NTP (Network Time Protocol) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use.

NTP is intended to synchronize all participating computers to within a few milliseconds of UTC. It uses the intersection algorithm, a modified version of Marzullo’s algorithm, to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.

The protocol is usually described in terms of a client-server model, but can as easily be used in peer-to-peer relationships where both peers consider the other to be a potential time source. Implementations send and receive timestamps using UDP on port number 123.

NTP supplies a warning of any impending leap second adjustment, but no information about local time zones or daylight saving time is transmitted.

The current protocol is version 4 (NTPv4), which is a proposed standard as documented in RFC 5905. It is backward compatible with version 3, specified in RFC 1305.


VyOS 1.4 uses chrony instead of ntpd (see T3008) which will no longer accept anonymous NTP requests as in VyOS 1.3. All configurations will be migrated to keep the anonymous functionality. For new setups if you have clients using your VyOS installation as NTP server, you must specify the allow-client directive.


set service ntp server <address>

Configure one or more servers for synchronisation. Server name can be either an IP address or FQDN.

There are 3 default NTP server set. You are able to change them.

  • time1.vyos.net

  • time2.vyos.net

  • time3.vyos.net

set service ntp server <address> <noselect | nts | pool | prefer>

Configure one or more attributes to the given NTP server.

  • noselect marks the server as unused, except for display purposes. The server is discarded by the selection algorithm.

  • nts enables Network Time Security (NTS) for the server as specified in RFC 8915

  • pool mobilizes persistent client mode association with a number of remote servers.

  • prefer marks the server as preferred. All other things being equal, this host will be chosen for synchronization among a set of correctly operating hosts.

set service ntp listen-address <address>

NTP process will only listen on the specified IP address. You must specify the <address> and optionally the permitted clients. Multiple listen addresses can be configured.

set service ntp allow-client address <address>

List of networks or client addresses permitted to contact this NTP server.

Multiple networks/client IP addresses can be configured.

set service ntp vrf <name>

Specify name of the VRF instance.

set service ntp leap-second [ignore|smear|system|timezone]

Define how to handle leap-seconds.

  • ignore: No correction is applied to the clock for the leap second. The clock will be corrected later in normal operation when new measurements are made and the estimated offset includes the one second error.

  • smear: When smearing a leap second, the leap status is suppressed on the server and the served time is corrected slowly by slewing instead of stepping. The clients do not need any special configuration as they do not know there is any leap second and they follow the server time which eventually brings them back to UTC. Care must be taken to ensure they use only NTP servers which smear the leap second in exactly the same way for synchronisation.

  • system: When inserting a leap second, the kernel steps the system clock backwards by one second when the clock gets to 00:00:00 UTC. When deleting a leap second, it steps forward by one second when the clock gets to 23:59:59 UTC.

  • timezone: This directive specifies a timezone in the system timezone database which chronyd can use to determine when will the next leap second occur and what is the current offset between TAI and UTC. It will periodically check if 23:59:59 and 23:59:60 are valid times in the timezone. This normally works with the right/UTC timezone which is the default