Common Features for Encrypted Recursive to Authoritative DNS
draft-pp-dprive-common-features-01

Document Type Active Internet-Draft (individual)
Authors Peter van Dijk  , Paul Hoffman 
Last updated 2021-05-19
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Network Working Group                                        P. van Dijk
Internet-Draft                                                  PowerDNS
Intended status: Experimental                                 P. Hoffman
Expires: 20 November 2021                                          ICANN
                                                             19 May 2021

      Common Features for Encrypted Recursive to Authoritative DNS
                   draft-pp-dprive-common-features-01

Abstract

   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
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   This Internet-Draft will expire on 20 November 2021.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
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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.  Serving with Encryption . . . . . . . . . . . . . . . . . . .   5
   5.  Resolvers Reporting Errors to Authoritative Servers . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   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
   [FULL-AUTH]:

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

   *  Order of processing discovered authoritative servers (Section 3)

   *  Serving with Encryption (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 is defined 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.

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.

   (( The question of how a parent zone knows the appropriate SCVB
   record for the child zone is outside the scope of this document.
   People have suggested an EPP extension and updating with CSYNC; these
   and other ideas might be standardized outside the DPRIVE Working
   Group. ))

   (( 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.

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

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(this_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:
        continue
      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:
      finished

  # 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.  Serving with Encryption

   An operator of an authoritative server following this protocol SHOULD
   publish SVCB records as described in Section 2.  If they cannot
   publish such records, the security properties of their authoritative
   servers will not be found.  If an operator wants to test serving
   using encryption, they can publish SVCB records with short TTLs and
   then stop serving with encryption after removing the SVCB records and
   waiting for the TTLs to expire.

   It is acceptable for an operator of authoritative servers to only
   offer encryption on some of the named authoritative servers, such as
   when the operator is determining how far to roll out encrypted
   service.

   A server MAY close an encrypted connection at any time.  For example,
   it can close the session if it has not received a DNS query in a
   defined length of time.  The server MAY close an encrypted session
   after it sends a DNS response; however, it might also want to keep
   the session open waiting for another DNS query from the resolver.
   [DNS-OVER-TCP] says "both clients and servers SHOULD support
   connection reuse" for TCP connections, and that advice could apply as

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   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.

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

5.  Resolvers Reporting Errors to Authoritative Servers

   Resolvers should have a method of telling authoritative servers that
   there are problems with the encrypted service they are offering.
   There is a proposal that the DNSOP Working Group might adopt
   [ERROR-REPORTING], which would enable such reporting.

   (( Clearly, more will need to go here. ))

6.  IANA Considerations

   (( Update registration for TCP/853 to also include ADoT ))

   (( Maybe other updates for DoH and DoQ ))

7.  Security Considerations

   An authoritative server that wants to only serve data to resolvers
   that use fully-authenticated encryption as described in [FULL-AUTH]
   cannot differentiate between those resolvers and resolvers using the
   mechanisms described in this document.

   (( Talk about requiring TLS 1.3 ))

8.  Acknowledgements

   The use of SVCB records for discovering whether an authoritative
   server supports encryption was first described by the authors of
   [FULL-AUTH].

   The DPRIVE Working Group has contributed many ideas that keep
   shifting the focus and content of this document.

9.  References

9.1.  Normative References

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   [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-
              schwartz-svcb-dns-03.txt>.

   [FULL-AUTH]
              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,
              <https://www.ietf.org/archive/id/draft-rescorla-dprive-
              adox-latest-00.txt>.

   [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,
              <https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
              https-05.txt>.

   [TLS-13]   Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [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-
              ietf-dprive-unauth-to-authoritative-00.txt>.

9.2.  Informative References

   [DNS-OVER-TCP]
              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,
              <https://www.rfc-editor.org/info/rfc7766>.

   [ERROR-REPORTING]
              Arends, R. and M. Larson, "DNS Error Reporting", Work in
              Progress, Internet-Draft, draft-arends-dns-error-
              reporting-00, 30 October 2020,
              <https://www.ietf.org/archive/id/draft-arends-dns-error-
              reporting-00.txt>.

Authors' Addresses

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   Peter van Dijk
   PowerDNS

   Email: peter.van.dijk@powerdns.com

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org

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