Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Experimental                                P. van Dijk
Expires: 1 April 2022                                           PowerDNS
                                                       28 September 2021


     Recursive to Authoritative DNS with Unauthenticated Encryption
              draft-ietf-dprive-unauth-to-authoritative-04

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.

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 1 April 2022.

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.  Discovery of Authoritative Server Encryption  . . . . . . . .   4
   3.  Processing Discovery Responses  . . . . . . . . . . . . . . .   5
     3.1.  Resolver Process as Pseudocode  . . . . . . . . . . . . .   6
     3.2.  Resolver Session Failures . . . . . . . . . . . . . . . .   7
   4.  Serving with Encryption . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  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 [OPPORTUN]) 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 [DNS-OVER-TLS] (DoT), DNS-
   over-HTTPS [DNS-OVER-HTTPS] (DoH), and/or DNS-over-QUIC
   [DNS-OVER-QUIC] (DoQ).

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

   See [FULL-AUTH] for a description of the use case and a proposed
   mechanism for fully-authenticated encryption.




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   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.  Ultimately,
   this effort has two two goals: to protect queries from failing in
   case authenticated encryption is not available, and to enable
   recursive resolver operators to encrypt without server
   authentication.

   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 [SVCB].  As described in [DNS-SVCB], 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 [DNS-SVCB].
      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 server
      authentication in the TLS session fails.

1.3.  Definitions

   The terms "recursive resolver", "authoritative server", and "classic
   DNS" are defined in [DNS-TERM].

   "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 [MUST-SHOULD-1] [MUST-SHOULD-2] when, and only when, they appear
   in all capitals, as shown here.

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.

   A resolver MAY also use port probing, although the mechanism for that
   is not described here.




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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 (and for the A/AAAA records of names
   mentioned in those 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 (and
   subsequent A/AAAA) records for some or all of the zone's
   authoritative servers so that future queries might be able to use DNS
   with encryption.

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

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.

   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
   attempts to use the encrypted transports that are in the associated
   SVCB record for the authoritative server.

   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



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   using encrypted session resumption.  Configuration for the maximum
   timeout, minimum timeout, and duration of encrypted sessions should
   take into consideration the recommendations given in [TCP-TIMEOUT],
   [EDNS-TCP], and (for DoH) [HTTP-1.1].

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

   A resolver following this protocol does not need to authenticate TLS
   servers.  Thus, when setting up a TLS connection, if the server's
   authentication credentials do not match those expected by the
   resolver, the resolver continues with the TLS connection.  Privacy-
   oriented resolvers (defined in [PRIVACY-REC]) following this protocol
   MUST NOT indicate that they are using encryption because this
   protocol is susceptible to on-path attacks.

   If the resolver gets a TLS failure (such as those listed in
   Section 3.2, the resolver instead uses classic DNS on any of the
   authoritative servers.

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.




















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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; finished

3.2.  Resolver Session Failures

   The following are some of 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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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
   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.  IANA Considerations

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

   (( Maybe other updates for DoH and DoQ ))

6.  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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   [TLS-1.3] specifically warns against anonymous connections because
   such connections only provide protection against passive
   eavesdropping while failing to protect against active on-path
   attacks.  Section C.5 of [TLS-1.3] explicitly states applications
   MUST NOT use TLS with unverifiable server authentication unless there
   is explicit configuration or a specific application profile to do so.
   This document is such an application profile.

   Encrypting the traffic between resolvers and authoritative servers
   does not solve all the privacy issues for resolution.  See
   [PRIVACY-REC] and [PRIVACY-CONS] for in-depth discussion of the
   associated privacy issues.

7.  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.

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-
              04, 26 July 2021, <https://www.ietf.org/archive/id/draft-
              schwartz-svcb-dns-04.txt>.

   [DNS-TERM] Hoffman, P. and K. Fujiwara, "DNS Terminology", Work in
              Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-03,
              28 September 2021, <https://www.ietf.org/archive/id/draft-
              ietf-dnsop-rfc8499bis-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>.

   [MUST-SHOULD-1]
              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>.




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   [MUST-SHOULD-2]
              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>.

   [OPPORTUN] 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>.

   [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-07, 5 August 2021,
              <https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
              https-07.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>.

8.2.  Informative References

   [DNS-OVER-HTTPS]
              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>.

   [DNS-OVER-QUIC]
              Huitema, C., Dickinson, S., and A. Mankin, "Specification
              of DNS over Dedicated QUIC Connections", Work in Progress,
              Internet-Draft, draft-ietf-dprive-dnsoquic-04, 3 September
              2021, <https://www.ietf.org/archive/id/draft-ietf-dprive-
              dnsoquic-04.txt>.

   [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>.

   [DNS-OVER-TLS]
              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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   [EDNS-TCP] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828,
              DOI 10.17487/RFC7828, April 2016,
              <https://www.rfc-editor.org/info/rfc7828>.

   [HTTP-1.1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [PRIVACY-CONS]
              Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
              DOI 10.17487/RFC9076, July 2021,
              <https://www.rfc-editor.org/info/rfc9076>.

   [PRIVACY-REC]
              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>.

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

   [TCP-TIMEOUT]
              Kristoff, J. and D. Wessels, "DNS Transport over TCP -
              Operational Requirements", Work in Progress, Internet-
              Draft, draft-ietf-dnsop-dns-tcp-requirements-12, 18 August
              2021, <https://www.ietf.org/archive/id/draft-ietf-dnsop-
              dns-tcp-requirements-12.txt>.

   [TLS-1.3]  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>.

Authors' Addresses

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org


   Peter van Dijk
   PowerDNS

   Email: peter.van.dijk@powerdns.com



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