Associating a DoH Server with a Resolver
draft-hoffman-resolver-associated-doh-01

Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                         August 25, 2018
Expires: February 26, 2019

                Associating a DoH Server with a Resolver
                draft-hoffman-resolver-associated-doh-01

Abstract

   Some clients will want to know if there are one or more DoH servers
   associated with the DNS recursive resolver that the client is already
   using.  This document describes a protocol for a resolver to tell a
   client what its associated DoH servers are.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Signalling the DoH Servers Associated with a Resolver . . . .   3
     3.1.  Signalling in the Resolver  . . . . . . . . . . . . . . .   3
     3.2.  Client Handling of the Signals  . . . . . . . . . . . . .   3
   4.  Design Choices  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   DoH [I-D.ietf-doh-dns-over-https] 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 clients who have one or more currently-
   configured DNS recursive resolvers wanting to use DoH for DNS
   resolution instead.  Clients 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 DNS
   recursive resolvers 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 web browser 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 it might want a DoH server that is
   associated with the resolver that the computer is currently using.

   To address this use case, this document defines a new special use
   domain name "resolver-associated-doh.arpa." and describes how it is
   used.  The design choices made are described in Section 4.

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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
   [I-D.ietf-doh-dns-over-https].

   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.  Signalling the DoH Servers Associated with a Resolver

   To find the DoH servers associated with a resolver, the client sends
   that resolver a query for resolver-associated-doh.arpa in class IN
   with the RRtype of TXT [RFC1035] (that is, the query is resolver-
   associated-doh.arpa/IN/TXT).

   The resolver replies with its associated DoH servers as URI Templates
   [RFC6570] in the TXT RRset in the Answer section.

3.1.  Signalling in the Resolver

   A resolver that understands this protocol MUST send a TXT RRset in
   the Answer section.  Each TXT record contains one URI Template.

   If a resolver that understands this protocol has no associated DoH
   servers, the TXT RRset contains exactly one record that has an empty
   string as the RDATA; that is, the RDLENGTH in that record is 1, and
   the RDATA contains just the byte 0x00.

   Note that the zone resolver-associated-doh.arpa, as it is delegated
   in Section 5, has no TXT records.  The resolver adds its own TXT
   records to the answer.

3.2.  Client Handling of the Signals

   The client uses the TXT records in the response to the resolver-
   associated-doh.arpa/IN/TXT query as a list of the URI Templates of
   the DoH servers associated with the resolver.  Note that TXT records
   can contain multiple "character-strings" [RFC1035]; for this

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   protocol, all characters-strings in a TXT record are concatenated to
   form a single URI Template.

   If a client sends the resolver-associated-doh.arpa/IN/TXT query over
   a transport that assures data integrity (such as DoT), and it
   receives a response that has the RCODE set to NOERROR and no relevant
   answers in the Answer section (also called a "NODATA" response in
   [RFC2308]), the client can assume that the resolver does not know
   this protocol.

   See Section 7 for warnings about sending the resolver-associated-
   doh.arpa/IN/TXT query over a transport that does not assure data
   integrity (such as Do53).

   The client SHOULD only use a DoH server listed in the response to
   resolver-associated-doh.arpa/IN/TXT for the length of time listed as
   the TXT RRset's TTL field.  Using an associated DoH server beyond the
   TTL can expose the client to problems such as loss of DNS service.
   The client SHOULD send a resolver-associated-doh.arpa/IN/TXT query
   before the expiration of the TTL in a previous response in order to
   allow the client to continue to use an associated DoH server without
   interruption.

   A client MUST issue a new resolver-associated-doh.arpa/IN/TXT query
   every time the configured resolver changes.

4.  Design Choices

   The primary use case for this protocol is a web browser 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.

   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 come into existence, they could co-exist.
   However, there is no current mechanism for a stub resolver to tell a
   web browser 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.

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   An earlier design suggestion was to use an EDNS0 [RFC6891] extension.
   The design chosen (the new RRtype and resolver-associated-
   doh.arpa/IN/TXT query) meets the use case better because if the stub
   resolver does not understand EDNS0, or there is a middlebox between
   the computer and the resolver that mishandles EDNS extensions, the
   information will not make it back to the web browser.

   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.  IANA Considerations

   IANA will record the domain name "resolver-associated-doh.arpa." in
   the "Special-Use Domain Names" registry [SUDN].

   IANA, with the approval of the IAB, will delegate "resolver-
   associated-doh.arpa." in the ".arpa." zone.

   This delegation MUST NOT include a DS record.

   This delegation MUST point to one or more black hole servers, for
   example, "blackhole-1.iana.org." and "blackhole-2.iana.org.".

   This delegation MUST NOT ever have a resource record with the RRtype
   "TXT".

6.  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.

7.  Security Considerations

   If a client sends the resolver-associated-doh.arpa/IN/TXT query over
   a transport that does not assure data integrity (such as Do53), an
   attacker between the client and the resolver can change the response.

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   o  A client who sends a query over such a transport and begins to use
      a DoH server based on the response MUST NOT assign a level of
      trust to that DoH server greater than to the trust it gave to the
      resolver itself.

   o  A client who sends a query over such a transport and receives a
      response that has an NXDOMAIN response code cannot be sure that
      the response comes from a resolver that does not know this
      protocol.  Instead, the client SHOULD assume that there could be
      an on-path attack where the attacker does not want the client to
      use DoH.

8.  References

8.1.  Normative References

   [I-D.ietf-doh-dns-over-https]
              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", draft-ietf-doh-dns-over-https-14 (work in
              progress), August 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>.

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

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8.2.  Informative References

   [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>.

   [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.

Author's Address

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org

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