Internet DRAFT - draft-dfranke-ntp-data-minimization
draft-dfranke-ntp-data-minimization
Network Working Group D. Franke
Internet-Draft Akamai
Updates: 5905 (if approved) A. Malhotra
Intended status: Standards Track Boston University
Expires: September 28, 2017 March 27, 2017
NTP Client Data Minimization
draft-dfranke-ntp-data-minimization-02
Abstract
This memo proposes backward-compatible updates to the Network Time
Protocol to strip unnecessary identifying information from client
requests and to improve resilience against blind spoofing of
unauthenticated server responses.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 28, 2017.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Client Packet Format . . . . . . . . . . . . . . . . . . . . 2
4. Security and Privacy Considerations . . . . . . . . . . . . . 3
4.1. Data Minimization . . . . . . . . . . . . . . . . . . . . 3
4.2. Transmit Timestamp Randomization . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Network Time Protocol (NTP) packets, as specified by RFC 5905
[RFC5905], carry a great deal of information about the state of the
NTP daemon which transmitted them. In the case of mode 4 packets
(responses sent from server to client), as well as in broadcast (mode
5) and symmetric peering modes (mode 1/2), most of this information
is essential for accurate and reliable time synchronizaton. However,
in mode 3 packets (requests sent from client to server), most of
these fields serve no purpose. Server implementations never need to
inspect them, and they can achieve nothing by doing so. Populating
these fields with accurate information is harmful to privacy of
clients because it allows a passive observer to fingerprint clients
and track them as they move across networks.
This memo updates RFC 5905 to redact unnecessary data from mode 3
packets. This is a fully backwards-compatible proposal. It calls
for no changes on the server side, and clients which implement these
updates will remain fully interoperable with existing servers.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Client Packet Format
In every client-mode packet sent by a Network Time Protocol [RFC5905]
implementation:
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The first octet, which contains the leap indicator, version
number, and mode fields, SHOULD be set to 0x23 (LI = 0, VN = 4,
Mode = 3).
The Transmit Timestamp field SHOULD be set uniformly at random,
generated by a mechanism suitable for cryptographic purposes.
[RFC4086] provides guidance on the generation of random values.
The Poll field MAY be set to the actual polling interval as
specified by RFC 5905, or else MAY be set to zero.
All other header fields, specifically the Stratum, Precision, Root
Delay, Root Dispersion, Reference ID, Reference Timestamp, Origin
Timestamp, and Receive Timestamp, SHOULD be set to zero.
Servers MUST allow client packets to conform to the above
recommendations. This requirement shall not be construed so as to
prohibit servers from rejecting conforming packets for unrelated
reasons, such as access control or rate limiting.
4. Security and Privacy Considerations
4.1. Data Minimization
Zeroing out unused fields in client requests prevents disclosure of
information that can be used for fingerprinting [RFC6973].
While populating any of these fields with authentic data reveals at
least some identifying information about the client, the Origin
Timestamp and Receive Timestamp fields constitute a particularly
severe information leak. RFC 5905 calls for clients to copy the
transmit timestamp and destination timestamp of the server's most
recent response into the origin timestamp and receive timestamp
(respectively) of their next request to that server. Therefore, when
a client moves between networks, a passive observer of both network
paths can determine with high confidence that the old and new IP
addresses belong to the same system by noticing that the transmit
timestamp of a response sent to the old IP matches the origin
timestamp of a request sent from the new one.
Zeroing the poll field is made optional (MAY rather than SHOULD) so
as not to preclude future development of schemes wherein the server
uses information about the client's current poll interval in order to
recommend adjustments back to the client. Putting accurate
information into this field has no significant impact on privacy
since an observer can already obtain this information simply by
observing the actual interval between requests.
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4.2. Transmit Timestamp Randomization
While this memo calls for most fields in client packets to be set to
zero, the transmit timestamp is randomized. This decision is
motivated by security as well as privacy.
NTP servers copy the transmit timestamp from the client's request
into the origin timestamp of the response; this memo calls for no
change in this behavior. Clients discard any response whose origin
timestamp does not match the transmit timestamp of any request
currently in flight.
In the absence of cryptographic authentication, verification of
origin timestamps is clients' primary defense against blind spoofing
of NTP responses. It is therefore important that clients' transmit
timestamps be unpredictable. Their role in this regard is closely
analagous to that of TCP Initial Sequence Numbers [RFC6528].
The traditional behavior of the NTP reference implementation is to
randomize only a few (typically 10-15 depending on the precision of
the system clock) low-order bits of transmit timestamp, with all
higher bits representing the system time, as measured just before the
packet was sent. This is suboptimal, because with so few random
bits, an adversary sending spoofed packets at high volume will have a
good chance of correctly guessing a valid origin timestamp.
5. IANA Considerations
[RFC EDITOR: DELETE PRIOR TO PUBLICATION]
This memo introduces no new IANA considerations.
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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
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6.2. Informative References
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <http://www.rfc-editor.org/info/rfc6528>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
Appendix A. Acknowledgements
The authors thank Prof. Sharon Goldberg and Miroslav Lichvar for
calling attention to the issues addressed in this memo.
Authors' Addresses
Daniel Fox Franke
Akamai Technologies, Inc.
150 Broadway
Cambridge, MA 02142
United States
Email: dafranke@akamai.com
URI: https://www.dfranke.us
Aanchal Malhotra
Boston University
111 Cummington St
Boston, MA 02215
United States
Email: aanchal4@bu.edu
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