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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 7230 (Obsoleted by RFC 9110, RFC 9112) ** Obsolete normative reference: RFC 7231 (Obsoleted by RFC 9110) ** Obsolete normative reference: RFC 7234 (Obsoleted by RFC 9111) ** Obsolete normative reference: RFC 7540 (Obsoleted by RFC 9113) -- Obsolete informational reference (is this intentional?): RFC 7719 (Obsoleted by RFC 8499) Summary: 5 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Hoffman 3 Internet-Draft ICANN 4 Intended status: Standards Track P. McManus 5 Expires: October 13, 2018 Mozilla 6 April 11, 2018 8 DNS Queries over HTTPS 9 draft-ietf-doh-dns-over-https-07 11 Abstract 13 This document describes how to run DNS service over HTTP (DOH) using 14 https:// URIs. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at https://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on October 13, 2018. 33 Copyright Notice 35 Copyright (c) 2018 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (https://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 52 3. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 3 53 3.1. Non-requirements . . . . . . . . . . . . . . . . . . . . 4 54 4. The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . . 4 55 4.1. The HTTP Request . . . . . . . . . . . . . . . . . . . . 4 56 4.1.1. HTTP Request Examples . . . . . . . . . . . . . . . . 5 57 4.2. The HTTP Response . . . . . . . . . . . . . . . . . . . . 6 58 4.2.1. HTTP Response Example . . . . . . . . . . . . . . . . 7 59 5. HTTP Integration . . . . . . . . . . . . . . . . . . . . . . 7 60 5.1. Cache Interaction . . . . . . . . . . . . . . . . . . . . 7 61 5.2. HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . . . 9 62 5.3. Server Push . . . . . . . . . . . . . . . . . . . . . . . 9 63 5.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 9 64 6. DNS Wire Format . . . . . . . . . . . . . . . . . . . . . . . 9 65 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 66 7.1. Registration of application/dns-message Media Type . . . 10 67 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 68 9. Operational Considerations . . . . . . . . . . . . . . . . . 13 69 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 70 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 71 11.1. Normative References . . . . . . . . . . . . . . . . . . 14 72 11.2. Informative References . . . . . . . . . . . . . . . . . 15 73 Appendix A. Previous Work on DNS over HTTP or in Other Formats . 16 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 76 1. Introduction 78 This document defines a specific protocol for sending DNS [RFC1035] 79 queries and getting DNS responses over HTTP [RFC7540] using https:// 80 (and therefore TLS [RFC5246] security for integrity and 81 confidentiality). Each DNS query-response pair is mapped into a HTTP 82 exchange. 84 The described approach is more than a tunnel over HTTP. It 85 establishes default media formatting types for requests and responses 86 but uses normal HTTP content negotiation mechanisms for selecting 87 alternatives that endpoints may prefer in anticipation of serving new 88 use cases. In addition to this media type negotiation, it aligns 89 itself with HTTP features such as caching, redirection, proxying, 90 authentication, and compression. 92 The integration with HTTP provides a transport suitable for both 93 traditional DNS clients and native web applications seeking access to 94 the DNS. 96 Two primary uses cases were considered during this protocol's 97 development. They included preventing on-path devices from 98 interfering with DNS operations and allowing web applications to 99 access DNS information via existing browser APIs in a safe way 100 consistent with Cross Origin Resource Sharing (CORS) [CORS]. There 101 are certainly other uses for this work. 103 2. Terminology 105 A server that supports this protocol on one or more URIs is called a 106 "DNS API server" to differentiate it from a "DNS server" (one that 107 uses the regular DNS protocol). Similarly, a client that supports 108 this protocol is called a "DNS API client". 110 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 111 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 112 "OPTIONAL" in this document are to be interpreted as described in BCP 113 14, RFC8174 [RFC8174] when, and only when, they appear in all 114 capitals, as shown here. 116 3. Protocol Requirements 118 The protocol described here bases its design on the following 119 protocol requirements: 121 o The protocol must use normal HTTP semantics. 123 o The queries and responses must be able to be flexible enough to 124 express every DNS query that would normally be sent in DNS over 125 UDP (including queries and responses that use DNS extensions, but 126 not those that require multiple responses). 128 o The protocol must permit the addition of new formats for DNS 129 queries and responses. 131 o The protocol must ensure interoperability by specifying a single 132 format for requests and responses that is mandatory to implement. 133 That format must be able to support future modifications to the 134 DNS protocol including the inclusion of one or more EDNS options 135 (including those not yet defined). 137 o The protocol must use a secure transport that meets the 138 requirements for HTTPS. 140 3.1. Non-requirements 142 o Supporting network-specific DNS64 [RFC6147] 144 o Supporting other network-specific inferences from plaintext DNS 145 queries 147 o Supporting insecure HTTP 149 4. The HTTP Exchange 151 4.1. The HTTP Request 153 A DNS API client encodes a single DNS query into an HTTP request 154 using either the HTTP GET or POST method and the other requirements 155 of this section. The DNS API server defines the URI used by the 156 request through the use of a URI Template [RFC6570]. Configuration 157 and discovery of the URI Template is done out of band from this 158 protocol. 160 The URI Template defined in this document is processed without any 161 variables when the HTTP method is POST. When the HTTP method is GET 162 the single variable "dns" is defined as the content of the DNS 163 request (as described in Section 6), encoded with base64url 164 [RFC4648]. 166 Future specifications for new media types MUST define the variables 167 used for URI Template processing with this protocol. 169 DNS API servers MUST implement both the POST and GET methods. 171 When using the POST method the DNS query is included as the message 172 body of the HTTP request and the Content-Type request header 173 indicates the media type of the message. POST-ed requests are 174 smaller than their GET equivalents. 176 Using the GET method is friendlier to many HTTP cache 177 implementations. 179 The DNS API client SHOULD include an HTTP "Accept" request header to 180 indicate what type of content can be understood in response. 181 Irrespective of the value of the Accept request header, the client 182 MUST be prepared to process "application/dns-message" (as described 183 in Section 6) responses but MAY also process any other type it 184 receives. 186 In order to maximize cache friendliness, DNS API clients using media 187 formats that include DNS ID, such as application/dns-message, SHOULD 188 use a DNS ID of 0 in every DNS request. HTTP correlates the request 189 and response, thus eliminating the need for the ID in a media type 190 such as application/dns-message. The use of a varying DNS ID can 191 cause semantically equivalent DNS queries to be cached separately. 193 DNS API clients can use HTTP/2 padding and compression in the same 194 way that other HTTP/2 clients use (or don't use) them. 196 4.1.1. HTTP Request Examples 198 These examples use HTTP/2 style formatting from [RFC7540]. 200 These examples use a DNS API service with a URI Template of 201 "https://dnsserver.example.net/dns-query{?dns}" to resolve IN A 202 records. 204 The requests are represented as application/dns-message typed bodies. 206 The first example request uses GET to request www.example.com 208 :method = GET 209 :scheme = https 210 :authority = dnsserver.example.net 211 :path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB 212 accept = application/dns-message 214 The same DNS query for www.example.com, using the POST method would 215 be: 217 :method = POST 218 :scheme = https 219 :authority = dnsserver.example.net 220 :path = /dns-query 221 accept = application/dns-message 222 content-type = application/dns-message 223 content-length = 33 225 <33 bytes represented by the following hex encoding> 226 00 00 01 00 00 01 00 00 00 00 00 00 03 77 77 77 227 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 228 01 230 Finally, a GET based query for a.62characterlabel-makes-base64url- 231 distinct-from-standard-base64.example.com is shown as an example to 232 emphasize that the encoding alphabet of base64url is different than 233 regular base64 and that padding is omitted. 235 The DNS query is 94 bytes represented by the following hex encoding 236 00 00 01 00 00 01 00 00 00 00 00 00 01 61 3e 36 237 32 63 68 61 72 61 63 74 65 72 6c 61 62 65 6c 2d 238 6d 61 6b 65 73 2d 62 61 73 65 36 34 75 72 6c 2d 239 64 69 73 74 69 6e 63 74 2d 66 72 6f 6d 2d 73 74 240 61 6e 64 61 72 64 2d 62 61 73 65 36 34 07 65 78 241 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 01 243 :method = GET 244 :scheme = https 245 :authority = dnsserver.example.net 246 :path = /dns-query? (no space or CR) 247 dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR) 248 bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR) 249 dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ 250 accept = application/dns-message 252 4.2. The HTTP Response 254 An HTTP response with a 2xx status code ([RFC7231] Section 6.3) 255 indicates a valid DNS response to the query made in the HTTP request. 256 A valid DNS response includes both success and failure responses. 257 For example, a DNS failure response such as SERVFAIL or NXDOMAIN will 258 be the message in a successful 2xx HTTP response even though there 259 was a failure at the DNS layer. Responses with non-successful HTTP 260 status codes do not contain DNS answers to the question in the 261 corresponding request. Some of these non-successful HTTP responses 262 (e.g., redirects or authentication failures) could allow clients to 263 make new requests to satisfy the original question. 265 Different response media types will provide more or less information 266 from a DNS response. For example, one response type might include 267 the information from the DNS header bytes while another might omit 268 it. The amount and type of information that a media type gives is 269 solely up to the format, and not defined in this protocol. 271 At the time this is published, the response types are works in 272 progress. The only response type defined in this document is 273 "application/dns-message", but it is possible that other response 274 formats will be defined in the future. 276 The DNS response for "application/dns-message" in Section 6 MAY have 277 one or more EDNS options, depending on the extension definition of 278 the extensions given in the DNS request. 280 Each DNS request-response pair is matched to one HTTP exchange. The 281 responses may be processed and transported in any order using HTTP's 282 multi-streaming functionality ([RFC7540] Section 5). 284 Section 5.1 discusses the relationship between DNS and HTTP response 285 caching. 287 A DNS API server MUST be able to process application/dns-message 288 request messages. 290 A DNS API server SHOULD respond with HTTP status code 415 291 (Unsupported Media Type) upon receiving a media type it is unable to 292 process. 294 4.2.1. HTTP Response Example 296 This is an example response for a query for the IN A records for 297 "www.example.com" with recursion turned on. The response bears one 298 record with an address of 192.0.2.1 and a TTL of 128 seconds. 300 :status = 200 301 content-type = application/dns-message 302 content-length = 64 303 cache-control = max-age=128 305 <64 bytes represented by the following hex encoding> 306 00 00 81 80 00 01 00 01 00 00 00 00 03 77 77 77 307 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 00 01 00 308 01 03 77 77 77 07 65 78 61 6d 70 6c 65 03 63 6f 309 6d 00 00 01 00 01 00 00 00 80 00 04 C0 00 02 01 311 5. HTTP Integration 313 This protocol MUST be used with the https scheme URI [RFC7230]. 315 5.1. Cache Interaction 317 A DNS API client may utilize a hierarchy of caches that include both 318 HTTP and DNS specific caches. HTTP cache entries may be bypassed 319 with HTTP mechanisms such as the "Cache-Control no-cache" directive; 320 however DNS caches do not have a similar mechanism. 322 The Answer section of a DNS response can contain zero or more RRsets. 323 (RRsets are defined in [RFC7719].) According to [RFC2181], each 324 resource record in an RRset has Time To Live (TTL) freshness 325 information. Different RRsets in the Answer section can have 326 different TTLs, although it is only possible for the HTTP response to 327 have a single freshness lifetime. The HTTP response freshness 328 lifetime ([RFC7234] Section 4.2) should be coordinated with the RRset 329 with the smallest TTL in the Answer section of the response. 330 Specifically, the HTTP freshness lifetime SHOULD be set to expire at 331 the same time any of the DNS resource records in the Answer section 332 reach a 0 TTL. The response freshness lifetime MUST NOT be greater 333 than that indicated by the DNS resoruce record with the smallest TTL 334 in the response. 336 If the DNS response has no records in the Answer section, and the DNS 337 response has an SOA record in the Authority section, the response 338 freshness lifetime MUST NOT be greater than the MINIMUM field from 339 that SOA record. (See [RFC2308].) Otherwise, the HTTP response MUST 340 set a freshness lifetime ([RFC7234] Section 4.2) of 0 by using a 341 mechanism such as "Cache-Control: no-cache" ([RFC7234] 342 Section 5.2.1.4). 344 A DNS API client that receives a response without an explicit 345 freshness lifetime MUST NOT assign that response a heuristic 346 freshness ([RFC7234] Section 4.2.2.) greater than that indicated by 347 the DNS Record with the smallest TTL in the response. 349 A DOH response that was previously stored in an HTTP cache will 350 contain the [RFC7234] Age response header indicating the elapsed time 351 between when the entry was placed in the HTTP cache and the current 352 DOH response. DNS API clients should subtract this time from the DNS 353 TTL if they are re-sharing the information in a non HTTP context 354 (e.g., their own DNS cache) to determine the remaining time to live 355 of the DNS record. 357 HTTP revalidation (e.g., via If-None-Match request headers) of cached 358 DNS information may be of limited value to DOH as revalidation 359 provides only a bandwidth benefit and DNS transactions are normally 360 latency bound. Furthermore, the HTTP response headers that enable 361 revalidation (such as "Last-Modified" and "Etag") are often fairly 362 large when compared to the overall DNS response size, and have a 363 variable nature that creates constant pressure on the HTTP/2 364 compression dictionary [RFC7541]. Other types of DNS data, such as 365 zone transfers, may be larger and benefit more from revalidation. 366 DNS API servers may wish to consider whether providing these 367 validation enabling response headers is worthwhile. 369 The stale-while-revalidate and stale-if-error cache control 370 directives may be well suited to a DOH implementation when allowed by 371 server policy. Those mechanisms allow a client, at the server's 372 discretion, to reuse a cache entry that is no longer fresh under some 373 extenuating circumstances defined in [RFC5861]. 375 All HTTP servers, including DNS API servers, need to consider cache 376 interaction when they generate responses that are not globally valid. 377 For instance, if a DNS API server customized a response based on the 378 client's identity then it would not want to globally allow reuse of 379 that response. This could be accomplished through a variety of HTTP 380 techniques such as a Cache-Control max-age of 0, or perhaps by the 381 Vary response header. 383 5.2. HTTP/2 385 The minimum version of HTTP used by DOH SHOULD be HTTP/2 [RFC7540]. 387 The messages in classic UDP based DNS [RFC1035] are inherently 388 unordered and have low overhead. A competitive HTTP transport needs 389 to support reordering, parallelism, priority, and header compression 390 to achieve similar performance. Those features were introduced to 391 HTTP in HTTP/2 [RFC7540]. Earlier versions of HTTP are capable of 392 conveying the semantic requirements of DOH but may result in very 393 poor performance. 395 5.3. Server Push 397 Before using DOH response data for DNS resolution, the client MUST 398 establish that the HTTP request URI is a trusted service for the DOH 399 query. For HTTP requests initiated by the DNS API client this trust 400 is implicit in the selection of URI. For HTTP server push ([RFC7540] 401 Section 8.2) extra care must be taken to ensure that the pushed URI 402 is one that the client would have directed the same query to if the 403 client had initiated the request. This specification does not extend 404 DNS resolution privileges to URIs that are not recognized by the 405 client as trusted DNS API servers. 407 5.4. Content Negotiation 409 In order to maximize interoperability, DNS API clients and DNS API 410 servers MUST support the "application/dns-message" media type. Other 411 media types MAY be used as defined by HTTP Content Negotiation 412 ([RFC7231] Section 3.4). 414 6. DNS Wire Format 416 The data payload is the DNS on-the-wire format defined in [RFC1035]. 417 The format is for DNS over UDP. Note that this is different than the 418 wire format used in [RFC7858]. Also note that while [RFC1035] says 419 "Messages carried by UDP are restricted to 512 bytes", that was later 420 updated by [RFC6891], and this protocol allows DNS on-the-wire format 421 payloads of any size. 423 When using the GET method, the data payload MUST be encoded with 424 base64url [RFC4648] and then provided as a variable named "dns" to 425 the URI Template expansion. Padding characters for base64url MUST 426 NOT be included. 428 When using the POST method, the data payload MUST NOT be encoded and 429 is used directly as the HTTP message body. 431 DNS API clients using the DNS wire format MAY have one or more EDNS 432 options [RFC6891] in the request. 434 The media type is "application/dns-message". 436 7. IANA Considerations 438 7.1. Registration of application/dns-message Media Type 439 To: ietf-types@iana.org 440 Subject: Registration of MIME media type 441 application/dns-message 443 MIME media type name: application 445 MIME subtype name: dns-message 447 Required parameters: n/a 449 Optional parameters: n/a 451 Encoding considerations: This is a binary format. The contents are a 452 DNS message as defined in RFC 1035. The format used here is for DNS 453 over UDP, which is the format defined in the diagrams in RFC 1035. 455 Security considerations: The security considerations for carrying 456 this data are the same for carrying DNS without encryption. 458 Interoperability considerations: None. 460 Published specification: This document. 462 Applications that use this media type: 463 Systems that want to exchange full DNS messages. 465 Additional information: 467 Magic number(s): n/a 469 File extension(s): n/a 471 Macintosh file type code(s): n/a 473 Person & email address to contact for further information: 474 Paul Hoffman, paul.hoffman@icann.org 476 Intended usage: COMMON 478 Restrictions on usage: n/a 480 Author: Paul Hoffman, paul.hoffman@icann.org 482 Change controller: IESG 484 8. Security Considerations 486 Running DNS over HTTPS relies on the security of the underlying HTTP 487 transport. This mitigates classic amplification attacks for UDP- 488 based DNS. Implementations utilizing HTTP/2 benefit from the TLS 489 profile defined in [RFC7540] Section 9.2. 491 Session level encryption has well known weaknesses with respect to 492 traffic analysis which might be particularly acute when dealing with 493 DNS queries. HTTP/2 provides further advice about the use of 494 compression (Section 10.6 of [RFC7540]) and padding (Section 10.7 of 495 [RFC7540]). 497 The HTTPS connection provides transport security for the interaction 498 between the DNS API server and client, but does not inherently ensure 499 the authenticity of DNS data. A DNS API client may also perform full 500 DNSSEC validation of answers received from a DNS API server or it may 501 choose to trust answers from a particular DNS API server, much as a 502 DNS client might choose to trust answers from its recursive DNS 503 resolver. This capability might be affected by the response media 504 type. 506 Section 5.1 describes the interaction of this protocol with HTTP 507 caching. An adversary that can control the cache used by the client 508 can affect that client's view of the DNS. This is no different than 509 the security implications of HTTP caching for other protocols that 510 use HTTP. 512 A server that is acting both as a normal web server and a DNS API 513 server is in a position to choose which DNS names it forces a client 514 to resolve (through its web service) and also be the one to answer 515 those queries (through its DNS API service). An untrusted DNS API 516 server can thus easily cause damage by poisoning a client's cache 517 with names that the DNS API server chooses to poison. A client MUST 518 NOT trust a DNS API server simply because it was discovered, or 519 because the client was told to trust the DNS API server by an 520 untrusted party. Instead, a client MUST only trust DNS API server 521 that is configured as trustworthy. 523 A client can use DNS over HTTPS as one of multiple mechanisms to 524 obtain DNS data. If a client of this protocol encounters an HTTP 525 error after sending a DNS query, and then falls back to a different 526 DNS retrieval mechanism, doing so can weaken the privacy and 527 authenticity expected by the user of the client. 529 9. Operational Considerations 531 Local policy considerations and similar factors mean different DNS 532 servers may provide different results to the same query: for instance 533 in split DNS configurations [RFC6950]. It logically follows that the 534 server which is queried can influence the end result. Therefore a 535 client's choice of DNS server may affect the responses it gets to its 536 queries. For example, in the case of DNS64 [RFC6147], the choice 537 could affect whether IPv6/IPv4 translation will work at all. 539 The HTTPS channel used by this specification establishes secure two 540 party communication between the DNS API client and the DNS API 541 server. Filtering or inspection systems that rely on unsecured 542 transport of DNS will not function in a DNS over HTTPS environment. 544 Some HTTPS client implementations perform real time third party 545 checks of the revocation status of the certificates being used by 546 TLS. If this check is done as part of the DNS API server connection 547 procedure and the check itself requires DNS resolution to connect to 548 the third party a deadlock can occur. The use of OCSP [RFC6960] 549 servers or AIA for CRL fetching ([RFC5280] Section 4.2.2.1) are 550 examples of how this deadlock can happen. To mitigate the 551 possibility of deadlock, DNS API servers SHOULD NOT rely on DNS based 552 references to external resources in the TLS handshake. For OCSP the 553 server can bundle the certificate status as part of the handshake 554 using a mechanism appropriate to the version of TLS, such as using 555 [RFC6066] Section 8 for TLS version 1.2. AIA deadlocks can be 556 avoided by providing intermediate certificates that might otherwise 557 be obtained through additional requests. 559 A DNS API client may face a similar bootstrapping problem when the 560 HTTP request needs to resolve the hostname portion of the DNS URI. 561 Just as the address of a traditional DNS nameserver cannot be 562 originally determined from that same server, a DNS API client cannot 563 use its DNS API server to initially resolve the server's host name 564 into an address. Alternative strategies a client might employ 565 include making the initial resolution part of the configuration, IP 566 based URIs and corresponding IP based certificates for HTTPS, or 567 resolving the DNS API server's hostname via traditional DNS or 568 another DNS API server while still authenticating the resulting 569 connection via HTTPS. 571 HTTP [RFC7230] is a stateless application level protocol and 572 therefore DOH implementations do not provide stateful ordering 573 guarantees between different requests. DOH cannot be used as a 574 transport for other protocols that require strict ordering. 576 If a DNS API server responds to a DNS API client with a DNS message 577 that has the TC (truncation) bit set in the header, that indicates 578 that the DNS API server was not able to retrieve a full answer for 579 the query and is providing the best answer it could get. This 580 protocol does not require that a DNS API server that cannot get an 581 untruncated answer send back such an answer; it can instead send back 582 an HTTP error to indicate that it cannot give a useful answer. 584 10. Acknowledgments 586 This work required a high level of cooperation between experts in 587 different technologies. Thank you Ray Bellis, Stephane Bortzmeyer, 588 Manu Bretelle, Tony Finch, Daniel Kahn Gilmor, Olafur Guomundsson, 589 Wes Hardaker, Rory Hewitt, Joe Hildebrand, David Lawrence, Eliot 590 Lear, John Mattson, Alex Mayrhofer, Mark Nottingham, Jim Reid, Adam 591 Roach, Ben Schwartz, Davey Song, Daniel Stenberg, Andrew Sullivan, 592 Martin Thomson, and Sam Weiler. 594 11. References 596 11.1. Normative References 598 [RFC1035] Mockapetris, P., "Domain names - implementation and 599 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 600 November 1987, . 602 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 603 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 604 . 606 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 607 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 608 . 610 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 611 (TLS) Protocol Version 1.2", RFC 5246, 612 DOI 10.17487/RFC5246, August 2008, 613 . 615 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 616 and D. Orchard, "URI Template", RFC 6570, 617 DOI 10.17487/RFC6570, March 2012, 618 . 620 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 621 Protocol (HTTP/1.1): Message Syntax and Routing", 622 RFC 7230, DOI 10.17487/RFC7230, June 2014, 623 . 625 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 626 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 627 DOI 10.17487/RFC7231, June 2014, 628 . 630 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 631 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 632 RFC 7234, DOI 10.17487/RFC7234, June 2014, 633 . 635 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 636 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 637 DOI 10.17487/RFC7540, May 2015, 638 . 640 [RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for 641 HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, 642 . 644 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 645 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 646 May 2017, . 648 11.2. Informative References 650 [CORS] "Cross-Origin Resource Sharing", n.d., 651 . 653 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 654 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 655 . 657 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 658 Housley, R., and W. Polk, "Internet X.509 Public Key 659 Infrastructure Certificate and Certificate Revocation List 660 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 661 . 663 [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale 664 Content", RFC 5861, DOI 10.17487/RFC5861, May 2010, 665 . 667 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 668 Extensions: Extension Definitions", RFC 6066, 669 DOI 10.17487/RFC6066, January 2011, 670 . 672 [RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van 673 Beijnum, "DNS64: DNS Extensions for Network Address 674 Translation from IPv6 Clients to IPv4 Servers", RFC 6147, 675 DOI 10.17487/RFC6147, April 2011, 676 . 678 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 679 for DNS (EDNS(0))", STD 75, RFC 6891, 680 DOI 10.17487/RFC6891, April 2013, 681 . 683 [RFC6950] Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba, 684 "Architectural Considerations on Application Features in 685 the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013, 686 . 688 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., 689 Galperin, S., and C. Adams, "X.509 Internet Public Key 690 Infrastructure Online Certificate Status Protocol - OCSP", 691 RFC 6960, DOI 10.17487/RFC6960, June 2013, 692 . 694 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 695 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 696 2015, . 698 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 699 and P. Hoffman, "Specification for DNS over Transport 700 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 701 2016, . 703 Appendix A. Previous Work on DNS over HTTP or in Other Formats 705 The following is an incomplete list of earlier work that related to 706 DNS over HTTP/1 or representing DNS data in other formats. 708 The list includes links to the tools.ietf.org site (because these 709 documents are all expired) and web sites of software. 711 o https://tools.ietf.org/html/draft-mohan-dns-query-xml 713 o https://tools.ietf.org/html/draft-daley-dnsxml 715 o https://tools.ietf.org/html/draft-dulaunoy-dnsop-passive-dns-cof 717 o https://tools.ietf.org/html/draft-bortzmeyer-dns-json 719 o https://www.nlnetlabs.nl/projects/dnssec-trigger/ 721 Authors' Addresses 723 Paul Hoffman 724 ICANN 726 Email: paul.hoffman@icann.org 728 Patrick McManus 729 Mozilla 731 Email: mcmanus@ducksong.com