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

DPRIVE                                                      J. Livingood
Internet-Draft                                                   Comcast
Intended status: Informational                              A. Mayrhofer
Expires: April 30, 2020                                      nic.at GmbH
                                                           B. Overeinder
                                                               NLnetLabs
                                                        October 28, 2019

 DNS Privacy Requirements for Exchanges between Recursive Resolvers and
                         Authoritative Servers
                draft-lmo-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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 30, 2020.

Copyright Notice

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

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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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Threat Model and Problem Statement  . . . . . . . . . . . . .   3
   4.  Core Requirements . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Prioritization of Requirements  . . . . . . . . . . . . .   5
     4.2.  Opportunistic Upgrade to Encryption . . . . . . . . . . .   5
     4.3.  Detection of Availability . . . . . . . . . . . . . . . .   5
     4.4.  Resistance to Downgrade Attack  . . . . . . . . . . . . .   6
     4.5.  End-User Policy Propagation . . . . . . . . . . . . . . .   6
   5.  Perspectives and Use Cases  . . . . . . . . . . . . . . . . .   7
     5.1.  The User Perspective and Use Cases  . . . . . . . . . . .   8
     5.2.  The Operator Perspective and Use Cases  . . . . . . . . .   8
     5.3.  The Implementor / Software Vendor Perspective and Use
           Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  Performance and Efficiency  . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  lmo-dprive-phase2-requirements-00 . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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).

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   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.  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]

3.  Threat Model and Problem Statement

   Currently, potentially privacy-protective protocols such as DoT
   provide encryption between the user's stub resolver and a recursive
   resolver.  This provides (1) protection from observation of end user
   DNS queries and responses as well as (2) protection from on-the-wire
   modification DNS queries or responses.  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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4.  Core Requirements

   The requirements of different interested stakeholders are outlined
   below.  The parenthetical risks and priority levels are intended only
   to spur discussion.  But at a high level the requirements may be
   summarized as follows:

   o  Implement DoT between a recursive resolver and the root servers
      (low risk, low priority)

   o  Implement DoT between a recursive resolver and TLD servers (low
      risk, low priority)

   o  Implement DoT between a recursive resolver and second level
      authoritative servers (high risk, high priority)

   o  Implement DoT between a recursive resolver and other authoritative
      servers

   o  Implement DoT in each case in a manner that enables operators to
      perform appropriate performance and security monitoring, conduct
      relevant research, and to comply with locally relevent law
      enforcement or regulatory requirements (high risk, high priority)

   o  Implement QNAME minimisation in all steps of recursion (medium
      risk, medium priority)

   o  Minimize the need for recursion through aggressive caching (medium
      risk, medium priority) *NOTING THAT CACHING IS CONTINGENT ON AUTH
      RR TTLs - SO IS THIS REALLY A REQUIREMENT?*

   o  Each implementing party should be able to independently take steps
      to meet requirements without the need for close coordination (e.g.
      loosely coupled) (low risk, high priority)

   o  The legacy unencrypted DNS protocol (e.g.  UDP/TCP port 53) MUST
      be supported in parallel to DoT (high risk, high priority)

   o  Recursive resolvers SHOULD opportunistically upgrade recursive
      query transmissions to DoT when an authoritative server is
      detected to support DoT (high risk, high priority)

   WG DISCUSS: What about DNSSEC validation?  WG DISCUSS: Risk levels
   and prioritization (see also below) WG DISCUSS: Provisioning impacts
   - operators and vendors say implementation must be zero-provisioning

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4.1.  Prioritization of Requirements

   The core requirements above each have varying levels of risk and so
   can be prioritized based on that risk.  As a result, the highest risk
   area is the one that involves the greatest potential for surveillance
   and modification based on the details of the specific step of
   recursion.  This suggests the highest risk and thus highest priority
   is between a recursive server and first level authoritative server.
   Lower risks are to TLDs and root servers, with correspondingly lower
   priority.  Support for monitoring and compliance are also high risk
   since this is operationally critical, and thus should also be
   considered high priority.

4.2.  Opportunistic Upgrade to Encryption

   Opportunistically upgrading to use encryption when it is supported
   has been the best practice for deploying encryption, such as when web
   browsers upgrade to use TLS connections.  This enables deployment to
   occur incrementally and without tighly coupled coordination across a
   diverse global group of very different potential implementors.  As
   such it is a good method to use here was well.

   The exact method by which a recursive resolver determines whether an
   authoritative server supports DoT has not been specified in this
   document.  But it seems reasonable to imagine that a recursive server
   might be able to probe authoritative servers on TCP/853 using the DoT
   protocol and then build a cached list of servers that support DoT so
   that subsequent queries will upgrade to use DoT (and can fallback if
   DoT connections subsequently fail).  It seems also possible to
   imagine a method might exist for an authoritative domain to use a TBD
   resource record or other method to specify whether DoT is supported.

4.3.  Detection of Availability

   EDITORIAL NOTE: This section was just moved up.  May need some better
   integration later on.

   Recursive resolvers typically communicate with many authoritative
   nameservers.  Not every authorititative nameserver will support DoT
   and not every recursive resolver will support every requirement.  How
   should a recursive resolver determine whether DoT is supported for
   example?  (There may be multiple ways, or none)

   What scope/granularity should such an availability marker have?

   o  by zone ("all authoritative nameservers in the "example.net" zone
      support private queries from resolvers")

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   o  by identified nameserver ("the nameserver "a.ns.example.net"
      supports private queries from resolvers")

   o  by IP address ("any namservers that resolve to 192.0.2.13 support
      private queries from resolvers")

   Note that if there is no signal for availability, recursors could
   still opportunistically try the DNS privacy mechanism, as this is
   employed by some stub resolvers when they contact their designated
   recursors.

   Should a signal of availability also indicate a preference for
   privacy over availability? i.e., are there distinct ways to signal
   "DNS-privacy is available" separately from "Only contact this server
   via DNS-privacy if you understand this signal (though we may continue
   to support non-private DNS queries for clients that don't understand
   it)".

4.4.  Resistance to Downgrade Attack

   When a connection is opportunistically upgraded to DoT, if a fallback
   to unencrypted DNS can be possible via a downgrade attack by blocking
   or modifying TCP/853 communications.  In such cases, it may be best
   to establish a mechanism whereby the authoritative domain can specify
   their preferred behaviour.  This may range from only use DoT and do
   not fallback to unencrypted DNS, to opportunistically use DoT but
   fallback in failure, to do not use DoT.  The email application layer
   protocols have similar methods for asserting how email from a
   particular domain should be treated, so following some of the lessons
   learned there is likely a good idea.  Compare HSTS [RFC6797]?

4.5.  End-User Policy Propagation

   EDITORIAL NOTE: This section was just moved up.  May need some better
   integration later on.

   Like any multi-party protocol (e.g.  SMTP), the end user's
   preferences or policies might or might not be respected by later hops
   in the chain.  But if we have a way to express those preferences, we
   offer cooperating resolvers at least an opportunity to respect them.

   WG DISCUSS: Is it better to let auth domains assert whether fallback
   should be permitted or is that an end user preference or both?  The
   email world might suggest the former while the DNSSEC world the
   latter.  Or specify the standardization of the preferences and their
   communication and leave it to implementors to decide whether or how
   to treat those signals?

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   What sorts of preferences or policy might an end-user want to
   express? for example:

   o  do not identify my general location (e.g. don't send my subnet
      information (ECS) [RFC7871] data about me when talking to
      authoritative servers), accepting that reduced localization may
      result in less localized responses from authoritative Content
      Delivery Network (CDN) servers and thus slower access to content

   o  prefer DNS privacy over reduced latency (i.e., do not try to do
      speedups - try opportunistic privacy first and fall back to
      cleartext only if that fails)

   o  never do non-private authoritative queries on my behalf (for any
      external queries you need to do to resolve this request, require
      strict, well-authenticated DNS privacy)

   How specifically are these preferences be expressed by the client?
   (e.g. new EDNS0 [RFC6891] options?)  Should the recursor have a way
   to indicate whether:

   o  they are capable of honoring them?

   o  they intend to honor them?

   o  they _did_ honor them over the course of a specific lookup?

   If a resolver merely forwards a request to another recursor, should
   it also propagate those preferences/policy?  if so, how?

   This seems similar to [I-D.ietf-uta-smtp-require-tls].

5.  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
   sections:

   o  Users

   o  Operators

   o  Implementors / Software Developers

   o  Researchers

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5.1.  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 authoritive 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:

5.2.  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
   nameserver.

   o  Maybe different technical perspectives for operators

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      *  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
         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

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      *  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"?

5.3.  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

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

5.4.  Performance and Efficiency

   o  Can authoritative server operators limit resource-exhaustion
      attacks against private DNS mechanisms from having an impact on
      traditional (non-private) authoritative DNS availability?  (JL:
      seems easy to implement per host connection limits and implement
      other standard DDoS protections - again for a later BCP doc)

   o  What are best practices for authoritative server operators that
      can minimize latency and unavailability?

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   o  What are best practices for recursors?

6.  Security Considerations

   At this point of the document, the authors have not yet discussed
   security considerations in detail, as the whole document tends to
   deal with user privacy, which can be considered part of security. :)

7.  IANA Considerations

   This document has no actions for IANA.

8.  Changelog

   Note to RFC editor: Remove this entire section before publication.

8.1.  lmo-dprive-phase2-requirements-00

   Initial version

9.  References

9.1.  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>.

9.2.  Informative References

   [I-D.ietf-uta-smtp-require-tls]
              Fenton, J., "SMTP Require TLS Option", draft-ietf-uta-
              smtp-require-tls-09 (work in progress), August 2019.

   [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>.

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   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <https://www.rfc-editor.org/info/rfc6797>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [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>.

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,
              <https://www.rfc-editor.org/info/rfc7871>.

   [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>.

9.3.  URIs

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

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

Acknowledgments

   TODO

Authors' Addresses

   Jason Livingood
   Comcast

   Email: Jason_Livingood@comcast.com

   Alexander Mayrhofer
   nic.at GmbH

   Email: alex.mayrhofer.ietf@gmail.com

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   Benno Overeinder
   NLnetLabs

   Email: benno@NLnetLabs.nl

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