NTP Client Data Minimization
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|Authors||Daniel Fox Franke , Aanchal Malhotra|
|RFC stream||Internet Engineering Task Force (IETF)|
|Additional resources||Mailing list discussion|
|Stream||WG state||WG Document|
|IESG||IESG state||I-D Exists|
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Network Working Group D. Franke Internet-Draft Akamai Updates: 5905 (if approved) A. Malhotra Intended status: Standards Track Boston University Expires: November 25, 2017 May 24, 2017 NTP Client Data Minimization draft-ietf-ntp-data-minimization-00 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. 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 http://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 November 25, 2017. Copyright Notice Copyright (c) 2017 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 (http://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. Franke & Malhotra Expires November 25, 2017 [Page 1] Internet-Draft NTP Client Data Minimization May 2017 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: Franke & Malhotra Expires November 25, 2017 [Page 2] Internet-Draft NTP Client Data Minimization May 2017 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. 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 Franke & Malhotra Expires November 25, 2017 [Page 3] Internet-Draft NTP Client Data Minimization May 2017 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 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>. Franke & Malhotra Expires November 25, 2017 [Page 4] Internet-Draft NTP Client Data Minimization May 2017 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: firstname.lastname@example.org URI: https://www.dfranke.us Aanchal Malhotra Boston University 111 Cummington St Boston, MA/ 02215 United States Email: email@example.com Franke & Malhotra Expires November 25, 2017 [Page 5]