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Franke 3 Internet-Draft Akamai 4 Updates: 5905 (if approved) October 17, 2016 5 Intended status: Standards Track 6 Expires: April 20, 2017 8 NTP Client Data Minimization 9 draft-dfranke-ntp-data-minimization-00 11 Abstract 13 This memo proposes backward-compatible updates to the Network Time 14 Protocol to strip unnecessary identifying information from client 15 requests and to improve resilience against blind spoofing of 16 unauthenticated server responses. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on April 20, 2017. 35 Copyright Notice 37 Copyright (c) 2016 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2 54 3. Client Packet Format . . . . . . . . . . . . . . . . . . . . 2 55 4. Security and Privacy Considerations . . . . . . . . . . . . . 3 56 4.1. Data Minimization . . . . . . . . . . . . . . . . . . . . 3 57 4.2. Transmit Timestamp Randomization . . . . . . . . . . . . 3 58 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 59 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 61 6.2. Informative References . . . . . . . . . . . . . . . . . 4 62 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5 63 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 65 1. Introduction 67 Network Time Protocol packets, as specified by RFC 5905 [RFC5905], 68 carry a great deal of information about the state of the NTP daemon 69 which transmitted them. In the case of mode 4 packets (responses 70 sent from server to client), as well as in broadcast and symmetric 71 modes, most of this information is essential for accurate and 72 reliable time synchronizaton. However, in mode 3 packets (requests 73 sent from client to server), these fields serve no purpose. Server 74 implementations never need to inspect them, and they can achieve 75 nothing by doing so. Populating these fields with accurate 76 information is harmful to privacy because it allows a passive 77 observer to fingerprint clients and track them as they move across 78 networks. 80 This memo updates RFC 5905 to redact unnecessary data from mode 3 81 packets. It calls for no changes on the server side, and clients 82 which implement these updates will remain fully interoperable with 83 existing servers. 85 2. Requirements Language 87 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 88 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 89 document are to be interpreted as described in RFC 2119 [RFC2119]. 91 3. Client Packet Format 93 In every client-mode packet sent by a Network Time Protocol [RFC5905] 94 implementation: 96 The first octet, which contains the leap indicator, version 97 number, and mode fields, SHALL be set to 0x23 (LI = 0, VN = 4, 98 Mode = 3). 100 The Transmit Timestamp field SHALL be set uniformly at random, 101 generated by a mechanism suitable for cryptographic purposes. 102 [RFC4086] provides guidance on the generation of random values. 104 All other header fields, specifically the Stratum, Poll, 105 Precision, Root Delay, Root Dispersion, Reference ID, Reference 106 Timestamp, Origin Timestamp, and Receive Timestamp, SHALL be set 107 to zero. 109 4. Security and Privacy Considerations 111 4.1. Data Minimization 113 Zeroing out unused fields in client requests prevents disclosure of 114 information that can be used for fingerprinting [RFC6973]. 116 While populating any of these fields with authentic data reveals at 117 least some identifying information about the client, the Origin 118 Timestamp and Receive Timestamp fields constitute a particularly 119 severe information leak. RFC 5905 calls for clients to copy the 120 transmit timestamp and destination timestamp of the server's most 121 recent response into the origin timestamp and receive timestamp 122 (respectively) of their next request to that server. Therefore, when 123 a client moves between networks, a passive observer of both network 124 paths can determine with high confidence that the old and new IP 125 addresses belong to the same system by noticing that the transmit 126 timestamp of a response sent to the old IP matches the origin 127 timestamp of a request sent from the new one. 129 4.2. Transmit Timestamp Randomization 131 While this memo calls for most fields in client packets to be set to 132 zero, the transmit timestamp is randomized. This decision is 133 motivated by security as well as privacy. 135 NTP servers copy the transmit timestamp from the client's request 136 into the origin timestamp of the response; this memo calls for no 137 change in this behavior. Clients discard any response whose origin 138 timestamp does not match the transmit timestamp of any request 139 currently in flight. 141 In the absence of cryptographic authentication, verification of 142 origin timestamps is clients' primary defense against blind spoofing 143 of NTP responses. It is therefore important that clients' transmit 144 timestamps be unpredictable. Their role in this regard is closely 145 analagous to that of TCP Initial Sequence Numbers [RFC6528]. 147 The traditional behavior of the NTP reference implementation is to 148 randomize only a few (typically 10-15 depending on the precision of 149 the system clock) low-order bits of transmit timestamp, with all 150 higher bits representing the system time, as measured just before the 151 packet was sent. This is suboptimal, because with so few random 152 bits, an adversary sending spoofed packets at high volume will have a 153 good chance of correctly guessing a valid origin timestamp. 155 5. IANA Considerations 157 [RFC EDITOR: DELETE PRIOR TO PUBLICATION] 159 This memo introduces no new IANA considerations. 161 6. References 163 6.1. Normative References 165 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 166 Requirement Levels", BCP 14, RFC 2119, 167 DOI 10.17487/RFC2119, March 1997, 168 . 170 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 171 "Randomness Requirements for Security", BCP 106, RFC 4086, 172 DOI 10.17487/RFC4086, June 2005, 173 . 175 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 176 "Network Time Protocol Version 4: Protocol and Algorithms 177 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 178 . 180 6.2. Informative References 182 [RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence 183 Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February 184 2012, . 186 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 187 Morris, J., Hansen, M., and R. Smith, "Privacy 188 Considerations for Internet Protocols", RFC 6973, 189 DOI 10.17487/RFC6973, July 2013, 190 . 192 Appendix A. Acknowledgements 194 The author thanks Prof. Sharon Goldberg, Miroslav Lichvar, and 195 Aanchal Malhotra for calling attention to the issues addressed in 196 this memo. 198 Author's Address 200 Daniel Fox Franke 201 Akamai Technologies, Inc. 202 150 Broadway 203 Cambridge, MA 02142 204 United States 206 Email: dafranke@akamai.com 207 URI: https://www.dfranke.us