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Malhotra 5 Intended status: Standards Track Boston University 6 Expires: May 2, 2017 October 29, 2016 8 NTP Client Data Minimization 9 draft-dfranke-ntp-data-minimization-01 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 May 2, 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 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 65 1. Introduction 67 Network Time Protocol (NTP) packets, as specified by RFC 5905 68 [RFC5905], carry a great deal of information about the state of the 69 NTP daemon which transmitted them. In the case of mode 4 packets 70 (responses sent from server to client), as well as in broadcast (mode 71 5) and symmetric peering modes (mode 1/2), most of this information 72 is essential for accurate and reliable time synchronizaton. However, 73 in mode 3 packets (requests sent from client to server), most of 74 these fields serve no purpose. Server implementations never need to 75 inspect them, and they can achieve nothing by doing so. Populating 76 these fields with accurate information is harmful to privacy of 77 clients because it allows a passive observer to fingerprint clients 78 and track them as they move across networks. 80 This memo updates RFC 5905 to redact unnecessary data from mode 3 81 packets. This is a fully backwards-compatible proposal. It calls 82 for no changes on the server side, and clients which implement these 83 updates will remain fully interoperable with 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 The Poll field MAY be set to the actual polling interval as 105 specified by RFC 5905, or else MAY be set to zero. 107 All other header fields, specifically the Stratum, Precision, Root 108 Delay, Root Dispersion, Reference ID, Reference Timestamp, Origin 109 Timestamp, and Receive Timestamp, SHALL be set to zero. 111 4. Security and Privacy Considerations 113 4.1. Data Minimization 115 Zeroing out unused fields in client requests prevents disclosure of 116 information that can be used for fingerprinting [RFC6973]. 118 While populating any of these fields with authentic data reveals at 119 least some identifying information about the client, the Origin 120 Timestamp and Receive Timestamp fields constitute a particularly 121 severe information leak. RFC 5905 calls for clients to copy the 122 transmit timestamp and destination timestamp of the server's most 123 recent response into the origin timestamp and receive timestamp 124 (respectively) of their next request to that server. Therefore, when 125 a client moves between networks, a passive observer of both network 126 paths can determine with high confidence that the old and new IP 127 addresses belong to the same system by noticing that the transmit 128 timestamp of a response sent to the old IP matches the origin 129 timestamp of a request sent from the new one. 131 Zeroing the poll field is made optional because this field conveys no 132 information that an observer could not otherwise obtain simply by 133 observing the actual interval between requests. Since in the NTP 134 reference implementation servers copy the poll field from the 135 client's request into their response, if clients rely on the value of 136 the poll field in the response then zeroing the poll field of the 137 request may result in adverse behavior. 139 4.2. Transmit Timestamp Randomization 141 While this memo calls for most fields in client packets to be set to 142 zero, the transmit timestamp is randomized. This decision is 143 motivated by security as well as privacy. 145 NTP servers copy the transmit timestamp from the client's request 146 into the origin timestamp of the response; this memo calls for no 147 change in this behavior. Clients discard any response whose origin 148 timestamp does not match the transmit timestamp of any request 149 currently in flight. 151 In the absence of cryptographic authentication, verification of 152 origin timestamps is clients' primary defense against blind spoofing 153 of NTP responses. It is therefore important that clients' transmit 154 timestamps be unpredictable. Their role in this regard is closely 155 analagous to that of TCP Initial Sequence Numbers [RFC6528]. 157 The traditional behavior of the NTP reference implementation is to 158 randomize only a few (typically 10-15 depending on the precision of 159 the system clock) low-order bits of transmit timestamp, with all 160 higher bits representing the system time, as measured just before the 161 packet was sent. This is suboptimal, because with so few random 162 bits, an adversary sending spoofed packets at high volume will have a 163 good chance of correctly guessing a valid origin timestamp. 165 5. IANA Considerations 167 [RFC EDITOR: DELETE PRIOR TO PUBLICATION] 169 This memo introduces no new IANA considerations. 171 6. References 173 6.1. Normative References 175 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 176 Requirement Levels", BCP 14, RFC 2119, 177 DOI 10.17487/RFC2119, March 1997, 178 . 180 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 181 "Network Time Protocol Version 4: Protocol and Algorithms 182 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 183 . 185 6.2. Informative References 187 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 188 "Randomness Requirements for Security", BCP 106, RFC 4086, 189 DOI 10.17487/RFC4086, June 2005, 190 . 192 [RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence 193 Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February 194 2012, . 196 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 197 Morris, J., Hansen, M., and R. Smith, "Privacy 198 Considerations for Internet Protocols", RFC 6973, 199 DOI 10.17487/RFC6973, July 2013, 200 . 202 Appendix A. Acknowledgements 204 The authors thank Prof. Sharon Goldberg and Miroslav Lichvar for 205 calling attention to the issues addressed in this memo. 207 Authors' Addresses 209 Daniel Fox Franke 210 Akamai Technologies, Inc. 211 150 Broadway 212 Cambridge, MA 02142 213 United States 215 Email: dafranke@akamai.com 216 URI: https://www.dfranke.us 218 Aanchal Malhotra 219 Boston University 220 111 Cummington St 221 Boston, MA 02215 222 United States 224 Email: aanchal4@bu.edu