ntpdate - set the date and time via NTP
ntpdate [ -Bbdpqsuv ] [ -a key# ] [ -e authdelay ] [ -k keyfile ]
[ -o version ] [ -p samples ] [ -t timeout ] server[ ... ]
ntpdate sets the local date and time by polling those 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 reliability of
ntpdate depends on the number of servers, the number of polls each
time it is run, and the interval between the 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 to run 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 xntpd, the accuracy using ntpdate is limited.
Time adjustments are made by ntpdate in one of two ways. If ntpdate
determines the clock is in error more than 0.5 seconds, it will simply
step the time by calling the clock_settime (see clocks(2)) system
routine. If the error is less than 0.5 seconds, it will slew the time
by calling the adjtime (see adjtime(2)) system 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 (see cron(1M))
every hour or two.
ntpdate will decline to set the date if an NTP server daemon (e.g.,
xntpd) is running on the same host. When running ntpdate on a regular
basis from cron 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.
COMMAND LINE OPTIONS
ntpdate supports the following options:
-a Enable the authentication function and specify the key
identifier to be used for authentication. The keys and
key identifiers must match in both the client and
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server key files. The default is to disable the
-B Force the time to always be slewed using the adjtime
system call, even if the measured offset is greater
than +-128 ms. The default is to step the time using
the clock_settime system call if the offset is greater
than +-128 ms. Note that, if the offset is much greater
than +-128 ms it can take a long time (hours) to slew
the clock to the correct value. During this time the
host should not be used to synchronize clients.
-b Force the time to be stepped using the clock_settime
system call, rather than slewed (default) using the
adjtime system call. This option should be used when
called from a startup file at boot time.
-d Enable the debugging mode, in which ntpdate will go
through all the steps, but not adjust the local clock.
Information useful for general debugging will also be
-e authdelay Specify the processing delay to perform an
authentication function as the value authdelay, in
seconds and fraction (see xntpd(1M) for 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.
-k keyfile Specify the path for the authentication key file as the
string keyfile. The default is /etc/ntp.keys. This
file should be in the format described in xntpd.
-o version Specify the NTP version for outgoing packets as the
integer version, which can be 1 or 2. The default is
3. This allows ntpdate to be used with older NTP
-p samples Specify the number of samples to be acquired from each
server as the integer samples, with values from 1 to 8
inclusive. The default is 4.
-q Prints the offset measurement, stratum of the server(s)
and delay measurement without adjusting the local
clock. This is similar to -d option which gives a more
detailed debugging information.
-s Divert logging output from the standard output
(default) to the system syslog (see syslog(3C))
facility. This is designed primarily for convenience of
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-t timeout Specify the maximum waiting time for a server response
as the value timeout, in seconds and fraction. The
value is rounded to a multiple of 0.2 seconds. The
default is 1 second, a value suitable for polling
across a LAN.
-u Direct ntpdate to use an unprivileged port for 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 -d option always uses unprivileged ports.
-v Prints the NTP version number and the offset
/etc/ntp.keys Contains the encryption keys used by ntpdate.
adjtime(2), clocks(2), cron(1M), syslog(3C), ntpq(1M), xntpd(1M),
DARPA Internet Request For Comments RFC1035 Assigned Numbers.
ntpdate was developed by Dennis Ferguson at the University of Toronto.
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