Alternative NTP port

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.¶

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 pool.ntp.org 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.¶

2. Alternative port - update to RFC 5905

The table in "Figure 6: Global Parameters" in Section 7.2 of [RFC5905] is extended with:¶

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.¶

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 supports 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 alternate 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.¶

6. References