ntpdate -- set the date and time via NTP
ntpdate [-bBdoqsuv] [-a key] [-e authdelay] [-k keyfile] [-o version]
[-p samples] [-t timeout] server ...
Note: The functionality of this program is now available in the ntpd(8)
program. See the -q command line option in the ntpd(8) page. After a
suitable period of mourning, the ntpdate utility is to be retired from
The ntpdate utility 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.
The following options are available:
-a key Enable the authentication function and specify the key identifier
to be used for authentication as the argument key. 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(2) system
call, even if the measured offset is greater than +-128 ms. The
default is to step the time using settimeofday(2) if the offset
is greater than +-128 ms. Note that, if the offset is much
greater than +-128 ms in this case, 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(2) system
call, rather than slewed (default) using the adjtime(2) 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 function
as the value authdelay, in seconds and fraction (see ntpd(8)
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.
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(8).
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.
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(3) facility. This is designed primarily for convenience
of cron(8) 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's version identification
string to be logged.
The ntpdate utility 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(8). It is also possible to run
ntpdate from a cron(8) script. However, it is important to note that
ntpdate with contrived cron(8) 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(8), 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(8) every hour or two.
The ntpdate utility will decline to set the date if an NTP server daemon
(e.g., ntpd(8)) is running on the same host. When running ntpdate on a
regular basis from cron(8) 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.
If NetInfo support is compiled into ntpdate, then the server argument is
optional if ntpdate can find a time server in the NetInfo configuration
/etc/ntp.keys contains the 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 kern.clockrate.tick and
FreeBSD 5.2.1 January 6, 2000 FreeBSD 5.2.1 [ Back ]