Internet-Draft Alternative NTP port October 2021
Lichvar Expires 21 April 2022 [Page]
Internet Engineering Task Force
5905 (if approved)
Intended Status:
Standards Track
M. Lichvar
Red Hat

Alternative NTP port


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.

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

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This Internet-Draft will expire on 21 April 2022.

Table of Contents

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.

Network operators have implemented different mitigations. They are not documented and may change over time. Some of the mitigations that have been observed are:

  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. However, this mitigation can be implemented only on devices which can inspect the UDP payload.

The number of public servers in the project has dropped since 2013, when the large-scale attacks started.

The length-specific filtering and rate limiting has an impact on the Network Time Security [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:

Table 1
Name Value Description
ALTPORT TBD Alternative NTP port

The following text from Section 9.1 of [RFC5905]:

is replaced with:

The following text is added to the Section 9.1:

3. IANA Considerations

IANA is requested to allocate the following port in the Service Name and Transport Protocol Port Number Registry [RFC6335]:

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, Ragnar Sundblad, and Steven Sommars for their useful comments.

6. References

6.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

6.2. Informative References

Haberman, B., "Control Messages Protocol for Use with Network Time Protocol Version 4", Work in Progress, Internet-Draft, draft-ietf-ntp-mode-6-cmds-10, , <>.
Haberman, B., Ed. and D. Mills, "Network Time Protocol Version 4: Autokey Specification", RFC 5906, DOI 10.17487/RFC5906, , <>.
Reilly, D., Stenn, H., and D. Sibold, "Network Time Protocol Best Current Practices", BCP 223, RFC 8633, DOI 10.17487/RFC8633, , <>.
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, , <>.

Author's Address

Miroslav Lichvar
Red Hat
Purkynova 115
612 00 Brno
Czech Republic