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NTPDATE(8) NetBSD System Manager's Manual NTPDATE(8)
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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 reliability of ntpdate depends on the number
of servers, the number of polls each time it is run and the interval
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 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 ntpd, 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 second it will simply step
the time by calling the system settimeofday(2) routine. If the error is
less than 0.5 seconds, it will slew the time by calling the system
adjtime(2) 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 daemon (e.g., ntpd
) 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
If NetInfo support is compiled into ntpdate, then the server argument is
optional if ntpdate can find a time server in the NetInfo configuration
COMMAND LINE OPTIONS
-a key Enable the authentication function and specify the key identifier
to be used for authentication as the argument key ntpdate. The
keys and key identifiers must match in both the client and server
key files. The default is to disable the authentication func-
-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 printed.
Specify the processing delay to perform an authentication func-
tion as the value authdelay , in seconds and fraction (see ntpd
for details). This number is usually small enough to be negligi-
ble for most purposes, though specifying 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 /etc/ntp.keys. This file should be in the
format described in ntpd
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.
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 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 conve-
nience of cron scripts.
Specify the maximum time waiting 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
-v Be verbose. This option will cause ntpdate string to be logged.
/etc/ntp.keys encryption keys used by ntpdate.
David L. Mills (email@example.com)
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 tickadj.
NetBSD 10.99 January 28, 2010 NetBSD 10.99