Internet DRAFT - draft-ietf-ntp-data-minimization
draft-ietf-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 26, 2019 March 25, 2019
NTP Client Data Minimization
draft-ietf-ntp-data-minimization-04
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 26, 2019.
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. Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 SHOULD be set to either the actual polling interval
as specified by RFC 5905 or zero.
The Precision field SHOULD be set to 0x20.
All other header fields, specifically the Stratum, 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
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since an observer can already obtain this information simply by
observing the actual interval between requests.
4.2. Transmit Timestamp Randomization
While this memo calls for most fields in client packets to be set to
zero, the transmit timestamp SHOULD be 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. Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC7942. The
description of implementations in this section is intended to assist
the IETF in its decision processes in progressing drafts to RFCs.
Please note that the listing of any individual implementation here
does not imply endorsement by the IETF. Furthermore, no effort has
been spent to verify the information presented here that was supplied
by IETF contributors. This is not intended as, and must not be
construed to be, a catalog of available implementations or their
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features. Readers are advised to note that other implementations may
exist.
As of today the following vendors have produced an implementation of
the NTP Client Data Minimization recommendations described in this
document.
OpenNTPD
7. References
7.1. 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>.
[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,
<https://www.rfc-editor.org/info/rfc5905>.
7.2. Informative References
[RFC2030] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 2030, DOI 10.17487/RFC2030,
October 1996, <https://www.rfc-editor.org/info/rfc2030>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://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, <https://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,
<https://www.rfc-editor.org/info/rfc6973>.
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7.3. URIs
[1] https://github.com/openbsd/src/
commit/1346900e6d0ac3aeb0e3f9eb60b94c66586978c6
Appendix A. Acknowledgements
The possibility of minimizing data in client packets was described in
RFC 2030 [RFC2030]. The authors would like to acknowledge Alexander
Guy for pioneering the idea of randomization of all bits of the
transmit timestamp in the rdate program of the OpenBSD project as
early as May 2004 [1].
The authors would also like to thank Prof. Sharon Goldberg and
Miroslav Lichvar for encouraging standardisation of the approach
described in this document.
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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