records for DNS 164 Servers, although AliasMode SVCB records can be used to direct 165 clients to the needed ServiceMode SVCB record per 166 [I-D.ietf-dnsop-svcb-https]. The formatting of these records, 167 including the DNS-unique parameters such as "dohpath", are defined by 168 [I-D.ietf-add-svcb-dns]. 170 The following is an example of an SVCB record describing a DoH server 171 discovered by querying for _dns.example.net: 173 _dns.example.net. 7200 IN SVCB 1 example.net. ( 174 alpn=h2 dohpath=/dns-query{?dns} ) 176 The following is an example of an SVCB record describing a DoT server 177 discovered by querying for _dns.example.net: 179 _dns.example.net 7200 IN SVCB 1 dot.example.net ( 180 alpn=dot port=8530 ) 182 If multiple Designated Resolvers are available, using one or more 183 encrypted DNS protocols, the resolver deployment can indicate a 184 preference using the priority fields in each SVCB record 185 [I-D.ietf-dnsop-svcb-https]. 187 If the client encounters a mandatory parameter in an SVCB record it 188 does not understand, it MUST NOT use that record to discover a 189 Designated Resolver. The client can still use others records in the 190 same response if the client can understand all of their mandatory 191 parameters. This allows future encrypted deployments to 192 simultaneously support protocols even if a given client is not aware 193 of all those protocols. For example, if the Unencrypted Resolver 194 returns three SVCB records, one for DoH, one for DoT, and one for a 195 yet-to-exist protocol, a client which only supports DoH and DoT 196 should be able to use those records while safely ignoring the third 197 record. 199 To avoid name lookup deadlock, Designated Resolvers SHOULD follow the 200 guidance in Section 10 of [RFC8484] regarding the avoidance of DNS- 201 based references that block the completion of the TLS handshake. 203 This document focuses on discovering DoH and DoT Designated 204 Resolvers. Other protocols can also use the format defined by 205 [I-D.ietf-add-svcb-dns]. However, if any protocol does not involve 206 some form of certificate validation, new validation mechanisms will 207 need to be defined to support validating designation as defined in 208 Section 4.2. 210 4. Discovery Using Resolver IP Addresses 212 When a DNS client is configured with an Unencrypted Resolver IP 213 address, it SHOULD query the resolver for SVCB records for 214 "dns://resolver.arpa" before making other queries. Specifically, the 215 client issues a query for _dns.resolver.arpa with the SVCB resource 216 record type (64) [I-D.ietf-dnsop-svcb-https]. 218 Because this query is for an SUDN, which no entity can claim 219 ownership over, the ServiceMode SVCB response MUST NOT use the "." 220 value for the TargetName. Instead, the domain name used for DoT or 221 used to construct the DoH template MUST be provided. 223 The following is an example of an SVCB record describing a DoH server 224 discovered by querying for _dns.resolver.arpa: 226 _dns.resolver.arpa 7200 IN SVCB 1 doh.example.net ( 227 alpn=h2 dohpath=/dns-query{?dns} ) 229 The following is an example of an SVCB record describing a DoT server 230 discovered by querying for _dns.resolver.arpa: 232 _dns.resolver.arpa 7200 IN SVCB 1 dot.example.net ( 233 alpn=dot port=8530 ) 235 If the recursive resolver that receives this query has one or more 236 Designated Resolvers, it will return the corresponding SVCB records. 237 When responding to these special queries for "dns://resolver.arpa", 238 the recursive resolver SHOULD include the A and AAAA records for the 239 name of the Designated Resolver in the Additional Answers section. 240 This will allow the DNS client to make queries over an encrypted 241 connection without waiting to resolve the Encrypted Resolver name per 242 [I-D.ietf-dnsop-svcb-https]. If no A/AAAA records or SVCB IP address 243 hints are included, clients will be forced to delay use of the 244 Encrypted Resolver until an additional DNS lookup for the A and AAAA 245 records can be made to the Unencrypted Resolver (or some other 246 resolver the DNS client has been configured to use). 248 If the recursive resolver that receives this query has no Designated 249 Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa" 250 SUDN. 252 4.1. Use of Designated Resolvers 254 When a client discovers Designated Resolvers from an Unencrypted 255 Resolver IP address, it can choose to use these Designated Resolvers 256 either automatically, or based on some other policy, heuristic, or 257 user choice. 259 This document defines two preferred methods to automatically use 260 Designated Resolvers: 262 * Verified Discovery Section 4.2, for when a TLS certificate can be 263 used to validate the resolver's identity. 265 * Opportunistic Discovery Section 4.3, for when a resolver is 266 accessed using a non-public IP address. 268 A client MAY additionally use a discovered Designated Resolver 269 without either of these methods, based on implementation-specific 270 policy or user input. Details of such policy are out of scope of 271 this document. Clients SHOULD NOT automatically use a Designated 272 Resolver without some sort of validation, such as the two methods 273 defined in this document or a future mechanism. 275 A client MUST NOT use a Designated Resolver designated by one 276 Unencrypted Resolver in place of another Unencrypted Resolver. As 277 these are known only by IP address, this means each unique IP address 278 used for unencrypted DNS requires its own designation discovery. 279 This ensures queries are being sent to a party designated by the 280 resolver originally being used. 282 Generally, clients also SHOULD NOT reuse the Designated Resolver 283 discovered from an Unencrypted Resolver over one network connection 284 in place of the same Unencrypted Resolver on another network 285 connection. Instead, clients SHOULD repeat the discovery process on 286 the other network connection. 288 However, if a given Unencrypted Resolver designates a Designated 289 Resolver that uses a public IP address and can be verified using the 290 mechanism described in Section 4.2, it MAY be used on different 291 network connections so long as the subsequent connections over other 292 networks can also be successfully verified using the mechanism 293 described in Section 4.2. This is a tradeoff between performance (by 294 having no delay in establishing an encrypted DNS connection on the 295 new network) and functionality (if the Unencrypted Resolver intends 296 to designate different Designated Resolvers based on the network from 297 which clients connect). 299 4.2. Verified Discovery 301 Verified Discovery is a mechanism that allows automatic use of a 302 Designated Resolver that supports DNS encryption that performs a TLS 303 handshake. 305 In order to be considered a verified Designated Resolver, the TLS 306 certificate presented by the Designated Resolver MUST contain the IP 307 address of the designating Unencrypted Resolver in a subjectAltName 308 extension. If the certificate can be validated, the client SHOULD 309 use the discovered Designated Resolver for any cases in which it 310 would have otherwise used the Unencrypted Resolver. If the 311 Designated Resolver has a different IP address than the Unencrypted 312 Resolver and the TLS certificate does not cover the Unencrypted 313 Resolver address, the client MUST NOT automatically use the 314 discovered Designated Resolver. Additionally, the client SHOULD 315 suppress any further queries for Designated Resolvers using this 316 Unencrypted Resolver for the length of time indicated by the SVCB 317 record's Time to Live (TTL). 319 If the Designated Resolver and the Unencrypted Resolver share an IP 320 address, clients MAY choose to opportunistically use the Designated 321 Resolver even without this certificate check (Section 4.3). 323 If resolving the name of a Designated Resolver from an SVCB record 324 yields an IP address that was not presented in the Additional Answers 325 section or ipv4hint or ipv6hint fields of the original SVCB query, 326 the connection made to that IP address MUST pass the same TLS 327 certificate checks before being allowed to replace a previously known 328 and validated IP address for the same Designated Resolver name. 330 4.3. Opportunistic Discovery 332 There are situations where Verified Discovery of encrypted DNS 333 configuration over unencrypted DNS is not possible. This includes 334 Unencrypted Resolvers on non-public IP addresses such as those 335 defined in [RFC1918] whose identity cannot be confirmed using TLS 336 certificates. 338 Opportunistic Privacy is defined for DoT in Section 4.1 of [RFC7858] 339 as a mode in which clients do not validate the name of the resolver 340 presented in the certificate. A client MAY use information from the 341 SVCB record for "dns://resolver.arpa" with this "opportunistic" 342 approach (not validating the names presented in the 343 SubjectAlternativeName field of the certificate) as long as the IP 344 address of the Encrypted Resolver does not differ from the IP address 345 of the Unencrypted Resolver. Clients SHOULD use this mode only for 346 resolvers using non-public IP addresses. This approach can be used 347 for any encrypted DNS protocol that uses TLS. 349 5. Discovery Using Resolver Names 351 A DNS client that already knows the name of an Encrypted Resolver can 352 use DDR to discover details about all supported encrypted DNS 353 protocols. This situation can arise if a client has been configured 354 to use a given Encrypted Resolver, or if a network provisioning 355 protocol (such as DHCP or IPv6 Router Advertisements) provides a name 356 for an Encrypted Resolver alongside the resolver IP address. 358 For these cases, the client simply sends a DNS SVCB query using the 359 known name of the resolver. This query can be issued to the named 360 Encrypted Resolver itself or to any other resolver. Unlike the case 361 of bootstrapping from an Unencrypted Resolver (Section 4), these 362 records SHOULD be available in the public DNS. 364 For example, if the client already knows about a DoT server 365 resolver.example.com, it can issue an SVCB query for 366 _dns.resolver.example.com to discover if there are other encrypted 367 DNS protocols available. In the following example, the SVCB answers 368 indicate that resolver.example.com supports both DoH and DoT, and 369 that the DoH server indicates a higher priority than the DoT server. 371 _dns.resolver.example.com. 7200 IN SVCB 1 resolver.example.com. ( 372 alpn=h2 dohpath=/dns-query{?dns} ) 373 _dns.resolver.example.com. 7200 IN SVCB 1 resolver.example.com. ( 374 alpn=dot ) 376 Clients MUST validate that for any Encrypted Resolver discovered 377 using a known resolver name, the TLS certificate of the resolver 378 contains the known name in a subjectAltName extension. In the 379 example above, this means that both servers need to have certificates 380 that cover the name resolver.example.com. Often, the various 381 supported encrypted DNS protocols will be specified such that the 382 SVCB TargetName matches the known name, as is true in the example 383 above. However, even when the TargetName is different (for example, 384 if the DoH server had a TargetName of doh.example.com), the clients 385 still check for the original known resolver name in the certificate. 387 Note that this resolver validation is not related to the DNS resolver 388 that provided the SVCB answer. 390 As another example, being able to discover a Designated Resolver for 391 a known Encrypted Resolver is useful when a client has a DoT 392 configuration for foo.resolver.example.com but is on a network that 393 blocks DoT traffic. The client can still send a query to any other 394 accessible resolver (either the local network resolver or an 395 accessible DoH server) to discover if there is a designated DoH 396 server for foo.resolver.example.com. 398 6. Deployment Considerations 400 Resolver deployments that support DDR are advised to consider the 401 following points. 403 6.1. Caching Forwarders 405 A DNS forwarder SHOULD NOT forward queries for "resolver.arpa" 406 upstream. This prevents a client from receiving an SVCB record that 407 will fail to authenticate because the forwarder's IP address is not 408 in the upstream resolver's Designated Resolver's TLS certificate SAN 409 field. A DNS forwarder which already acts as a completely blind 410 forwarder MAY choose to forward these queries when the operator 411 expects that this does not apply, either because the operator knows 412 the upstream resolver does have the forwarder's IP address in its TLS 413 certificate's SAN field or that the operator expects clients of the 414 unencrypted resolver to use the SVCB information opportunistically. 416 Operators who choose to forward queries for "resolver.arpa" upstream 417 should note that client behavior is never guaranteed and use of DDR 418 by a resolver does not communicate a requirement for clients to use 419 the SVCB record when it cannot be verified. 421 6.2. Certificate Management 423 Resolver owners that support Verified Discovery will need to list 424 valid referring IP addresses in their TLS certificates. This may 425 pose challenges for resolvers with a large number of referring IP 426 addresses. 428 6.3. Server Name Handling 430 Clients MUST NOT use "resolver.arpa" as the server name either in the 431 TLS Server Name Indication (SNI) ([RFC8446]) for DoT or DoH 432 connections, or in the URI host for DoH requests. 434 When performing discovery using resolver IP addresses, clients MUST 435 use the IP address as the URI host for DoH requests. 437 Note that since IP addresses are not supported by default in the TLS 438 SNI, resolvers that support discovery using IP addresses will need to 439 be configured to present the appropriate TLS certificate when no SNI 440 is present for both DoT and DoH. 442 7. Security Considerations 444 Since clients can receive DNS SVCB answers over unencrypted DNS, on- 445 path attackers can prevent successful discovery by dropping SVCB 446 packets. Clients should be aware that it might not be possible to 447 distinguish between resolvers that do not have any Designated 448 Resolver and such an active attack. To limit the impact of discovery 449 queries being dropped either maliciously or unintentionally, clients 450 can re-send their SVCB queries periodically. 452 DoH resolvers that allow discovery using DNS SVCB answers over 453 unencrypted DNS MUST NOT provide differentiated behavior based on the 454 HTTP path alone, since an attacker could modify the "dohpath" 455 parameter. 457 While the IP address of the Unencrypted Resolver is often provisioned 458 over insecure mechanisms, it can also be provisioned securely, such 459 as via manual configuration, a VPN, or on a network with protections 460 like RA guard [RFC6105]. An attacker might try to direct Encrypted 461 DNS traffic to itself by causing the client to think that a 462 discovered Designated Resolver uses a different IP address from the 463 Unencrypted Resolver. Such a Designated Resolver might have a valid 464 certificate, but be operated by an attacker that is trying to observe 465 or modify user queries without the knowledge of the client or 466 network. 468 If the IP address of a Designated Resolver differs from that of an 469 Unencrypted Resolver, clients applying Verified Discovery 470 (Section 4.2) MUST validate that the IP address of the Unencrypted 471 Resolver is covered by the SubjectAlternativeName of the Designated 472 Resolver's TLS certificate. 474 Clients using Opportunistic Discovery (Section 4.3) MUST be limited 475 to cases where the Unencrypted Resolver and Designated Resolver have 476 the same IP address. 478 The constraints on the use of Designated Resolvers specified here 479 apply specifically to the automatic discovery mechanisms defined in 480 this document, which are referred to as Verified Discovery and 481 Opportunistic Discovery. Clients MAY use some other mechanism to 482 verify and use Designated Resolvers discovered using the DNS SVCB 483 record. However, use of such an alternate mechanism needs to take 484 into account the attack scenarios detailed here. 486 8. IANA Considerations 488 8.1. Special Use Domain Name "resolver.arpa" 490 This document calls for the addition of "resolver.arpa" to the 491 Special-Use Domain Names (SUDN) registry established by [RFC6761]. 492 This will allow resolvers to respond to queries directed at 493 themselves rather than a specific domain name. While this document 494 uses "resolver.arpa" to return SVCB records indicating designated 495 encrypted capability, the name is generic enough to allow future 496 reuse for other purposes where the resolver wishes to provide 497 information about itself to the client. 499 The "resolver.arpa" SUDN is similar to "ipv4only.arpa" in that the 500 querying client is not interested in an answer from the authoritative 501 "arpa" name servers. The intent of the SUDN is to allow clients to 502 communicate with the Unencrypted Resolver much like "ipv4only.arpa" 503 allows for client-to-middlebox communication. For more context, see 504 the rationale behind "ipv4only.arpa" in [RFC8880]. 506 9. References 508 9.1. Normative References 510 [I-D.ietf-add-svcb-dns] 511 Schwartz, B., "Service Binding Mapping for DNS Servers", 512 Work in Progress, Internet-Draft, draft-ietf-add-svcb-dns- 513 01, 21 October 2021, 514 . 517 [I-D.ietf-dnsop-svcb-https] 518 Schwartz, B., Bishop, M., and E. Nygren, "Service binding 519 and parameter specification via the DNS (DNS SVCB and 520 HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf- 521 dnsop-svcb-https-08, 12 October 2021, 522 . 525 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. 526 J., and E. Lear, "Address Allocation for Private 527 Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, 528 February 1996, . 530 [RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names", 531 RFC 6761, DOI 10.17487/RFC6761, February 2013, 532 . 534 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 535 and P. Hoffman, "Specification for DNS over Transport 536 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 537 2016, . 539 [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS 540 (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, 541 . 543 9.2. Informative References 545 [I-D.ietf-tls-esni] 546 Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS 547 Encrypted Client Hello", Work in Progress, Internet-Draft, 548 draft-ietf-tls-esni-13, 12 August 2021, 549 . 552 [I-D.schinazi-httpbis-doh-preference-hints] 553 Schinazi, D., Sullivan, N., and J. Kipp, "DoH Preference 554 Hints for HTTP", Work in Progress, Internet-Draft, draft- 555 schinazi-httpbis-doh-preference-hints-02, 13 July 2020, 556 . 559 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 560 Requirement Levels", BCP 14, RFC 2119, 561 DOI 10.17487/RFC2119, March 1997, 562 . 564 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 565 Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, 566 . 568 [RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch, 569 Ed., "Design Choices When Expanding the DNS", RFC 5507, 570 DOI 10.17487/RFC5507, April 2009, 571 . 573 [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J. 574 Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, 575 DOI 10.17487/RFC6105, February 2011, 576 . 578 [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 579 "IPv6 Router Advertisement Options for DNS Configuration", 580 RFC 8106, DOI 10.17487/RFC8106, March 2017, 581 . 583 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 584 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 585 May 2017, . 587 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 588 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 589 . 591 [RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name 592 'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August 593 2020, . 595 Appendix A. Rationale for using SVCB records 597 This mechanism uses SVCB/HTTPS resource records 598 [I-D.ietf-dnsop-svcb-https] to communicate that a given domain 599 designates a particular Designated Resolver for clients to use in 600 place of an Unencrypted Resolver (using a SUDN) or another Encrypted 601 Resolver (using its domain name). 603 There are various other proposals for how to provide similar 604 functionality. There are several reasons that this mechanism has 605 chosen SVCB records: 607 * Discovering encrypted resolver using DNS records keeps client 608 logic for DNS self-contained and allows a DNS resolver operator to 609 define which resolver names and IP addresses are related to one 610 another. 612 * Using DNS records also does not rely on bootstrapping with higher- 613 level application operations (such as 614 [I-D.schinazi-httpbis-doh-preference-hints]). 616 * SVCB records are extensible and allow definition of parameter 617 keys. This makes them a superior mechanism for extensibility as 618 compared to approaches such as overloading TXT records. The same 619 keys can be used for discovering Designated Resolvers of different 620 transport types as well as those advertised by Unencrypted 621 Resolvers or another Encrypted Resolver. 623 * Clients and servers that are interested in privacy of names will 624 already need to support SVCB records in order to use Encrypted TLS 625 Client Hello [I-D.ietf-tls-esni]. Without encrypting names in 626 TLS, the value of encrypting DNS is reduced, so pairing the 627 solutions provides the largest benefit. 629 * Clients that support SVCB will generally send out three queries 630 when accessing web content on a dual-stack network: A, AAAA, and 631 HTTPS queries. Discovering a Designated Resolver as part of one 632 of these queries, without having to add yet another query, 633 minimizes the total number of queries clients send. While 634 [RFC5507] recommends adding new RRTypes for new functionality, 635 SVCB provides an extension mechanism that simplifies client 636 behavior. 638 Authors' Addresses 640 Tommy Pauly 641 Apple Inc. 642 One Apple Park Way 643 Cupertino, California 95014, 644 United States of America 646 Email: tpauly@apple.com 648 Eric Kinnear 649 Apple Inc. 650 One Apple Park Way 651 Cupertino, California 95014, 652 United States of America 654 Email: ekinnear@apple.com 655 Christopher A. Wood 656 Cloudflare 657 101 Townsend St 658 San Francisco, 659 United States of America 661 Email: caw@heapingbits.net 663 Patrick McManus 664 Fastly 666 Email: mcmanus@ducksong.com 668 Tommy Jensen 669 Microsoft 671 Email: tojens@microsoft.com