Associating a DoH Server with a Resolver
draft-hoffman-resolver-associated-doh-03

Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                        October 22, 2018
Expires: April 25, 2019

                Associating a DoH Server with a Resolver
                draft-hoffman-resolver-associated-doh-03

Abstract

   Browsers and web applications may want to know if there are one or
   more DoH servers associated with the DNS recursive resolver that the
   operating system is already using.  This would allow them to get DNS
   responses from a resolver that the user (or, more likely, the user's
   network administrator) has already chosen.  This document describes a
   protocol for a resolver to tell a client what its associated DoH
   servers are.

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   This Internet-Draft will expire on April 25, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Finding the DoH Servers Associated with a Resolver  . . . . .   3
     3.1.  Finding the IP Addresses of a Resolver  . . . . . . . . .   3
     3.2.  Finding the DoH Servers Associated with a Resolver  . . .   4
   4.  User Interface  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Design Choices  . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   DoH [RFC8484] requires that one or more DoH servers be configured for
   the DoH client.  That document does not say how the DoH servers are
   found, nor how to select from a list of possible DoH servers, nor
   what the user interface (UI) for the configuration should be.

   There is a use case for browsers and web applications who have one or
   more currently-configured DNS recursive resolvers wanting to use DoH
   for DNS resolution instead.  For example, the recursive resolver
   knows how to give correct answers to DNS queries that contain names
   that are only resolvable in the local context.  Users typically
   configure their DNS recursive resolvers with through manual
   configuration (such as manually editing a /etc/named.conf file) or
   through automatic configuration from a protocol such as DHCP.

   The client that wants to change from its currently-configured Do53
   recursive resolver(s) to one or more DoH servers might be the stub
   resolver in an operating system, although at this time it is rare
   that such stub resolvers can use DoH.  A much more likely use case is
   a browser or web application that is getting name resolution through
   the stub resolver on the computer on which it is running.  The user
   of the browser might have a preference for using a DoH server, and
   they might need to use a DoH server that is associated with the
   resolver that the computer is currently using so that its queries for
   non-global names are answered correctly.  They may also be required

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   to use only resolvers that are approved by their organization's
   network operators.

   To address these use cases, this document defines a new special use
   domain name (described in [RFC6761]) and a well-known URI
   [I-D.nottingham-rfc5785bis].  When combined, they allow an
   application that can use the POSIX "getaddrinfo()" function and
   resolve HTTP and HTTPS URLs to get a list of the DoH servers
   associated with at least one of the resolvers being used by the
   operating system on the system on which the application is being run.

   The design choices for this protocol, particularly earlier designs
   that were deemed unusable, are described in Section 5.

2.  Terminology

   In this document, "DoT" is used to indicate DNS over TLS as defined
   in [RFC7858].

   In this document, "Do53" is used to indicate DNS over UDP or TCP as
   defined in [RFC1035].

   "DoH client" and "DoH server" are defined in [RFC8484].

   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.  Finding the DoH Servers Associated with a Resolver

   To find the DoH Servers associated with a resolver, an application
   uses a special use domain name that causes a resolver to return its
   IP addresses.  It uses those IP addresses as part of a well-known URI
   to find out the URI templates [RFC6570] to use for the DoH server(s)
   associated with the resolver.

3.1.  Finding the IP Addresses of a Resolver

   A browser is able to use the POSIX "getaddrinfo()" function to
   convert host names into IP addresses through the stub resolver in the
   operating system on which it is running.  It can also send queries to
   a resolver, but it would need to have the address of that resolver
   first.  The same is true for web applications.

   In order for a browser (or other application) to find the address of
   the resolver that the operating system is using, it uses the POSIX

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   "getaddrinfo()" function (or some equivalent) with the special use
   name "resolver-addresses.arpa".  When a resolver that understands
   this special use domain name receives a query for either resolver-
   addresses.arpa/IN/A or resolver-addresses.arpa/IN/AAAA, it returns
   its own IP addresses in the answer.

3.2.  Finding the DoH Servers Associated with a Resolver

   To find the DoH servers associated with a resolver, the client uses
   the addresses returned from the query to resolver-addresses.arpa and
   sends a query to

   https://IPADDRESSGOESHERE/.well-known/doh-servers-associated/

   The resolver replies with its associated DoH servers as URI Templates
   [RFC6570].

   [[ Need to describe the media types; likely JSON ]]

   [[ Need to talk about what a response with an empty list means ]]

   [[ Need to talk about what happens if authentication fails.  This is
   complicated by the fact that the application doesn't know if the OS-
   to-resolver communication is authenticated. ]]

   [[ Need to talk about HTTP caching ]]

   A client MUST try to establish a new list of DoH servers associated
   with a resolver every time the configured resolver in the operating
   system changes.

4.  User Interface

   For this protocol to be useful in a browser, the browser needs to
   have an entry in its configuration interface where the allowed DoH
   servers are listed that indicates that a DoH server from the
   configured Do53 or DoT resolver is allowed.  That wording might say
   something like "DoH server associated with my current resolver".

5.  Design Choices

   The primary use case for this protocol is a browser or web
   application that is getting name resolution through the stub resolver
   on the computer on which it is running wanting to switch its name
   resolution to DoH.  A secondary use case is an OS that wants to make
   a similar switch.

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   An earlier design suggestion was to use a new RRtype with a query to
   ./IN/NEWRRTYPE.  However, it was pointed out that this would not work
   going through stub resolvers that validate DNSSEC.

   An earlier design suggestion was to use DHCP to tell the OS the DoH
   servers that the stub resolver might use.  That protocol is
   orthogonal to the one in this document in that it addresses a
   different use case.  If both the protocol in this document and a
   DHCP-based protocol are standardized, they could co-exist.  However,
   there is no current mechanism for a stub resolver to tell a browser,
   or a web application, what DoH server the stub resolver is using, so
   DoH configuration in the stub resolver would not prevent the browser
   from trying to find a DoH server on its own.

   An earlier design suggestion was to use an EDNS0 [RFC6891] extension.
   The design chosen in this document meets the use case better because
   applications cannot communicate EDNS0 extensions to the stub
   resolver.

   An earlier design suggestion used a special use domain name of
   resolver-associated-doh.arpa with an RRtype of TXT.  The design
   chosen in this document meets the use case better because
   applications cannot query the stub resolver for types other than
   address records.

6.  IANA Considerations

   IANA will record the domain name "resolver-addresses.arpa." in the
   "Special-Use Domain Names" registry [SUDN].  IANA MUST NOT delegate
   resolver-addresses.arpa in the .arpa zone.

   [[ When this document settles down, need to register ".well-known/
   doh-servers-associated" as specified in [I-D.nottingham-rfc5785bis].
   ]]

7.  Privacy Considerations

   Allowing a user to use DoH instead of Do53 increases communication
   privacy because of the TLS protection.

   When a Do53 or DoT server indicates that a particular DoH server is
   associated with it, the client might assume that the DoH server has
   the same information privacy policies as the Do53 or DoT server.
   Therefore, a Do53 or DoT server SHOULD NOT recommend a DoH server
   unless that DoH server has the same (or better) information privacy
   policy as the Do53 or DoT server.

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8.  Security Considerations

   There is currently no way for an application to know whether the
   operating system's stub resolver is using a transport that assures
   data integrity such as DoT.

   Even is an application could determine the use of a transport like
   DoT, the application would also need to know whether the transport
   was authenticated or was simply chosen opportunistically.

9.  References

9.1.  Normative References

   [I-D.nottingham-rfc5785bis]
              Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", draft-nottingham-rfc5785bis-08 (work in
              progress), October 2018.

   [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>.

   [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>.

   [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>.

   [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>.

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   [SUDN]     "Special-Use Domain Names", n.d.,
              <https://www.iana.org/assignments/
              special-use-domain-names/>.

9.2.  Informative References

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [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>.

Acknowledgments

   The use case in this document was inspired by discussions and the
   DRIU BoF at IETF 102 and later in the DNSOP Working Group.  Vladimir
   Cunat, Philip Homburg, and Shumon Huque offered useful advice to
   greatly improve an early version of the protocol.

Author's Address

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org

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