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Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                         August 23, 2018
Expires: February 24, 2019


                Associating a DoH Server with a Resolver
                draft-hoffman-resolver-associated-doh-00

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

   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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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on February 24, 2019.

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
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   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.  RAD RRtype Description  . . . . . . . . . . . . . . . . . . .   3
   4.  Use of RAD  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Server Use of RAD . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Client Use of RAD . . . . . . . . . . . . . . . . . . . .   4
   5.  Design Choices  . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Root Zone . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.2.  Registration for RAD RRtype . . . . . . . . . . . . . . .   5
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 RRtype and
   describes how it is used.  The design choices made are described in
   Section 5.




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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.  RAD RRtype Description

   The mnemonic for the new RRtype is RAD (for "router associated DoH").
   The Type value for the RAD RR is TBD; see Section 6.

   The RDATA of the resource record consists of one field: a URI
   Template [RFC6570] that is enclosed in ASCII quotation marks.

   The display format of the RDATA is simply the value of the RDATA.
   For example, here is an example RAD RRset in the display format:

   . 3600 IN RAD "https://dnsserver.example.net/dns-query{?dns}"
   . 3600 IN RAD "https://192.0.2.78/doh{?dns}"

   The wire format of the RDATA is simply the bytes that represent the
   RDATA, including the required enclosing quotation marks.  For
   example, the above RRset is encoded as follows (with the example
   value for TBD being 42424 decimal, or 0xA5B3):

   00 A5B3 0001 00000E10 002F
   2268747470733A2F2F646E737365727665722E6578616D706C652E6E6574
   2F646E732D71756572797B3F646E737D22
   00 A5B3 0001 00000E10 001E
   2268747470733A2F2F3139322E302E322E37382F646F687B3F646E737D22

4.  Use of RAD

   To find the DoH servers associated with a resolver, the client sends
   that resolver a query for the root zone in class IN with the RRtype
   of RAD (that is, the query is ./IN/RAD).  The server replies with its
   associated DoH servers in the RAD RRset in the Answer section.



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4.1.  Server Use of RAD

   A resolver that understands this protocol MUST send a RAD RRset in
   the Answer section.  If a resolver that understands this protocol has
   no associated DoH servers, the RRset contains exactly one record that
   has just "" as the RDATA; that is, the RDLENGTH in that record is 2,
   and the RDATA contains just the two bytes 0x2222.

4.2.  Client Use of RAD

   If a client sends the ./IN/RAD query over a transport that assures
   data integrity (such as DoT), and it receives a response that has an
   NXDOMAIN response code, the client can assume that the resolver does
   not know this protocol.

   See Section 8 for warnings about sending the ./IN/RAD query over a
   transport that does not assure data integrity (such as Do53).

   This protocol only defines the use of the RAD RRtype in queries that
   are exactly ./IN/RAD.  Clients MUST NOT send DNS queries using the
   RAD RRtype with any QNAME other than that of the root and any QCLASS
   other than IN.

   The client SHOULD only use a DoH server listed in the RAD response
   for the length of time listed as the RAD RRset's TTL.  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 ./IN/
   RAD 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 ./IN/RAD query every time the configured
   resolver changes.

5.  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 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



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   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 (the new RRtype and ./IN/RAD 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.

   An earlier design suggestion was to use a new special-use TLD
   [RFC6761] instead of the root in the query.  However, [RFC6761]
   continues to be contentious and using such a TLD has pretty much all
   of the problems as using the root itself.

   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".

6.  IANA Considerations

6.1.  Root Zone

   IANA MUST NOT ever include an RRset for the RAD RRtype in the root
   zone.

6.2.  Registration for RAD RRtype

   @@@ This will actually be a submission to the RRtypes expert sometime
   during the progress of this draft (if this draft progresses), and
   then will turn into a stub section just saying "IANA assigned Type
   value X to RAD in the RRtypes registry". @@@

   A.  Submission Date: (sometime in the future)

   B.1 Submission Type:

   New RRTYPE

   B.2 Kind of RR:

   Data RR

   C.  Contact Information for submitter (will be publicly posted):

   Paul Hoffman, paul.hoffman@icann.org



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   D.  Motivation for the new RRTYPE application

   To support recursive resolvers who want to associate DoH servers with
   the resolvers

   E.  Description of the proposed RR type.

   See Section 3

   F.  What existing RRTYPE or RRTYPEs come closest to filling that need
   and why are they unsatisfactory?

   URI, but it is unsatisfactory because the semantics are different
   (This is not associating the URI with a hostname) and because it has
   a priority and weight.

   G.  What mnemonic is requested for the new RRTYPE (optional)?

   RAD

   H.  Does the requested RRTYPE make use of any existing IANA registry
   or require the creation of a new IANA subregistry in DNS Parameters?

   No

   I.  Does the proposal require/expect any changes in DNS servers/
   resolvers that prevent the new type from being processed as an
   unknown RRTYPE (see [RFC3597])?

   No

   J.  Comments:

   None

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

   If a client sends the ./IN/RAD 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.

   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.

9.  References

9.1.  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>.

   [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>.







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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>.

   [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

   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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