Adaptive DNS Resolver Discovery
draft-pauly-add-resolver-discovery-00

Network Working Group                                           T. Pauly
Internet-Draft                                                E. Kinnear
Intended status: Standards Track                              Apple Inc.
Expires: 21 November 2020                                      C.A. Wood
                                                              Cloudflare
                                                              P. McManus
                                                                  Fastly
                                                               T. Jensen
                                                               Microsoft
                                                             20 May 2020

                    Adaptive DNS Resolver Discovery
                 draft-pauly-add-resolver-discovery-00

Abstract

   This document defines a method for dynamically discovering resolvers
   that support encrypted transports, and introduces the concept of a
   designating a resolver to be used for a subset of client queries
   based on domain.  This method is intended to work both for locally-
   hosted resolvers and resolvers accessible over the broader Internet.

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 21 November 2020.

Copyright Notice

   Copyright (c) 2020 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.

Pauly, et al.           Expires 21 November 2020                [Page 1]
Internet-Draft               ADNS Discovery                     May 2020

   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.  Specification of Requirements . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Designated Resolvers  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Designating with Service Binding DNS Records  . . . . . .   4
     3.2.  Additional Designation with PvD JSON  . . . . . . . . . .   5
     3.3.  Mutual Confirmation with PvD JSON . . . . . . . . . . . .   6
   4.  Explicit Discovery of Local Resolvers . . . . . . . . . . . .   7
   5.  Discovery of DoH Capabilities for Direct Resolvers  . . . . .   8
   6.  Server Deployment Considerations  . . . . . . . . . . . . . .   9
     6.1.  Single Content Provider . . . . . . . . . . . . . . . . .   9
     6.2.  Multiple Content Providers  . . . . . . . . . . . . . . .   9
     6.3.  Avoid Narrow Deployments  . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  DoH Template PvD Key  . . . . . . . . . . . . . . . . . .  11
     9.2.  Trusted Names PvD Key . . . . . . . . . . . . . . . . . .  11
     9.3.  DoH URI Template DNS Parameter  . . . . . . . . . . . . .  11
     9.4.  Special Use Domain Name "resolver.arpa" . . . . . . . . .  12
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   When clients need to resolve names into addresses in order to
   establish networking connections, they traditionally use by default
   the DNS resolver that is provisioned by the local network along with
   their IP address [RFC2132] [RFC8106].  Alternatively, they can use a
   resolver indicated by a tunneling service such as a VPN.

Pauly, et al.           Expires 21 November 2020                [Page 2]
Internet-Draft               ADNS Discovery                     May 2020

   However, privacy-sensitive clients might prefer to use an encrypted
   DNS service other than the one locally provisioned in order to
   prevent interception, profiling, or modification by entities other
   than the operator of the name service for the name being resolved.
   Protocols that can improve the transport security of a client when
   using DNS or creating TLS connections include DNS-over-TLS (DoT)
   [RFC7858], DNS-over-HTTPS (DoH) [RFC8484], and Encrypted TLS Client
   Hellos [I-D.ietf-tls-esni].

   This document defines a method for dynamically discovering resolvers
   that support encrypted transports, and introduces the concept of a
   designating a resolver to be used for a subset of client queries
   based on domain.  This method is intended to work both for locally-
   hosted resolvers and resolvers accessible over the broader Internet.

1.1.  Specification of Requirements

   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.  Terminology

   This document defines the following terms:

   Direct Resolver:  A DNS resolver using any transport, encrypted or
      unencrypted, that is provisioned directly by a local router or a
      VPN.

   Designated Resolver:  A DNS resolver that is designated as a
      responsible resolver for a given domain or zone.  Designated
      resolvers use encrypted transports.

   Companion DoH Server:  A DNS resolver that provides connectivity over
      HTTPS (DoH) that is designated as equivalent to querying a
      particular Direct Resolver.

3.  Designated Resolvers

   An encrypted DNS resolver, such as a DoH or DoT server, can be
   designated for use in resolving names within one or more zones.  This
   means that clients can learn about an explicit mapping from a given
   domain or zone to one or more Designated Resolvers, and use that
   mapping to select the best resolver for a given query.

Pauly, et al.           Expires 21 November 2020                [Page 3]
Internet-Draft               ADNS Discovery                     May 2020

   Designating a resolver MUST rely on mutual agreement between the
   entity managing a zone (the Domain Owner) and the entity operating
   the resolver.  These entities can be one and the same, or a Domain
   Owner can choose to designate a third-party resolver to handle its
   traffic.  Proof of this mutual agreement asserts to clients that
   sending any query to the designated resolver exposes no more
   information than sending that query to the entity managing the
   corresponding zone.

   As an example with only one entity, a company that runs many sites
   within "enterprise.example.com" can provide its own DoH resolver,
   "doh.enterprise.example.com", and designate only that resolver for
   all names that fall within "enterprise.example.com".  This means that
   no other resolver would be designated for those names, and clients
   would only resolve names with the same entity that would service TLS
   connections.

   As an example with several entities, the organization that operates
   sites within "example.org" may work with two different Content
   Delivery Networks (CDNs) to serve its sites.  It might designate
   names under "example.com" to two different entities, "doh.cdn-a.net"
   and "doh.cdn-b.net".  These are CDNs that have an existing
   relationship with the organization that runs "example.org", and have
   agreements with that organization about how data with information on
   names and users is handled.

   There are several methods that can be used to designate a resolver:

   *  Based on SVCB DNS records issued to another resolver (Section 3.1)

   *  Based on information from Designated DoH Resolver that is
      confirmed via SVCB DNS records (Section 3.2)

   *  Based on mutual agreement through confirmation of domains over
      HTTPS (Section 3.3)

   Note that clients MUST NOT accept designations for effective top-
   level domains (eTLDs), such as ".com".

3.1.  Designating with Service Binding DNS Records

   The primary source for discovering Designated DoH Server
   configurations is from properties stored in a SVCB (or a SVCB-
   conformant type like HTTPSSVC) DNS Record
   [I-D.ietf-dnsop-svcb-httpssvc].  This record provides the URI
   Template of a DoH server that is designated for a specific domain.  A
   specific domain may have more than one such record.

Pauly, et al.           Expires 21 November 2020                [Page 4]
Internet-Draft               ADNS Discovery                     May 2020

   In order to designate a DoH server for a domain, a SVCB record can
   contain the "dohuri" (Section 9).  The value stored in the parameter
   is a URI, which is the DoH URI template [RFC8484].

   The following example shows a record containing a DoH URI, as
   returned by a query for the HTTPSSVC variant of the SVCB record type
   on "foo.example.com", where the response indicates a DoH Resolver
   that is designated for names under "example.com".

      foo.example.com.  7200  IN HTTPSSVC 1 example.com. (
                              dohuri=https://doh.example.net/dns-query )

   If this record is DNSSEC-signed [RFC4033], clients can immediately
   create a mapping that indicates the server (doh.example.net) as a
   Designated Resolver for the name in the SVCB record
   (foo.example.com).

   If this record is not DNSSEC-signed, clients MUST perform other
   validation to determine that the zone designation is permitted, as
   described in Section 3.3.

3.2.  Additional Designation with PvD JSON

   A provisioning domain (PvD) defines a coherent set of information
   that can be used to access a network and resolve names.
   [I-D.ietf-intarea-provisioning-domains] defines a JSON dictionary
   format that can be fetched over HTTPS at the well-known URI "/.well-
   known/pvd".

   Designated Resolvers that support DoH SHOULD provide a PvD JSON
   dictionary available at the well-known PvD URI with the path of the
   DoH server's URI template appended.

   For example, the PvD JSON for the DoH server
   "https://doh.example.net/dns-query" would be available at
   "https://doh.example.net/.well-known/pvd/dns-query".

   Names that are listed in the "dnsZones" key in the JSON dictionary
   indicate other names that designate the resolver.  For each of those
   domains, clients SHOULD issue an SVCB query to the DoH resolver.  If
   this record confirms the designation and is DNSSEC-signed, clients
   can create a mapping to designate the resolver.  In order to optimize
   the validation of these domains, servers MAY use HTTP Server Push to
   deliver the records prior to the request being made.

Pauly, et al.           Expires 21 November 2020                [Page 5]
Internet-Draft               ADNS Discovery                     May 2020

   The key "dohTemplate" is also defined within the JSON dictionary
   (Section 9) to point back to the DoH URI Template itself.  This is
   used for confirming the DoH server when the PvD is discovered locally
   or during mutual confirmation (Section 3.3).

3.3.  Mutual Confirmation with PvD JSON

   Designated DoH Resolvers that provide the PvD JSON described in
   Section 3.2 can also provide information to allow validation of zone
   designations without DNSSEC.

   The JSON dictionary MAY contain a key "trustedNames" that is an array
   of strings containing domains that can be used for mutual
   confirmation of resolver designation.

   For example, the JSON dictionary retrieved at
   "https://doh.example.net/.well-known/pvd/dns-query" can contain the
   following contents:

      {
        "identifier": "doh.example.net.",
        "dohTemplate": "https://doh.example.net/dns-query",
        "dnsZones": ["example.com"],
        "trustedNames": ["example.com"]
      }

   This indicates that "example.com" should be treated as a designated
   domain, and that it can be validated by checking with the
   "example.com" server rather than using DNSSEC.

   Clients MUST validate the resolver designation by checking a resource
   hosted by the name indicated in "trustedNames".  The client first
   issues an HTTP GET request by appending "/.well-known/pvd" to the
   trusted name, using the "https" scheme.  In this example, the
   resulting URI is "https://example.com/.well-known/pvd".  In order to
   trust the designation, this request must return valid JSON with the
   "dohTemplate" key matching the original DoH resolver.  For example,
   this dictionary could contain the following contents:

      {
        "identifier": "example.com.",
        "dohTemplate": "https://doh.example.net/dns-query",
      }

   A client MUST NOT trust a designation if the JSON content is not
   present, does not contain a "dohTemplate" key, or the value in the
   "dohTemplate" key does not match.  The following result would not be
   acceptable for the example above:

Pauly, et al.           Expires 21 November 2020                [Page 6]
Internet-Draft               ADNS Discovery                     May 2020

      {
        "identifier": "example.com.",
        "dohTemplate": "https://not-the-doh-youre-looking-for.example.net/dns-query",
      }

   Note that the domains listed in "trustedNames" may be broader than
   the zones that designate the resolver.  In the following example,
   names under "foo.example.com" and "bar.example.com" designate the DoH
   server "https://doh.example.net/dns-query", and use the PvD JSON from
   "example.com" to validate the designation.  However, the client would
   not designate the DoH server for all names under "example.com".

      {
        "identifier": "doh.example.net.",
        "dohTemplate": "https://doh.example.net/dns-query",
        "dnsZones": ["foo.example.com", "bar.example.com"],
        "trustedNames": ["example.com"]
      }

4.  Explicit Discovery of Local Resolvers

   If the local network provides configuration with an Explicit
   Provisioning Domain (PvD), as defined by
   [I-D.ietf-intarea-provisioning-domains], clients can learn about
   domains for which the local network's resolver is authoritative.  The
   keys for DoH resolvers described in Section 3.2 also allow this local
   PvD to be used for resolver discovery.

   If an RA provided by the router on the network defines an Explicit
   PvD that has additional information, and this additional information
   JSON dictionary contains the key "dohTemplate", then the client
   SHOULD add this DoH server to its list of known DoH configurations.
   The domains that the DoH server claims authority for are listed in
   the "dnsZones" key.  Clients MUST use one of the methods for
   validating a designation described in Section 3.1 or Section 3.3.

   Local deployments that want to designate a resolver for a private
   name that is not easily signed with DNSSEC MUST provide an alternate
   method of validating a designation, particularly the one described in
   Section 3.3.

Pauly, et al.           Expires 21 November 2020                [Page 7]
Internet-Draft               ADNS Discovery                     May 2020

5.  Discovery of DoH Capabilities for Direct Resolvers

   Direct Resolvers can advertise a Companion DoH server that offers
   equivalent services and is controlled by the same entity.  To do
   this, a DNS server returns an SVCB record for the "resolver.arpa"
   domain with "ipv4hint" and/or "ipv6hint" set to a valid IP address
   and the "dohuri" key set to a valid DoH URI template as with the
   Designated DoH Server SVCB record.  The TLS certificate used with the
   DoH URI MUST have the IP addresses for each of its DNS endpoints,
   classic or DoH, within the SubjectAlternativeName field to allow the
   client to verify ownership.

   Once a client is configured to query a Direct Resolver, it SHOULD
   query the resolver for SVCB records for the "resolver.arpa" domain
   before making other queries.  This will help the client avoid leaking
   queries that could go over DoH once the Companion DoH Server is
   discovered.  If an SVCB record is returned, its "dohip" field
   designates an IP address the client can send DoH queries to in lieu
   of sending classic DNS queries to the Direct Resolver.  The "dohuri"
   field contains the DoH URI similarly to the SVCB record for a
   Designated DoH Server.

   To validate the Companion DoH Server and the resolver that advertised
   it are related, the client MUST check the SubjectAlternativeName
   field of the Companion DoH Server's TLS certificate for the original
   resolver's IP address and the advertised IP address for the Companion
   DoH server.  If both are present, the discovered Companion DoH Server
   MUST be used whenever the original Direct Resolver would be used.
   Otherwise, the client SHOULD suppress queries for Companion DoH
   Servers against this resolver for the TTL of the negative or invalid
   response and continue to use the original Direct Resolver.

   The following example shows a record containing a Companion DoH URI,
   as returned by a query for the HTTPSSVC variant of the SVCB record
   type on the "resolver.arpa" domain.

      resolver.arpa  7200  IN HTTPSSVC 1 doh.example.net (
                           ipv4hint=x.y.z.w
                           dohuri=https://doh.example.net/dns-query )

   A DNS resolver MAY return more than one SVCB record of this form to
   advertise multiple Companion DoH Servers that are valid as a
   replacement for itself.  Any or all of these servers may have the
   same IP address as the DNS resolver itself.  In this case, clients
   will only have one IP address to check for when verifying ownership
   of the Companion DoH server.

Pauly, et al.           Expires 21 November 2020                [Page 8]
Internet-Draft               ADNS Discovery                     May 2020

6.  Server Deployment Considerations

   When servers designate DoH servers for their names, the specific
   deployment model can impact the effective privacy and performance
   characteristics.

6.1.  Single Content Provider

   If a name always resolves to server IP addresses that are hosted by a
   single content provider, the name ought to designate a single DoH
   server.  This DoH server will be most optimal when it is designated
   by many or all names that are hosted by the same content provider.
   This ensures that clients can increase connection reuse to reduce
   latency in connection setup.

   A DoH server that corresponds to the content provider that hosts
   content has an opportunity to tune the responses provided to a client
   based on the location inferred by the client IP address.

6.2.  Multiple Content Providers

   Some hostnames may resolve to server IP addresses that are hosted by
   multiple content providers.  In such scenarios, the deployment may
   want to be able to control the percentage of traffic that flows to
   each content provider.

   In these scenarios, there can either be:

   *  multiple designated DoH servers that are advertised via SVCB DNS
      Records; or,

   *  a single designated DoH server that can be referenced by one or
      more SVCB DNS Records, operated by a party that is aware of both
      content providers and can manage splitting the traffic.

   If a server deployment wants to easily control the split of traffic
   between different content providers, it ought to use the latter model
   of using a single designated DoH server that can better control which
   IP addresses are provided to clients.  Otherwise, if a client is
   aware of multiple DoH servers, it might use a single resolver
   exclusively, which may lead to inconsistent behavior between clients
   that choose different resolvers.

Pauly, et al.           Expires 21 November 2020                [Page 9]
Internet-Draft               ADNS Discovery                     May 2020

6.3.  Avoid Narrow Deployments

   Using designated DoH servers can improve the privacy of name
   resolution whenever a DoH server is designated by many different
   names within one or more domains.  This limits the amount of
   information leaked to an attacker observing traffic between a client
   and a DoH server: the attacker only learns that the client might be
   resolving one of the many names for which the server is designated.

   However, if a deployment designates a given DoH server for only one
   name, or a very small set of names, then it becomes easier for an
   attacker to infer that a specific name is being accessed by a client.
   For this reason, deployments are encouraged to avoid deploying a DoH
   server that is only designated by a small number of names.  Clients
   can also choose to only whitelist DoH servers that are associated
   with many names.

   Beyond the benefits to privacy, having a larger number of names
   designate a given DoH server improves the opportunity for DoH
   connection reuse, which can improve the performance of name
   resolutions.

7.  Security Considerations

   In order to avoid interception and modification of the information
   sent between clients and Designated Resolvers, all exchanges between
   clients and servers are performed over encrypted connections, e.g.,
   TLS.

   Malicious adversaries may block client connections to a Designated
   Resolver as a Denial-of-Service (DoS) measure.  Clients which cannot
   connect these resolvers may be forced to, if local policy allows,
   fall back to unencrypted DNS if this occurs.

8.  Privacy Considerations

   Clients must be careful in determining to which DoH servers they send
   queries directly.  A malicious resolver that can direct queries to
   itself can track or profile client activity.  In order to avoid the
   possibility of a spoofed SVCB record designating a malicious DoH
   server for a name, clients MUST ensure that such records validate
   using DNSSEC (Section 3.1) or using mutual confirmation
   (Section 3.3).

   Even servers that are validly designated can risk leaking or logging
   information about client lookups.  Such risk can be mitigated by
   further restricting the list of resolvers that are whitelisted for
   direct use based on client policy.

Pauly, et al.           Expires 21 November 2020               [Page 10]
Internet-Draft               ADNS Discovery                     May 2020

   An adversary able to see traffic on each path segment of a DoH query
   (e.g., from client to a Designated Resolver, and the Designated
   Resolver to an authoritative DNS server) can link queries to specific
   clients with high probability.  Failure to observe traffic on any one
   of these path segments makes this linkability increasingly difficult.
   For example, if an adversary can only observe traffic between a
   client and proxy and egress traffic from a target, then it may be
   difficult identify a specific client's query among the recursive
   queries generated by the target.

9.  IANA Considerations

9.1.  DoH Template PvD Key

   This document adds a key to the "Additional Information PvD Keys"
   registry [I-D.ietf-intarea-provisioning-domains].

   +------------+-------------+------+---------------------------------+
   | JSON key   | Description | Type | Example                         |
   +============+=============+======+=================================+
   |dohTemplate | DoH URI     |String| "https://dnsserver.example.net/ |
   |            | Template    |      | dns-query{?dns}"                |
   |            | [RFC8484]   |      |                                 |
   +------------+-------------+------+---------------------------------+

                                  Table 1

9.2.  Trusted Names PvD Key

   This document adds a key to the "Additional Information PvD Keys"
   registry [I-D.ietf-intarea-provisioning-domains].

    +--------------+-----------------------+---------+---------------+
    | JSON key     | Description           | Type    | Example       |
    +==============+=======================+=========+===============+
    | trustedNames | Names of servers that | Array   | [             |
    |              | can validate resolver | of      | "example.com" |
    |              | designation.          | Strings | ]             |
    +--------------+-----------------------+---------+---------------+

                                 Table 2

9.3.  DoH URI Template DNS Parameter

   If present, this parameters indicates the URI template of a DoH
   server that is designated for use with the name being resolved.  This
   is a string encoded as UTF-8 characters.

Pauly, et al.           Expires 21 November 2020               [Page 11]
Internet-Draft               ADNS Discovery                     May 2020

   Name:  dohuri

   SvcParamKey:  TBD

   Meaning:  URI template for a designated DoH server

   Reference:  This document.

9.4.  Special Use Domain Name "resolver.arpa"

   This document calls for the creation of the "resolver.arpa" SUDN.
   This will allow resolvers to respond to queries directed at
   themselves rather than a specific domain name.  While this document
   uses "resolver.arpa" to return SVCB records indicating DoH
   capability, the name is generic enough to allow future reuse for
   other purposes where the resolver wishes to provide information about
   itself to the client.

10.  Acknowledgments

   Thanks to Erik Nygren, Lorenzo Colitti, Mikael Abrahamsson, Ben
   Schwartz, Ask Hansen, Leif Hedstrom, Tim McCoy, Stuart Cheshire,
   Miguel Vega, Joey Deng, Ted Lemon, and Elliot Briggs for their
   feedback and input on this document.

11.  References

11.1.  Normative References

   [I-D.ietf-dnsop-svcb-httpssvc]
              Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPSSVC)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-httpssvc-02, 9 March 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-dnsop-
              svcb-httpssvc-02.txt>.

   [I-D.ietf-intarea-provisioning-domains]
              Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W.
              Shao, "Discovering Provisioning Domain Names and Data",
              Work in Progress, Internet-Draft, draft-ietf-intarea-
              provisioning-domains-11, 31 January 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-intarea-
              provisioning-domains-11.txt>.

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
              "Encrypted Server Name Indication for TLS 1.3", Work in

Pauly, et al.           Expires 21 November 2020               [Page 12]
Internet-Draft               ADNS Discovery                     May 2020

              Progress, Internet-Draft, draft-ietf-tls-esni-06, 9 March
              2020, <http://www.ietf.org/internet-drafts/draft-ietf-tls-
              esni-06.txt>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [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>.

   [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>.

11.2.  Informative 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>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.

   [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>.

Authors' Addresses

   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America

   Email: tpauly@apple.com

Pauly, et al.           Expires 21 November 2020               [Page 13]
Internet-Draft               ADNS Discovery                     May 2020

   Eric Kinnear
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America

   Email: ekinnear@apple.com

   Christopher A. Wood
   Cloudflare
   101 Townsend St
   San Francisco,
   United States of America

   Email: caw@heapingbits.net

   Patrick McManus
   Fastly

   Email: mcmanus@ducksong.com

   Tommy Jensen
   Microsoft

   Email: tojens@microsoft.com

Pauly, et al.           Expires 21 November 2020               [Page 14]