ntpdate - set the date and time via NTP
Adventures in Wonderland, Lewis Carroll
I told you it was eyeball and wristwatch.
Disclaimer: The functionality of this program is now available
in the ntpd program. See the -q command line
option in the ntpd - Network Time
Protocol (NTP) daemon page. After a suitable period of
mourning, the ntpdate program is to be retired from this
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 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 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() routine. If the error is less than 0.5
seconds, it will slew the time by calling the system
adjtime() 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 for ntpd.
Command Line Options
- -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
- 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.
- 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
- 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 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 outgoint 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.
- Query only - don't set the clock.
- 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.
- Direct ntpdate to use an unprivileged port or 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.
- Be verbose. This option will cause ntpdate's version
identification string to be logged.
/etc/ntp.keys - 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 tickadj.
David L. Mills