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ntpdate \- set the date and time via NTP
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sets the local date and time by polling the Network Time
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Protocol (NTP) server(s) given as the
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arguments to determine
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the correct time. It must be run as root on the local host. A number
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of samples are obtained from each of the servers specified and a
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subset of the NTP clock filter and selection algorithms are applied to
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select the best of these. Note that the accuracy and reliability of
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ntpdate depends on the number of servers, the number of polls each
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time it is run and the interval between runs.
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ntpdate can be run manually as necessary to set the host clock, or it
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can be run from the host startup script to set the clock at boot time.
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This is useful in some cases to set the clock initially before
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starting the NTP daemon ntpd. It is also possible to run ntpdate from
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a cron script. However, it is important to note that ntpdate with
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contrived cron scripts is no substitute for the NTP daemon, which uses
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sophisticated algorithms to maximize accuracy and reliability while
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minimizing resource use. Finally, since ntpdate does not discipline
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the host clock frequency as does ntpd, the accuracy using ntpdate is
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Time adjustments are made by ntpdate in one of two ways. If ntpdate
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determines the clock is in error more than 0.5 second it will simply
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step the time by calling the system settimeofday() routine. If the
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error is less than 0.5 seconds, it will slew the time by calling the
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system adjtime() routine. The latter technique is less disruptive and
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more accurate when the error is small, and works quite well when
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ntpdate is run by cron every hour or two.
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ntpdate will decline to set the date if an NTP server daemon (e.g.,
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ntpd) is running on the same host. When running ntpdate on a regular
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basis from cron as an alternative to running a daemon, doing so once
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every hour or two will result in precise enough timekeeping to avoid
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Enable the authentication function and specify the key
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identifier to be used for authentication as the argument
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keyntpdate. The keys and key identifiers must match in both the
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client and server key files. The default is to disable the
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authentication function.
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Force the time to always be slewed using the adjtime() system
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call, even if the measured offset is greater than +-128 ms. The
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default is to step the time using settimeofday() if the offset
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is greater than +-128 ms. Note that, if the offset is much
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greater than +-128 ms in this case, that it can take a long
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time (hours) to slew the clock to the correct value. During
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this time. the host should not be used to synchronize clients.
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Force the time to be stepped using the settimeofday() system
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call, rather than slewed (default) using the adjtime() system
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call. This option should be used when called from a startup
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Enable the debugging mode, in which ntpdate will go through all
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the steps, but not adjust the local clock. Information useful
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for general debugging will also be printed.
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Specify the processing delay to perform an authentication
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function as the value authdelay, in seconds and fraction (see
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ntpd for details). This number is usually small enough to be
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negligible for most purposes, though specifying a value may
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improve timekeeping on very slow CPU's.
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Specify the path for the authentication key file as the string
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keyfile. The default is /etc/ntp.keys. This file should be in
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the format described in ntpd.
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Specify the NTP version for outgoint packets as the integer
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version, which can be 1 or 2. The default is 3. This allows
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ntpdate to be used with older NTP versions.
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Specify the number of samples to be acquired from each server
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as the integer samples, with values from 1 to 8 inclusive. The
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Query only - don't set the clock.
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Divert logging output from the standard output (default) to the
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system syslog facility. This is designed primarily for
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convenience of cron scripts.
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Specify the maximum time waiting for a server response as the
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value timeout, in seconds and fraction. The value is is rounded
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to a multiple of 0.2 seconds. The default is 1 second, a value
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suitable for polling across a LAN.
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Direct ntpdate to use an unprivileged port or outgoing packets.
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This is most useful when behind a firewall that blocks incoming
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traffic to privileged ports, and you want to synchronise with
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hosts beyond the firewall. Note that the -d option always uses
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Be verbose. This option will cause ntpdate's version
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identification string to be logged.
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\- encryption keys used by ntpdate.
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The slew adjustment is actually 50% larger than the measured offset,
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since this (it is argued) will tend to keep a badly drifting clock
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more accurate. This is probably not a good idea and may cause a
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troubling hunt for some values of the kernel variables tick and
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David L. Mills (mills@udel.edu)
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This manpage converted from html to roff by
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Fabrizio Polacco <fpolacco@debian.org>
added
removed
debian/ntpdate.1
