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78 lines
2.9 KiB
ReStructuredText
78 lines
2.9 KiB
ReStructuredText
.. _ntp:
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###
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NTP
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###
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:abbr:`NTP (Network Time Protocol`) is a networking protocol for clock
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synchronization between computer systems over packet-switched, variable-latency
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data networks. In operation since before 1985, NTP is one of the oldest Internet
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protocols in current use.
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NTP is intended to synchronize all participating computers to within a few
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milliseconds of :abbr:`UTC (Coordinated Universal Time)`. It uses the
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intersection algorithm, a modified version of Marzullo's algorithm, to select
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accurate time servers and is designed to mitigate the effects of variable
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network latency. NTP can usually maintain time to within tens of milliseconds
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over the public Internet, and can achieve better than one millisecond accuracy
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in local area networks under ideal conditions. Asymmetric routes and network
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congestion can cause errors of 100 ms or more.
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The protocol is usually described in terms of a client-server model, but can as
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easily be used in peer-to-peer relationships where both peers consider the other
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to be a potential time source. Implementations send and receive timestamps using
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:abbr:`UDP (User Datagram Protocol)` on port number 123.
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NTP supplies a warning of any impending leap second adjustment, but no
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information about local time zones or daylight saving time is transmitted.
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The current protocol is version 4 (NTPv4), which is a proposed standard as
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documented in :rfc:`5905`. It is backward compatible with version 3, specified
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in :rfc:`1305`.
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Configuration
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=============
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.. cfgcmd:: set system ntp server <address>
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Configure one or more servers for synchronisation. Server name can be either
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an IP address or :abbr:`FQDN (Fully Qualified Domain Name)`.
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There are 3 default NTP server set. You are able to change them.
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* ``0.pool.ntp.org``
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* ``1.pool.ntp.org``
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* ``2.pool.ntp.org``
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.. cfgcmd:: set system ntp server <address> <noselect | pool | preempt | prefer>
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Configure one or more attributes to the given NTP server.
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* ``noselect`` marks the server as unused, except for display purposes. The
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server is discarded by the selection algorithm.
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* ``pool`` mobilizes persistent client mode association with a number of
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remote servers.
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* ``preempt`` a preemptable association is expendable.
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* ``prefer`` marks the server as preferred. All other things being equal,
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this host will be chosen for synchronization among a set of correctly
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operating hosts.
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.. cfgcmd:: set system ntp listen-address <address>
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NTP process will only listen on the specified IP address. You must specify
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the `<address>` and optionally the permitted clients. Multiple listen
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addresses can be configured.
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.. cfgcmd:: set system ntp allow-clients address <address>
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List of networks or client addresses permitted to contact this NTP server.
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Multiple networks can be configured.
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.. cfgcmd:: set system ntp vrf <name>
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Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.
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