Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Experimental                                P. van Dijk
Expires: 18 December 2021                                       PowerDNS
                                                            16 June 2021

     Recursive to Authoritative DNS with Unauthenticated Encryption


   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 [FULL-AUTH].

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 18 December 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
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
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   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 . . . . . . . . . . . . . . . .   5
   4.  Serving with Encryption . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 [DNSOTLS] (DoT), DNS-over-
   HTTPS [DNSOHTTPS] (DoH), and/or DNS-over-QUIC [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.

   See [FULL-AUTH] for a description of the use case and a proposed
   mechanism for fully-authenticated encryption.  See [COMMON] for a
   definition of the features that are in common between this document
   and [FULL-AUTH].

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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.

   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

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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

      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 the
      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",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [MUSTSHOULD1] [MUSTSHOULD2] 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 using the discovery mechanism
   described in Section 2.1 of [COMMON].  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.  Resolving with Encryption

   A resolver following this protocol processes the discovery response
   using the processing mechanism described in [COMMON].

   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.

3.1.  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

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4.  Serving with Encryption

   An authoritative server following this protocol publishes the
   discovery records using the serving mechanism described in [COMMON].

5.  IANA Considerations

   Relevant IANA considerations are covered in [COMMON].

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.

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

   [COMMON]   Dijk, P. V. and P. Hoffman, "Common Features for Encrypted
              Recursive to Authoritative DNS", Work in Progress,
              Internet-Draft, draft-pp-dprive-common-features-01, 19 May
              2021, <

   [DNS-SVCB] Schwartz, B., "Service Binding Mapping for DNS Servers",
              Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
              03, 19 April 2021, <

   [DNS-TERM] Hoffman, P. and K. Fujiwara, "DNS Terminology", Work in
              Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-01,
              20 November 2020, <

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              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,

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

              Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [OPPORTUN] Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

   [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-06, 16 June 2021,

8.2.  Informative References

              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [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, <

   [DNSOTLS]  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, <>.

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              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, <>.

              "Statement on DNS Encryption", 2021, <https://root-

Authors' Addresses

   Paul Hoffman


   Peter van Dijk


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