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Versions: 00 01 02                                                      
Network Working Group                                        P. van Dijk
Internet-Draft                                                  PowerDNS
Intended status: Experimental                                 P. Hoffman
Expires: 3 November 2021                                           ICANN
                                                              2 May 2021

      Common Features for Encrypted Recursive to Authoritative DNS


   Encryption between recursive and authoritative DNS servers is
   currently being defined in two modes: unauthenticated and fully-
   authenticated.  These two modes have some features in common, and
   this document defines those common features so that the documents
   defining the modes do not need to point to each other.

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
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   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 3 November 2021.

Copyright Notice

   Copyright (c) 2021 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/
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   Please review these documents carefully, as they describe your rights
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Discovery of Authoritative Server Encryption  . . . . . . . .   3
     2.1.  DNS SVCB Records in the Parent Zone . . . . . . . . . . .   3
   3.  Processing Discovery Responses  . . . . . . . . . . . . . . .   3
     3.1.  Resolver Process as Pseudocode  . . . . . . . . . . . . .   4
   4.  TLS Requirements for Encrypting Resolver to Authoritative
           Server Sessions . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The DPRIVE Working Group in the IETF is working on standardizing
   methods for encrypted communication between DNS recursive resolvers
   and authoritative servers.  At the time of this writing, [UNAUTH] is
   a work item in the working group, and [FULL-AUTH] has been widely
   discussed.  The working group expressed a desire that the modes share
   as much design as possible to simplify the working group's process of
   evaluating the security and operational aspects of the methods.  If
   the DPRIVE Working Group later adopts other modes, those modes should
   be considered in this document.

   This document lists the major technical features that are shared by
   [UNAUTH] and [FULL-AUTH].  Differences from the common features in
   this document are listed in the respective method documents.  The
   following are the features in common between and [UNAUTH] and

   *  Discovery of an authoritative server's encryption support
      (Section 2)

   *  Order of processing discovered authoritative servers (Section 3)

   *  TLS requirements (Section 4)

   Other topics might be added as the working group discusses [UNAUTH]
   and [FULL-AUTH] (and maybe other methods).

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2.  Discovery of Authoritative Server Encryption

   An authoritative server that supports DNS with encryption makes
   itself discoverable by publishing one or more DNS SVCB records that
   contain "alpn" parameter keys.  SVCB records are defined in [SVCB],
   and the DNS extension to those records are define in [DNS-SVCB].

   A recursive resolver discovers whether an authoritative server
   supports DNS with encryption by looking for cached SVCB records for
   the name of the authoritative server with a positive answer.  A
   cached DNS SVCB record with a negative answer indicates that the
   authoritative server does not support any encrypted transport.

   If the cache has no positive or negative answers for any DNS SVCB
   record for any of a zone's authoritative servers, the resolver needs
   to send queries for the DNS SVCB records for some or all of the
   zone's authoritative servers.

   Because some authoritative servers or middleboxes are misconfigured,
   requests for unknown RRtypes might be ignored by them.  Resolvers
   should be ready to deal with timeouts or other bad responses to their
   SVCB queries.

2.1.  DNS SVCB Records in the Parent Zone

   DNS SVCB records act as advisory information for resolvers about the
   encrypted protocols that are supported.  They can be thought of as
   similar to NS records on the parent side of a zone cut: advisory
   enough to act on, but not authoritative.  Given this, authoritative
   servers that know the DNS SCVB records associated with NS records for
   any child zones MAY include those DNS SCVB records in the Additional
   section of responses to queries to a parent authoritative server.

   (( Before this is published for real, it would be useful to check
   whether any resolvers freak out or fall over when they receive SVCB
   records in the Additional section. ))

3.  Processing Discovery Responses

   After a resolver has DNS SCVB records in its cache (possibly due to
   having just queried for them), it needs to use those records to try
   to find an authoritative server that uses DNS with encryption.  This
   section describes how the resolver can make that selection.

   A resolver MUST NOT attempt encryption for a server that has a
   negative response in its cache for the associated DNS SVCB record.

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   After sending out all requests for SVCB records for the authoritative
   servers in the NS RRset for a name, if all of the SVCB records for
   those authoritative servers in the cache are negative responses, the
   resolver MUST use classic (unencrypted) DNS instead of encryption.
   Similarly, if none of the DNS SVCB records for the authoritative
   servers in the cache have supported "alpn" parameters, the resolver
   MUST use classic (unencrypted) DNS instead of encryption.

   If there are any DNS SVCB records in the cache for the authoritative
   servers for a zone with supported "alpn" parameters, the resolver
   MUST try each indicated authoritative server using DNS with
   encryption until it successfully sets up a connection.  The resolver
   only attempts to use the encrypted transports that are in the
   associated SVCB record for the authoritative server. (( Note that
   this completely prohibits "simple port 853 probing" even though that
   is what some operators are currently doing.  Does the WG want to be
   this strict? ))

   A resolver SHOULD keep a DNS with encryption session to a particular
   server open if it expects to send additional queries to that server
   in a short period of time.  [DNS-OVER-TCP] says "both clients and
   servers SHOULD support connection reuse" for TCP connections, and
   that advice could apply as well for DNS with encryption, especially
   as DNS with encryption has far greater overhead for re-establishing a
   connection.  If the server closes the DNS with encryption session,
   the resolver can possibly re-establish a DNS with encryption session
   using encrypted session resumption.

3.1.  Resolver Process as Pseudocode

   This section is meant as an informal clarification of the protocol,
   and is not normative.  The pseudocode here is designed to show the
   intent of the protocol, so it is not optimized for things like
   intersection of sets and other shortcuts.

   In this code, "signal_rrset(name)" means an "SVCB" query for the
   "'_dns'" prefix of "this_name".  The "Query over secure transport
   until successful" section ignores differences in name server
   selection and retry behaviour in different resolvers.  The pseudocode
   was written to roughly cover the shared behaviour between [UNAUTH]
   and [FULL-AUTH].  Specifically, whether an implementation waits for
   the resolution of "queue a query" would differ between the two.

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  # Inputs
  ns_names = List of NS Rdatas from the NS RRset for the queried name
  can_do_secure = List of secure transports supported by resolver
  secure_names_and_transports = Empty list, filled in below

  # Fill secure_names_and_transports with (name, transport) tuples
  for this_name in ns_names:
    if signal_rrset(this_name) is in the resolver cache:
      if signal_rrset(this_name) positively does not exist:
      for this_transport in signal_rrset(this_name):
        if this_transport in can_do_secure:
          add (this_name, this_transport) to secure_names_and_transports
    else: # signal_rrset(this_name) is not in the resolver cache
      queue a query for signal_rrset(this_name) for later caching

  # Query over secure transport until successful
  for (this_name, this_transport) tuple in secure_names_and_transports:
    query using this_transport on this_name
    if successful:

  # Got here if no this_name/this_transport query was successful
  #   or if secure_names_and_transports was empty
  query using classic DNS on any/all ns_names; finished

4.  TLS Requirements for Encrypting Resolver to Authoritative Server

   All protocols for DNS with encryption rely on TLS.  This section
   defines requirements for the TLS use of DNS with encryption clients
   and servers.

   For any DNS with encryption protocols, TLS version 1.3 [TLS-13] or
   later MUST be used.

   (( There are other requirements, surely? ))

5.  IANA Considerations

   This document contains no changes to IANA registries.

6.  Security Considerations

   (( Talk about requiring TLS 1.3 ))

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7.  Acknowledgements

   The use of SVCB records for discovering whether an authoritative
   server supports encryption was first described by the authors of

8.  References

8.1.  Normative References

   [DNS-SVCB] Schwartz, B., "Service Binding Mapping for DNS Servers",
              Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
              03, 19 April 2021, <https://www.ietf.org/archive/id/draft-

              Pauly, T., Rescorla, E., Schinazi, D., and C. A. Wood,
              "Signaling Authoritative DNS Encryption", Work in
              Progress, Internet-Draft, draft-rescorla-dprive-adox-
              latest-00, 26 February 2021,

   [SVCB]     Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-https-05, 21 April 2021,

   [TLS-13]   Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [UNAUTH]   Hoffman, P. and P. V. Dijk, "Recursive to Authoritative
              DNS with Unauthenticated Encryption", Work in Progress,
              Internet-Draft, draft-ietf-dprive-unauth-to-authoritative-
              00, 12 April 2021, <https://www.ietf.org/archive/id/draft-

8.2.  Informative References

              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,

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Authors' Addresses

   Peter van Dijk

   Email: peter.van.dijk@powerdns.com

   Paul Hoffman

   Email: paul.hoffman@icann.org

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