Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                              P. McManus
Expires: October 13, 2018                                        Mozilla
                                                          April 11, 2018

                         DNS Queries over HTTPS


   This document describes how to run DNS service over HTTP (DOH) using
   https:// URIs.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 13, 2018.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Requirements . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Non-requirements  . . . . . . . . . . . . . . . . . . . .   4
   4.  The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  The HTTP Request  . . . . . . . . . . . . . . . . . . . .   4
       4.1.1.  HTTP Request Examples . . . . . . . . . . . . . . . .   5
     4.2.  The HTTP Response . . . . . . . . . . . . . . . . . . . .   6
       4.2.1.  HTTP Response Example . . . . . . . . . . . . . . . .   7
   5.  HTTP Integration  . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Cache Interaction . . . . . . . . . . . . . . . . . . . .   7
     5.2.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Server Push . . . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  Content Negotiation . . . . . . . . . . . . . . . . . . .   9
   6.  DNS Wire Format . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Registration of application/dns-message Media Type  . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  13
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Previous Work on DNS over HTTP or in Other Formats .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This document defines a specific protocol for sending DNS [RFC1035]
   queries and getting DNS responses over HTTP [RFC7540] using https://
   (and therefore TLS [RFC5246] security for integrity and
   confidentiality).  Each DNS query-response pair is mapped into a HTTP

   The described approach is more than a tunnel over HTTP.  It
   establishes default media formatting types for requests and responses
   but uses normal HTTP content negotiation mechanisms for selecting
   alternatives that endpoints may prefer in anticipation of serving new
   use cases.  In addition to this media type negotiation, it aligns
   itself with HTTP features such as caching, redirection, proxying,
   authentication, and compression.

   The integration with HTTP provides a transport suitable for both
   traditional DNS clients and native web applications seeking access to
   the DNS.

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   Two primary uses cases were considered during this protocol's
   development.  They included preventing on-path devices from
   interfering with DNS operations and allowing web applications to
   access DNS information via existing browser APIs in a safe way
   consistent with Cross Origin Resource Sharing (CORS) [CORS].  There
   are certainly other uses for this work.

2.  Terminology

   A server that supports this protocol on one or more URIs is called a
   "DNS API server" to differentiate it from a "DNS server" (one that
   uses the regular DNS protocol).  Similarly, a client that supports
   this protocol is called a "DNS API client".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC8174 [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Protocol Requirements

   The protocol described here bases its design on the following
   protocol requirements:

   o  The protocol must use normal HTTP semantics.

   o  The queries and responses must be able to be flexible enough to
      express every DNS query that would normally be sent in DNS over
      UDP (including queries and responses that use DNS extensions, but
      not those that require multiple responses).

   o  The protocol must permit the addition of new formats for DNS
      queries and responses.

   o  The protocol must ensure interoperability by specifying a single
      format for requests and responses that is mandatory to implement.
      That format must be able to support future modifications to the
      DNS protocol including the inclusion of one or more EDNS options
      (including those not yet defined).

   o  The protocol must use a secure transport that meets the
      requirements for HTTPS.

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3.1.  Non-requirements

   o  Supporting network-specific DNS64 [RFC6147]

   o  Supporting other network-specific inferences from plaintext DNS

   o  Supporting insecure HTTP

4.  The HTTP Exchange

4.1.  The HTTP Request

   A DNS API client encodes a single DNS query into an HTTP request
   using either the HTTP GET or POST method and the other requirements
   of this section.  The DNS API server defines the URI used by the
   request through the use of a URI Template [RFC6570].  Configuration
   and discovery of the URI Template is done out of band from this

   The URI Template defined in this document is processed without any
   variables when the HTTP method is POST.  When the HTTP method is GET
   the single variable "dns" is defined as the content of the DNS
   request (as described in Section 6), encoded with base64url

   Future specifications for new media types MUST define the variables
   used for URI Template processing with this protocol.

   DNS API servers MUST implement both the POST and GET methods.

   When using the POST method the DNS query is included as the message
   body of the HTTP request and the Content-Type request header
   indicates the media type of the message.  POST-ed requests are
   smaller than their GET equivalents.

   Using the GET method is friendlier to many HTTP cache

   The DNS API client SHOULD include an HTTP "Accept" request header to
   indicate what type of content can be understood in response.
   Irrespective of the value of the Accept request header, the client
   MUST be prepared to process "application/dns-message" (as described
   in Section 6) responses but MAY also process any other type it

   In order to maximize cache friendliness, DNS API clients using media
   formats that include DNS ID, such as application/dns-message, SHOULD

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   use a DNS ID of 0 in every DNS request.  HTTP correlates the request
   and response, thus eliminating the need for the ID in a media type
   such as application/dns-message.  The use of a varying DNS ID can
   cause semantically equivalent DNS queries to be cached separately.

   DNS API clients can use HTTP/2 padding and compression in the same
   way that other HTTP/2 clients use (or don't use) them.

4.1.1.  HTTP Request Examples

   These examples use HTTP/2 style formatting from [RFC7540].

   These examples use a DNS API service with a URI Template of
   "{?dns}" to resolve IN A

   The requests are represented as application/dns-message typed bodies.

   The first example request uses GET to request

   :method = GET
   :scheme = https
   :authority =
   :path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB
   accept = application/dns-message

   The same DNS query for, using the POST method would

   :method = POST
   :scheme = https
   :authority =
   :path = /dns-query
   accept = application/dns-message
   content-type = application/dns-message
   content-length = 33

   <33 bytes represented by the following hex encoding>
   00 00 01 00 00 01 00 00  00 00 00 00 03 77 77 77
   07 65 78 61 6d 70 6c 65  03 63 6f 6d 00 00 01 00

   Finally, a GET based query for a.62characterlabel-makes-base64url- is shown as an example to
   emphasize that the encoding alphabet of base64url is different than
   regular base64 and that padding is omitted.

   The DNS query is 94 bytes represented by the following hex encoding

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   00 00 01 00 00 01 00 00  00 00 00 00 01 61 3e 36
   32 63 68 61 72 61 63 74  65 72 6c 61 62 65 6c 2d
   6d 61 6b 65 73 2d 62 61  73 65 36 34 75 72 6c 2d
   64 69 73 74 69 6e 63 74  2d 66 72 6f 6d 2d 73 74
   61 6e 64 61 72 64 2d 62  61 73 65 36 34 07 65 78
   61 6d 70 6c 65 03 63 6f  6d 00 00 01 00 01

   :method = GET
   :scheme = https
   :authority =
   :path = /dns-query? (no space or CR)
           dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR)
           bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR)
   accept = application/dns-message

4.2.  The HTTP Response

   An HTTP response with a 2xx status code ([RFC7231] Section 6.3)
   indicates a valid DNS response to the query made in the HTTP request.
   A valid DNS response includes both success and failure responses.
   For example, a DNS failure response such as SERVFAIL or NXDOMAIN will
   be the message in a successful 2xx HTTP response even though there
   was a failure at the DNS layer.  Responses with non-successful HTTP
   status codes do not contain DNS answers to the question in the
   corresponding request.  Some of these non-successful HTTP responses
   (e.g., redirects or authentication failures) could allow clients to
   make new requests to satisfy the original question.

   Different response media types will provide more or less information
   from a DNS response.  For example, one response type might include
   the information from the DNS header bytes while another might omit
   it.  The amount and type of information that a media type gives is
   solely up to the format, and not defined in this protocol.

   At the time this is published, the response types are works in
   progress.  The only response type defined in this document is
   "application/dns-message", but it is possible that other response
   formats will be defined in the future.

   The DNS response for "application/dns-message" in Section 6 MAY have
   one or more EDNS options, depending on the extension definition of
   the extensions given in the DNS request.

   Each DNS request-response pair is matched to one HTTP exchange.  The
   responses may be processed and transported in any order using HTTP's
   multi-streaming functionality ([RFC7540] Section 5).

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   Section 5.1 discusses the relationship between DNS and HTTP response

   A DNS API server MUST be able to process application/dns-message
   request messages.

   A DNS API server SHOULD respond with HTTP status code 415
   (Unsupported Media Type) upon receiving a media type it is unable to

4.2.1.  HTTP Response Example

   This is an example response for a query for the IN A records for
   "" with recursion turned on.  The response bears one
   record with an address of and a TTL of 128 seconds.

   :status = 200
   content-type = application/dns-message
   content-length = 64
   cache-control = max-age=128

   <64 bytes represented by the following hex encoding>
   00 00 81 80 00 01 00 01  00 00 00 00 03 77 77 77
   07 65 78 61 6d 70 6c 65  03 63 6f 6d 00 00 01 00
   01 03 77 77 77 07 65 78  61 6d 70 6c 65 03 63 6f
   6d 00 00 01 00 01 00 00  00 80 00 04 C0 00 02 01

5.  HTTP Integration

   This protocol MUST be used with the https scheme URI [RFC7230].

5.1.  Cache Interaction

   A DNS API client may utilize a hierarchy of caches that include both
   HTTP and DNS specific caches.  HTTP cache entries may be bypassed
   with HTTP mechanisms such as the "Cache-Control no-cache" directive;
   however DNS caches do not have a similar mechanism.

   The Answer section of a DNS response can contain zero or more RRsets.
   (RRsets are defined in [RFC7719].)  According to [RFC2181], each
   resource record in an RRset has Time To Live (TTL) freshness
   information.  Different RRsets in the Answer section can have
   different TTLs, although it is only possible for the HTTP response to
   have a single freshness lifetime.  The HTTP response freshness
   lifetime ([RFC7234] Section 4.2) should be coordinated with the RRset
   with the smallest TTL in the Answer section of the response.
   Specifically, the HTTP freshness lifetime SHOULD be set to expire at
   the same time any of the DNS resource records in the Answer section

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   reach a 0 TTL.  The response freshness lifetime MUST NOT be greater
   than that indicated by the DNS resoruce record with the smallest TTL
   in the response.

   If the DNS response has no records in the Answer section, and the DNS
   response has an SOA record in the Authority section, the response
   freshness lifetime MUST NOT be greater than the MINIMUM field from
   that SOA record.  (See [RFC2308].)  Otherwise, the HTTP response MUST
   set a freshness lifetime ([RFC7234] Section 4.2) of 0 by using a
   mechanism such as "Cache-Control: no-cache" ([RFC7234]

   A DNS API client that receives a response without an explicit
   freshness lifetime MUST NOT assign that response a heuristic
   freshness ([RFC7234] Section 4.2.2.) greater than that indicated by
   the DNS Record with the smallest TTL in the response.

   A DOH response that was previously stored in an HTTP cache will
   contain the [RFC7234] Age response header indicating the elapsed time
   between when the entry was placed in the HTTP cache and the current
   DOH response.  DNS API clients should subtract this time from the DNS
   TTL if they are re-sharing the information in a non HTTP context
   (e.g., their own DNS cache) to determine the remaining time to live
   of the DNS record.

   HTTP revalidation (e.g., via If-None-Match request headers) of cached
   DNS information may be of limited value to DOH as revalidation
   provides only a bandwidth benefit and DNS transactions are normally
   latency bound.  Furthermore, the HTTP response headers that enable
   revalidation (such as "Last-Modified" and "Etag") are often fairly
   large when compared to the overall DNS response size, and have a
   variable nature that creates constant pressure on the HTTP/2
   compression dictionary [RFC7541].  Other types of DNS data, such as
   zone transfers, may be larger and benefit more from revalidation.
   DNS API servers may wish to consider whether providing these
   validation enabling response headers is worthwhile.

   The stale-while-revalidate and stale-if-error cache control
   directives may be well suited to a DOH implementation when allowed by
   server policy.  Those mechanisms allow a client, at the server's
   discretion, to reuse a cache entry that is no longer fresh under some
   extenuating circumstances defined in [RFC5861].

   All HTTP servers, including DNS API servers, need to consider cache
   interaction when they generate responses that are not globally valid.
   For instance, if a DNS API server customized a response based on the
   client's identity then it would not want to globally allow reuse of
   that response.  This could be accomplished through a variety of HTTP

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   techniques such as a Cache-Control max-age of 0, or perhaps by the
   Vary response header.

5.2.  HTTP/2

   The minimum version of HTTP used by DOH SHOULD be HTTP/2 [RFC7540].

   The messages in classic UDP based DNS [RFC1035] are inherently
   unordered and have low overhead.  A competitive HTTP transport needs
   to support reordering, parallelism, priority, and header compression
   to achieve similar performance.  Those features were introduced to
   HTTP in HTTP/2 [RFC7540].  Earlier versions of HTTP are capable of
   conveying the semantic requirements of DOH but may result in very
   poor performance.

5.3.  Server Push

   Before using DOH response data for DNS resolution, the client MUST
   establish that the HTTP request URI is a trusted service for the DOH
   query.  For HTTP requests initiated by the DNS API client this trust
   is implicit in the selection of URI.  For HTTP server push ([RFC7540]
   Section 8.2) extra care must be taken to ensure that the pushed URI
   is one that the client would have directed the same query to if the
   client had initiated the request.  This specification does not extend
   DNS resolution privileges to URIs that are not recognized by the
   client as trusted DNS API servers.

5.4.  Content Negotiation

   In order to maximize interoperability, DNS API clients and DNS API
   servers MUST support the "application/dns-message" media type.  Other
   media types MAY be used as defined by HTTP Content Negotiation
   ([RFC7231] Section 3.4).

6.  DNS Wire Format

   The data payload is the DNS on-the-wire format defined in [RFC1035].
   The format is for DNS over UDP.  Note that this is different than the
   wire format used in [RFC7858].  Also note that while [RFC1035] says
   "Messages carried by UDP are restricted to 512 bytes", that was later
   updated by [RFC6891], and this protocol allows DNS on-the-wire format
   payloads of any size.

   When using the GET method, the data payload MUST be encoded with
   base64url [RFC4648] and then provided as a variable named "dns" to
   the URI Template expansion.  Padding characters for base64url MUST
   NOT be included.

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   When using the POST method, the data payload MUST NOT be encoded and
   is used directly as the HTTP message body.

   DNS API clients using the DNS wire format MAY have one or more EDNS
   options [RFC6891] in the request.

   The media type is "application/dns-message".

7.  IANA Considerations

7.1.  Registration of application/dns-message Media Type

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   Subject: Registration of MIME media type

   MIME media type name: application

   MIME subtype name: dns-message

   Required parameters: n/a

   Optional parameters: n/a

   Encoding considerations: This is a binary format. The contents are a
   DNS message as defined in RFC 1035. The format used here is for DNS
   over UDP, which is the format defined in the diagrams in RFC 1035.

   Security considerations:  The security considerations for carrying
   this data are the same for carrying DNS without encryption.

   Interoperability considerations:  None.

   Published specification:  This document.

   Applications that use this media type:
     Systems that want to exchange full DNS messages.

   Additional information:

   Magic number(s):  n/a

   File extension(s):  n/a

   Macintosh file type code(s):  n/a

   Person & email address to contact for further information:
      Paul Hoffman,

   Intended usage:  COMMON

   Restrictions on usage:  n/a

   Author:  Paul Hoffman,

   Change controller:  IESG

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8.  Security Considerations

   Running DNS over HTTPS relies on the security of the underlying HTTP
   transport.  This mitigates classic amplification attacks for UDP-
   based DNS.  Implementations utilizing HTTP/2 benefit from the TLS
   profile defined in [RFC7540] Section 9.2.

   Session level encryption has well known weaknesses with respect to
   traffic analysis which might be particularly acute when dealing with
   DNS queries.  HTTP/2 provides further advice about the use of
   compression (Section 10.6 of [RFC7540]) and padding (Section 10.7 of

   The HTTPS connection provides transport security for the interaction
   between the DNS API server and client, but does not inherently ensure
   the authenticity of DNS data.  A DNS API client may also perform full
   DNSSEC validation of answers received from a DNS API server or it may
   choose to trust answers from a particular DNS API server, much as a
   DNS client might choose to trust answers from its recursive DNS
   resolver.  This capability might be affected by the response media

   Section 5.1 describes the interaction of this protocol with HTTP
   caching.  An adversary that can control the cache used by the client
   can affect that client's view of the DNS.  This is no different than
   the security implications of HTTP caching for other protocols that
   use HTTP.

   A server that is acting both as a normal web server and a DNS API
   server is in a position to choose which DNS names it forces a client
   to resolve (through its web service) and also be the one to answer
   those queries (through its DNS API service).  An untrusted DNS API
   server can thus easily cause damage by poisoning a client's cache
   with names that the DNS API server chooses to poison.  A client MUST
   NOT trust a DNS API server simply because it was discovered, or
   because the client was told to trust the DNS API server by an
   untrusted party.  Instead, a client MUST only trust DNS API server
   that is configured as trustworthy.

   A client can use DNS over HTTPS as one of multiple mechanisms to
   obtain DNS data.  If a client of this protocol encounters an HTTP
   error after sending a DNS query, and then falls back to a different
   DNS retrieval mechanism, doing so can weaken the privacy and
   authenticity expected by the user of the client.

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9.  Operational Considerations

   Local policy considerations and similar factors mean different DNS
   servers may provide different results to the same query: for instance
   in split DNS configurations [RFC6950].  It logically follows that the
   server which is queried can influence the end result.  Therefore a
   client's choice of DNS server may affect the responses it gets to its
   queries.  For example, in the case of DNS64 [RFC6147], the choice
   could affect whether IPv6/IPv4 translation will work at all.

   The HTTPS channel used by this specification establishes secure two
   party communication between the DNS API client and the DNS API
   server.  Filtering or inspection systems that rely on unsecured
   transport of DNS will not function in a DNS over HTTPS environment.

   Some HTTPS client implementations perform real time third party
   checks of the revocation status of the certificates being used by
   TLS.  If this check is done as part of the DNS API server connection
   procedure and the check itself requires DNS resolution to connect to
   the third party a deadlock can occur.  The use of OCSP [RFC6960]
   servers or AIA for CRL fetching ([RFC5280] Section are
   examples of how this deadlock can happen.  To mitigate the
   possibility of deadlock, DNS API servers SHOULD NOT rely on DNS based
   references to external resources in the TLS handshake.  For OCSP the
   server can bundle the certificate status as part of the handshake
   using a mechanism appropriate to the version of TLS, such as using
   [RFC6066] Section 8 for TLS version 1.2.  AIA deadlocks can be
   avoided by providing intermediate certificates that might otherwise
   be obtained through additional requests.

   A DNS API client may face a similar bootstrapping problem when the
   HTTP request needs to resolve the hostname portion of the DNS URI.
   Just as the address of a traditional DNS nameserver cannot be
   originally determined from that same server, a DNS API client cannot
   use its DNS API server to initially resolve the server's host name
   into an address.  Alternative strategies a client might employ
   include making the initial resolution part of the configuration, IP
   based URIs and corresponding IP based certificates for HTTPS, or
   resolving the DNS API server's hostname via traditional DNS or
   another DNS API server while still authenticating the resulting
   connection via HTTPS.

   HTTP [RFC7230] is a stateless application level protocol and
   therefore DOH implementations do not provide stateful ordering
   guarantees between different requests.  DOH cannot be used as a
   transport for other protocols that require strict ordering.

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   If a DNS API server responds to a DNS API client with a DNS message
   that has the TC (truncation) bit set in the header, that indicates
   that the DNS API server was not able to retrieve a full answer for
   the query and is providing the best answer it could get.  This
   protocol does not require that a DNS API server that cannot get an
   untruncated answer send back such an answer; it can instead send back
   an HTTP error to indicate that it cannot give a useful answer.

10.  Acknowledgments

   This work required a high level of cooperation between experts in
   different technologies.  Thank you Ray Bellis, Stephane Bortzmeyer,
   Manu Bretelle, Tony Finch, Daniel Kahn Gilmor, Olafur Guomundsson,
   Wes Hardaker, Rory Hewitt, Joe Hildebrand, David Lawrence, Eliot
   Lear, John Mattson, Alex Mayrhofer, Mark Nottingham, Jim Reid, Adam
   Roach, Ben Schwartz, Davey Song, Daniel Stenberg, Andrew Sullivan,
   Martin Thomson, and Sam Weiler.

11.  References

11.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

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   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

11.2.  Informative References

   [CORS]     "Cross-Origin Resource Sharing", n.d.,

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

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   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC6950]  Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
              "Architectural Considerations on Application Features in
              the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

Appendix A.  Previous Work on DNS over HTTP or in Other Formats

   The following is an incomplete list of earlier work that related to
   DNS over HTTP/1 or representing DNS data in other formats.

   The list includes links to the site (because these
   documents are all expired) and web sites of software.






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Authors' Addresses

   Paul Hoffman


   Patrick McManus


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