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rfc:rfc8484

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.

Hoffman & McManus Standards Track [Page 1] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

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

Hoffman & McManus Standards Track [Page 2] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

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.

Hoffman & McManus Standards Track [Page 3] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

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.

Hoffman & McManus Standards Track [Page 4] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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

Hoffman & McManus Standards Track [Page 5] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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

Hoffman & McManus Standards Track [Page 6] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

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.

Hoffman & McManus Standards Track [Page 7] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

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.

Hoffman & McManus Standards Track [Page 8] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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-

Hoffman & McManus Standards Track [Page 9] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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.

Hoffman & McManus Standards Track [Page 10] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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.

Hoffman & McManus Standards Track [Page 11] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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.

Hoffman & McManus Standards Track [Page 12] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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.

Hoffman & McManus Standards Track [Page 13] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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.

Hoffman & McManus Standards Track [Page 14] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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.

Hoffman & McManus Standards Track [Page 15] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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>.

Hoffman & McManus Standards Track [Page 16] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 [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>.

Hoffman & McManus Standards Track [Page 17] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 [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>.

Hoffman & McManus Standards Track [Page 18] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 [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>.

Hoffman & McManus Standards Track [Page 19] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

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>

Hoffman & McManus Standards Track [Page 20] RFC 8484 DNS Queries over HTTPS (DoH) October 2018

 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

Hoffman & McManus Standards Track [Page 21]

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