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Versions: (draft-lmo-dprive-phase2-requirements) 00

DPRIVE                                                      J. Livingood
Internet-Draft                                                   Comcast
Intended status: Informational                              A. Mayrhofer
Expires: June 16, 2020                                       nic.at GmbH
                                                           B. Overeinder
                                                              NLnet Labs
                                                       December 14, 2019


 DNS Privacy Requirements for Exchanges between Recursive Resolvers and
                         Authoritative Servers
                draft-ietf-dprive-phase2-requirements-00

Abstract

   This document provides requirements for adding confidentiality to DNS
   exchanges between recursive resolvers and authoritative servers.

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 June 16, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Mandatory Requirements  . . . . . . . . . . . . . . . . .   4
     5.2.  Optional Requirements . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   9.  APPENDIX: Perspectives and Use Cases  . . . . . . . . . . . .   5
     9.1.  The User Perspective and Use Cases  . . . . . . . . . . .   6
     9.2.  The Operator Perspective and Use Cases  . . . . . . . . .   6
     9.3.  The Implementor / Software Vendor Perspective and Use
           Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   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 resolver and authoritative name
   servers.

   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
      document).

      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




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   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
   https://github.com/alex-nicat/ietf-dprive-phase2-requirements.
   Feedback via pull requests and issues are invited there.

3.  Terminology

   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.

   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.




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5.  Requirements

   The requirements of different interested stakeholders are outlined
   below.

5.1.  Mandatory Requirements

   1.   Each implementing party should 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 recursive resolver and
        authoritative servers

   3.   Use a secure transport protocol between a recursive resolver and
        TLD servers

   4.   Use a secure transport protocol between a recursive resolver and
        the root servers

   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 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.

   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.






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

   10.  For the secure transport, 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

   1.  QNAME minimisation SHOULD be implemented in all steps of
       recursion

   2.  DNSSEC validation SHOULD be performed

   3.  If an authoritative domain owner or their administrator indicates
       that (1) multiple secure transport protocols are available or
       that (2) a secure transport and insecure transport are available,
       then per the recommendations in [RFC8305] (aka Happy Eyeballs) a
       recursive server SHOULD initiate concurrent connections to
       available protocols.  Consistent with Section 2 of [RFC8305] this
       would be: (1) Initiation of asynchronous DNS queries to determine
       what transport protocols are supported, (2) Sorting of resolved
       destination transport protocols, (3) Initiation of asynchronous
       connection attempts, and (4) Establishment of one connection,
       which cancels all other attempts.

6.  Security Considerations

   This entire document concerns the security of DNS traffic, so a
   specific section on security is superfluous.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Changelog

   Version 00: Updated prior individual draft following IETF-106
   feedback

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




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   be identified, and their interests are described in the following
   sections:

   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
      which 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 has never a chance to identify which 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
   point

   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:



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   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
   nameserver.

   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"
         perspective?

   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
      messages)

   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



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         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
         considerations

      *  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
         signaling

      *  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
   perspectives:

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

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

   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





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   *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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc7816>.

   [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,
              <https://www.rfc-editor.org/info/rfc8305>.

10.3.  URIs

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

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






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Acknowledgments

   TODO

Authors' Addresses

   Jason Livingood
   Comcast

   Email: Jason_Livingood@comcast.com


   Alexander Mayrhofer
   nic.at GmbH

   Email: alex.mayrhofer.ietf@gmail.com


   Benno Overeinder
   NLnet Labs

   Email: benno@NLnetLabs.nl





























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