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Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Standards Track                        19 November 2020
Expires: 23 May 2021


      Recursive to Authoritative DNS with Opportunistic Encryption
             draft-pp-recursive-authoritative-opportunistic-02

Abstract

   This document describes a use case and a method for a DNS recursive
   resolver to use opportunistic encryption when communicating with
   authoritative servers.  A motivating use case for this method is that
   more encryption on the Internet is better, and opportunistic
   encryption is better than no encryption at all.  The method here is
   optional for both the recursive resolver and the authoritative
   server.  Nothing in this method prevents use cases and methods that
   require authenticated 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
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   This Internet-Draft will expire on 23 May 2021.

Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Method for Opportunistic Encryption . . . . . . . . . . . . .   3
   3.  Transport Caches  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   A recursive resolver using traditional DNS over port 53 may wish
   instead to use encrypted communication with authoritative servers in
   order to prevent passive snooping of its DNS traffic.  The recursive
   resolver can use opportunistic encryption (defined in [RFC7435] to
   achieve this goal.

   This document describes a use case and a method for recursive
   resolvers to use opportunistic encryption.  The use case is described
   in Section 1.1.  The method uses DNS-over-TLS [RFC7858] with
   authoritative servers in an efficient manner.

1.1.  Use Case

   The use case in this document is recursive resolver operators who are
   happy to use TLS [RFC8446] 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.

   Both parties 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:



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   *  Extra round trips to establish TCP for every session

   *  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

1.2.  Definitions

   The terms "recursive resolver" and "authoritative server" are defined
   in [RFC8499].

   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.  Method for Opportunistic Encryption

   [RFC7435] defines opportunistic encryption.  The method described
   here uses DNS-over-TLS [RFC7858] between resolvers and authoritative
   servers.

   In this document, the only difference between normal TLS session
   establishment and opportunistic encryption is that the the TLS client
   (the recursive resolver) optionally authenticates the server.  In
   normal TLS, the client is required to authenticate the server and the
   TLS connection fails if the authentication is not successful.

   [[ The following might be changed.  A possible use case to be added,
   depending on WG discussion, would be optional authentication in order
   for caches to promote parent-side NS records to a higher standing
   than they currently have in the caches. ]]

   In the opportunistic encryption described here, there is no need for
   the recursive resolver to authenticate the authoritative server
   because any certificate authentication failure does not cause the TLS
   session from being set up.  Note that other authentication failures
   in TLS, such as incorrect algorithm choices or TLS record failures,
   MUST cause the TLS session from being set up.  If it is easier
   programmatically for the recursive resolver to authenticate the
   authoritative server and then ignore the negative result for
   certificate authentication, than to just not authenticate, the
   recursive resolver MAY do that.  The recursive resolver MAY note a



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   certificate authentication failure and act on it (such as by logging
   it or noting it in the cache), as long as the failure does not
   prevent the TLS session from completing.

   Note that later protocols for encrypted resolver-to-authoritative
   communication might to require normal TLS authentication.  Because of
   this, authoritative servers SHOULD use TLS certificates that can be
   used in authenticated TLS authentication, such as those issued by
   trusted third parties or self-issued certificates that can be
   authenticated with DANE [RFC6698] records.  However, if an
   authoritative server does not care about the use cases for such
   future protocols, it MAY use self-issued certificates that cannot be
   authenticated.

3.  Transport Caches

   [[ Because this section probably applies to use cases for
   authenticated encryption as well, it should probably be moved to a
   separate document. ]]

   A recursive resolver that attempted to use encrypted transport every
   time it connected to any authoritative server would inherently be
   slower than one that did not.  Similarly, a recursive resolver that
   made an external lookup of what secure transports every authoritative
   server supports each time it connected would also inherently be
   slower than one that did not.  Recursive resolver operators desire to
   give answers to stub resolvers as quickly as possible, so neither of
   these two strategies would make sense.

   Instead, recursive resolvers following the method described in this
   document MUST keep a cache of relevant information about how DNS-
   over-TLS is supported by authoritative servers.  This is called a
   "transport cache" in this document.  The relevant information could
   include things such as support for encryption, authentication
   information such as public keys discovered from TLSA records, which
   records in a server's NS RRset support DNS-over-TLS, and so on.
   Future specifications might describe how to use other secure DNS
   transports for encryption, and thus would also have to describe ways
   that a resolver could discover whether an authoritative server
   supports them.











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   This document only DNS-over-TLS for encryption.  Thus, a recursive
   resolver can test whether an authoritative server supports DNS-over-
   TLS by attempting to open a TLS session on port 853 or through
   discovery of appropriate TLSA records (to be described later and/or
   in different documents).  The transport cache should store both
   positive and negative information about whether the authoritative
   server supports DNS-over-TLS and about authentication uses by
   particular name servers.

   The recursive resolver MUST look in its transport cache before
   sending DNS queries to an authoritative server.  If there is no entry
   for an authoritative server in its transport cache, the recursive
   resolver MUST use plain, unencrypted DNS over port 53.  It MAY then
   probe for encrypted transports, and cache that information for later
   connections.

   This document explicitly does not mandate the contents of the
   transport cache.  Different recursive resolver implementers are
   likely to have different cache structures based on many factors, such
   as research results, active measurements, secure protocols supported,
   and customer feedback, There will likely be different strategies for
   the time-to-live for parts of the transport cache, such as how often
   to refresh the data in the cache, how often to refresh negative data,
   whether to prioritize refreshing certain zones or types of zones, and
   so on.

   This document also explicitly doesn't mandate how the strategy for
   filling transport caches.  Some strategies might include one or more
   of "try to send a refresh query over DoT", "use external data",
   "trust a third-party service for filling the transport cache", and so
   on.

   There are no interoperability issues with different implementors
   making different choices for the contents and fill strategies of
   their transport caches, and having many different options available
   will likely cause the cache designs to get better over time.

4.  Security Considerations

   The method described in this document explicitly allows a stub to
   perform DNS communications over traditional unencrypted,
   unauthenticated DNS on port 53.

   The method described in this document explicitly allows a stub to
   choose to allow unauthenticated TLS.  In this case, 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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5.  Acknowledgements

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

6.  References

6.1.  Normative References

   [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>.

   [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>.

   [RFC8446]  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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

6.2.  Informative References

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

Author's Address

   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org




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