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+.. _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 | 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.
+
+   * ``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.