DNS Privacy (dprive) Working Group                         S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Standards Track                       December 20, 2016
Expires: June 23, 2017

              Next step for DPRIVE: resolver-to-auth link


   This document examines the possible future work for the DPRIVE (DNS
   privacy) working group, specially in securing the resolver-to-
   authoritative name server link with TLS under DNS.

   It is not intended to be published as a RFC.

   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

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   This Internet-Draft will expire on June 23, 2017.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   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.  TLS or not TLS  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Possible solutions  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Encode key in name  . . . . . . . . . . . . . . . . . . .   4
     3.2.  Key in DNS  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  PKIX  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  "reverse" DNS . . . . . . . . . . . . . . . . . . . . . .   6
     3.5.  CGA . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.6.  Lax security  . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
     8.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction and background

   To improve the privacy of the DNS user ([RFC7626]), the standard
   solution is to encrypt the requests with TLS ([RFC7858]).  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 have
   currently no standard way to authenticate the authoritative name
   server, in the resolver-to-auth link.

   The two cases are quite different: a stub 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 server.

   Another difference is that resolvers are typically known by IP
   address (obtained by DHCP or manual configuration) while
   authoritative name servers are known by name (obtained from zone

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   delegation).  This makes things easier for techniques similar to
   DANE: the manager of the ns1.example.net name server can always add a
   TLSA record under example.net while she may have problems modifying
   the zone under in-addr.arpa or ip6.arpa.

   Note that, despite the fact that resolvers are in general configured
   by IP address, [I-D.ietf-dprive-dtls-and-tls-profiles] does not use
   it.  It describes several techniques to get the domain name of the
   resolver, and authenticates using this name.

   (On the other hand, the resolver knows also the IP address of the
   authoritative server, so it can authenticate based on this, the
   fourth and fifth solutions in Section 3.)

   A third difference between the stub-to-resolver and resolver-to-auth
   links is that, in the second case, it is more difficult to report
   authentication problems to the end-user.  (For better or for worse,
   the DNS is not end-to-end.)

   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.  TLS or not TLS

   There are several criteria to take into account before choosing an
   encryption protocol: client latency, server memory, client memory,
   novelty vs. maturity of design, maturity of actual code (libraries),
   support by other IETF groups, design complexity, cryptographic
   protocol agility ([RFC7696])...

   At first glance, the obvious protocol choice for encrypting the
   resolver-to-auth link is to use DNS over TLS or DTLS ([RFC7858],

      Already standardised

      Relies on a well-know security protocol (and inventing a new
      security protocol is quite dangerous)

   On the other hand, the DNS has some special properties.  While a stub
   resolver talks to only a few few resolvers (and therefore can afford
   a few TCP+TLS connections), a resolver may hesitate in front of the

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   task of managing hundreds of connections to remote authoritative
   servers [tdns].  TCP Fast Open [RFC7413], or persistent TCP
   connections [RFC7766], both requiring client-side state, will mean a
   lot of state to maintain on the resolver.  Some may think that a
   specially-designed protocol like [I-D.krecicki-dprive-dnsenc] or
   [I-D.wijngaards-dnsop-confidentialdns] is better.

   If we choose TLS, we may require a minimum version of 1.3.  TLS 1.3
   [I-D.ietf-tls-tls13] will probably be standardised before this
   document, and, specially combined with TCP Fast Open [RFC7413], it
   promises a minimum latency when contacting a new server.

   One last possibility is to wait for QUIC [I-D.ietf-quic-transport],
   which can use TLS, but QUIC is far from ready.  (One advantage of
   QUIC over DTLS is that it allows for messages of arbitrary length.
   That means there is no need to fall back to TCP when the message
   exceeds a certain size.)

3.  Possible solutions

   We can express the problem this way: if we want to use TLS-over-DNS
   to secure the link between the resolver and the authoritative server,
   it would be important to have a standard way to authenticate the
   authoritative server.  Basically, the client will get the public key
   of the server in the TLS session, how will it know that this key is
   the right one?

   Here is a comprehensive list of the six possible solutions to this
   problem.  First, the two where the key (or a hash of it) is available
   somewhere else than in the TLS session.

3.1.  Encode key in name

   We could encode the key in the authoritative server name (as in
   DNScurve [dnscurve] [I-D.dempsky-dnscurve]).  Here is an example of a
   domain using DNScurve: the names of the authoritative name servers
   are a Base-32 encoded form of the server's Curve25519 public key.

   % dig +short NS yp.to

   Securely transmitting the key would therefore be a by-product of
   delegation.  Among the limits of this solution, the length of these
   names limit the number of possible name servers, if we want to keep
   the delegation short.  Also, it requires a cryptographic algorithm
   where keys are short (which means no RSA).

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   If we want to add cryptographic agility to this solution, we will
   need a few bytes before the key itself, to indicate, for instance,
   the algorithm it uses.  It will reduce the possible key size even
   more.  (There was a suggestion to use the prefix "djb--" to clearly
   indicate this name doubles as a key...)

3.2.  Key in DNS

   We could publish keys in the DNS, secured with DNSSEC (as in DANE
   [RFC6698]).  This raises an interesting bootstrap problem: we need
   the key to get information privately with the DNS but we need the DNS
   to do so.  A possible solution is to have an "unsecure" mode to
   retrieve the initial key material.  The algorithm could be:

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

      (1)When the resolver needs to talk to a server (say
      ns1.example.net) for which it does not know the key, it does a
      TLSA request for _853._tcp.ns1.example.net,

      (2)If the resolution of this request requires to talk to the very
      server we search the key for, the resolver connects to this server
      with TLS to port 853, does not authenticate, and sends the query.
      This step offers no authentication ("opportunistic"?).

   (See also [I-D.ietf-dprive-dtls-and-tls-profiles], section 9.2.)  The
   real algorithm will need to be more complicated since there are
   several servers per zone.  A resolver may use the knowledge of TLS
   authentication it has to choose an authoritative name server among a
   NS RRset.

   Another solution is for the authoritative name server to use
   [I-D.ietf-tls-dnssec-chain-extension] to send all the necessary
   DNSSEC records over TLS.

3.3.  PKIX

   We could use the X.509 security model [RFC5280]).  The certificates
   for authoritative name servers would be signed by regular CAs, with
   the name of the server in the Subject Alternative Name (or may be its
   IP address in this field, but, as far as the author knows, few CAs
   issue certificates for IP addresses).

   One of the problems is that resolvers will probably have different
   sets of trusted CA so an authoritative name server will not know in
   advance what percentage of the resolvers may authenticate it.

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   Of course, this technique would not work if the server used raw
   public keys ([RFC7250].

3.4.  "reverse" DNS

   The resolver could start from the IP address of the authoritative
   name server, and get a key from the name in the in-addr.arpa or
   ip6.arpa zone.  For instance, if the authoritative server will be
   queried at 2001:db8:42:cafe::1:53, the request will be done for 3.5.0

   There is also a boostrapping problem here too, but since there are
   only five RIR that manage the reverse DNS some sort of hard-coded or
   semi-hard-coded setup might be possible.

   Unfortunately the operators for the space are often different from
   the operators of the DNS, so this is often not a reasonable solution

3.5.  CGA

   Another solution starting from the IP address of the authoritative
   name server would be to use CGA ([RFC3972]).  The IPv6 address
   encodes the public key in the lower 64-bits of the address.  So we
   could use the IPv6 address as the public key of the servers.

   This only works for IPv6, doesn't have (much) cryptographic agility,
   and raises serious layering violation issues.

3.6.  Lax security

   Finally, we could simply not check the keys at all, accepting
   anything.  This would break privacy promises, when there is an active
   attacker, able to pose as the authoritative name server.  But it is
   still better, privacy-wise, than the current situation where DNS
   requests are sent in clear text.

   Note that some of these proposals to authenticate the server also
   have the side-effect of signaling if the server is able and willing
   to do encryption.  This is the case with key in the name and of
   course of DANE.  In the other cases, the only way for a resolver to
   know if a server is supposed to accept encryption is to try it.

4.  Miscellaneous

   A resolver may use a combination of these solutions.  For instance,
   trying PKIX authentication (it does not require an extra lookup,
   except may be OCSP), if it fails, search a TLSA record, if there is

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   none, depending on the resolver's policy, accept anyway.  Clearly,
   trying all six solutions is unrealistic so some guidance on how to
   combine them will be necessary.

   Trying the various solutions in sequence may seriously increase the
   latency, specially if there are timeouts involved.  Using parallel
   attemps ("happy eyeballs", [RFC6555]) seem therefore crucial.

   All these solutions can be improved by things like automatic key
   pinning ([RFC6797]).

   An authoritative name server cannot know if the resolver authentified
   it, and how.  In the future, it may be interesting to have a 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 solutions.
   We may use front-end systems dispatching requests to port 53 and 853
   to different servers.  Or we may introduce a new delegation RR type,
   PNS (for Privacy Name Server), located only in the child zone (to
   avoid depending on a change of the provisioning software in the

5.  IANA Considerations

   There is currently nothing to do for IANA.  The future chosen
   solution may require some IANA action, such as a registry.

6.  Security Considerations

   For the time being, refer to each subsection under Section 3 to have
   an analysis of its security.

   A general problem with all (or most?) encryption protocols will be
   the state to be kept in the server.  It may make some denial-of-
   service attacks easier, of the protocol is not properly designed.

7.  Acknowledgments

   Shane Kerr for the ideas on authentication by IP address and John
   Heidemann for the list of criteria for evaluation.

8.  References

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8.1.  Normative References

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

              Dickinson, S., Gillmor, D., and T. Reddy, "Authentication
              and (D)TLS Profile for DNS-over-(D)TLS", draft-ietf-
              dprive-dtls-and-tls-profiles-07 (work in progress),
              October 2016.

8.2.  Informative References

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.

   [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, <http://www.rfc-editor.org/info/rfc6698>.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

   [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, <http://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,

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

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 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,

              Dempsky, M., "DNSCurve: Link-Level Security for the Domain
              Name System", draft-dempsky-dnscurve-01 (work in
              progress), February 2010.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-00 (work
              in progress), November 2016.

              Krecicki, W., "Stateless DNS Encryption", draft-krecicki-
              dprive-dnsenc-01 (work in progress), October 2015.

              Wijngaards, W. and G. Wiley, "Confidential DNS", draft-
              wijngaards-dnsop-confidentialdns-03 (work in progress),
              March 2015.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-18 (work in progress),
              October 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-01 (work in
              progress), July 2016.

              Reddy, T., Wing, D., and P. Patil, "Specification for DNS
              over Datagram Transport Layer Security (DTLS)", draft-
              ietf-dprive-dnsodtls-13 (work in progress), November 2016.

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              Bernstein, D., "DNSCurve: Usable security for DNS", June
              2009, <http://dnscurve.org/>.

   [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,

8.3.  URIs

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