idnits 2.17.1 draft-ietf-ntp-alternative-port-00.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC2606-compliant FQDNs in the document. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year (Using the creation date from RFC5905, updated by this document, for RFC5378 checks: 2005-07-11) -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (Oct 12, 2020) is 1291 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-11) exists of draft-ietf-ntp-mode-6-cmds-10 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force M. Lichvar 3 Internet-Draft Red Hat 4 Updates: 5905 (if approved) Oct 12, 2020 5 Intended status: Standards Track 6 Expires: April 15, 2021 8 Alternative NTP port 9 draft-ietf-ntp-alternative-port-00 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 April 15, 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 . . . . . . . . . . . . . . . . . . . . . 5 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 [RFC5906], which allows an NTP response to be 77 longer than the request, e.g. packets containing the Certificate 78 Message or Cookie Message extension field. Autokey is rarely used. 79 If it is enabled on a publicly accessible server, the access needs to 80 be tightly controlled to limit denial-of-service (DoS) attacks 81 exploiting the amplification. 83 The modes 6 and 7 of NTP allow significant traffic amplification, 84 which has been exploited in large-scale DoS attacks on the Internet. 85 Publicly accessible servers that support these modes need to be 86 configured to not respond to requests using the modes, as recommended 87 in BCP 233 [RFC8633], but the number of servers that still do that is 88 significant enough to require specific mitigations. 90 Over time, network operators have been observed to implement the 91 following mitigations: 93 1. Blocked UDP packets with destination or source port 123 95 2. Blocked UDP packets with destination or source port 123 and 96 specific length (e.g. longer than 48 octets) 98 3. Blocked UDP packets with destination or source port 123 and NTP 99 mode 6 or 7 101 4. Limited rate of UDP packets with destination or source port 123 103 From those, only the 3rd approach does not have an impact on 104 synchronization of clocks with NTP. 106 The number of public servers in the pool.ntp.org project has dropped 107 in large part due to the mitigations (citation?). 109 The length-specific filtering and rate limiting has an impact on the 110 Network Time Security NTS [RFC8915] authentication, which uses 111 extension fields in NTPv4 packets. 113 This document specifies an alternative port for NTP which is 114 restricted to a subset of the NTP protocol which does not allow 115 amplification in order to enable safe synchronization of clocks in 116 networks where the port 123 is blocked or rate limited. 118 1.1. Requirements Language 120 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 121 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 122 "OPTIONAL" in this document are to be interpreted as described in BCP 123 14 [RFC2119] [RFC8174] when, and only when, they appear in all 124 capitals, as shown here. 126 2. Alternative port - update to RFC 5905 128 The table in "Figure 6: Global Parameters" in Section 7.2 of 129 [RFC5905] is extended with: 131 +---------+-------+----------------------+ 132 | Name | Value | Description | 133 +---------+-------+----------------------+ 134 | ALTPORT | TBD | Alternative NTP port | 135 +---------+-------+----------------------+ 137 The following text from Section 9.1 of [RFC5905]: 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) assigned by the 143 IANA. In other modes, it can contain any number consistent with 144 local policy. 146 is replaced with: 148 srcport: UDP port number of the server or reference clock. This 149 becomes the destination port number in packets sent from this 150 association. When operating in symmetric modes (1 and 2), this 151 field must contain the NTP port number PORT (123) or the 152 alternative NTP port ALTPORT (TBD) assigned by the IANA. In other 153 modes, it can contain any number consistent with local policy. 155 The following text is added to the Section 9.1: 157 The port ALTPORT (TBD) is an alternative port to the port PORT 158 (123). The protocol and the format of NTP packets sent from and 159 to this port is unchanged. Both NTP requests and responses MAY be 160 sent from the alternative port. An NTP packet MUST NOT be sent 161 from the alternative port if it is a response which has a longer 162 UDP payload than the request, or the number of NTP packets in a 163 single response is larger than one. 165 Only modes 1 (active), 2 (passive), 3 (client), 4 (server), and 5 166 (broadcast) are generally usable on this port. 168 An NTP server that supports the alternative port MUST receive 169 requests in the client mode on both the PORT (123) and ALTPORT 170 (TBD) ports. If it responds, it MUST send the response from the 171 port which received the request. If the server support an NTP 172 extension field, it MUST verify for each response that it is not 173 longer than the request. 175 When an NTP client is started, it SHOULD send the first request to 176 the alternative port. The client SHOULD be switching between the 177 two ports until a valid response is received. The client MAY send 178 a limited number of requests to both ports at the same time in 179 order to speed up the discovery of the responding port. When both 180 ports are responding, the client SHOULD prefer the alternative 181 port. 183 An NTP server which supports NTS SHOULD include the NTPv4 Port 184 Negotiation record in NTS-KE responses to specify the alternative 185 port as the port to which the client should send NTP requests. 187 In the symmetric modes (active and passive) NTP packets are 188 considered to be requests and responses at the same time. 189 Therefore, two peers using the alternative port MUST send packets 190 with an equal length in order to synchronize with each other. The 191 peers MAY still use different polling intervals as packets sent at 192 subsequent polls are considered to be separate requests and 193 responses. 195 3. IANA Considerations 197 IANA is requested to allocate the following port in the Service Name 198 and Transport Protocol Port Number Registry [RFC6335]: 200 Service Name: ntp-alt 202 Transport Protocol: udp 204 Assignee: IESG 206 Contact: IETF Chair 208 Description: Network Time Protocol 210 Reference: [[this memo]] 212 Port Number: [[TBD]], selected by IANA from the System Port range 214 4. Security Considerations 216 A Man-in-the-middle (MITM) attacker can selectively block requests 217 sent to the alternative port to force a client to select the original 218 port and get a degraded NTP service with a significant packet loss. 219 The client needs to periodically try the alternative port to recover 220 from the degraded service when the attack stops. 222 5. Acknowledgements 224 The author would like to thank Daniel Franke, Dhruv Dhody, and Ragnar 225 Sundblad for their useful comments. 227 6. References 229 6.1. Normative References 231 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 232 Requirement Levels", BCP 14, RFC 2119, 233 DOI 10.17487/RFC2119, March 1997, 234 . 236 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 237 "Network Time Protocol Version 4: Protocol and Algorithms 238 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 239 . 241 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 242 Cheshire, "Internet Assigned Numbers Authority (IANA) 243 Procedures for the Management of the Service Name and 244 Transport Protocol Port Number Registry", BCP 165, 245 RFC 6335, DOI 10.17487/RFC6335, August 2011, 246 . 248 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 249 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 250 May 2017, . 252 6.2. Informative References 254 [I-D.ietf-ntp-mode-6-cmds] 255 Haberman, B., "Control Messages Protocol for Use with 256 Network Time Protocol Version 4", draft-ietf-ntp-mode- 257 6-cmds-10 (work in progress), September 2020. 259 [RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol 260 Version 4: Autokey Specification", RFC 5906, 261 DOI 10.17487/RFC5906, June 2010, 262 . 264 [RFC8633] Reilly, D., Stenn, H., and D. Sibold, "Network Time 265 Protocol Best Current Practices", BCP 223, RFC 8633, 266 DOI 10.17487/RFC8633, July 2019, 267 . 269 [RFC8915] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. 270 Sundblad, "Network Time Security for the Network Time 271 Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020, 272 . 274 Author's Address 276 Miroslav Lichvar 277 Red Hat 278 Purkynova 115 279 Brno 612 00 280 Czech Republic 282 Email: mlichvar@redhat.com