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Versions: 00 01 02

Network Working Group                                         A. Ghedini
Internet-Draft                                          Cloudflare, Inc.
Intended status: Standards Track                       February 25, 2020
Expires: August 28, 2020


                    Using Early Data in DNS over TLS
                   draft-ghedini-dprive-early-data-02

Abstract

   This document illustrates the risks of using TLS 1.3 early data with
   DNS over TLS, and specifies behaviors that can be adopted by clients
   and servers to reduce those risks.

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
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   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 August 28, 2020.

Copyright Notice

   Copyright (c) 2020 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   2
   3.  Early Data in DNS over TLS  . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
     4.1.  Information Exposure  . . . . . . . . . . . . . . . . . .   3
     4.2.  Denial of Service . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Registry for DNS Resource Record (RR) TYPEs for TLS Early
           Data  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   TLS 1.3 [TLS13] defines a mechanism, called 0-RTT session resumption
   or early data, that allows clients to send data to servers in the
   first round-trip of a resumed connection without having to wait for
   the TLS handshake to complete.

   This can be used to send DNS queries to DNS over TLS [DOT] servers
   without incurring in the cost of the additional round-trip required
   by the TLS handshake.  This can provide significant performance
   improvements in cases where new DNS over TLS connections need to be
   established often such as on mobile clients where the network might
   not be stable, or on resolvers where keeping an open connection to
   many authoritative servers might not be practical.

   However the use of early data allows an attacker to capture and
   replay the encrypted DNS queries carried on the TLS connection.  This
   can have unwanted consequences and help in recovering information
   about those queries.  While [TLS13] describes tecniques to reduce the
   likelihood of a replay attack, they are not perfect and still leave
   some potential for exploitation.

2.  Notational Conventions

   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.





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3.  Early Data in DNS over TLS

   Early data forms a single stream of data along with other application
   data, meaning that one or more DNS queries can either be partially or
   fully contained within early data.  Once the TLS handshake has
   completed, the early data is known to not be a replayed copy of that
   data, but this doesn't mean that it can't be replayed, or that it
   hasn't already been replayed, in another connection.

   A server can signal to clients whether it is willing to accept early
   data in future connections by providing the "early_data" TLS
   extension as part of a TLS session ticket, as well as limit the
   amount of early data it is willing to accept using the
   "max_early_data_size" field of the "early_data" extension.

   In addition to the mitigation mechanisms mandated in [TLS13] that
   reduce the ability of an attacker to replay early data, but may not
   completely eliminate it, a server that decided to offer early data to
   clients MAY reject early data at the TLS layer, or delay the
   processing of early data after the handshake is completed.

   If the server rejects early data at the TLS layer, a client MUST
   forget information it optmisitically assumed about the onnection when
   sending early data, such as the negotiated protocol [ALPN].  Any DNS
   queries sent in early data will need to be sent again, unless the
   client decides to abandon them.

   Not all types of DNS messages are safe to be sent as early data, as
   they might modfify the server's state, or expose sensitive data,
   through replay.  Clients MUST NOT use early data to send messages
   that make use of opcodes other than "Query" and RR types not listed
   in the registry defined in Section 5.1.  Servers receiving any of
   those messages MUST reply with a "FormErr" response code.

4.  Security Considerations

4.1.  Information Exposure

   By replaying DNS queries that were captured when transmitted over
   early data, an attacker might be able to expose information about
   those queries, even if encrypted.

   For example, it's a common behavior for DNS servers to statefully
   rotate the order of RRs when replying to DNS queries for an RRSet
   that contains multiple RRs.  If the order of rotation is predictable,
   replaying a captured early data DNS query and observing the order of
   RRs in DNS responses before and after the replayed query, might allow




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   the attacker to confirm whether the query targeted a specific name
   that was suspected of being queried.

   When accepting early data, servers SHOULD either use fixed ordering
   for multiple RRs in the same DNS response or shuffle the RRs at
   random, but MUST NOT use stateful and deterministic ordering across
   multiple queries.

4.2.  Denial of Service

   Accepting early data exposes a server to potential denial of service
   through the replay of queries that might be expensive to handle.

   When under load, a server MAY reject TLS early data such that the
   client is forced to retry them after the handshake is completed.

5.  IANA Considerations

   This document has no actions for IANA.

5.1.  Registry for DNS Resource Record (RR) TYPEs for TLS Early Data

   This document establishes a registry of DNS RR types that can be used
   within TLS early data, titled "DNS Resource Record (RR) TYPEs for Use
   with TLS Early Data", under the existing "Domain Name System (DNS)
   Parameters" heading.

   The entries in the registry are:























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                       +--------+-----------------+
                       | TYPE   | Reference       |
                       +--------+-----------------+
                       | A      | [this document] |
                       |        |                 |
                       | NS     | [this document] |
                       |        |                 |
                       | CNAME  | [this document] |
                       |        |                 |
                       | SOA    | [this document] |
                       |        |                 |
                       | PTR    | [this document] |
                       |        |                 |
                       | MX     | [this document] |
                       |        |                 |
                       | TXT    | [this document] |
                       |        |                 |
                       | AAAA   | [this document] |
                       |        |                 |
                       | SRV    | [this document] |
                       |        |                 |
                       | DNAME  | [this document] |
                       |        |                 |
                       | DS     | [this document] |
                       |        |                 |
                       | DNSKEY | [this document] |
                       +--------+-----------------+

   The values in this registry MUST correspond to existing entries in
   the "Resource Record (RR) TYPEs" registry.  Specifically, the value
   of the "TYPE" column for each entry in this new registry MUST match
   the value of the "TYPE" column of an entry in the "Resource Record
   (RR) TYPEs" registry.

6.  References

6.1.  Normative References

   [DOT]      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>.

   [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>.




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   [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>.

   [TLS13]    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>.

6.2.  Informative References

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <https://www.rfc-editor.org/info/rfc8470>.

Appendix A.  Acknowledgments

   Thanks to Martin Thomson, Mark Nottingham and Willy Tarreau for
   writing [RFC8470] which heavily inspired this document, and to Daniel
   Kahn Gillmor and Colm MacCarthaigh who also provided important ideas
   and contributions.

Author's Address

   Alessandro Ghedini
   Cloudflare, Inc.

   Email: alessandro@cloudflare.com



















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