DNS Queries over HTTPS (DoH)
RFC 8484

Internet Engineering Task Force (IETF)                        P. Hoffman
Request for Comments: 8484                                         ICANN
Category: Standards Track                                     P. McManus
ISSN: 2070-1721                                                  Mozilla
                                                            October 2018

                      DNS Queries over HTTPS (DoH)

Abstract

   This document defines a protocol for sending DNS queries and getting
   DNS responses over HTTPS.  Each DNS query-response pair is mapped
   into an HTTP exchange.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8484.

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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Selection of DoH Server . . . . . . . . . . . . . . . . . . .   4
   4.  The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  The HTTP Request  . . . . . . . . . . . . . . . . . . . .   4
       4.1.1.  HTTP Request Examples . . . . . . . . . . . . . . . .   5
     4.2.  The HTTP Response . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Handling DNS and HTTP Errors  . . . . . . . . . . . .   7
       4.2.2.  HTTP Response Example . . . . . . . . . . . . . . . .   8
   5.  HTTP Integration  . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Cache Interaction . . . . . . . . . . . . . . . . . . . .   8
     5.2.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Server Push . . . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  Content Negotiation . . . . . . . . . . . . . . . . . . .  10
   6.  Definition of the "application/dns-message" Media Type  . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Registration of the "application/dns-message" Media Type   11
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
     8.1.  On the Wire . . . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  In the Server . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Protocol Development . . . . . . . . . . . . . . . .  20
   Appendix B.  Previous Work on DNS over HTTP or in Other Formats .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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1.  Introduction

   This document defines a specific protocol, DNS over HTTPS (DoH), for
   sending DNS [RFC1035] queries and getting DNS responses over HTTP
   [RFC7540] using https [RFC2818] URIs (and therefore TLS [RFC8446]
   security for integrity and confidentiality).  Each DNS query-response
   pair is mapped into an 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
   existing DNS clients and native web applications seeking access to
   the DNS.

   Two primary use cases were considered during this protocol's
   development.  These use cases are preventing on-path devices from
   interfering with DNS operations, and also allowing web applications
   to access DNS information via existing browser APIs in a safe way
   consistent with Cross Origin Resource Sharing (CORS) [FETCH].  No
   special effort has been taken to enable or prevent application to
   other 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 is called a "DoH server" to
   differentiate it from a "DNS server" (one that only provides DNS
   service over one or more of the other transport protocols
   standardized for DNS).  Similarly, a client that supports this
   protocol is called a "DoH 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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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3.  Selection of DoH Server

   The DoH client is configured with a URI Template [RFC6570], which
   describes how to construct the URL to use for resolution.
   Configuration, discovery, and updating of the URI Template is done
   out of band from this protocol.  Note that configuration might be
   manual (such as a user typing URI Templates in a user interface for
   "options") or automatic (such as URI Templates being supplied in
   responses from DHCP or similar protocols).  DoH servers MAY support
   more than one URI Template.  This allows the different endpoints to
   have different properties, such as different authentication
   requirements or service-level guarantees.

   A DoH client uses configuration to select the URI, and thus the DoH
   server, that is to be used for resolution.  [RFC2818] defines how
   HTTPS verifies the DoH server's identity.

   A DoH client MUST NOT use a different URI simply because it was
   discovered outside of the client's configuration (such as through
   HTTP/2 server push) or because a server offers an unsolicited
   response that appears to be a valid answer to a DNS query.  This
   specification does not extend DNS resolution privileges to URIs that
   are not recognized by the DoH client as configured URIs.  Such
   scenarios may create additional operational, tracking, and security
   hazards that require limitations for safe usage.  A future
   specification may support this use case.

4.  The HTTP Exchange

4.1.  The HTTP Request

   A DoH 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 DoH server defines the URI used by the request through
   the use of a URI Template.

   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
   [RFC4648].

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

   DoH servers MUST implement both the POST and GET methods.

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

   Using the GET method is friendlier to many HTTP cache
   implementations.

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

   In order to maximize HTTP cache friendliness, DoH clients using media
   formats that include the ID field from the DNS message header, 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.

   DoH clients can use HTTP/2 padding and compression [RFC7540] 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 DoH service with a URI Template of
   "https://dnsserver.example.net/dns-query{?dns}" to resolve IN A
   records.

   The requests are represented as bodies with media type "application/
   dns-message".

   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

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   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

   In this example, the 33 bytes are the DNS message in DNS wire format
   [RFC1035], starting with the DNS header.

   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, expressed in DNS wire format, is 94 bytes represented
   by the following:

   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 Carriage Return (CR))
           dns=AAABAAABAAAAAAAAAWE-NjJjaGFyYWN0ZXJsYWJl (no space or CR)
           bC1tYWtlcy1iYXNlNjR1cmwtZGlzdGluY3QtZnJvbS1z (no space or CR)
           dGFuZGFyZC1iYXNlNjQHZXhhbXBsZQNjb20AAAEAAQ
   accept = application/dns-message

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4.2.  The HTTP Response

   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.  A DoH server MUST be able to process
   "application/dns-message" request messages.

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

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

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

4.2.1.  Handling DNS and HTTP Errors

   DNS response codes indicate either success or failure for the DNS
   query.  A successful HTTP response with a 2xx status code (see
   Section 6.3 of [RFC7231]) is used for any valid DNS response,
   regardless of the DNS response code.  For example, a successful 2xx
   HTTP status code is used even with a DNS message whose DNS response
   code indicates failure, such as SERVFAIL or NXDOMAIN.

   HTTP responses with non-successful HTTP status codes do not contain
   replies to the original DNS question in the HTTP request.  DoH
   clients need to use the same semantic processing of non-successful
   HTTP status codes as other HTTP clients.  This might mean that the
   DoH client retries the query with the same DoH server, such as if
   there are authorization failures (HTTP status code 401; see
   Section 3.1 of [RFC7235]).  It could also mean that the DoH client
   retries with a different DoH server, such as for unsupported media
   types (HTTP status code 415; see Section 6.5.13 of [RFC7231]), or
   where the server cannot generate a representation suitable for the
   client (HTTP status code 406; see Section 6.5.6 of [RFC7231]), and so
   on.

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4.2.2.  HTTP Response Example

   This is an example response for a query for the IN AAAA records for
   "www.example.com" with recursion turned on.  The response bears one
   answer record with an address of 2001:db8:abcd:12:1:2:3:4 and a TTL
   of 3709 seconds.

   :status = 200
   content-type = application/dns-message
   content-length = 61
   cache-control = max-age=3709

   <61 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 1c 00
   01 c0 0c 00 1c 00 01 00  00 0e 7d 00 10 20 01 0d
   b8 ab cd 00 12 00 01 00  02 00 03 00 04

5.  HTTP Integration

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

   Sections 8 and 9 discuss additional considerations for the
   integration with HTTP.

5.1.  Cache Interaction

   A DoH exchange can pass through a hierarchy of caches that include
   both HTTP- and DNS-specific caches.  These caches may exist between
   the DoH server and client, or they may exist on the DoH client
   itself.  HTTP caches are generic by design; that is, they do not
   understand this protocol.  Even if a DoH 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, DoH servers need to carefully consider the HTTP caching
   metadata they send in response to GET requests (responses to POST
   requests are not cacheable unless specific response header fields are
   sent; this is not widely implemented and is not advised for DoH).

   In particular, DoH servers SHOULD assign an explicit HTTP freshness
   lifetime (see Section 4.2 of [RFC7234]) so that the DoH 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 Section 4.2.2 of [RFC7234]), which would take
   control of the cache contents out of the hands of the DoH server.

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   The assigned freshness lifetime of a DoH HTTP response MUST be less
   than or equal to the smallest TTL in the Answer section of the DNS
   response.  A freshness lifetime equal to the smallest TTL in the
   Answer section is RECOMMENDED.  For example, if a 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").  This requirement helps prevent expired
   RRsets in messages in an HTTP cache from unintentionally being
   served.

   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]).

   The stale-while-revalidate and stale-if-error Cache-Control
   directives [RFC5861] could be well suited to a DoH implementation
   when allowed by server policy.  Those mechanisms allow a client, at
   the server's discretion, to reuse an HTTP cache entry that is no
   longer fresh.  In such a case, the client reuses either all of a
   cached entry or none of it.

   DoH servers also need to consider HTTP caching when generating
   responses that are not globally valid.  For instance, if a DoH server
   customizes a response 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 field (see
   Section 7.1.4 of [RFC7231]) to establish a secondary cache key (see
   Section 4.1 of [RFC7234]).

   DoH clients MUST account for the Age response header field's value
   [RFC7234] when calculating the DNS TTL of a response.  For example,
   if an RRset is received with a DNS TTL of 600, but the Age header
   field indicates that the response has been cached for 250 seconds,
   the remaining lifetime of the RRset is 350 seconds.  This requirement
   applies to both DoH client HTTP caches and DoH client DNS caches.

   DoH clients can request an uncached copy of a HTTP response by using
   the "no-cache" request Cache-Control directive (see Section 5.2.1.4
   of [RFC7234]) and similar controls.  Note that some caches might not
   honor these directives, either due to configuration or interaction
   with traditional DNS caches that do not have such a mechanism.

   HTTP conditional requests [RFC7232] 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 header fields that enable revalidation (such as "Last-

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   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.

5.2.  HTTP/2

   HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP 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.  Server Push

   Before using DoH response data for DNS resolution, the client MUST
   establish that the HTTP request URI can be used for the DoH query.
   For HTTP requests initiated by the DoH client, this is implicit in
   the selection of URI.  For HTTP server push (see Section 8.2 of
   [RFC7540]), 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 (in addition to the other security
   checks normally needed for server push).

5.4.  Content Negotiation

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

6.  Definition of the "application/dns-message" Media Type

   The data payload for the "application/dns-message" media type is a
   single message of the DNS on-the-wire format defined in Section 4.2.1
   of [RFC1035], which in turn refers to the full wire format defined in
   Section 4.1 of that RFC.

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   Although [RFC1035] says "Messages carried by UDP are restricted to
   512 bytes", that was later updated by [RFC6891].  This media type
   restricts the maximum size of the DNS message to 65535 bytes.

   Note that the wire format used in this media type is different than
   the wire format used in [RFC7858] (which uses the format defined in
   Section 4.2.2 of [RFC1035] that includes two length bytes).

   DoH clients using this media type MAY have one or more Extension
   Mechanisms for DNS (EDNS) options [RFC6891] in the request.  DoH
   servers using this media type MUST ignore the value given for the
   EDNS UDP payload size in DNS requests.

   When using the GET method, the data payload for this media type 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 for this media type MUST
   NOT be encoded and is used directly as the HTTP message body.

7.  IANA Considerations

7.1.  Registration of the "application/dns-message" Media Type

   Type name: application

   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: See RFC 8484.  The content is a DNS message
      and thus not executable code.

   Interoperability considerations: None.

   Published specification: RFC 8484.

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

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   Additional information:

      Deprecated alias names for this type: N/A
      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.  Privacy Considerations

   [RFC7626] discusses DNS privacy considerations in both "on the wire"
   (Section 2.4 of [RFC7626]) and "in the server" (Section 2.5 of
   [RFC7626]) contexts.  This is also a useful framing for DoH's privacy
   considerations.

8.1.  On the Wire

   DoH encrypts DNS traffic and requires authentication of the server.
   This mitigates both passive surveillance [RFC7258] and active attacks
   that attempt to divert DNS traffic to rogue servers (see
   Section 2.5.1 of [RFC7626]).  DNS over TLS [RFC7858] provides similar
   protections, while direct UDP- and TCP-based transports are
   vulnerable to this class of attack.  An experimental effort to offer
   guidance on choosing the padding length can be found in [RFC8467].

   Additionally, the use of the HTTPS default port 443 and the ability
   to mix DoH traffic with other HTTPS traffic on the same connection
   can deter unprivileged on-path devices from interfering with DNS
   operations and make DNS traffic analysis more difficult.

8.2.  In the Server

   The DNS wire format [RFC1035] contains no client identifiers;
   however, various transports of DNS queries and responses do provide
   data that can be used to correlate requests.  HTTPS presents new
   considerations for correlation, such as explicit HTTP cookies and
   implicit fingerprinting of the unique set and ordering of HTTP
   request header fields.

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   A DoH implementation is built on IP, TCP, TLS, and HTTP.  Each layer
   contains one or more common features that can be used to correlate
   queries to the same identity.  DNS transports will generally carry
   the same privacy properties of the layers used to implement them.
   For example, the properties of IP, TCP, and TLS apply to
   implementations of DNS over TLS.

   The privacy considerations of using the HTTPS layer in DoH are
   incremental to those of DNS over TLS.  DoH is not known to introduce
   new concerns beyond those associated with HTTPS.

   At the IP level, the client address provides obvious correlation
   information.  This can be mitigated by use of a NAT, proxy, VPN, or
   simple address rotation over time.  It may be aggravated by use of a
   DNS server that can correlate real-time addressing information with
   other personal identifiers, such as when a DNS server and DHCP server
   are operated by the same entity.

   DNS implementations that use one TCP connection for multiple DNS
   requests directly group those requests.  Long-lived connections have
   better performance behaviors than short-lived connections; however,
   they group more requests, which can expose more information to
   correlation and consolidation.  TCP-based solutions may also seek
   performance through the use of TCP Fast Open [RFC7413].  The cookies
   used in TCP Fast Open allow servers to correlate TCP sessions.

   TLS-based implementations often achieve better handshake performance
   through the use of some form of session resumption mechanism, such as
   Section 2.2 of [RFC8446].  Session resumption creates trivial
   mechanisms for a server to correlate TLS connections together.

   HTTP's feature set can also be used for identification and tracking
   in a number of different ways.  For example, Authentication request
   header fields explicitly identify profiles in use, and HTTP cookies
   are designed as an explicit state-tracking mechanism between the
   client and serving site and often are used as an authentication
   mechanism.

   Additionally, the User-Agent and Accept-Language request header
   fields often convey specific information about the client version or
   locale.  This facilitates content negotiation and operational work-
   arounds for implementation bugs.  Request header fields that control
   caching can expose state information about a subset of the client's
   history.  Mixing DoH requests with other HTTP requests on the same
   connection also provides an opportunity for richer data correlation.

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   The DoH protocol design allows applications to fully leverage the
   HTTP ecosystem, including features that are not enumerated here.
   Utilizing the full set of HTTP features enables DoH to be more than
   an HTTP tunnel, but it is at the cost of opening up implementations
   to the full set of privacy considerations of HTTP.

   Implementations of DoH clients and servers need to consider the
   benefit and privacy impact of these features, and their deployment
   context, when deciding whether or not to enable them.
   Implementations are advised to expose the minimal set of data needed
   to achieve the desired feature set.

   Determining whether or not a DoH implementation requires HTTP cookie
   [RFC6265] support is particularly important because HTTP cookies are
   the primary state tracking mechanism in HTTP.  HTTP cookies SHOULD
   NOT be accepted by DOH clients unless they are explicitly required by
   a use case.

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 Section 9.2 of [RFC7540].

   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 (see Section 10.6 of [RFC7540]) and padding (see
   Section 10.7 of [RFC7540]).  DoH servers can also add DNS padding
   [RFC7830] if the DoH client requests it in the DNS query.  An
   experimental effort to offer guidance on choosing the padding length
   can be found in [RFC8467].

   The HTTPS connection provides transport security for the interaction
   between the DoH server and client, but it does not provide the
   response integrity of DNS 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 to either
   perform full DNSSEC validation of answers or to trust the DoH server
   to 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 4.2, different response media types will
   provide more or less information from a DNS response, so this choice
   may be affected by the response media type.

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   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.

   In the absence of DNSSEC information, a DoH server can give a client
   invalid data in response to a DNS query.  Section 3 disallows the use
   of DoH DNS responses that do not originate from configured servers.
   This prohibition does not guarantee protection against invalid data,
   but it does reduce the risk.

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 that 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 DoH client and the DoH server.
   Filtering or inspection systems that rely on unsecured transport of
   DNS will not function in a DNS over HTTPS environment due to the
   confidentiality and integrity protection provided by TLS.

   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 DoH server connection
   procedure and the check itself requires DNS resolution to connect to
   the third party, a deadlock can occur.  The use of Online Certificate
   Status Protocol (OCSP) [RFC6960] servers or Authority Information
   Access (AIA) for Certificate Revocation List (CRL) fetching (see
   Section 4.2.2.1 of [RFC5280]) are examples of how this deadlock can
   happen.  To mitigate the possibility of deadlock, the authentication
   given DoH 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 Section 4.4.2.1 of
   [RFC8446] for TLS version 1.3.  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 servers that a DoH server might redirect to.

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   A DoH 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 DoH client cannot use its DoH
   server to initially resolve the server's host name into an address.
   Alternative strategies a client might employ include 1) making the
   initial resolution part of the configuration, 2) IP-based URIs and
   corresponding IP-based certificates for HTTPS, or 3) resolving the
   DNS API server's hostname via traditional DNS or another DoH 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.

   A DoH server is allowed to answer queries with any valid DNS
   response.  For example, a valid DNS response might have the TC
   (truncation) bit set in the DNS header to indicate that the server
   was not able to retrieve a full answer for the query but is providing
   the best answer it could get.  A DoH server can reply to queries with
   an HTTP error for queries that it cannot fulfill.  In this same
   example, a DoH server could use an HTTP error instead of a 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 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>.

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   [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>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [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>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [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>.

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   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,
              <https://www.rfc-editor.org/info/rfc7626>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

11.2.  Informative References

   [FETCH]    "Fetch Living Standard", August 2018,
              <https://fetch.spec.whatwg.org/>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [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>.

   [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>.

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   [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>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [RFC7828]  Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828,
              DOI 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>.

   [RFC8467]  Mayrhofer, A., "Padding Policies for Extension Mechanisms
              for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
              October 2018, <https://www.rfc-editor.org/info/rfc8467>.

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Appendix A.  Protocol Development

   This appendix describes the requirements used to design DoH.  These
   requirements are listed here to help readers understand the current
   protocol, not to limit how the protocol might be developed in the
   future.  This appendix is non-normative.

   The protocol described in this document based 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.

   The following were considered non-requirements:

   o  Supporting network-specific DNS64 [RFC6147]

   o  Supporting other network-specific inferences from plaintext DNS
      queries

   o  Supporting insecure HTTP

Appendix B.  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>

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   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/>

Acknowledgments

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

Authors' Addresses

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org

   Patrick McManus
   Mozilla

   Email: mcmanus@ducksong.com

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