ntpdate- set the date and time via NTP
ntpdate [ -bBdoqsuv ] [ -a key ] [ -e authdelay ] [
-k keyfile ] [ -o version ] [ -p samples ] [ -t
timeout ] server [ ... ]
ntpdate sets the local date and time by polling the
Network Time Protocol (NTP) server(s) given as the server
arguments to determine the correct time. It must be run as root on the
local host. A number of samples are obtained from each of the servers
specified and a subset of the NTP clock filter and selection algorithms
are applied to select the best of these. Note that the accuracy and
ntpdate depends on the number of servers,
the number of polls each time it is run and the interval between runs.
ntpdate can be run manually as necessary to set the host
clock, or it can be run from the host startup script to set the clock at
boot time. This is useful in some cases to set the clock initially
before starting the NTP daemon
xntpd. It is also possible
ntpdate from a
cron script. However, it
is important to note that
ntpdate with contrived
cron scripts is no substitute for the NTP daemon, which
uses sophisticated algorithms to maximize accuracy and reliability while
minimizing resource use. Finally, since
ntpdate does not
discipline the host clock frequency as does
ntpdate is limited.
Time adjustments are made by
ntpdate in one of two ways.
ntpdate determines the clock is in error more than 0.5
second it will simply step the time by calling the system
settimeofday() routine. If the error is less than 0.5
seconds, it will slew the time by calling the system
adjtime() routine. The latter technique is less disruptive
and more accurate when the error is small, and works quite well when
ntpdate is run by
cron every hour or two.
ntpdate will decline to set the date if an NTP server
xntpd) is running on the same host. When
ntpdate on a regular basis from
as an alternative to running a daemon, doing so once every hour or two
will result in precise enough timekeeping to avoid stepping the clock.
ntpdate. The keys and key identifiers must match in both the client and server key files. The default is to disable the authentication function.
ntpdatewill go through all the steps, but not adjust the local clock. Information useful for general debugging will also be printed.
xntpdfor details). This number is usually small enough to be negligible for most purposes, though specifying a value may improve timekeeping on very slow CPU's.
/etc/ntp.keys. This file should be in the format described in
ntpdateto be used with older NTP versions.
syslogfacility. This is designed primarily for convenience of
ntpdateto use an unprivileged port or outgoing packets. This is most useful when behind a firewall that blocks incoming traffic to privileged ports, and you want to synchronise with hosts beyond the firewall. Note that the
-doption always uses unprivileged ports.
ntpdate's version identification string to be logged.
/etc/ntp.keys - encryption keys used by
The slew adjustment is actually 50% larger than the measured offset,
since this (it is argued) will tend to keep a badly drifting clock more
accurate. This is probably not a good idea and may cause a troubling
hunt for some values of the kernel variables