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Lichvar 3 Internet-Draft Red Hat 4 Updates: 5905 (if approved) Sep 16, 2020 5 Intended status: Standards Track 6 Expires: March 20, 2021 8 Alternative NTP port 9 draft-mlichvar-ntp-alternative-port-02 11 Abstract 13 This document updates RFC 5905 to specify an alternative port for the 14 Network Time Protocol (NTP) which is restricted to NTP messages that 15 do not allow traffic amplification in order to make NTP safe for the 16 Internet. 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 https://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 March 20, 2021. 35 Copyright Notice 37 Copyright (c) 2020 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 (https://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 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 54 2. Alternative port - update to RFC 5905 . . . . . . . . . . . . 3 55 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 56 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 57 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 58 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 60 6.2. Informative References . . . . . . . . . . . . . . . . . 6 61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 63 1. Introduction 65 There are several modes specified for NTP. NTP packets in versions 66 2, 3, and 4 have a 3-bit field for the mode. Modes 1 (active), 2 67 (passive), 3 (client), 4 (server), and 5 (broadcast) are used for 68 synchronization of clocks. They are specified in RFC 5905 [RFC5905]. 69 Modes 6 and 7 are used for other purposes, like monitoring and remote 70 management of NTP servers and clients. The mode 6 is specified in 71 Control Messages Protocol for Use with Network Time Protocol Version 72 4 [I-D.ietf-ntp-mode-6-cmds]. 74 The first group of modes typically does not allow any traffic 75 amplification, i.e. the response is not larger than the request. An 76 exception is Autokey specified in RFC 5906 [RFC5906]. Autokey is 77 rarely supported on public NTP servers. 79 However, the modes 6 and 7 allow significant traffic amplification, 80 which has been exploited in large-scale denial-of-service (DoS) 81 attacks over the Internet. 83 Over time, network operators have been observed to implement the 84 following mitigations: 86 1. Blocked UDP packets with destination or source port 123 88 2. Blocked UDP packets with destination or source port 123 and 89 specific length (e.g. longer than 48 octets) 91 3. Blocked UDP packets with destination or source port 123 and NTP 92 mode 6 or 7 94 4. Limited rate of UDP packets with destination or source port 123 96 From those, only the 3rd approach does not have an impact on 97 synchronization of clocks with NTP. 99 The number of public servers in the pool.ntp.org project has dropped 100 in large part due to the mitigations (citation?). 102 Longer NTP packets (using extension fields) are needed by NTS 103 [I-D.ietf-ntp-using-nts-for-ntp]. 105 This document specifies an alternative port for NTP which is 106 restricted to the safe modes in order to enable synchronization of 107 clocks in networks where the port 123 is blocked or rate limited. 109 1.1. Requirements Language 111 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 112 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 113 "OPTIONAL" in this document are to be interpreted as described in BCP 114 14 [RFC2119] [RFC8174] when, and only when, they appear in all 115 capitals, as shown here. 117 2. Alternative port - update to RFC 5905 119 The table in "Figure 6: Global Parameters" in Section 7.2 of 120 [RFC5905] is extended with: 122 +---------+-------+----------------------+ 123 | Name | Value | Description | 124 +---------+-------+----------------------+ 125 | ALTPORT | TBD | Alternative NTP port | 126 +---------+-------+----------------------+ 128 The following text from Section 9.1 of [RFC5905]: 130 srcport: UDP port number of the server or reference clock. This 131 becomes the destination port number in packets sent from this 132 association. When operating in symmetric modes (1 and 2), this 133 field must contain the NTP port number PORT (123) assigned by the 134 IANA. In other modes, it can contain any number consistent with 135 local policy. 137 is replaced with: 139 srcport: UDP port number of the server or reference clock. This 140 becomes the destination port number in packets sent from this 141 association. When operating in symmetric modes (1 and 2), this 142 field must contain the NTP port number PORT (123) or the 143 alternative NTP port ALTPORT (TBD) assigned by the IANA. In other 144 modes, it can contain any number consistent with local policy. 146 The following text is added to the Section 9.1: 148 The port ALTPORT (TBD) is an alternative port to the port PORT 149 (123). The protocol and the format of NTP packets sent from and 150 to this port is unchanged. Both NTP requests and responses MAY be 151 sent from the alternative port. An NTP packet MUST NOT be sent 152 from the alternative port if it is a response which has a longer 153 UDP payload than the request, or the number of NTP packets in a 154 single response is larger than one. 156 Only modes 1 (active), 2 (passive), 3 (client), 4 (server), and 5 157 (broadcast) are generally usable on this port. 159 An NTP server SHOULD receive requests in the client mode on both 160 the PORT (123) and ALTPORT (TBD) ports. If it responds, it MUST 161 send the response from the port which received the request. If 162 the server supports any extension fields in NTP packets, it MUST 163 verify that each response is not larger than the request, even if 164 the number of extension fields is constant and they have a 165 constant length. 167 When an NTP client is started, it SHOULD send the first request to 168 the alternative port. The client SHOULD be switching between the 169 two ports until a valid response is received. The client MAY send 170 a limited number of requests to both ports at the same time in 171 order to speed up the discovery of the responding port. When both 172 ports are responding, the client SHOULD prefer the alternative 173 port. 175 An NTP server which supports NTS SHOULD include the NTPv4 Port 176 Negotiation record in NTS-KE responses to specify the alternative 177 port as the port to which the client should send NTP requests. 179 In the symmetric modes (active and passive) NTP packets are 180 considered to be requests and responses at the same time. 181 Therefore, the peers MUST send packets with an equal length in 182 order to synchronize with each other. The peers MAY use different 183 polling intervals (packets sent at subsequent polls are considered 184 to be separate requests and responses). 186 3. IANA Considerations 188 IANA is requested to allocate the following port in the Service Name 189 and Transport Protocol Port Number Registry [RFC6335]: 191 Service Name: ntp-alt 193 Transport Protocol: udp 195 Assignee: IESG 196 Contact: IETF Chair 198 Description: Network Time Protocol 200 Reference: [[this memo]] 202 Port Number: [[TBD]], selected by IANA from the System Port range 204 4. Security Considerations 206 A Man-in-the-middle (MITM) attacker can selectively block requests 207 sent to the alternative port to force a client to select the original 208 port and get a degraded NTP service with a significant packet loss. 209 The client needs to periodically try the alternative port to recover 210 from the degraded service when the attack stops. 212 5. Acknowledgements 214 The author would like to thank Daniel Franke, Dhruv Dhody, and Ragnar 215 Sundblad for their useful comments. 217 6. References 219 6.1. Normative References 221 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 222 Requirement Levels", BCP 14, RFC 2119, 223 DOI 10.17487/RFC2119, March 1997, 224 . 226 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 227 "Network Time Protocol Version 4: Protocol and Algorithms 228 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 229 . 231 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 232 Cheshire, "Internet Assigned Numbers Authority (IANA) 233 Procedures for the Management of the Service Name and 234 Transport Protocol Port Number Registry", BCP 165, 235 RFC 6335, DOI 10.17487/RFC6335, August 2011, 236 . 238 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 239 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 240 May 2017, . 242 6.2. Informative References 244 [I-D.ietf-ntp-mode-6-cmds] 245 Haberman, B., "Control Messages Protocol for Use with 246 Network Time Protocol Version 4", draft-ietf-ntp-mode- 247 6-cmds-09 (work in progress), June 2020. 249 [I-D.ietf-ntp-using-nts-for-ntp] 250 Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. 251 Sundblad, "Network Time Security for the Network Time 252 Protocol", draft-ietf-ntp-using-nts-for-ntp-28 (work in 253 progress), March 2020. 255 [RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol 256 Version 4: Autokey Specification", RFC 5906, 257 DOI 10.17487/RFC5906, June 2010, 258 . 260 Author's Address 262 Miroslav Lichvar 263 Red Hat 264 Purkynova 115 265 Brno 612 00 266 Czech Republic 268 Email: mlichvar@redhat.com