Alternative NTP port

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. 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. 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 specified in RFC 5906. Autokey is rarely supported on public NTP servers. 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: Blocked UDP packets with destination or source port 123 Blocked UDP packets with destination or source port 123 and specific length (e.g. longer than 48 octets) Blocked UDP packets with destination or source port 123 and NTP mode 6 or 7 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. 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.

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 when, and only when, they appear in all capitals, as shown here.

The table in "Figure 6: Global Parameters" in Section 7.2 of is extended with: Name Value Description ALTPORT TBD Alternative NTP port The following text from Section 9.1 of : 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 SHOULD 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 any extension fields in NTP packets, it MUST verify that each response is not larger than the request, even if the number of extension fields is constant and they have a constant length. 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, the peers MUST send packets with an equal length in order to synchronize with each other. The peers MAY use different polling intervals (packets sent at subsequent polls are considered to be separate requests and responses).

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

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.

The author would like to thank Daniel Franke, Dhruv Dhody, and Ragnar Sundblad for their useful comments.