Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                              P. McManus
Expires: June 1, 2018                                            Mozilla
                                                       November 28, 2017

                         DNS Queries over HTTPS


   DNS queries sometimes experience problems with end to end
   connectivity at times and places where HTTPS flows freely.

   HTTPS provides the most practical mechanism for reliable end to end
   communication.  Its use of TLS provides integrity and confidentiality
   guarantees and its use of HTTP allows it to interoperate with
   proxies, firewalls, and authentication systems where required for

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

   [[ There is a repository for this draft at
   draft-ietf-doh-dns-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

   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 June 1, 2018.

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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Protocol Requirements . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Non-requirements  . . . . . . . . . . . . . . . . . . . .   5
   5.  The HTTP Request  . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  DNS Wire Format . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  The HTTP Response . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  HTTP Integration  . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Registration of Well-Known URI  . . . . . . . . . . . . .   9
     8.2.  Registration of application/dns-udpwireformat Media Type    9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  12
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Previous Work on DNS over HTTP or in Other Formats .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The Internet does not always provide end to end reachability for
   native DNS.  On-path network devices may spoof DNS responses, block
   DNS requests, or just redirect DNS queries to different DNS servers
   that give less-than-honest answers.

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   Over time, there have been many proposals for using HTTP and HTTPS as
   a substrate for DNS queries and responses.  To date, none of those
   proposals have made it beyond early discussion, partially due to
   disagreement about what the appropriate formatting should be and
   partially because they did not follow HTTP best practices.

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

   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, proxying, and compression.

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

2.  Terminology

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

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119

3.  Use Cases

   There are two initial use cases for this protocol.

   The primary use case is to prevent on-path network devices from
   interfering with DNS operations.  This interference includes, but is
   not limited to, spoofing DNS responses, blocking DNS requests, and

   In this use, clients - whether operating systems or individual
   applications - will be explicitly configured to use a DOH server as a
   recursive resolver by its user (or administrator).  They might use

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   the DOH server for all queries, or only for a subset of them.  The
   specific configuration mechanism is out of scope for this document.

   A secondary use case is allowing web applications to access DNS
   information, by using existing APIs in browsers to access it over
   HTTP in a safe way consistent with Cross Origin Resource Sharing
   (CORS) [CORS].

   This is technically already possible (since the server controls both
   the HTTP resources it exposes and the use of browser APIs by its
   content), but standardisation might make this easier to accomplish.

   Note that in this second use, the browser does not consult the DOH
   server or use its responses for any DNS lookups outside the scope of
   the application using them; i.e., there is (currently) no API that
   allows a Web site to poison DNS for others.

   [[ This paragraph is to be removed when this document is published as
   an RFC ]] Note that these use cases are different than those in a
   similar protocol described at [I-D.ietf-dnsop-dns-wireformat-http].
   The use case for that protocol is proxying DNS queries over HTTP
   instead of over DNS itself.  The use cases in this document all
   involve query origination instead of proxying.

4.  Protocol Requirements

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

   o  The protocol must use normal HTTP semantics.

   o  The queries and responses must be able to be flexible enough to
      express every normal DNS query.

   o  The protocol must allow implementations to use HTTP's content
      negotiation mechanism.

   o  The protocol must ensure interoperable media formats through a
      mandatory to implement format wherein a query must be able to
      contain one or more EDNS extensions, including those not yet

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

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4.1.  Non-requirements

   o  Supporting network-specific DNS64 [RFC6147]

   o  Supporting other network-specific inferences from plaintext DNS

   o  Supporting insecure HTTP

   o  Supporting legacy HTTP versions

5.  The HTTP Request

   To make a DNS API query, a DNS API client sends an HTTP request to
   the URI of the DNS API.

   The URI scheme MUST be https.

   A client can be configured with a DNS API URI, or it can discover the
   URI.  This document defines a well-known URI path of "/.well-known/
   dns-query" so that a discovery process that produces a domain name or
   domain name and port can be used to construct the DNS API URI.  (See
   Section 8 for the registration of this in the well-known URI
   registry.)  DNS API servers SHOULD use this well-known path to help
   contextualize DNS Query requests that use server push [RFC7540].

   A DNS API Client encodes a single DNS query into the HTTP request
   using either the HTTP GET or POST 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.

   When using the GET method the URI path MUST contain a query parameter
   with the name of ct and a value indicating the media-format used for
   the body parameter.  The value may either be an explicit media type
   (e.g. ct=application/dns-udpwireformat&body=...) or it may be empty.
   An empty value indicates the default application/dns-udpwireformat
   type (e.g. ct&body=...).

   When using the GET method the URI path MUST contain a query parameter
   with the name of body.  The value of the parameter is the content of
   the request encoded with base64url [RFC4648].  Using the GET method
   is friendlier to many HTTP cache implementations.

   The DNS API Client SHOULD include an HTTP "Accept:" request header to
   say what type of content can be understood in response.  The client

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   MUST be prepared to process "application/dns-udpwireformat"
   Section 5.1 responses but MAY process any other type it receives.

   In order to maximize cache friendliness, DNS API clients using media
   formats that include DNS ID, such as application/dns-udpwireformat,
   SHOULD use a DNS ID of 0 in every DNS request.  HTTP correlates
   request and response, thus eliminating the need for the ID in a media
   type such as application/dns-udpwireformat and the use of a varying
   DNS ID can cause semantically equivalent DNS queries to be cached

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

5.1.  DNS Wire Format

   The media type is "application/dns-udpwireformat".  The body is the
   DNS on-the-wire format is defined in [RFC1035].

   When using the GET method, the body MUST be encoded with base64url
   [RFC4648].  Padding characters for base64url MUST NOT be included.

   When using the POST method, the body is not encoded.

   DNS API clients using the DNS wire format MAY have one or more
   EDNS(0) extensions [RFC6891] in the request.

5.2.  Examples

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

   For this example assume a DNS API server is following this
   specification on origin and the well-
   known path.  The DNS API client chooses to send its requests in
   application/dns-udpwirefomat but indicates it can parse replies in
   that format or as a hypothetical JSON-based content type.  The
   application/simpledns+json type used by this example is currently

   :method = GET
   :scheme = https
   :authority =
   :path = /.well-known/dns-query?ct&  (no CR)
   accept = application/dns-udpwireformat, application/simpledns+json

   The same DNS query, using the POST method would be:

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   :method = POST
   :scheme = https
   :authority =
   :path = /.well-known/dns-query
   accept = application/dns-udpwireformat, application/simpledns+json
   content-type = application/dns-udpwireformat
   content-length = 33

   <33 bytes represented by the following hex encoding>
   abcd 0100 0001 0000 0000 0000 0377 7777
   0765 7861 6d70 6c65 0363 6f6d 0000 0100

6.  The HTTP Response

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

   At the time this is published, the response types are works in
   progress.  The only known response type is "application/dns-
   udpwireformat", but it is possible that at least one JSON-based
   response format will be defined in the future.

   The DNS response for "application/dns-udpwireformat" in Section 5.1
   MAY have one or more EDNS(0) extensions, depending on the extension
   definition of the extensions given in the DNS request.

   Native HTTP methods are used to correlate requests and responses.
   Responses may be returned in a different temporal order than requests
   were made using the protocols native multi-streaming functionality.

   The Answer section of a DNS response contains one or more RRsets.
   (RRsets are defined in [RFC7719].)  According to [RFC2181], each
   resource record in an RRset is supposed to have the Time To Live
   (TTL) freshness information.  Different RRsets in the Answer section
   can have different TTLs, though it is only possible for the HTTP
   response to have a single freshness lifetime.  The HTTP response
   freshness lifetime ([RFC7234] Section 4.2) should be coordinated with
   the Resource Record bearing the smallest TTL in the Answer section of
   the response.  The HTTP freshness lifetime SHOULD be set to expire at
   the same time any of the DNS Records reach a 0 TTL.  The response
   freshness lifetime MUST NOT be greater than that indicated by the DNS
   Record with the smallest TTL in the response.

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

   A DNS API Server MUST be able to process application/dns-
   udpwireformat request messages.

   A DNS API Server SHOULD respond with HTTP status code 415 upon
   receiving a media type it is unable to process.

   This document does not change the definition of any HTTP response
   codes or otherwise proscribe their use.

   HTTP revalidation of cached DNS information may be of limited value
   as revalidation provides only a bandwidth benefit and DNS
   transactions are normally latency bound instead.  Furthermore, the
   HTTP response headers that enable revalidation (such as "Last-
   Modified" and "Etag") are often fairly large when compared to the
   overall DNS response size, and have a variable nature that creates
   constant pressure on the HTTP/2 compression dictionary [RFC7541].
   Other types of DNS data, such as zone transfers, may be larger and
   benefit more from revalidation.  DNS API servers may wish to consider
   whether providing these optional response headers is worthwhile.

6.1.  Example

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

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

   <64 bytes represented by the following hex encoding>
   abcd 8180 0001 0001 0000 0000 0377 7777
   0765 7861 6d70 6c65 0363 6f6d 0000 0100

   0103 7777 7707 6578 616d 706c 6503 636f
   6d00 0001 0001 0000 0080 0004 5db8 d822

7.  HTTP Integration

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

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7.1.  HTTP/2

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

   The messages in classic UDP based DNS [RFC1035] are inherently
   unordered and have low overhead.  A competitive HTTP transport needs
   to support reordering, parallelism, priority, and header compression
   to acheive 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 would result in very
   poor performance for many uses cases.

8.  IANA Considerations

8.1.  Registration of Well-Known URI

   This specification registers a Well-Known URI [RFC5785]:

   o  URI Suffix: dns-query

   o  Change Controller: IETF

   o  Specification Document(s): [this specification]

8.2.  Registration of application/dns-udpwireformat Media Type

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   Subject: Registration of MIME media type

   MIME media type name: application

   MIME subtype name: dns-udpwireformat

   Required parameters: n/a

   Optional parameters: n/a

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

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

   Interoperability considerations:  None.

   Published specification:  This document.

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

   Additional information:

   Magic number(s):  n/a

   File extension(s):  n/a

   Macintosh file type code(s):  n/a

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

   Intended usage:  COMMON

   Restrictions on usage:  n/a

   Author:  Paul Hoffman,

   Change controller:  IESG

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9.  Security Considerations

   Running DNS over HTTPS relies on the security of the underlying HTTP
   transport.  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.  Sections 10.6 (Compression) and 10.7 (Padding) of
   [RFC7540] provide some further advice on mitigations within an HTTP/2

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

   [[ From the WG charter:

   The working group will analyze the security and privacy issues that
   could arise from accessing DNS over HTTPS.  In particular, the
   working group will consider the interaction of DNS and HTTP caching.


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

   [[ From the WG charter:

   The working group may define mechanisms for discovery of DOH servers
   similar to existing mechanisms for discovering other DNS servers if
   the chairs determine that there is both sufficient interest and
   working group consensus.


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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

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

   Many HTTPS implementations perform real time third party checks of
   the revocation status of the certificates being used by TLS.  If this
   check is done as part of the DNS API server connection procedure and
   the check itself requires DNS resolution to connect to the third
   party a deadlock can occur.  The use of an OCSP [RFC6960] server is
   one example of how this can happen.  DNS API servers SHOULD utilize
   OCSP Stapling [RFC6961] to provide the client with certificate
   revocation information that does not require contacting a third

11.  Acknowledgments

   Joe Hildebrand contributed lots of material for a different iteration
   of this document.  Helpful early comments were given by Ben Schwartz
   and Mark Nottingham.

12.  References

12.1.  Normative References

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

   [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-

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

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   [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-

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010, <https://www.rfc-

   [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,

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013, <https://www.rfc-

   [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,

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

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

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

12.2.  Informative References

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

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              Song, L., Vixie, P., Kerr, S., and R. Wan, "DNS wire-
              format over HTTP", draft-ietf-dnsop-dns-wireformat-http-01
              (work in progress), March 2017.

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

   [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-

   [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-

   [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,

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

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

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

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






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Authors' Addresses

   Paul Hoffman


   Patrick McManus


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