NAME
ntpdate — set the date and time via NTP
SYNOPSIS
ntpdate
[
-Bbdpqsuv
] [
-a
key#
] [
-e
authdelay
] [
-k
keyfile
]
[
-o
version
] [
-p
samples
] [
-t
timeout
]
server[ ... ]
DESCRIPTION
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 server key files. The
default is to disable the authentication 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 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 printed.
- -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 versions.
- -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
cron
scripts.
- -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 measurement information.
AUTHOR
ntpdate
was developed by Dennis Ferguson at the University of Toronto.
FILES
- /etc/ntp.keys
Contains the encryption keys used by
ntpdate.
Printable version
