Internet Engineering Task Force M. Lichvar Internet-Draft Red Hat Updates: 5905 (if approved) Oct 12, 2020 Intended status: Standards Track Expires: April 15, 2021 Alternative NTP port draft-ietf-ntp-alternative-port-00 Abstract This document updates RFC 5905 to specify an alternative port for the Network Time Protocol (NTP) which is restricted to NTP messages that do not allow traffic amplification in order to make NTP safe for the Internet. 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 April 15, 2021. 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 April 15, 2021 [Page 1]
Internet-Draft Alternative NTP port Oct 2020 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. Alternative port - update to RFC 5905 . . . . . . . . . . . . 3 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction There are several modes specified for NTP. NTP packets in versions 2, 3, and 4 have a 3-bit field for the mode. Modes 1 (active), 2 (passive), 3 (client), 4 (server), and 5 (broadcast) 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 typically does not allow any traffic amplification, i.e. the response is not larger than the request. An exception is Autokey [RFC5906], which allows an NTP response to be longer than the request, e.g. packets containing the Certificate Message or Cookie Message extension field. Autokey is rarely used. If it is enabled on a publicly accessible server, the access needs to be tightly controlled to limit denial-of-service (DoS) attacks exploiting the amplification. The modes 6 and 7 of NTP allow significant traffic amplification, which has been exploited in large-scale DoS attacks on the Internet. Publicly accessible servers that support these modes need to be configured to not respond to requests using the modes, as recommended in BCP 233 [RFC8633], but the number of servers that still do that is significant enough to require specific mitigations. 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) Lichvar Expires April 15, 2021 [Page 2]
Internet-Draft Alternative NTP port Oct 2020 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?). The length-specific filtering and rate limiting has an impact on the Network Time Security NTS [RFC8915] authentication, which uses extension fields in NTPv4 packets. This document specifies an alternative port for NTP which is restricted to a subset of the NTP protocol which does not allow amplification in order to enable safe 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", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Alternative port - update to RFC 5905 The table in "Figure 6: Global Parameters" in Section 7.2 of [RFC5905] is extended with: +---------+-------+----------------------+ | Name | Value | Description | +---------+-------+----------------------+ | ALTPORT | TBD | Alternative NTP port | +---------+-------+----------------------+ The following text from Section 9.1 of [RFC5905]: srcport: UDP port number of the server or reference clock. This becomes the destination port number in packets sent from this association. When operating in symmetric modes (1 and 2), this field must contain the NTP port number PORT (123) assigned by the IANA. In other modes, it can contain any number consistent with local policy. Lichvar Expires April 15, 2021 [Page 3]
Internet-Draft Alternative NTP port Oct 2020 is replaced with: srcport: UDP port number of the server or reference clock. This becomes the destination port number in packets sent from this association. When operating in symmetric modes (1 and 2), this field must contain the NTP port number PORT (123) or the alternative NTP port ALTPORT (TBD) assigned by the IANA. In other modes, it can contain any number consistent with local policy. The following text is added to the Section 9.1: The port ALTPORT (TBD) is an alternative port to the port PORT (123). The protocol and the format of NTP packets sent from and to this port is unchanged. Both NTP requests and responses MAY be sent from the alternative port. An NTP packet MUST NOT be sent from the alternative port if it is a response which has a longer UDP payload than the request, or the number of NTP packets in a single response is larger than one. Only modes 1 (active), 2 (passive), 3 (client), 4 (server), and 5 (broadcast) are generally usable on this port. An NTP server that supports the alternative port MUST receive requests in the client mode on both the PORT (123) and ALTPORT (TBD) ports. If it responds, it MUST send the response from the port which received the request. If the server support an NTP extension field, it MUST verify for each response that it is not longer than the request. 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. In the symmetric modes (active and passive) NTP packets are considered to be requests and responses at the same time. Therefore, two peers using the alternative port MUST send packets with an equal length in order to synchronize with each other. The peers MAY still use different polling intervals as packets sent at subsequent polls are considered to be separate requests and responses. Lichvar Expires April 15, 2021 [Page 4]
Internet-Draft Alternative NTP port Oct 2020 3. IANA Considerations IANA is requested to allocate the following port in the Service Name and Transport Protocol Port Number Registry [RFC6335]: Service Name: ntp-alt Transport Protocol: udp Assignee: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org> Description: Network Time Protocol Reference: [[this memo]] Port Number: [[TBD]], selected by IANA from the System Port range 4. Security Considerations A Man-in-the-middle (MITM) attacker can selectively block requests sent to the alternative port to force a client to select the original port and get a degraded NTP service with a significant packet loss. The client needs to periodically try the alternative port to recover from the degraded service when the attack stops. 5. Acknowledgements The author would like to thank Daniel Franke, Dhruv Dhody, and Ragnar Sundblad for their useful comments. 6. References 6.1. 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>. Lichvar Expires April 15, 2021 [Page 5]
Internet-Draft Alternative NTP port Oct 2020 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011, <https://www.rfc-editor.org/info/rfc6335>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. 6.2. Informative References [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-10 (work in progress), September 2020. [RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol Version 4: Autokey Specification", RFC 5906, DOI 10.17487/RFC5906, June 2010, <https://www.rfc-editor.org/info/rfc5906>. [RFC8633] Reilly, D., Stenn, H., and D. Sibold, "Network Time Protocol Best Current Practices", BCP 223, RFC 8633, DOI 10.17487/RFC8633, July 2019, <https://www.rfc-editor.org/info/rfc8633>. [RFC8915] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. Sundblad, "Network Time Security for the Network Time Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020, <https://www.rfc-editor.org/info/rfc8915>. Author's Address Miroslav Lichvar Red Hat Purkynova 115 Brno 612 00 Czech Republic Email: mlichvar@redhat.com Lichvar Expires April 15, 2021 [Page 6]
