Internet DRAFT - draft-schinazi-httpbis-doh-preference-hints

draft-schinazi-httpbis-doh-preference-hints







Network Working Group                                        D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Experimental                                N. Sullivan
Expires: January 9, 2020                                         J. Kipp
                                                              Cloudflare
                                                           July 08, 2019


                     DoH Preference Hints for HTTP
             draft-schinazi-httpbis-doh-preference-hints-00

Abstract

   When using a publicly available DNS-over-HTTPS (DoH) server, some
   clients may suffer poor performance when the authoritative DNS server
   is located far from the DoH server.  For example, a publicly
   available DoH server provided by a Content Delivery Network (CDN)
   should be able to resolve names hosted by that CDN with good
   performance but might take longer to resolve names provided by other
   CDNs, or might provide suboptimal results if that CDN is using DNS-
   based load balancing and returns different address records depending
   or where the DNS query originated from.  This document attempts to
   lessen these issues by allowing the web server to indicate to the
   client which DoH server can best resolve its addresses.  This
   document defines an HTTP header field that enables web host operators
   to inform user agents of the preferred DoH servers to use for
   subsequent DNS lookups for the host's domain.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 2020.







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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  The DoH-Preference Header Field . . . . . . . . . . . . . . .   3
     2.1.  The max-age Directive . . . . . . . . . . . . . . . . . .   4
     2.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Considerations For Choosing a Preferred DoH Server  . . .   4
   4.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Fallback  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Internationalization Considerations . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   When using a publicly available DNS-over-HTTPS (DoH) server, some
   clients may suffer poor performance when the authoritative DNS server
   is located far from the DoH server.  For example, a publicly
   available DoH server provided by a Content Delivery Network (CDN)
   should be able to resolve names hosted by that CDN with good
   performance but might take longer to resolve names provided by other
   CDNs, or might provide suboptimal results if that CDN is using DNS-
   based load balancing and returns different address records depending
   or where the DNS query originated from.  This document attempts to
   lessen these issues by allowing the web server to indicate to the
   client which DoH server can best resolve its addresses.  This



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   document defines an HTTP header field that enables web host operators
   to inform user agents of the preferred DoH servers to use for
   subsequent DNS lookups for the host's domain.

   When a web server wishes its client to use a specific DoH server to
   resolve its addresses, it can send the DoH-Preference header to
   indicate that preference to the user agent.  The header is not
   prescriptive, it only indicates the server's preference to the user.
   It also only applies to the web server's current hostname.

   The header defined in this document is not intended to be used as a
   discovery mechanism for clients learning about the existence of new
   DoH servers.  Instead, it is intended to be used as an optimization
   technique for clients with support for multiple DoH servers who wish
   to choose the most performant DNS server for a given query.

1.1.  Conventions and Definitions

   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.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405] along with the "#rule" extension defined in Section 7 of
   [RFC7230].  The rules below are defined in [RFC5234], [RFC7230], and
   [RFC7234]:

     OWS           = <OWS, see {{RFC7230}}, Section 3.2.3>
     delta-seconds = <delta-seconds; see {{RFC7234}}, Section 1.2.1>
     quoted-string = <quoted-string, see {{RFC7230}}, Section 3.2.6>
     token         = <token, see {{RFC7230}}, Section 3.2.6>

2.  The DoH-Preference Header Field

   An HTTPS origin can indicate its preference regarding DoH servers to
   the client by adding an DoH-Preference header field to responses.

     DoH-Preference = doh-uri *( OWS ";" OWS parameter )
     doh-uri        = quoted-string
     parameter      = token "=" ( token / quoted-string )

   The "doh-uri" component consists of the DoH URI Template as defined
   in [RFC8484].






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   Sending multiple DoH-Preference header fields indicates that the
   server prefers multiple DoH servers.  They are sent in decreasing
   order of preference.

2.1.  The max-age Directive

   The REQUIRED "max-age" directive specifies the number of seconds,
   after the reception of the DoH-Preference header field, during which
   the user agent caches the server's DoH preferences.

   The syntax of the max-age directive's REQUIRED value (after quoted-
   string unescaping, if necessary) is defined as:

       max-age-value = delta-seconds

   A max-age value of zero (i.e., "max-age=0") signals the user agent to
   remove the DoH URI template from its cache.

2.2.  Examples

   The header below indicates that the user agent should consider
   querying DNS results for the web server's hostname using
   "dnsserver.example.net" for approximately six months.  (Lines are
   folded to fit.)

   DoH-Preference: "https://dnsserver.example.net/dns-query{?dns}";
       max-age=15768000

3.  Server Behavior

   Web servers MAY send a DoH-Preference header to indicate to clients
   that it would prefer they use that DoH server when resolving
   addresses for the hostname of the web server.  Web servers MAY send
   multiple DoH-Preference headers.  Web servers MUST NOT send the DoH-
   Preference header in HTTP responses conveyed over a non-secure
   transport.

3.1.  Considerations For Choosing a Preferred DoH Server

   The choice of DoH server can affect overall performance and
   responsiveness as perceived by the client.  Some example
   considerations in choosing a preferred DoH server are:

   o  A DoH host specified as a host name rather than an IP address will
      require one or more additional DNS resolutions when the cached DNS
      entries for the resolver or resolvers expire.

   o  Support for extension mechanisms (e.g.  EDNS(0)) may be desired.



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   o  Clients, particularly mobile device clients, may frequently move
      between networks that have different network paths to the DoH
      server.

4.  Client Behavior

   If a client chooses to act on received DoH-Preference headers, it
   SHOULD cache the server's hostname and the corresponding DoH URI
   template and lifetime.  It SHOULD then send subsequent DNS requests
   for A and AAAA records for that host name to the provided DoH server,
   until the cache entry expires after the time specified in the "max-
   age" directive.  Any received DoH-Preference header replaces and
   overrides any and all information received in a previous DoH-
   Preference header for the same host name and DoH URI template.

   Clients MAY decide to only respect the DoH-Preference header for a
   subset of vetted DoH servers.

   Clients MUST NOT use the contents of the DoH-Preference header to
   impact how it resolves other domain names.  Clients MUST ignore the
   DoH-Preference header in HTTP responses conveyed over a non-secure
   transport.

   If the DoH-Preference URI contains a host expressed as a host name
   rather than as an IP address and that host name is resolved via DoH,
   the DoH server might also specify a DoH-Preference header.  This
   means that respecting the DoH server recommendation could result in
   an excessively long chain of DoH queries or a loop of DoH servers.
   Clients SHOULD be capable of detecting a loop or an excessively long
   chain of DoH servers and treat these conditions as a query failure.

4.1.  Fallback

   If resolution using the recommended DoH server fails, clients MUST
   fall back and retry their query using another DNS resolution
   mechanism.

5.  Internationalization Considerations

   An internationalized domain name that appears in the header field
   MUST be expressed using A-labels; see Section 2.3.2.1 of [RFC5890].

6.  Security Considerations

   The DoH-Preference header allows a web server to impact how a user
   agent resolves DNS A and AAAA records for its own host name.  Since
   the web server has proven ownership of the domain name via its TLS
   certificate and the DNS result that led to the initial connection,



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   impacting future DNS resolutions to the same host name has limited
   security impact.

   The potential exists for the DoH-Preference header to be used as a
   form of web tracking.  Because a DoH URI is chosen by the server,
   cached by the client, and then subsequently contacted by the client,
   a uniquely chosen DoH URI could identify a client even after other
   client-side state has expired or been removed.  Clients SHOULD expire
   cached DoH URIs when other client state expires unless the URIs refer
   to vetted DoH servers or match common DoH URI patterns that preclude
   client-unique URIs.

7.  IANA Considerations

   This document, if approved, requests IANA to register the DoH-
   Preference header in the "Permanent Message Header Field Names"
   registry maintained at https://www.iana.org/assignments/message-
   headers/ [1].

     +-------------------+----------+----------+------------+
     | Header Field Name | Protocol | Status   | Reference  |
     +-------------------+----------+----------+------------+
     | DoH-Preference    | http     | standard | Section 2  |
     +-------------------+----------+----------+------------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

8.  References

8.1.  Normative References

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.





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   [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>.

   [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>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [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>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

8.2.  URIs

   [1] https://www.iana.org/assignments/message-headers/

Acknowledgments

   The authors would like to thank many members of the IETF community,
   as this document is the fruit of many hallway conversations.

Authors' Addresses

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America

   Email: dschinazi.ietf@gmail.com


   Nick Sullivan
   Cloudflare

   Email: nick@cloudflare.com





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   Jesse Kipp
   Cloudflare

   Email: jkipp@cloudflare.com















































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