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Versions: (draft-hoffman-resolver-associated-doh) 00 01 02 03 draft-sah-resolver-information

Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                          March 06, 2019
Expires: September 7, 2019


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

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
   two protocols for a resolver to tell a client what its associated DoH
   servers are.  It also describes a protocol for a client to find out
   the address of the resolver it is using, if it cannot find that
   address by an operating system API or some other means.

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 https://datatracker.ietf.org/drafts/current/.

   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 September 7, 2019.

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



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   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Finding the URI Templates of the DoH Servers Associated with
       a Resolver  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  DoH Servers by TXT  . . . . . . . . . . . . . . . . . . .   4
     2.2.  DoH Servers by Addresses  . . . . . . . . . . . . . . . .   5
     2.3.  Issues Common to "DoH Servers by TXT" and "Resolver
           Addresses by SUDN"  . . . . . . . . . . . . . . . . . . .   6
   3.  Finding the Resolver Addresses Without Operating System APIs    6
   4.  User Interface  . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Design Choices  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 to want the DNS
   recursive resolver(s) configured in the operating system to use DoH
   for DNS resolution instead of normal DNS, but to do so to at a DoH
   server specified by the configured resolver.  For example, a
   recursive resolver configured by the operating system may know how to
   give correct answers to DNS queries that contain names that are only
   resolvable in the local context, or resolve differently in the local
   context.  Similarly, the recursive resolver configured in the
   operating system may implement security policies such as malware
   prevention that are not implemented in the same way in DoH servers
   not affiliated with the user's organization.  Users typically
   configure their DNS recursive resolvers with through automatic
   configuration from a protocol such as DHCP; much less often, they use




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   manual configuration (such as manually editing a /etc/resolv.conf
   file).

   The expected use cases for DoH are browsers and web applications that
   would otherwise get their DNS service from the resolver configured by
   the operating system.  The user of the client might have a preference
   for using a DoH server for the benefits that DoH brings, and they
   might need to use a DoH server that is associated with the resolver
   that the computer is currently using for the reasons listed above.
   In a common scenario, user may be required to use only resolvers that
   are approved by their organization's network operators.

   To address these use cases, this document defines two protocols to
   get the list of URI templates [RFC6570] for 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.

   o  The first, called "DoH servers by TXT" and described in
      Section 2.1, is a new special use domain name (SUDN) [RFC6761]
      that can be queried for a TXT RRset.  This protocol is most likely
      useful only to browsers that can call operating system functions
      that in turn query the DNS for text records; web applications can
      only query for IP addresses.

   o  The second, called "DoH servers by addresses" and described in
      Section 2.2, is a well-known URI [I-D.nottingham-rfc5785bis] that
      can be resolved to return the URI templates.  This is useful if a
      browser can call operating system functions that will return the
      address of the recursive DNS resolver that the operating system is
      currently using.

   This document also defines a third protocol, called "resolver
   addresses by SUDN" and described in Section 3, that is a new SUDN
   that that can be queried for the IP address(es) of a resolver.  This
   protocol is useful for a client that can query for the addresses
   associated with a domain name (such as using the POSIX
   "getaddrinfo()" function) but cannot use an operating system function
   to find those addresses.  For browsers, it is only needed if the
   browser cannot use an API to determine the configured resolver IP
   address(es).

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








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1.1.  Terminology

   In this document, "client" means either a web browser or application.
   When one or the other is named explicitly,

   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.

2.  Finding the URI Templates of the DoH Servers Associated with a
    Resolver

   A client (a browser or web application) uses either the protocol in
   Section 2.1 or Section 2.2 to get a list of URI templates for the DoH
   servers associated with a resolver.  The following sub-sections
   describe the protocols and have notes that are common to both
   protocols.

2.1.  DoH Servers by TXT

   To find the DoH Servers associated with a resolver, an application
   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).

   As described in Section 6, the zone resolver-associated-doh.arpa is
   not actually delegated and never will be.  The resolver acts as if it
   is delegated, and adds its own TXT records to the answer.  The
   resolver replies with its associated DoH servers as URI templates in
   the TXT RRset in the Answer section.  The resolver can generate this
   reply with special code to capture queries for "resolver-associated-
   doh.arpa"; if the resolver can be configured to also be authoritative
   for some zones, it can use that configuration to actually be
   authoritative for "resolver-associated-doh.arpa".

   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



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   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.

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

   The URI templates of the DoH servers associated with a resolver might
   be hosted on the resolver itself, or a resolver hosted by the same
   operator, or even hosted somewhere else.  The latter could be used by
   resolver operators who don't want to host DoH servers but trust
   another operator to do so.

2.2.  DoH Servers by Addresses

   To find the DoH servers associated with a resolver, a browser or web
   application uses either an operating system function (if such a
   function is available to it) or the process described in Section 3 to
   find one or more IP addresses for the resolver.  It uses one or more
   of 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.

   To find the DoH servers associated with a resolver, the client 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.



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   The result of sending this query can be an HTTP redirect to a
   different server, such as when a resolver operator doesn't want to
   run its own DoH server.

2.3.  Issues Common to "DoH Servers by TXT" and "Resolver Addresses by
      SUDN"

   See Section 8 for warnings about sending the DNS queries over a
   transport that does not assure data integrity (such as Do53), and
   over a transport that does assure data integrity (such as DoT) but in
   circumstances where the browser or web application doesn't know the
   type of DNS transport being used.

   A client MUST re-issue the queries in {#doh_by_txt} and
   {#resolver_by_sudn} every time the configured resolver in the
   operating system changes.

   [[ What if there is a list of DoH servers?  Pick one (how?) or jump
   around? ]]

3.  Finding the Resolver Addresses Without Operating System APIs

   Browsers can often, but not always, get the IP address(es) of the
   resolver configured by the operating system using APIs.  Browsers
   which cannot are still able to use an operating system function such
   as gethostbyname() or its equivalents to convert host names into IP
   addresses through the stub resolver in the operating system on which
   they are running.  Web applications also can convert host names to IP
   addresses.  Either can use a new protocol to find the address(es) of
   the resolvers configured by the operating system.

   In this protocol, the browser or web application uses it normal
   interface for getting addresses for a hostname, and uses the SUDN
   "resolver-addresses.arpa" as the hostname.  As described in
   Section 6, the zone resolver-addresses.arpa is not actually delegated
   and never will be.  The resolver acts as if that name is delegated,
   and returns its own A or AAAA addresses in the records in the answer.
   The resolver can generate this reply with special code to capture
   queries for "resolver-addresses.arpa"; if the resolver can be
   configured to also be authoritative for some zones, it can use that
   configuration to actually be authoritative for "resolver-
   addresses.arpa".

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



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   configured Do53 or DoT resolver is allowed.  That wording might say
   something like "DoH server associated with my current resolver" (or
   "servidor DoH asociado con mi resolucion actual" or "serveur DoH
   associe a mon resolveur actuel").

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 operating system 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 operating
   system 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.

6.  IANA Considerations

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

   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.

   Before this draft is complete, mail will be sent to wellknown-uri-
   review@ietf.org in order to be registered in the "Well-Known URIs"
   registry at IANA.  The mail will contain the following:






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   URI suffix:  doh-servers-associated
   Change controller:  IETF
   Specification document(s):  draft-hoffman-resolver-associated-doh
      (or its successor, if it is adopted in a WG)
   Status:  permanent

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.

   A browser that has both a stub resolver stack and a TLS stack that is
   independent of HTTP could make a DOT connection to the resolver being
   used by the operating system.

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-09 (work in
              progress), February 2019.

   [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>.







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   [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>.

   [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, Shumon Huque, Martin Thomson, Eric Rescorla,
   and Tony Finch offered useful advice to improve versions of the
   protocol before it came to the DOH Working Group.






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Author's Address

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org













































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