Script :

service ntp stop
/usr/sbin/ntpdate 2>&1
service ntp start

Reference :

Man page for ntpdate :

ntpdate(8) ntpdate(8)

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 (unless the option -q is used). 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 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 ntpd. It is also possible to run ntp-
date 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 maxi-
mize accuracy and reliability while minimizing resource use. Finally, since ntpdate does
not discipline the host clock frequency as does ntpd, the accuracy using ntpdate is lim-

Time adjustments are made by ntpdate in one of two ways. If ntpdate determines the clock
is in error more than 0.5 second it will simply step the time by calling the system set-
timeofday() routine. If the error is less than 0.5 seconds, it will slew the time by call-
ing the system adjtime() routine. The latter technique is less disruptive and more accu-
rate 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 daemon (e.g., ntpd) is running on
the same host. When running ntpdate on a regular basis from cron as an alternative to run-
ning a daemon, doing so once every hour or two will result in precise enough timekeeping
to avoid stepping the clock.

-a key Enable the authentication function and specify the key identifier to be used for
authentication as the argument keyntpdate. The keys and key identifiers must match
in both the client and server key files. The default is to disable the authentica-
tion function.

-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
settimeofday() if the offset is greater than +-128 ms. Note that, if the offset is
much greater than +-128 ms in this case, that 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 settimeofday() 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 ntpd 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 ntpd.

-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 versions.

-p samples
Specify the number of samples to be acquired from each server as the integer sam-
ples, with values from 1 to 8 inclusive. The default is 4.

-q Query only – don’t set the clock.

-s Divert logging output from the standard output (default) to the system syslog
facility. This is designed primarily for convenience of cron scripts.

-t timeout
Specify the maximum time waiting for a server response as the value timeout, in
seconds and fraction. The value is 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 use-
ful 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 Be verbose. This option will cause ntpdate’s version identification string to be

ntpdate’s exit status is zero if it found a server and could update the clock, and nonzero

– encryption keys used by ntpdate.

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 tick and

David L. Mills (
This manpage converted from html to roff by Fabrizio Polacco


Synchronize NTP clock

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