ntpdate(ADMN)

Syntax

Description

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(ADMN). It is also possible to run ntpdate from a cron script. However, it is important to note that ntpdate with contrived cron(C) 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(S) routine. If the error is less than 0.5 seconds, it will slew the time by calling the system adjtime(S) 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. In the latter case, the slew adjustment is actually 50% larger than the measured offset, since this will tend to keep a badly drifting clock more accurate.

ntpdate will decline to set the date if an NTP server daemon (for example, 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 clock.

Options

-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 always to be slewed using the adjtime system call, even if the measured offset is greater than ±128ms. The default is to step the time using settimeofday if the offset is greater than ±128ms. Note that, if the offset is much greater than ±128ms 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 do 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(ADMN) for details). This number is usually small enough to be negligible for most purposes, though specifying a value may improve timekeeping on very slow CPUs.

-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 on the ntpd(ADMN) manual page.

-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

Query only - do not set the clock.

-r

The interval (in seconds) to spend measuring the drift rate.

-s

Divert logging output from the standard output (default) to the system syslog(SLIB) 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 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

Verbose option. Causes the ntpdate version identification string to be logged.

Files

/etc/ntp.keys

encryption keys used by ntpdate

See also