Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                              P. McManus
Expires: October 13, November 17, 2018                                       Mozilla
                                                          April 11,
                                                            May 16, 2018

                      DNS Queries over HTTPS
                   draft-ietf-doh-dns-over-https-07 (DOH)
                    draft-ietf-doh-dns-over-https-08

Abstract

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

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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, November 17, 2018.

Copyright Notice

   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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Requirements . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Non-requirements  . . . . . . . . . . . . . . . . . . . .   4
   4.  Selection of DNS API Server . . . . . . . . . . . . . . . . .   4
   5.  The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.
     5.1.  The HTTP Request  . . . . . . . . . . . . . . . . . . . .   4
       4.1.1.
       5.1.1.  HTTP Request Examples . . . . . . . . . . . . . . . .   5
     4.2.
     5.2.  The HTTP Response . . . . . . . . . . . . . . . . . . . .   6
       4.2.1.
       5.2.1.  HTTP Response Example . . . . . . . . . . . . . . . .   7
   5.
   6.  HTTP Integration  . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.   8
     6.1.  Cache Interaction . . . . . . . . . . . . . . . . . . . .   7
     5.2.   8
     6.2.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  10
     6.3.  Server Push . . . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  10
     6.4.  Content Negotiation . . . . . . . . . . . . . . . . . . .   9
   6.  10
   7.  DNS Wire Format . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  11
     8.1.  Registration of application/dns-message Media Type  . . .  10
   8.  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  13
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  13
   10. Acknowledgments  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . .  14
   11. . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . .  14
     11.1.  Normative References . . . . . . . . . .  16
   Acknowledgments . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  17
   Previous Work on DNS over HTTP or in Other Formats  .  16 . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17  18

1.  Introduction

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

   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
   existing DNS clients and native web applications seeking access to
   the DNS.

   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  No
   special effort has been taken to enable or prevent application to
   other uses for this work. use cases.  This document focuses on communication between DNS
   clients (such as operating system stub resolvers) and recursive
   resolvers.

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 only provides DNS protocol).
   service over one or more of the other transport protocols
   standardized for DNS).  Similarly, a client that supports this
   protocol is called a "DNS API client".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC8174
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Protocol Requirements

   [[ RFC Editor: Please remove this entire section before publication.
   ]]

   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.

3.1.  Non-requirements

   o  Supporting network-specific DNS64 [RFC6147]

   o  Supporting other network-specific inferences from plaintext DNS
      queries

   o  Supporting insecure HTTP

4.  The HTTP Exchange

4.1.  The HTTP Request

   A  Selection of DNS API client encodes Server

   Before using a single DNS query into an HTTP request
   using either API server for DNS resolution, the client MUST
   establish that the HTTP GET or POST method and request URI is a trusted service for the DOH
   query, in other requirements
   of this section.  The words, a DNS API client MUST only use a DNS API
   server defines the URI used by the
   request through the that is configured as trustworthy.

   A client MUST NOT use of a URI Template [RFC6570].  Configuration
   and discovery of DNS API server simply because it was
   discovered, or because the URI Template is done out client was told to use the DNS API server
   by an untrusted party.

   This specification does not extend DNS resolution privileges to URIs
   that are not recognized by the DNS API client as trusted DNS API
   servers.  As such, use of untrusted servers is out of scope of this
   document.

5.  The HTTP Exchange

5.1.  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 protocol.  DNS API Servers MAY support more than one URI.
   This allows the different endpoints to have different properties such
   as different authentication requirements or service level guarantees.

   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), 7), encoded with base64url
   [RFC4648].

   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
   implementations.

   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) 7) responses but MAY also process any other type it
   receives.

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

5.1.1.  HTTP Request Examples

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

   These examples use a DNS API service with a URI Template of
   "https://dnsserver.example.net/dns-query{?dns}" to resolve IN A
   records.

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

   The first example request uses GET to request www.example.com

   :method = GET
   :scheme = https
   :authority = dnsserver.example.net
   :path = /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB
   accept = application/dns-message
   The same DNS query for www.example.com, using the POST method would
   be:

   :method = POST
   :scheme = https
   :authority = dnsserver.example.net
   :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
   01

   Finally, a GET based query for a.62characterlabel-makes-base64url-
   distinct-from-standard-base64.example.com 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

   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 = dnsserver.example.net
   :path = /dns-query? (no space or CR)
           dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR)
           bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR)
           dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ
   accept = application/dns-message

4.2.

5.2.  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 mean that clients
   need 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", "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 7 MAY have
   one or more EDNS options, options [RFC6891], 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).

   Section 5.1 6.1 discusses the relationship between DNS and HTTP response
   caching.

   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
   process.

4.2.1.

5.2.1.  HTTP Response Example

   This is an example response for a query for the IN A records for
   "www.example.com" with recursion turned on.  The response bears one
   record with an address of 192.0.2.1 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.

6.  HTTP Integration

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

5.1.

6.1.  Cache Interaction

   A DNS API client may utilize DOH exchange can pass through a hierarchy of caches that include
   both HTTP and DNS specific caches.  HTTP cache entries  These caches may be bypassed
   with HTTP mechanisms such as exist beteen the
   DNS API server and client, or on the "Cache-Control no-cache" directive;
   however DNS API client itself.  HTTP
   caches are by design generic; that is, they do not have understand this
   protocol.  Even if a similar mechanism.

   The Answer section DNS API client has modified its cache
   implementation to be aware of DOH semantics, it does not follow that
   all upstream caches (for example, inline proxies, server-side
   gateways and Content Delivery Networks) will be.

   As a result, DNS response can contain zero or more RRsets.
   (RRsets are defined in [RFC7719].)  According API servers need 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 carefully consider the HTTP
   caching metadata they send in response to GET requests (POST requests
   are not cacheable unless specific response to
   have a single freshness lifetime.  The HTTP response headers are sent; this is
   not widely implemented, and not advised for DOH).

   In particular, DNS API servers SHOULD assign an explicit freshness
   lifetime ([RFC7234] Section 4.2) should be coordinated with the RRset
   with so that the smallest TTL DNS API client is more
   likely to use fresh DNS data.  This requirement is due to HTTP caches
   being able to assign their own heuristic freshness (such as that
   described in [RFC7234] Section 4.2.2), which would take control of
   the Answer section cache contents out of the response.
   Specifically, hands of the HTTP DNS API server.

   The assigned freshness lifetime of a DOH HTTP response SHOULD be set to expire at the same time any of the DNS resource records
   smallest TTL in the Answer section
   reach of the DNS response.  For example,
   if a 0 TTL.  The HTTP response carries three RRsets with TTLs of 30, 600, and
   300, the HTTP freshness lifetime should be 30 seconds (which could be
   specified as "Cache-Control: max-age=30").  The assigned freshness
   lifetime MUST NOT be greater than that indicated by the DNS resoruce record with the smallest TTL in the Answer
   section of the DNS response.  This requirement helps assure that none
   of the RRsets contained in a DNS response are served stale from an
   HTTP cache.

   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 record (see [RFC2308]).

   The stale-while-revalidate and stale-if-error Cache-Control
   directives ([RFC5861]) could be well suited to a freshness lifetime ([RFC7234] Section 4.2) of 0 DOH implementation
   when allowed by using server policy.  Those mechanisms allow a
   mechanism client, at
   the server's discretion, to reuse a cache entry that is no longer
   fresh.  In such as "Cache-Control: no-cache" ([RFC7234]
   Section 5.2.1.4).

   A a case, the client reuses all of a cached entry, or
   none of it.

   DNS API client servers also need to consider caching when generating
   responses that receives are not globally valid.  For instance, if a DNS API
   server customizes a response without an explicit
   freshness lifetime MUST NOT assign based on the client's identity, it would
   not want to allow global reuse of that response.  This could be
   accomplished through a variety of HTTP techniques such as a Cache-
   Control max-age of 0, or by using the Vary response header ([RFC7231]
   Section 7.1.4) to establish a heuristic
   freshness secondary cache key ([RFC7234]
   Section 4.2.2.) greater than that indicated by
   the 4.1).

   DNS Record with API clients MUST account for the smallest TTL in Age response header's value
   ([RFC7234]) when calculating the DNS TTL of a response.

   A DOH response that was previously stored in an HTTP cache will
   contain  For example,
   if a RRset is received with a DNS TTL of 600, but the [RFC7234] Age response header indicating the elapsed time
   between when
   indicates that the entry was placed in response has been cached for 250 seconds, the HTTP cache and
   remaining lifetime of the current
   DOH response. RRset is 350 seconds.

   DNS API clients should subtract this time from the DNS
   TTL if they are re-sharing the information in can request an uncached copy of a non HTTP context
   (e.g., their own DNS cache) to determine response by using
   the remaining time "no-cache" request cache control directive ([RFC7234],
   Section 5.2.1.4) and similar controls.  Note that some caches might
   not honor these directives, either due to live
   of the configuration or
   interaction with traditional DNS record. caches that do not have such a
   mechanism.

   HTTP revalidation (e.g., via If-None-Match request headers) of cached
   DNS information conditional requests ([RFC7232]) may be of limited value to DOH 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

6.2.  HTTP/2

   HTTP/2 [RFC7540] 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
   techniques such as a Cache-Control max-age of 0, or perhaps by the
   Vary response header.

5.2.  HTTP/2

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

   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.

6.3.  Server Push

   Before using DOH response data for DNS resolution, the client MUST
   establish that the HTTP request URI is a trusted service may be used 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.

6.4.  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.  Those media types MUST 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).

7.  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 [RFC6891].  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.

   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.

8.  IANA Considerations

7.1.

8.1.  Registration of application/dns-message Media Type
   To: ietf-types@iana.org
   Subject: Registration of MIME media type
            application/dns-message

   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, paul.hoffman@icann.org

   Intended usage:  COMMON

   Restrictions on usage:  n/a

   Author:  Paul Hoffman, paul.hoffman@icann.org

   Change controller:  IESG

8.

9.  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]) ([RFC7540] Section 10.6) and padding (Section ([RFC7540]
   Section 10.7 of
   [RFC7540]). ).  DNS API Servers can also add DNS padding [RFC7830]
   if the DNS API requests it in the DNS query.

   The HTTPS connection provides transport security for the interaction
   between the DNS API server and client, but does not inherently ensure provide the authenticity
   response integrity of DNS data.  A DNS API data provided by DNSSEC.  DNSSEC and DOH
   are independent and fully compatible protocols, each solving
   different problems.  The use of one does not diminish the need nor
   the usefulness of the other.  It is the choice of a client may also to either
   perform full DNSSEC validation of answers received from a DNS API server or it may
   choose to trust answers from a particular the DNS API server, much as a
   DNS client might choose
   server to trust answers do DNSSEC validation and inspect the AD (Authentic Data)
   bit in the returned message to determine whether an answer was
   authentic or not.  As noted in Section 5.2, different response media
   types will provide more or less information from its recursive a DNS
   resolver.  This capability might response so
   this choice may be affected by the response media type.

   Section 5.1 6.1 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

   In the absence of DNSSEC information, a DNS API server is in a position to choose which DNS names it forces can give a
   client invalid data in response 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. query.  A client MUST NOT trust a DNS API server simply because it was discovered, or
   because the client was told to trust the
   use arbitrary DNS API server by an
   untrusted party. servers.  Instead, a client MUST only trust use DNS
   API server
   that is configured servers specified using mechanisms such as trustworthy. explicit
   configuration.  This does not guarantee protection against invalid
   data but reduces the risk.

   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.

9.

10.  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 4.2.2.1) 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.  Note that these deadlocks
   also need to be considered for server that a DNS API server might
   redirect to.

   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.

   If a

   A DNS API server responds is allowed to a DNS API client answer queries with any valid DNS
   response.  For example, a valid DNS message
   that has response might have the TC
   (truncation) bit set in the header, that indicates
   that the DNS API header to indicate that the server
   was not able to retrieve a full answer for the query and but is providing
   the best answer it could get.  This
   protocol does not require that a  A DNS API server that cannot get an
   untruncated answer send back such an answer; it can instead send back reply to queries
   with an HTTP error to indicate for queries that it cannot give fulfill.  In this same
   example, a useful answer.

10.  Acknowledgments

   This work required DNS API server could use an HTTP error instead of a high level non-
   error response that has the TC bit set.

   Many extensions to DNS, using [RFC6891], have been defined over the
   years.  Extensions that are specific to the choice 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. transport, such
   as [RFC7828], are not applicable to DOH.

11.  References

11.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <https://www.rfc-editor.org/info/rfc2308>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.

   [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,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [CORS]     "Cross-Origin Resource Sharing", n.d.,
              <https://fetch.spec.whatwg.org/#http-cors-protocol>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
              <https://www.rfc-editor.org/info/rfc5861>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [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,
              <https://www.rfc-editor.org/info/rfc6147>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [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,
              <https://www.rfc-editor.org/info/rfc6950>.

   [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,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC7719]  Hoffman,

   [RFC7828]  Wouters, P., Sullivan, A., Abley, J., Dickinson, S., and K. Fujiwara, "DNS
              Terminology", R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7719, 7828,
              DOI 10.17487/RFC7719, December
              2015, <https://www.rfc-editor.org/info/rfc7719>. 10.17487/RFC7828, April 2016,
              <https://www.rfc-editor.org/info/rfc7828>.

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,
              <https://www.rfc-editor.org/info/rfc7830>.

   [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, <https://www.rfc-editor.org/info/rfc7858>.

Appendix A.

Acknowledgments

   This work required a high level of cooperation between experts in
   different technologies.  Thank you Ray Bellis, Stephane Bortzmeyer,
   Manu Bretelle, Sara Dickinson, 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.

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 tools.ietf.org site (because these
   documents are all expired) and web sites of software.

   o  https://tools.ietf.org/html/draft-mohan-dns-query-xml

   o  https://tools.ietf.org/html/draft-daley-dnsxml

   o  https://tools.ietf.org/html/draft-dulaunoy-dnsop-passive-dns-cof

   o  https://tools.ietf.org/html/draft-bortzmeyer-dns-json

   o  https://www.nlnetlabs.nl/projects/dnssec-trigger/

Authors' Addresses

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org

   Patrick McManus
   Mozilla

   Email: mcmanus@ducksong.com