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Versions: (draft-pauly-add-requirements) 00 01

ADD                                                               C. Box
Internet-Draft                                                        BT
Intended status: Informational                                  T. Pauly
Expires: 6 May 2021                                                Apple
                                                               C.A. Wood
                                                                T. Reddy
                                                              D. Migault
                                                         2 November 2020

                Requirements for Adaptive DNS Discovery


   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, that of discovering the
   encrypted DNS resolver that corresponds to the Do53 resolver offered
   by a network.  It lists requirements that any proposed discovery
   mechanisms should 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 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 6 May 2021.

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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.
   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  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Equivalence . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Local addressing  . . . . . . . . . . . . . . . . . . . .   5
   4.  Network-identified encrypted resolvers  . . . . . . . . . . .   5
   5.  Resolver-identified encrypted resolvers . . . . . . . . . . .   5
   6.  Privacy and security requirements . . . . . . . . . . . . . .   6
   7.  Statement of Requirements . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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.

   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.

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   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 currently focusses on one common use case, that of
   discovering the encrypted DNS resolver that corresponds to the Do53
   resolver offered by a network.  Additional use cases can be added in
   future versions.  As well as describing the use case, it lists
   requirements that any proposed discovery mechanisms should address.
   They can do this either by providing a solution, or by explicitly
   stating why it is not in scope.

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].

   Equivalent: See Section 3.1.

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 has no previous knowledge about.  In such a case the
   usual existing behaviour is to dynamically discover the network's
   recommended 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 recommended unencrypted 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

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   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 that is equivalent to
   the one provisioned by the network 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 an encrypted DNS resolver that provides
   equivalent responses as the ones served by the network's recommended
   unencrypted resolver.  It is not a requirement that these two
   resolvers are the same physical or logical machine.  Often they will
   be, but they could equally be separated, perhaps by hundreds of
   miles.  However it is deployed, the key is that they are equivalent.

3.1.  Equivalence

   Given two resolvers A and B, equivalence is the claim that A and B
   can provide the same upper-layer DNS function to the client.  This
   does not include the DNS transport protocol (e.g.  Do53 or DNS-over-
   HTTPS) which can differ between equivalent resolvers.  To provide
   equivalence it is frequently likely to be the case that A and B are
   operated by the same administrative domain, but this document does
   not require that.

   There are two possible ways to claim equivalence.

   *  The local network can claim that one or more encrypted DNS
      resolvers (B, C, etc) are equivalent to the Do53 resolver (A) it
      has offered.  This is known as network-identified.

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

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   Network-identified is preferred since it comes from the same source
   of information, 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 must support being able to announce an equivalent
   resolver 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.

4.  Network-identified encrypted 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 equivalent encrypted DNS resolvers,
   or states that there are none.

5.  Resolver-identified encrypted 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
   equivalent encrypted DNS resolvers by communicating with the network-
   recommended unencrypted Do53 resolver.  This should involve an
   exchange that uses standard DNS messages that can be handled, or
   forwarded, by existing deployed software.

   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

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   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.

   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.

6.  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
      authenticated delegation from a client-known entity.

   *  Influence automatic discovery mechanisms such that a client uses
      one or more resolvers that are not otherwise involved with
      providing service to the client, such as: a network provider, a
      VPN server, a content provider being accessed, or a server that
      the client has manually configured.

   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.

7.  Statement of Requirements

   This section lists requirements that flow from the above sections.

     | Requirement | Description                                     |
     | R1.1        | Discovery MUST provide a local network the      |
     |             | ability to announce to clients a set of, or     |
     |             | absence of, equivalent resolvers.               |
     | R1.2        | Discovery MUST provide a resolver the ability   |
     |             | to announce to clients a set of, or absence of, |
     |             | equivalent resolvers.                           |
     | R1.3        | Discovery MUST support at least one encrypted   |
     |             | DNS protocol.                                   |
     | R1.4        | Discovery SHOULD support all standardised       |
     |             | encrypted DNS protocols.                        |
     | R2.1        | Networks MUST be able to announce one or more   |
     |             | equivalent encrypted DNS resolvers using        |
     |             | existing mechanisms such as DHCPv4, DHCPv6,     |
     |             | IPv6 Router Advertisement, and the Point-to-    |
     |             | Point Protocol.                                 |

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     | R2.2        | The format for resolver information MUST be     |
     |             | specified such that provisioning mechanisms     |
     |             | defined outside of the IETF can advertise       |
     |             | encrypted DNS resolvers.                        |
     | R3.1        | When discovery is instantiated from a resolver  |
     |             | (R1.2), that resolver MAY be encrypted or not.  |
     | R3.2        | When discovery is instantiated from a resolver  |
     |             | (R1.2), that resolver MAY be locally or         |
     |             | globally reachable.  Both options MUST be       |
     |             | supported.                                      |
     | R4.1        | In a home network use case, if the local        |
     |             | network forwarder does not offer encrypted DNS  |
     |             | service, the ISP's encrypted DNS server         |
     |             | information MUST be retrievable via a query     |
     |             | sent to a local network forwarder.              |
     | R4.2        | Encrypted DNS server discovery MUST 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.         |
     | R5.5        | An attacker in the network MUST NOT be able to  |
     |             | override or interfere with the resolver         |
     |             | preferences of a user or administrator.         |
     | R5.6        | Attackers MUST NOT be able to influence         |
     |             | automatic discovery mechanisms such that a      |
     |             | client uses one or more resolvers that are not  |
     |             | otherwise involved with providing service to    |
     |             | the client, including a network provider, a VPN |
     |             | server, a content provider being accessed, or a |

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     |             | server that the client has manually configured. |

                                  Table 1

8.  Security Considerations

   See Section 6.

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  Normative References

   [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, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  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, <http://www.ietf.org/internet-drafts/draft-ietf-

   [RFC1035]  Mockapetris, P.V., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [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, <https://www.rfc-editor.org/info/rfc1918>.

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   [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, <https://www.rfc-editor.org/info/rfc7858>.

   [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, the subsequent interims, 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
   and Andrew Campling.

Authors' Addresses

   Chris Box
   2000 Park Avenue
   United Kingdom

   Email: chris.box@bt.com

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

   Email: tpauly@apple.com

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

   Email: caw@heapingbits.net

   Tirumaleswar Reddy
   Embassy Golf Link Business Park

   Email: TirumaleswarReddy_Konda@McAfee.com

   Daniel Migault
   8275 Trans Canada Route
   Saint Laurent, QC

   Email: daniel.migault@ericsson.com

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