Recursive to Authoritative DNS with Unauthenticated Encryption
draft-ietf-dprive-unauth-to-authoritative-00

Document Type Active Internet-Draft (dprive WG)
Authors Paul Hoffman  , Peter van Dijk 
Last updated 2021-04-12
Replaces draft-ietf-dprive-opportunistic-adotq
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Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Experimental                                P. van Dijk
Expires: 14 October 2021                                        PowerDNS
                                                           12 April 2021

     Recursive to Authoritative DNS with Unauthenticated Encryption
             draft-ietf-dprive-unauth-to-authoritative-00

Abstract

   This document describes a use case and a method for a DNS recursive
   resolver to use unauthenticated encryption when communicating with
   authoritative servers.  The motivating use case for this method is
   that more encryption on the Internet is better, and some resolver
   operators believe that unauthenticated encryption is better than no
   encryption at all.  The method described here is optional for both
   the recursive resolver and the authoritative server.  This method
   supports unauthenticated encryption using the same mechanism for
   discovery of encryption support for the server as
   [I-D.rescorla-dprive-adox-latest].

   NOTE: The file name for this draft, draft-ietf-dprive-opportunistic-
   adotq, is now incorrect.  This draft only covers unauthenticated
   encryption, not opportunistic encryption.

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
   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 14 October 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Use Case for Unauthenticated Encryption . . . . . . . . .   3
     1.2.  Summary of Protocol . . . . . . . . . . . . . . . . . . .   3
     1.3.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Discovering Whether an Authoritative Server Uses
           Encryption  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Resolving with Encryption . . . . . . . . . . . . . . . . . .   5
     3.1.  Resolver Session Failures . . . . . . . . . . . . . . . .   6
     3.2.  Resolver Process as Pseudocode  . . . . . . . . . . . . .   7
   4.  Serving with Encryption . . . . . . . . . . . . . . . . . . .   7
   5.  Resolvers Reporting Errors to Authoritative Servers . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A recursive resolver using traditional DNS over port 53 may wish
   instead to use encrypted communication with authoritative servers in
   order to limit snooping of its DNS traffic by passive or on-path
   attackers.  The recursive resolver can use unauthenticated encryption
   (defined in [RFC7435]) to achieve this goal.

   This document describes the use case for unauthenticated encryption
   in recursive resolvers in Section 1.1.  The encryption method with
   authoritative servers can be DNS-over-TLS [RFC7858] (DoT), DNS-over-
   HTTPS [RFC8484] (DoH), and/or DNS-over-QUIC
   [I-D.ietf-dprive-dnsoquic] (DoQ), as described in Section 3.

   The document also describes a discovery method that shows if an
   authoritative server supports encryption in Section 2.

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   See [I-D.rescorla-dprive-adox-latest] for a description of the use
   case and a proposed mechanism for fully-authenticated encryption.

   NOTE: The draft uses the SVCB record as a discovery mechanism for
   encryption by a particular authoritative server.  Any record type
   that can show multiple types of encryption (currently DoT, DoH, and
   DoQ) can be used for discovery.  Thus, this record type might change
   in the future, depending on the discussion in the DPRIVE WG.

1.1.  Use Case for Unauthenticated Encryption

   The use case in this document for unauthenticated encryption is
   recursive resolver operators who are happy to use encryption with
   authoritative servers if doing so doesn't significantly slow down
   getting answers, and authoritative server operators that are happy to
   use encryption with recursive resolvers if it doesn't cost much.  In
   this use case, resolvers do not want to return an error for requests
   that were sent over an encrypted channel if they would have been able
   to give a correct answer using unencrypted transport.

   Resolvers and authoritative servers understand that using encryption
   costs something, but are willing to absorb the costs for the benefit
   of more Internet traffic being encrypted.  The extra costs (compared
   to using traditional DNS on port 53) include:

   *  Extra round trips to establish TCP for every session (but not
      necessarily for every query)

   *  Extra round trips for TLS establishment

   *  Greater CPU use for TLS establishment

   *  Greater CPU use for encryption after TLS establishment

   *  Greater memory use for holding TLS state

   This use case is not expected to apply to all resolvers or
   authoritative servers.  For example, according to [RSO_STATEMENT],
   some root server operators do not want to be the early adopters for
   DNS with encryption.  The protocol in this document explicitly allows
   authoritative servers to signal when they are ready to begin offering
   DNS with encryption.

1.2.  Summary of Protocol

   This summary gives an overview of how the parts of the protocol work
   together.

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   *  The resolver discovers whether any authoritative server of
      interest supports DNS with encryption by querying for the SVCB
      records [I-D.ietf-dnsop-svcb-https].  As described in
      [I-D.schwartz-svcb-dns], SVCB records can indicate that a server
      supports encrypted transport of DNS queries.

      NOTE: In this document, the term "SVCB record" is used _only_ for
      SVCB records that indicate encryption as described in
      [I-D.schwartz-svcb-dns].  SVCB records that do not have these
      indicators in the RDATA are not included in the term "SVCB record"
      in this document.

   *  The resolver uses any authoritative server with a SVCB record that
      indicates encryption to perform unauthenticated encryption.

   *  The resolver does not fail to set up encryption if the
      authentication in the TLS session fails.

1.3.  Definitions

   The terms "recursive resolver", "authoritative server", and "classic
   DNS" are defined in [I-D.ietf-dnsop-rfc8499bis].

   "DNS with encryption" means transport of DNS over any of DoT, DoH, or
   DoQ.  A server that supports DNS with encryption supports transport
   over one or more of DoT, DoH, or DoQ.

   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.

2.  Discovering Whether an Authoritative Server Uses Encryption

   A recursive resolver discovers whether an authoritative server
   supports DNS with encryption by looking for a cached SVCB record for
   the name of the authoritative server (with "_dns" prefix) with a
   positive answer.  A cached SVCB record with a negative answer
   indicates that the authoritative server does not support any
   encrypted transport.  Positive and negative responses for SVCB
   queries are cached the same way as for all other DNS resource
   records.

   See [I-D.rescorla-dprive-adox-latest] for examples of querying for NS
   records and for SVCB records, and the interpretation of positive
   answers.

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   If the cache has no positive or negative answers for any SVCB record
   for any of a zone's authoritative servers, the resolver MAY send
   queries for the SVCB records for some or all of the zone's
   authoritative servers and wait for a positive response so that the
   resolver can use DNS with encryption for the original query.  In this
   situation, the resolver MAY instead just use classic DNS for the
   original query but simultaneously queue queries for the SVCB records
   for some or all of the zone's authoritative servers so that future
   queries might be able to use DNS with encryption.

   Discovery using SVCB records differs between resolvers using
   unauthenticated encryption and those using fully-authenticated
   encryption (described in [I-D.rescorla-dprive-adox-latest]).  If the
   resolver is using unauthenticated encryption, the SVCB records do not
   need to be DNSSEC-signed.

   DNSSEC validation of SVCB RRsets used strictly for this discovery
   mechanism is not mandated.

   As described in [I-D.rescorla-dprive-adox-latest], these records may
   be in the resolver's cache because they came in the Additional
   section of a query for the NS records of a zone.  This document does
   not rely on that feature being standardized or operationally present
   to work.

   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.

3.  Resolving with Encryption

   A resolver following this protocol MUST use SVCB records in its cache
   to decide whether to use classic DNS or encryption to contact
   authoritative servers for a zone.  If any of the SVCB records in the
   cache for the authoritative servers for a zone are positive
   responses, the resolver uses any of those servers for encryption.  A
   resolver MUST NOT attempt encryption for a server that has a negative
   response in its cache for the associated SVCB record.

   If all of the SVCB records for the authoritative servers in the cache
   for a zone are negative responses, the resolver MUST use classic
   (unencrypted) DNS instead of encryption.  Similarly, if none of the
   SVCB records for the authoritative servers in the cache have
   information about encrypted services as described in
   [I-D.schwartz-svcb-dns], the resolver MUST use classic (unencrypted)
   DNS instead of encryption.

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   If there are any SVCB records in the cache for the authoritative
   servers for a zone with a positive response, 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.  Reasons for TLS failures are listed in
   Section 3.1.

   After a DNS with encryption session is set up, the resolver uses that
   authoritative server for whatever query about the zone it was going
   to send.  If a resolver cannot set up a DNS with encryption session
   with any of the authoritative servers, it MUST attempt to perform the
   resolution over classic (unencrypted) DNS as it would have without
   encryption.

   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.  If the server closes the DNS with
   encryption session, the resolver can possibly re-establish a DNS with
   encryption session using encrypted session resumption.  [RFC7766]
   says "both clients and servers SHOULD support connection reuse" for
   TCP connections, and that advice could apply as well for DNS with
   encryption even though DNS with encryption has greater overhead for
   saving state.

   Privacy-oriented resolvers (defined in [RFC8932]) following this
   protocol MUST NOT indicate that they are using encryption because
   this protocol is susceptible to on-path attacks.

3.1.  Resolver Session Failures

   The following are the reasons that a DNS with encryption session
   might fail to be set up:

   *  The resolver receives a TCP RST response

   *  The resolver does not receive replies to TCP or TLS setup (such as
      getting the TCP SYN message, the first TLS message, or completing
      TLS handshakes)

   *  The TLS handshake gets a definitive failure

   *  The encrypted session fails for reasons other than for
      authentication, such as incorrect algorithm choices or TLS record
      failures

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3.2.  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.

  # 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

  # Does this resolver support any secure transports?
  if length of can_do_secure is 0:
    query using classic DNS on any/all ns_names; finished

  # 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) is NXDOMAIN:
        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: # if 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.

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   An operator of authoritative servers for a zone that is following
   this protocol MAY support encryption towards any IP address on which
   it offers service for classic DNS on port 53.  It is acceptable for
   such an operator 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.
   [RFC7766] says "both clients and servers SHOULD support connection
   reuse" for TCP connections, and that advice could apply as well for
   DNS with encryption even though DNS with encryption has greater
   overhead for saving state.

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
   [I-D.arends-dns-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

   The method described in this document explicitly allows a resolver to
   perform DNS communications over traditional unencrypted,
   unauthenticated DNS on port 53, if it cannot find an authoritative
   server that advertises that it supports encryption.  The method
   described in this document explicitly allows a resolver using
   encryption to choose to allow unauthenticated encryption.  In either
   of these cases, the resulting communication will be susceptible to
   obvious and well-understood attacks from an attacker in the path of
   the communications.

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   An authoritative server that wants to only serve data to resolvers
   that using fully-authenticated encryption as described in
   [I-D.rescorla-dprive-adox-latest] cannot differentiate between those
   resolvers and resolvers using the mechanisms described in this
   document.

8.  Acknowledgements

   Puneet Sood contributed many ideas to early drafts of this document.

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

9.  References

9.1.  Normative References

   [I-D.ietf-dnsop-rfc8499bis]
              Hoffman, P. and K. Fujiwara, "DNS Terminology", Work in
              Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-01,
              20 November 2020, <https://www.ietf.org/archive/id/draft-
              ietf-dnsop-rfc8499bis-01.txt>.

   [I-D.ietf-dnsop-svcb-https]
              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-04, 17 March 2021,
              <https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
              https-04.txt>.

   [I-D.rescorla-dprive-adox-latest]
              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>.

   [I-D.schwartz-svcb-dns]
              Schwartz, B., "Service Binding Mapping for DNS Servers",
              Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
              02, 17 February 2021, <https://www.ietf.org/archive/id/
              draft-schwartz-svcb-dns-02.txt>.

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   [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>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

   [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,
              <https://www.rfc-editor.org/info/rfc7766>.

   [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>.

   [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>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

9.2.  Informative References

   [I-D.arends-dns-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>.

   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", Work in Progress,
              Internet-Draft, draft-ietf-dprive-dnsoquic-02, 22 February
              2021, <https://www.ietf.org/archive/id/draft-ietf-dprive-
              dnsoquic-02.txt>.

   [RFC8932]  Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and
              A. Mankin, "Recommendations for DNS Privacy Service
              Operators", BCP 232, RFC 8932, DOI 10.17487/RFC8932,
              October 2020, <https://www.rfc-editor.org/info/rfc8932>.

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   [RSO_STATEMENT]
              "Statement on DNS Encryption", 2021, <https://root-
              servers.org/media/news/Statement_on_DNS_Encryption.pdf>.

Authors' Addresses

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org

   Peter van Dijk
   PowerDNS

   Email: peter.van.dijk@powerdns.com

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