DPRIVE                                                      J. Livingood
Internet-Draft                                                   Comcast
Intended status: Informational                              A. Mayrhofer
Expires: December 18, 2020 May 6, 2021                                         nic.at GmbH
                                                           B. Overeinder
                                                              NLnet Labs
                                                           June 16,
                                                       November 02, 2020

 DNS Privacy Requirements for Exchanges between Recursive Resolvers and
                         Authoritative Servers


   This document provides describes requirements and considerations for adding
   confidentiality to DNS exchanges between recursive resolvers and
   authoritative servers.  The intent of this document is to guide
   Internet Drafts in the DNS Private Exchange (DPRIVE) Working Group
   pertaining to recursive to authorized name servers, with the stated
   requirements and considerations.

Status of This Memo

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   This Internet-Draft will expire on December 18, 2020. May 6, 2021.

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Table of Contents

   1.  Introduction & Scope  . . . . . . . . . . . . . . . . . . . .   2
   2.  Document Work Via GitHub  . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Threat Model and Problem Statement  . . . . . . . . . . . . .   3
   5.  Requirements  . . . . . .  Features to Provide Confidentiality . . . . . . . . . . . . .   4
     5.1.  Requirements  . . . . .   4
     5.1.  Mandatory Requirements . . . . . . . . . . . . . . . . .   4
     5.2.  Optional Requirements Features . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5   6
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .   5   6
   9.  APPENDIX: Perspectives and Use Cases  . . . . . . . . . . . .   5   6
     9.1.  The User Perspective and Use Cases  . . . . . . . . . . .   6
     9.2.  The Operator Perspective and Use Cases  . . . . . . . . .   6   7
     9.3.  The Implementor / Software Vendor Perspective and Use
           Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction & Scope

   The 2018 approved charter of the IETF DPRIVE Working Group [1]
   contains milestones related to confidentiality aspects of DNS
   transactions between the iterative recursive resolver and authoritative name

   This is also reflected in the DPRIVE milestones [2], which (as of
   October 2019) contains two relevant milestones:

      Develop requirements for adding confidentiality to DNS exchanges
      between recursive resolvers and authoritative servers (unpublished

      Investigate potential solutions for adding confidentiality to DNS
      exchanges involving authoritative servers (Experimental).

   This document intends to cover the first milestone for defining
   requirements for adding confidentiality to DNS exchanges between
   recursive resolvers and authoritative servers.  This may in turn lead
   to progress in investigating, developing and standardizing potential
   experimental methods of meeting those requirements.

   The motivation for this work is to extend the confidentiality methods
   used between a user's stub resolver and a recursive resolver to the
   recursive queries sent by recursive resolvers in response to a DNS
   lookup (when a cache miss occurs and the server must perform
   recursion to obtain a response to the query).  A recursive resolver
   will send queries to root servers, to Top Level Domain (TLD) servers,
   to authoritative second level domain servers and potentially to other
   authoritative DNS servers and each of these query/response
   transactions presents an opportunity to extend the confidentiality of
   user DNS queries.

2.  Document Work Via GitHub

   The authors are working on this document via GitHub at
   Feedback via pull requests and issues are invited there.

3.  Terminology

   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.

   This document also makes use of DNS Terminology defined in [RFC8499]

4.  Threat Model and Problem Statement

   Currently, protocols such as DoT provide encryption between the
   user's stub resolver and a recursive resolver.  This potentially
   provides (1) protection from observation of end user DNS queries and
   responses, (2) protection from on-the-wire modification DNS queries
   or responses (including potentially forcing a downgrade to an
   unencrypted communication).  Of course, observation and modification
   are still possible when performed by the recursive resolver, which
   decrypts queries, serves a response from cache or performs recursion
   to obtain a response (or synthesizes a response), and then encrypts
   the response and sends it back to the user's stub resolver.

   But observation and modification threats still exist when a recursive
   resolver must perform DNS recursion, from the root to TLD to
   authoritative servers.  This document specifies requirements for
   filling those gaps.

5.  Requirements

   The requirements  Features to Provide Confidentiality

   Confidentialty can be provided using a combination of different interested stakeholders are outlined
   below. techniques.
   This section describes the protocol implementation requirements and
   optional features that can be used to provide confidentiality.

5.1.  Mandatory  Requirements

   1.   Each implementing party should MUST be able to independently take
        incremental steps to meet requirements without the need for
        close coordination (e.g. loosely coupled)

   2.   Use a secure transport protocol between a   A recursive resolver and that supports recursive-to-authoritative
        DNS encryption MUST be able to determine whether or not a given
        authoritative servers name server to which it intends to connect also
        supports recursive-to-authoritative DNS encryption.

   3.   Use a secure transport protocol between   An authoritative name server that supports recursive-to-
        authoritative DNS encryption MUST be able to indicate that it
        supports recursive-to-authoritative DNS encryption in a recursive resolver and
        TLD servers way that
        facilitates (2).

   4.   Use a secure transport protocol between a recursive resolver and
        the root servers   An authoritative name server that does not support recursive-to-
        authoritative MUST NOT have to make any changes to facilitate

   5.   The secure transport MUST only be established when referential
        integrity can be verified, MUST NOT have circular dependencies,
        and MUST be easily analyzed for diagnostic purposes.

   6.   Use   Each implementing party MUST be able to negotiate use of a
        secure transport protocol or other DNS privacy protections in a
        manner that enables operators to perform appropriate performance
        and security monitoring, conduct relevant research, etc.

   7.   The authoritative domain owner or their administrator MUST have
        the option to specify their secure transport preferences (e.g.
        what specific protocols are supported).  This SHALL include a
        method to publish a list of secure transport protocols (e.g.
        DoH, DoT and other future protocols not yet developed).  In
        addition this SHALL include whether a secure transport protocol
        MUST always be used (non-downgradable) or whether a secure
        transport protocol MAY be used on an opportunistic (not strict)
        basis in recognition that some servers for a domain might use a
        secure transport protocol and others might not.

   8.   The authoritative domain owner or their administrator MUST have
        the option to vary their preferences on an authoritative
        nameserver to nameserver basis, due to the fact that
        administration of a particular DNS zone may be delegated to
        multiple parties (such as several CDNs), each of which may have
        different technical capabilities.  This includes that some
        servers for a domain may use secure transport and others may
        not, as it is common for a given name server to be authoritative
        for multiple zones.

   9.   A given name server may be authoritative for multiple zones.  As
        such, a name server MAY support use of a secure transport
        protocol for one zone, but not for another.

   10.  The specification of secure transport preferences MUST be
        performed using the DNS and MUST NOT depend on non-DNS


   11.  For the secure transport, transports using TLS, TLS 1.3 (or later versions)
        MUST be supported and downgrades from TLS 1.3 to prior versions
        MUST not occur.

5.2.  Optional Requirements Features

   1.  QNAME minimisation SHOULD be implemented in all steps of

   2.  DNSSEC validation SHOULD be performed

   3.  If an authoritative domain owner or their administrator indicates
       that (1) multiple secure transport protocols are available available, or
       that (2) a secure transport and insecure transport are available,
       or that (3) no secure transport is available, then per the recommendations in [RFC8305] (aka Happy Eyeballs) a recursive
       server SHOULD initiate concurrent connections to
       available protocols.  Consistent with Section 2 negotiate selection of [RFC8305] this
       would be: (1) Initiation of asynchronous DNS queries to determine
       what an available transport protocols are supported, (2) Sorting

6.  Security Considerations

   Authoritative name servers will need to perform additional processing
   steps, such as completing key exchanges and maintaining persistent
   connections, when responding to queries from a recursive resolver
   that requests use of resolved
       destination a secure transport protocols, (3) Initiation of asynchronous
       connection attempts, protocol.  These additional
   processing steps can have an impact on server availability if they
   are abused.  As such, negotiation and (4) Establishment use of one connection,
       which cancels all other attempts.

6.  Security Considerations

   This entire document concerns a secure transport
   protocol should be done in a manner that does not increase the security risk
   of DNS traffic, so an authoritative name server outage or lead a
   specific section on security is superfluous. recursive server to
   fail to communicate with an authoritative name server.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Changelog

   Version 00: Updated prior individual draft following IETF-106
   feedback Version 01: Small editorial changes Version 02: Incorporate
   feedback and suggestions from Scott Hollenbeck, Duane Wessels and
   email discussions.

9.  APPENDIX: Perspectives and Use Cases

   The DNS resolving process involves several entities.  These entities
   have different interests/requirements, and hence it does make sense
   to examine the interests of those entities separately - though in
   many cases their interests are aligned.  Four different entities can
   be identified, and their interests are described in the following

   o  Users

   o  Operators

   o  Implementors / Software Developers

   o  Researchers

9.1.  The User Perspective and Use Cases

   The privacy and confidentiality of Users (that is, users as in
   clients of recursive resolvers, which in turn forward/resolve the
   user's DNS requests by contacting authoritative servers) can be
   improved in several ways.  We call this "minimisation of exposure",
   and there are currently three ways to reduce that exposure:

   o  Qname minimisation [RFC7816], reducing the amount of information
      to what is absolutely necessary to resolve a query

   o  Aggressive NSEC/local auth cache [RFC8198], reducing the amount of
      outgoing queries in the first place

   o  Encryption, removing exposure of information while in transit

   As recursors typically forwards queries received from the user to
   authoritative servers.  This creates a transitive trust between the
   user and the recursor, as well as the authoritative server, since
   information created by the user is exposed to the authoritative
   server.  However, the user never has a chance to identify what data
   was exposed to which authoritative party (via which path).

   Also, Users would want to be informed about the status of the
   connections which were made on their behalf, which adds a fourth

   Encryption/privacy status signaling

   *TODO*: Actual requirements - what do users "want"?  Start below:

9.2.  The Operator Perspective and Use Cases

   Operators of authoritative services have to provide stable and fast
   DNS services, and interact with a wide range of clients, not all of
   them authoritative servers.  The operator side actually consists of
   two sides:

   o  The "upstream" facing side of recursive resolvers

   o  The "downstream" side of authoritative servers

   Those two sides are typically operated by different entities, but
   many entities operate "both sides".  Even though that is discouraged
   (*TODO* source), the two sides might even be operated on the same

   o  Maybe different technical perspectives for operators

      *  Intelligence (sharing information)

      *  SLD popularity for marketing

   o  Focus initially on Second Level Domains (SLDs) initially

      *  Is there a difference for TLDs vs. SLDs from a "protocol"

   o  Monitoring and aggregated data analysis

   o  Signaling provisioning information

      *  New record type for finding authoritative server key and
         authentication?  Use SRV?  (Being able to use different servers
         for serving up DNS-over-{TCP,UDP} vs DNS-over-TLS responses may
         be valuable.

      *  Signal secure transport details (DNS-over-TLS, DNS-over-QUIC,
         EncryptedSNI, connectionless, etc.), perhaps in an extensible
         manner?  Minimize RTTs and reduce need for trials.

      *  Large provider use cases where the NS names are out of
         bailiwick for the zone (e.g. small number of distinct NS
         records serving 100k+ zones)

   o  EDNS client subnet (JL: Not sure ECS crosses the cost/benefit
      threshold to be included as a requirement and many CDNs that run
      auth servers will likely say ECS is quite operationally important)

   o  Decide between TLS and connectionless (such as COSE-based

   o  Costs of TLS connection vs. connectionless

      *  Technical solution, e.g. encryption of the DNS query, shouldn't
         enable an attack vector for DDoS or resource exhaustion.  For
         example, only if the client uses DNS-over-TLS, the upstream
         query to the authoritative will be over DNS-over-TLS also.  If
         the client uses UDP, the resolver won't invest resources in
         DNS-over-TLS to prevent a potential resource exhaustion attack.

      *  Reuse connection state (if any) and examine resumption

      *  Minimize server-side state (eg, with session tickets)

      *  Need empirical studies on capacity, traffic, attack vectors

      *  Evaluate impact on architecture and footprint expansion

      *  Analyze optimal persistent connection time/time-out

      *  Analyze optimal number of persistent connections recursive
         resolvers should maintain

      *  Consider operational concerns with respect to capabilities

      *  Develop a profile that has operational advantages for operators

   *TODO*: Actual requirements - what do operators "want"?

9.3.  The Implementor / Software Vendor Perspective and Use Cases

   Implementer requirements follows requirements from user and operator

   o  Non-functional requirements, e.g. diversity of implementations

   o  Horizontal vs. vertical scaling, for example similar to http

   o  Use of DANE [RFC6698] for authentication: strict vs. opportunistic

   o  Incremental deployment

   o  Cache reuse vs. downgrade?  Does the cache need to be partitioned?
      When can an in-cache answer retrieved via cleartext be served
      encrypted to a recursive query?

   o  (Use of TCP fast open) - but this might be a requirement for the
      actual encryption protocol

   *TODO*: Actual requirements of implementors - essentially, they
   follow what Operators need?

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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

10.2.  Informative References

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <https://www.rfc-editor.org/info/rfc8198>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

10.3.  URIs

   [1] https://datatracker.ietf.org/doc/charter-ietf-dprive/

   [2] https://datatracker.ietf.org/wg/dprive/about/



   The authors would like to thank Scott Hollenbeck for his early
   feedback and providing text for the Internet Draft.  We would also
   like to thank Duane Wessels for the feedback on the mailing list, and
   Peter van Dijk for his comments in personal conversations.

Authors' Addresses

   Jason Livingood

   Email: Jason_Livingood@comcast.com

   Alexander Mayrhofer
   nic.at GmbH

   Email: alex.mayrhofer.ietf@gmail.com

   Benno Overeinder
   NLnet Labs

   Email: benno@NLnetLabs.nl