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DNS Privacy (dprive) Working Group                         S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Standards Track                          March 20, 2018
Expires: September 21, 2018

   Encryption and authentication of the DNS resolver-to-authoritative


   This document proposes a mechanism for securing (privacy-wise) the
   communication between the DNS resolver and the authoritative name

   REMOVE BEFORE PUBLICATION: this document should be discussed in the
   IETF DPRIVE group, through its mailing list.  The source of the
   document, as well as a list of open issues, is currently kept at
   Github [1].

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
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   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 September 21, 2018.

Copyright Notice

   Copyright (c) 2018 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

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   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 and background . . . . . . . . . . . . . . . . .   2
   2.  Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Operational considerations  . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     6.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Alternatives . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction and background

   To improve the privacy of the DNS user ([RFC7626]), the standard
   solution is to encrypt the requests with TLS ([RFC7858]).  We use
   this DNS-over-TLS solution as well here, since it is standardized,
   already implemented in many programs, and relies on a well-known
   security protocol (inventing a new security protocol is quite
   dangerous).  But just encrypting, without authenticating the remote
   server, leaves the user's privacy vulnerable to active man-in-the-
   middle attacks.  [RFC7858] and
   [I-D.ietf-dprive-dtls-and-tls-profiles] describe how to authenticate
   the DNS resolver, in the stub-to-resolver link.  We describe here
   authentication of the authoritative name server, in the resolver-to-
   authoritative link.

   A stub DNS resolver has only a few resolvers, and there is typically
   a pre-existing relationship.  But a resolver speaks to many
   authoritative name servers, without any prior relationship.  This
   means that, for instance, having a static key for the resolver makes
   sense while it would be clearly unrealistic for the authoritative

   Instead, we rely on DANE ([RFC6698]).  Authoritative name servers are
   known by name (obtained from zone delegation).  The manager of the
   ns1.example.net name server adds a TLSA record under example.net.
   The client establishes the TLS session, then authenticate in the
   normal DANE way.

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   The original charter of the DPRIVE working group, in force at the
   time of this draft, says "The primary focus of this Working Group is
   to develop mechanisms that provide confidentiality between DNS
   Clients and Iterative Resolvers" and adds "but it may also later
   consider mechanisms that provide confidentiality between Iterative
   Resolvers and Authoritative Servers".  This document is here for this
   second step, "between Iterative Resolvers and Authoritative Servers".
   It will probably require a rechartering of the group.

2.  Rules

   A DNS full-service resolver who needs to query an authoritative name
   server establishes a TLS-over-TCP session with this authoritative
   name server.  If the DNS material to perform DANE authentication is
   sent in the TLS session ([I-D.ietf-tls-dnssec-chain-extension]), it
   uses it.  Otherwise, the resolver queries TLSA records ([RFC6698])
   for this name server and authenticates the key or certificate of the
   server this way.  If the name server is ns1.example.net, the TLSA
   record to query is _853._tcp.ns1.example.net.

   Note that the server MAY use raw public keys ([RFC7250]) and so there
   is not always a certificate.  If the server uses raw public keys, the
   TLSA record's Selector field must be 1 (SPKI, SubjectPublicKeyInfo).

   The recommended order is to try TLS before querying the TLSA records.
   True, DANE signals if the server is willing to make DNS-over-TLS (and
   can therefore save a TLS attempt) but cannot guarantee that it will
   work (for instance if a middlebox blocks port 853).  Also, the DANE
   records may be transferred in the TLS session, not through the DNS.

   If the TLS session establishement fails, or if the DANE
   authentication fails, the result depends on whether the resolver runs
   in strict or opportunistic mode
   ([I-D.ietf-dprive-dtls-and-tls-profiles]).  In strict mode, the
   resolver MUST stop using this authoritative name server, and MUST try
   other servers of the DNS zone.  In opportunistic mode, the resolver
   MUST use the authoritative name server despite the failure.  It MAY
   try other name servers of the zone before, in the hope they will
   accept TLS and be authenticated.  To avoid a chicken-and-egg problem,
   the resolver, even in strict mode, MAY use unsecure servers for the
   meta-queries (getting the TLSA records).  More specifically:

      (0)The resolver remembers the keys of the authoritative name
      servers (in the same way it remembers the lowest RTT among an NS

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      (1)When the resolver needs to talk to a server (say
      ns2.example.net) for which it does not know the key, it does a
      TLSA request for _853._tcp.ns2.example.net,

      (2)If the resolution of this request requires that we talk to the
      same server for which we're searching for the TLSA record, the
      resolver connects to this server with TLS to port 853, does not
      bother to authenticate, and sends the query.  This step offers no

   (See also [I-D.ietf-dprive-dtls-and-tls-profiles], section 5.)  A
   resolver MAY use the knowledge of TLS authentication it has to choose
   an authoritative name server among a NS RRset.

   As of this revision, we do not expect resolvers to use strict mode,
   since the encryption and authentication modes described in this
   document are not yet supported in authoritative name servers.

3.  Operational considerations

   DNS-over-TLS depends on TCP, and the resolver and the authoritative
   name server must therefore support persistent TCP connections
   ([RFC7766], specially section 6.2.1).

   A resolver may have a lot of client-side state, when managing
   hundreds of connections to remote authoritative servers ([tdns]).

   The latency when connecting to a authoritative name server is
   certainly an issue.  TLS 1.3 and TCP Fast Open ([RFC7413]) may help.

   Open question: do we require a minimum TLS version of 1.3?

   Because the resolver cannot know in advance if the TLS connection
   will work (even if there is a DANE record), using parallel attempts
   ("happy eyeballs", [RFC8305]) is important.  A resolver working in
   opportunistic mode should try ports 53 and 853 in parallel.

   An authoritative name server cannot know if the resolver
   authenticated it, nor how.  In the future, it may be interesting to
   have an EDNS option to signal a successful authentication, or a
   failure, but this is out of scope currently.

   If it is a concern that the same authoritative name servers are used
   for ordinary DNS and for encrypted DNS, there are several ways to
   address this concern.  A server operator may use front-end systems
   dispatching requests to ports 53 and 853 to different servers.

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   A resolver must be configurable to operate in strict or opportunistic
   modes.  Until the features described herein are widely supported,
   opportunistic mode should not be the default since strict mode would
   yield frequent failures.  A resolver may have a configuration
   mechanism to be in strict mode only for some domains.

4.  IANA Considerations

   No action for IANA.  This section can be deleted.

5.  Security Considerations

   The state to be kept in both the client and the server may make some
   denial-of-service attacks easier.  Following the advice contained in
   section 10 of [RFC7766] is recommended.

   In opportunistic mode, there is no guarantee to have a secure use of
   the DNS, or even a guarantee to be informed of a problem.
   Opportunistic mode is a "best effort" privacy service.  Even in
   strict mode, some leaks may occur, through the DANE meta-queries, and
   through SNI indication ([I-D.ietf-tls-sni-encryption]) in the TLS

   Neither transport encryption nor authentication protect DNS users
   from authentic servers which nonetheless abuse users' privacy once
   they've received their queries.  These techniques must therefore be
   combined with data minimization techniques ([RFC7816]).

6.  References

6.1.  Normative References

              Dickinson, S., Gillmor, D., and T. Reddy, "Usage and
              (D)TLS Profiles for DNS-over-(D)TLS", draft-ietf-dprive-
              dtls-and-tls-profiles-11 (work in progress), September

   [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>.

   [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>.

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6.2.  Informative References

              Bortzmeyer, S., "Next step for DPRIVE: resolver-to-auth
              link", draft-bortzmeyer-dprive-step-2-05 (work in
              progress), December 2016.

              Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
              Record and DNSSEC Authentication Chain Extension for TLS",
              draft-ietf-tls-dnssec-chain-extension-06 (work in
              progress), January 2018.

              Huitema, C. and E. Rescorla, "SNI Encryption in TLS
              Through Tunneling", draft-ietf-tls-sni-encryption-02 (work
              in progress), March 2018.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-27 (work in progress),
              March 2018.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,

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

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

   [tdns]     Liang, Z., Wessels, D., Zi, H., Heidemann, J., Mankin, A.,
              and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve
              Privacy and Security; USC/ISI Technical Report ISI-TR-
              706", August 2014,

6.3.  URIs

   [1] https://github.com/bortzmeyer/ietf-dprive-step-2

Appendix A.  Acknowledgments

   Thanks to Bill Woodcock for a detailed review.

Appendix B.  Alternatives

   A number of other possible solutions to this problem may be found in
   in [I-D.bortzmeyer-dprive-step-2].

Author's Address

   Stephane Bortzmeyer
   1, rue Stephenson
   Montigny-le-Bretonneux  78180

   Phone: +33 1 39 30 83 46
   Email: bortzmeyer+ietf@nic.fr
   URI:   http://www.afnic.fr/

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