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Versions: (draft-hoffman-dns-over-https) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                              P. McManus
Expires: December 17, 2018                                       Mozilla
                                                           June 15, 2018


                      DNS Queries over HTTPS (DoH)
                    draft-ietf-doh-dns-over-https-11

Abstract

   This document describes how to make DNS queries over 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 December 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.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Requirements . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Non-requirements  . . . . . . . . . . . . . . . . . . . .   4
   4.  Selection of DoH Server . . . . . . . . . . . . . . . . . . .   4
   5.  The HTTP Exchange . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  The HTTP Request  . . . . . . . . . . . . . . . . . . . .   4
       5.1.1.  HTTP Request Examples . . . . . . . . . . . . . . . .   5
     5.2.  The HTTP Response . . . . . . . . . . . . . . . . . . . .   7
       5.2.1.  Handling DNS and HTTP Errors  . . . . . . . . . . . .   7
       5.2.2.  HTTP Response Example . . . . . . . . . . . . . . . .   7
   6.  HTTP Integration  . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Cache Interaction . . . . . . . . . . . . . . . . . . . .   8
     6.2.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  Server Push . . . . . . . . . . . . . . . . . . . . . . .  10
     6.4.  Content Negotiation . . . . . . . . . . . . . . . . . . .  10
   7.  Definition of the application/dns-message media type  . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Registration of application/dns-message Media Type  . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  13
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Previous Work on DNS over HTTP or in Other Formats  . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   This document defines a specific protocol for sending DNS [RFC1035]
   queries and getting DNS responses over HTTP [RFC7540] using 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.






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   The integration with HTTP provides a transport suitable for both
   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].  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.

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.



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      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.  Selection of DoH Server

   Configuration, discovery, and updating of the URI Template [RFC6570]
   (see Section 5.1) 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.  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, 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.

5.  The HTTP Exchange

5.1.  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.



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   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 7), encoded with base64url
   [RFC4648].

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

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

   In order to maximize cache friendliness, DoH 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.

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

5.1.1.  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 application/dns-message typed bodies.

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




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   :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







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5.2.  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 is solely
   up to the format, and not defined in this protocol.

   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 6.1 discusses the relationship between DNS and HTTP response
   caching.

5.2.1.  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 ([RFC7231]
   Section 6.3) can be 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
   authorization failures (HTTP status code 401 [RFC7235] Section 3.1).
   It could also mean that the DoH client retries with a different DoH
   server, such as for unsupported media types (HTTP status code 415,
   [RFC7231] Section 6.5.13), or where the server cannot generate a
   representation suitable for the client (HTTP status code 406,
   [RFC7231] Section 6.5.6), and so on.

5.2.2.  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.





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   :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

6.  HTTP Integration

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

6.1.  Cache Interaction

   A DoH exchange can pass through a hierarchy of caches that include
   both HTTP and DNS specific caches.  These caches may exist beteen the
   DoH server and client, or on the DoH client itself.  HTTP caches are
   by design generic; 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 (POST requests are not
   cacheable unless specific response headers are sent; this is not
   widely implemented, and not advised for DoH).

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

   The assigned freshness lifetime of a DoH HTTP response SHOULD be the
   smallest TTL in the Answer section of the DNS response.  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").  The assigned freshness
   lifetime MUST NOT be greater than 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.




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   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 a cache entry that is no longer
   fresh.  In such a case, the client reuses all of a cached entry, or
   none of it.

   DoH servers also need to consider 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 ([RFC7231]
   Section 7.1.4) to establish a secondary cache key ([RFC7234]
   Section 4.1).

   DoH clients MUST account for the Age response header's value
   ([RFC7234]) when calculating the DNS TTL of a response.  For example,
   if a RRset is received with a DNS TTL of 600, but the Age header
   indicates that the response has been cached for 250 seconds, the
   remaining lifetime of the RRset is 350 seconds.

   DoH clients can request an uncached copy of a response by using the
   "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 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 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.








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6.2.  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.

6.3.  Server Push

   Before using DoH response data for DNS resolution, the client MUST
   establish that the HTTP request URI may 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 ([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.

6.4.  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 ([RFC7231]
   Section 3.4).  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.  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].  The format was originally for DNS over UDP.  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]).

   DoH clients using this media type MAY have one or more 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.



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   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.

8.  IANA Considerations

8.1.  Registration of application/dns-message Media Type








































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   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







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

   The HTTPS connection provides transport security for the interaction
   between the DoH server and client, but 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 5.2, 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.

   Section 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.

   In the absence of DNSSEC information, a DoH server can give a client
   invalid data in response to a DNS query.  Section 4 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 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.



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   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.

   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 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, 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
   [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 DoH server might
   redirect to.

   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 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 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.





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   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>.

   [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>.







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   [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>.

   [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>.

   [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>.





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   [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>.

   [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>.

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 Mattsson, Alex Mayrhofer, Mark Nottingham,



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   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


















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