Network Working Group A. Ghedini
Internet-Draft Cloudflare, Inc.
Intended status: Standards Track July 06, 2019
Expires: January 7, 2020
Using Early Data in DNS over TLS
draft-ghedini-dprive-early-data-01
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
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 January 7, 2020.
Copyright Notice
Copyright (c) 2019 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
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.
Ghedini Expires January 7, 2020 [Page 1]
Internet-Draft DNS Early Data July 2019
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
4.3. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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.
Ghedini Expires January 7, 2020 [Page 2]
Internet-Draft DNS Early Data July 2019
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 queries are safe to be sent as early data.
Clients MUST NOT use early data to send DNS Updates ([RFC2136]) or
Zone Transfers ([RFC5936]) messages. Servers receiving any of those
messages MUST reply with a "FormErr" response code.
[[TODO: forbid other types? use a different status code? should we
define a whitelist instead of a blacklist?]]
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
Ghedini Expires January 7, 2020 [Page 3]
Internet-Draft DNS Early Data July 2019
the attacker to confirm whether the query targeted a specific name
that was suspected of being queried.
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.
4.3. Privacy
TBD
[[TODO: linkability (e.g. clients changing network, ...) and more?]]
5. IANA Considerations
This document has no actions for IANA.
6. 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.
7. References
7.1. 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>.
Ghedini Expires January 7, 2020 [Page 4]
Internet-Draft DNS Early Data July 2019
[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>.
7.2. 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>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[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>.
Author's Address
Alessandro Ghedini
Cloudflare, Inc.
Email: alessandro@cloudflare.com
Ghedini Expires January 7, 2020 [Page 5]