Internet Engineering Task Force M. Lichvar Internet-Draft Red Hat Updates: RFC5905 (if approved) Apr 06, 2020 Intended status: Standards Track Expires: October 8, 2020 Alternative NTP port draft-mlichvar-ntp-alternative-port-00 Abstract This document specifies an alternative port for the Network Time Protocol (NTP) which is restricted in modes in order to avoid amplification attacks. 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 https://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 October 8, 2020. Copyright Notice Copyright (c) 2020 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 (https://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. Lichvar Expires October 8, 2020 [Page 1]
Internet-Draft Alternative NTP port Apr 2020 1. Introduction There are several modes specified for NTP. NTP packets have a 3-bit field for the mode. Modes 1, 2, 3, 4, and 5 are used for synchronization of clocks. They are specified in RFC 5905 [RFC5905]. Modes 6 and 7 are used for other purposes, like monitoring and remote management of NTP servers and clients. The mode 6 is specified in Control Messages Protocol for Use with Network Time Protocol Version 4 [I-D.ietf-ntp-mode-6-cmds]. The first group of modes does not allow traffic amplification. (Explain why?) However, the modes 6 and 7 allow significant traffic amplification, which has been exploited in large-scale denial-of-service (DoS) attacks over the Internet. Over time, network operators have been observed to implement the following mitigations: 1. Blocked UDP packets with destination or source port 123 2. Blocked UDP packets with destination or source port 123 and specific length (e.g. longer than 48 octets) 3. Blocked UDP packets with destination or source port 123 and NTP mode 6 or 7 4. Limited rate of UDP packets with destination or source port 123 From those, only the 3rd approach does not have an impact on synchronization of clocks with NTP. The number of public servers in the pool.ntp.org project has dropped in large part due to the mitigations (citation?). Longer NTP packets (using extension fields) are needed by NTS [I-D.ietf-ntp-using-nts-for-ntp]. This document specifies an alternative port for NTP which is restricted to the safe modes in order to enable synchronization of clocks in networks where the port 123 is blocked or rate limited. 1.1. 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]. Lichvar Expires October 8, 2020 [Page 2]
Internet-Draft Alternative NTP port Apr 2020 2. Alternative port The port TBD is an alternative port for NTP. The protocol and the format of NTP packets sent from or to this port is unchanged. The port MAY be used in modes 1 (active), 2 (passive), 3 (client), 4 (server), and 5 (broadcast). The port MUST NOT be used in modes 0 (unspecified), 6 (control), and 7 (private). An NTP server SHOULD receive requests on both the original NTP port (123) and the alternative port (TBD). It MUST respond from the same port which received the request. If a request is received on the alternative port in a mode which is not allowed on this port, the request MUST be discarded with no response. When an NTP client is started, it SHOULD send the first request to the alternative port. The client SHOULD be switching between the two ports until a valid response is received. The client MAY send a limited number of requests to both ports at the same time in order to speed up the discovery of the responding port. When both ports are responding, the client SHOULD prefer the alternative port. An NTP server which supports NTS SHOULD include the NTPv4 Port Negotiation record in NTS-KE responses to specify the alternative port as the port to which the client should send NTP requests. 3. IANA Considerations IANA is requested to allocate a (system?) UDP/TCP port. 4. References 4.1. 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, <https://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, <https://www.rfc-editor.org/info/rfc5905>. 4.2. Informative References Lichvar Expires October 8, 2020 [Page 3]
Internet-Draft Alternative NTP port Apr 2020 [I-D.ietf-ntp-mode-6-cmds] Haberman, B., "Control Messages Protocol for Use with Network Time Protocol Version 4", draft-ietf-ntp-mode- 6-cmds-07 (work in progress), November 2019. [I-D.ietf-ntp-using-nts-for-ntp] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. Sundblad, "Network Time Security for the Network Time Protocol", draft-ietf-ntp-using-nts-for-ntp-28 (work in progress), March 2020. Author's Address Miroslav Lichvar Red Hat Purkynova 115 Brno 612 00 Czech Republic Email: mlichvar@redhat.com Lichvar Expires October 8, 2020 [Page 4]