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114 lines
4.7 KiB
ReStructuredText
114 lines
4.7 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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.. note:: VyOS 1.4 uses chrony instead of ntpd (see :vytask:`T3008`) which will
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no longer accept anonymous NTP requests as in VyOS 1.3. All configurations
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will be migrated to keep the anonymous functionality. For new setups if you
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have clients using your VyOS installation as NTP server, you must specify
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the `allow-client` directive.
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Configuration
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=============
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.. cfgcmd:: set service 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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* ``time1.vyos.net``
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* ``time2.vyos.net``
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* ``time3.vyos.net``
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.. cfgcmd:: set service ntp server <address> <noselect | nts | pool | 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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* ``nts`` enables Network Time Security (NTS) for the server as specified
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in :rfc:`8915`
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* ``pool`` mobilizes persistent client mode association with a number of
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remote servers.
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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 service 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 service ntp allow-client address <address>
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List of networks or client addresses permitted to contact this NTP server.
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Multiple networks/client IP addresses can be configured.
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.. cfgcmd:: set service ntp vrf <name>
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Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.
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.. cfgcmd:: set service ntp leap-second [ignore|smear|system|timezone]
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Define how to handle leap-seconds.
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* `ignore`: No correction is applied to the clock for the leap second. The
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clock will be corrected later in normal operation when new measurements are
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made and the estimated offset includes the one second error.
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* `smear`: When smearing a leap second, the leap status is suppressed on the
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server and the served time is corrected slowly by slewing instead of
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stepping. The clients do not need any special configuration as they do not
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know there is any leap second and they follow the server time which
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eventually brings them back to UTC. Care must be taken to ensure they use
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only NTP servers which smear the leap second in exactly the same way for
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synchronisation.
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* `system`: When inserting a leap second, the kernel steps the system clock
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backwards by one second when the clock gets to 00:00:00 UTC. When deleting
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a leap second, it steps forward by one second when the clock gets to
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23:59:59 UTC.
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* `timezone`: This directive specifies a timezone in the system timezone
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database which chronyd can use to determine when will the next leap second
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occur and what is the current offset between TAI and UTC. It will
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periodically check if 23:59:59 and 23:59:60 are valid times in the
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timezone. This normally works with the right/UTC timezone which is the
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default
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