.. _ntp:

###
NTP
###

:abbr:`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 :abbr:`UTC (Coordinated Universal Time)`. 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
:abbr:`UDP (User Datagram Protocol)` 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`.

.. note:: VyOS 1.4 uses chrony instead of ntpd (see :vytask:`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.

Configuration
=============

.. cfgcmd:: set service ntp server <address>

   Configure one or more servers for synchronisation. Server name can be either
   an IP address or :abbr:`FQDN (Fully Qualified Domain Name)`.

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

   * ``0.pool.ntp.org``
   * ``1.pool.ntp.org``
   * ``2.pool.ntp.org``

.. cfgcmd:: 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.

.. cfgcmd:: 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.

.. cfgcmd:: 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.

.. cfgcmd:: set service ntp vrf <name>

   Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.

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

   Define how to handle leaf-seonds.

   * `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