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Requirements for Discovering Designated Resolvers

Document Type Replaced Internet-Draft (add WG)
Authors Chris Box , Tommy Pauly , Christopher A. Wood , Tirumaleswar Reddy.K , Daniel Migault
Last updated 2021-03-01 (Latest revision 2021-01-24)
Replaces draft-pauly-add-requirements
Replaced by draft-ietf-add-requirements
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ADD                                                               C. Box
Internet-Draft                                                        BT
Intended status: Informational                                  T. Pauly
Expires: 28 July 2021                                              Apple
                                                               C.A. Wood
                                                                T. Reddy
                                                              D. Migault
                                                         24 January 2021

           Requirements for Discovering Designated Resolvers


   Adaptive DNS Discovery is chartered to define mechanisms that allow
   clients to discover and select encrypted DNS resolvers.  This
   document describes one common use case, namely that of clients that
   connect to a network but where they cannot securely authenticate the
   identity of that network.  In such cases the client would like to
   learn which encrypted DNS resolvers are designated by that network or
   by the Do53 resolver offered by that network.  It lists requirements
   that any proposed discovery mechanisms should seek to address.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at

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

   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."

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   This Internet-Draft will expire on 28 July 2021.

Copyright Notice

   Copyright (c) 2021 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 (
   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 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.  Requirements language . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use case description  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Designation . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Local addressing  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Use of designation information  . . . . . . . . . . . . .   5
     3.4.  Network-identified designated resolvers . . . . . . . . .   6
     3.5.  Resolver-identified designated resolvers  . . . . . . . .   6
       3.5.1.  Local to local  . . . . . . . . . . . . . . . . . . .   7
       3.5.2.  Local to upstream . . . . . . . . . . . . . . . . . .   7
       3.5.3.  Public to public  . . . . . . . . . . . . . . . . . .   8
     3.6.  Identification over an encrypted channel  . . . . . . . .   8
   4.  Privacy and security requirements . . . . . . . . . . . . . .   8
   5.  Statement of Requirements . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Several protocols for protecting DNS traffic with encrypted
   transports have been defined, such as DNS-over-TLS (DoT) [RFC7858]
   and DNS-over-HTTPS (DoH) [RFC8484].  Encrypted DNS can provide many
   security and privacy benefits for network clients.

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   While it is possible for clients to statically configure encrypted
   DNS resolvers to use, dynamic discovery and provisioning of encrypted
   resolvers can expand the usefulness and applicability of encrypted
   DNS to many more use cases.

   The Adaptive DNS Discovery (ADD) Working Group is chartered to define
   mechanisms that allow clients to automatically discover and select
   encrypted DNS resolvers in a wide variety of network environments.
   This document describes one common use case, namely that of clients
   that connect to a network but where they cannot securely authenticate
   that network.  Whether the network required credentials before the
   client was permitted to join is irrelevant; the client still cannot
   be sure that it has connected to the network it was expecting.

   In such cases the client would like to learn which encrypted DNS
   resolvers are designated by that network, or by the Do53 resolver
   offered by that network.  It lists requirements that any proposed
   discovery mechanisms should seek to address.  They can do this either
   by providing a solution, or by explicitly stating why it is not in

1.1.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 makes use of the following terms.

   Encrypted DNS: DNS-over-HTTPS [RFC8484], DNS-over-TLS [RFC7858], or
   any other encrypted DNS technology that the IETF may publish, such as
   DNS-over-QUIC [I-D.ietf-dprive-dnsoquic].

   Do53: Unencrypted DNS over UDP port 53, or TCP port 53 [RFC1035].

   Designated: See Section 3.1.

   Designator: The network or resolver that issued the designation.

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3.  Use case description

   It is often the case that a client possesses no specific
   configuration for how to operate DNS, and at some point joins a
   network that it cannot authenticate.  It may have no prior knowledge
   of the network, or it may have connected previously to a network that
   looked the same.  In either case the usual behaviour, because of lack
   of specific configuration, is to dynamically discover the network's
   designated Do53 resolver and use it.  This long-standing practice
   works in nearly all networks, but presents a number of privacy and
   security risks that were the motivation for the development of
   encrypted DNS.

   The network's designated Do53 resolver may have a number of
   properties that differ from a generic resolver.  It may be able to
   answer names that are not known globally, it may exclude some names
   (for positive or negative reasons), and it may provide address
   answers that have improved proximity.  In this use case it is assumed
   that the user who chose to join this network would also like to make
   use of these properties of the network's unencrypted resolver, at
   least some of the time.  However they would like to use an encrypted
   DNS protocol rather than Do53.

   Using an encrypted and authenticated resolver can provide several
   benefits that are not possible if only unencrypted DNS is used:

   *  Prevent other devices on the network from observing client DNS

   *  Authenticate that the DNS resolver is the correct one

   *  Verify that answers come from the selected DNS resolver

   To meet this case there should be a means by which the client can
   learn how to contact a set of encrypted DNS resolvers that are
   designated by the network it has joined.

3.1.  Designation

   Designation is the process by which a local network or a resolver can
   point clients towards a particular set of resolvers.  This is not a
   new concept, as networks have been able to dynamically designate Do53
   resolvers for decades (see Section 3.4).  However here we extend the
   concept in two ways:

   *  To allow resolvers to designate other resolvers

   *  The inclusion of support for encrypted DNS

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   The designated set could be empty, or it could list the contact
   details (such as DoH URI Template) of DNS resolvers that it
   recommends.  It is not required that there be any relationship
   between the resolvers in the set, simply that all of them are options
   that the designator asserts are safe and appropriate for the client
   to use without user intervention.

   There are two possible sources of designation.

   *  The local network can designate one or more encrypted DNS
      resolvers (B, C, etc) in addition to any Do53 resolver (A) it may
      offer.  This is known as network-identified.

   *  During communication with the (often unencrypted) resolver (A),
      this resolver can designate one or more encrypted DNS resolvers
      (B, C, etc).  This is known as resolver-identified.

   Network-identified has the advantages that it derives from the same
   source of information as the network's Do53 announcement, and removes
   the need to talk to the Do53 resolver at all.  However it cannot be
   the sole mechanism, at least for several years, since there is a
   large installed base of local network equipment that is difficult to
   upgrade with new features.  Hence the second mechanism should support
   being able to designate resolvers using only existing widely-deployed
   DNS features.

3.2.  Local addressing

   Many networks offer a Do53 resolver on an address that is not
   globally meaningful, e.g.  [RFC1918], link-local or unique local
   addresses.  To support the discovery of encrypted DNS in these
   environments, a means is needed for the discovery process to work
   from a locally-addressed Do53 resolver to an encrypted DNS resolver
   that is accessible either at the same (local) address, or at a
   different global address.  Both options need to be supported.

3.3.  Use of designation information

   After the client receives designation information, it must come to a
   decision on whether and when to use any of the designated resolvers.

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   In the case of resolver-identified designation, it would be
   advantageous for a solution to enable the client to validate the
   source of the assertion in some way.  For example it may be possible
   to verify that the designation comes from an entity who already has
   full control of the client's Do53 queries.  Network-identified
   designation should not require this, unless the network-identified
   resolver in turn initiated a new resolver-identified designation.  It
   would be beneficial to extend such a verification process to defend
   against attackers that have only transient control of such queries.

   Clients may also seek to validate the identity of the designated
   resolver, beyond what is required by the relevant protocol.  Authors
   of solution specifications should be aware that clients may impose
   arbitrary additional requirements and heuristics as they see fit.

3.4.  Network-identified designated resolvers

   DNS servers are often provisioned by a network as part of DHCP
   options [RFC2132], IPv6 Router Advertisement (RA) options [RFC8106],
   Point-to-Point Protocol (PPP) [RFC1877], or 3GPP Protocol
   Configuration Options (TS24.008).  Historically this is usually one
   or more Do53 resolver IP addresses, to be used for traditional
   unencrypted DNS.

   A solution is required that enhances the set of information delivered
   to include details of one or more designated encrypted DNS resolvers,
   or states that there are none.  Such resolvers could be on the local
   network, somewhere upstream, or on the public Internet.

3.5.  Resolver-identified designated resolvers

   To support cases where the network is unable to identify an encrypted
   resolver, it should be possible to learn the details of one or more
   designated encrypted DNS resolvers by communicating with the
   network's designated Do53 resolver.  This should involve an exchange
   that uses standard DNS messages that can be handled, or forwarded, by
   existing deployed software.

   Each resolver in the set may be at a different network location,
   which leads to several subcases for the mapping from Do53 to a
   particular designated resolver.

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3.5.1.  Local to local

   If the local resolver has been upgraded to support encrypted DNS, the
   client may not initially be aware that its local resolver supports
   it.  Discovering this may require communication with the local
   resolver, or an upstream resolver, over Do53.  Clients that choose to
   use this local encrypted DNS gain the benefits of encryption while
   retaining the benefits of a local caching resolver with knowledge of
   the local topology.

   Clients will be aware when the designated resolver has the same IP
   address as the Do53 (after looking up its name if required).  They
   can use this information in their decision-making as to the level of
   trust to place in the designated resolver.  In some networks it will
   not be possible to deploy encrypted DNS on the same IP address, e.g.
   because of the increased resource requirements of encrypted DNS.
   Discovery solutions should work in the presence of a change to a
   different local IP address.

   An additional benefit of using a local resolver occurs with IoT
   devices.  A common usage pattern for such devices is for it to "call
   home" to a service that resides on the public Internet, where that
   service is referenced through a domain name.  As discussed in
   Manufacturer Usage Description Specification [RFC8520], because these
   devices tend to require access to very few sites, all other access
   should be considered suspect.  However, if the query is not
   accessible for inspection, it becomes quite difficult for the
   infrastructure to suspect anything.

3.5.2.  Local to upstream

   It is frequently the case that Do53 resolvers announced by home
   networks are difficult to upgrade to support encrypted operation.  In
   such cases it is possible that the only option for encrypted
   operation is to refer to a separate globally-addressed encrypted DNS
   resolver, somewhere upstream.  Other networks may choose deploy their
   encrypted DNS resolver away from the local network, for other

   The use of an upstream resolver can mean the loss of local knowledge,
   such as the ability to respond to queries for locally-relevant names.
   Solutions should consider how to guide clients when to direct their
   queries to the local Do53.  For example this could be through pre-
   emptive communication ("if you ever need to query *, use
   your local Do53"), or reactively ("I don't know the answer to that,
   but your local Do53 should know").

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3.5.3.  Public to public

   In cases where the local network has designated a Do53 resolver on
   the public Internet, this resolver may designate its own or another
   public encrypted DNS service.  Since public IP addresses may appear
   in TLS certificates, solutions may use this as one way to validate
   that the designated encrypted resolver is legitimately associated
   with the original Do53.

3.6.  Identification over an encrypted channel

   In cases where the designation is delivered over an authenticated and
   encrypted channel, such as when one encrypted DNS resolver designates
   another, one form of attack is removed.  Specifically, clients may be
   more confident that the received designation was actually sent by the
   designator.  Clients may take this into account when deciding whether
   to follow the designation.

4.  Privacy and security requirements

   Encrypted (and authenticated) DNS improves the privacy and security
   of DNS queries and answers in the presence of malicious attackers.
   Such attackers are assumed to interfere with or otherwise impede DNS
   traffic and corresponding discovery mechanisms.  They may be on-path
   or off-path between the client and entities with which the client
   communicates [RFC3552].  These attackers can inject, tamper, or
   otherwise interfere with traffic as needed.  Given these
   capabilities, an attacker may have a variety of goals, including,
   though not limited to:

   *  Monitor and profile clients by observing unencrypted DNS traffic

   *  Modify unencrypted DNS traffic to filter or augment the user

   *  Block encrypted DNS

   Given this type of attacker, resolver discovery mechanisms must be
   designed carefully to not worsen a client's security or privacy
   posture.  In particular, attackers under consideration must not be
   able to:

   *  Redirect secure DNS traffic to themselves when they would not
      otherwise handle DNS traffic.

   *  Override or interfere with the resolver preferences of a user or

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   *  Cause clients to use a discovered resolver which has no
      designation from a client-known entity.

   When discovering DNS resolvers on a local network, clients have no
   mechanism to distinguish between cases where an active attacker with
   the above capabilities is interfering with discovery, and situations
   wherein the network has no encrypted resolver.  Absent such a
   mechanism, an attacker can always succeed in these goals.  Therefore,
   in such circumstances, viable solutions for local DNS resolver
   discovery should consider weaker attackers, such as those with only
   passive eavesdropping capabilities.  It is unknown whether such
   relaxations represent a realistic attacker in practice.  Thus, local
   discovery solutions designed around this threat model may have
   limited value.

5.  Statement of Requirements

   This section lists requirements that flow from the above sections.

    | Requirement | Description                                       |
    | R1.1        | Discovery SHOULD provide a local network the      |
    |             | ability to announce to clients a set of, or       |
    |             | absence of, designated resolvers.                 |
    | R1.2        | Discovery SHOULD provide a resolver the ability   |
    |             | to announce to clients a set of, or absence of,   |
    |             | designated resolvers.                             |
    | R1.3        | Discovery SHOULD support all encrypted DNS        |
    |             | protocols standardised by the IETF.               |
    | R2.1        | Networks SHOULD be able to announce one or more   |
    |             | designated encrypted DNS resolvers using existing |
    |             | mechanisms such as DHCPv4, DHCPv6, IPv6 Router    |
    |             | Advertisement, and the Point-to-Point Protocol.   |
    | R2.2        | The format for resolver designation SHOULD be     |
    |             | specified such that provisioning mechanisms       |
    |             | defined outside of the IETF can advertise         |
    |             | encrypted DNS resolvers.                          |
    | R2.3        | This format SHOULD convey, at minimum, the        |
    |             | information the client needs to make contact with |
    |             | each designated resolver.                         |
    | R2.4        | This format MAY convey additional resolver        |

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    |             | information.                                      |
    | R3.1        | In resolver-identified designation (R1.2), the    |
    |             | communication with the designator MAY be          |
    |             | encrypted or not, depending on the capability of  |
    |             | the resolver.                                     |
    | R3.2        | In resolver-identified designation (R1.2), that   |
    |             | resolver MAY be locally or globally reachable.    |
    |             | Both options SHOULD be supported.                 |
    | R4.1        | If the local network resolver is a forwarder that |
    |             | does not offer encrypted DNS service, an upstream |
    |             | encrypted resolver SHOULD be retrievable via      |
    |             | queries sent to that forwarder.                   |
    | R4.2        | Achieving requirement 4.1 SHOULD NOT require any  |
    |             | changes to DNS forwarders hosted on non-          |
    |             | upgradable legacy network devices.                |
    | R5.1        | Discovery MUST NOT worsen a client's security or  |
    |             | privacy posture.                                  |
    | R5.2        | Threat modelling MUST assume that there is a      |
    |             | passive eavesdropping attacker on the local       |
    |             | network.                                          |
    | R5.3        | Threat modelling MUST assume that an attacker can |
    |             | actively attack from outside the local network.   |
    | R5.4        | Attackers MUST NOT be able to redirect encrypted  |
    |             | DNS traffic to themselves when they would not     |
    |             | otherwise handle DNS traffic.                     |

                                  Table 1

6.  Security Considerations

   See Section 4.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

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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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

8.2.  Informative References

              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", Work in Progress,
              Internet-Draft, draft-ietf-dprive-dnsoquic-01, 20 October
              2020, <

   [RFC1035]  Mockapetris, P.V., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1877]  Cobb, S., "PPP Internet Protocol Control Protocol
              Extensions for Name Server Addresses", RFC 1877,
              DOI 10.17487/RFC1877, December 1995,

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,

   [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, <>.

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   [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,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,


   This document was started based on discussion during the ADD meeting
   of IETF108, subsequent meetings, on the list, and with text from
   draft-pauly-add-requirements.  In particular this document was
   informed by contributions from Martin Thomson, Eric Rescorla, Tommy
   Jensen, Ben Schwartz, Paul Hoffman, Ralf Weber, Michael Richardson,
   Mohamed Boucadair, Sanjay Mishra, Jim Reid, Neil Cook, Nic Leymann,
   Andrew Campling, Eric Orth, Ted Hardie, Paul Vixie, Vittorio Bertola,
   and Vinny Parla.

Authors' Addresses

   Chris Box
   2000 Park Avenue
   United Kingdom


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


   Christopher A. Wood
   101 Townsend St

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   San Francisco,
   United States of America


   Tirumaleswar Reddy
   Embassy Golf Link Business Park


   Daniel Migault
   8275 Trans Canada Route
   Saint Laurent, QC


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