ntpdate(1M)ntpdate(1M)NAMEntpdate - set the date and time via NTP
SYNOPSIS
[ ] [ key# ] [ authdelay ] [ keyfile ]
[ version ] [ samples ] [ timeout ] server[ ... ]
DESCRIPTION
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 depends on the number
of servers, the number of polls each time it is run, and the interval
between the runs.
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
It is also possible to run from a cron script. However, it is important
to note that 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 does not dis‐
cipline the host clock frequency as does the accuracy using is limited.
Time adjustments are made by in one of two ways. If determines the
clock is in error more than 0.5 seconds, it will simply step the time
by calling the (see clocks(2)) system routine. If the error is less
than 0.5 seconds, it will slew the time by calling the (see adjtime(2))
system routine. The latter technique is less disruptive and more accu‐
rate when the error is small, and works quite well when is run by (see
cron(1M)) every hour or two.
will decline to set the date if an NTP server daemon (e.g., is running
on the same host. When running on a regular basis from as an alterna‐
tive 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
supports the following options:
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 server key files. The
default is to disable the authentication function.
Force the time to always be slewed using the
system call, even if the measured offset is greater than
The default is to step the time using the system call if
the offset is greater than Note that, if the offset is
much greater than 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.
Force the time to be stepped using the
system call, rather than slewed (default) using the sys‐
tem call. This option should be used when called from a
startup file at boot time.
Enable the debugging mode, in which
will go through all the steps, but not adjust the local
clock. Information useful for general debugging will
also be printed.
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 speci‐
fying a value may improve timekeeping on very slow
CPU's.
Specify the path for the authentication key file as the string
keyfile. The default is This file should be in the for‐
mat described in
Specify the NTP version for outgoing packets as the integer version,
which can be 1 or 2. The default is 3. This allows to
be used with older NTP versions.
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.
Prints the offset measurement, stratum of the server(s) and delay
measurement without adjusting the local clock. This is
similar to option which gives a more detailed debugging
information.
Divert logging output from the standard output (default) to the system
(see syslog(3C)) facility. This is designed primarily
for convenience of scripts.
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.
Direct 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 option always uses unprivileged ports.
Prints the version number and the offset measurement information.
AUTHOR
was developed by Dennis Ferguson at the University of Toronto.
FILES
Contains the encryption keys used by
SEE ALSOadjtime(2), clocks(2), cron(1M), syslog(3C), ntpq(1M), xntpd(1M), xnt‐
pdc(1M).
DARPA Internet Request For Comments RFC1035 Assigned Numbers.
ntpdate(1M)
