ntpdate - set the date and time via NTP (Network Time Protocol)
/usr/sbin/ntpdate [-bdqsuv] [-a key#] [-e authdelay] [-k
keyfile] [-o version] [-p samples] [-t timeout] server1
server2 server3...
Tells ntpdate to step the system time immediately to match
NTP. Use this option only when booting the system. Prints
configuration and debugging information. Queries the
server(s) and prints the information received; the date
and time are not set. Tells ntpdate to log its actions
through the syslog(3) facility rather than to the standard
output. This is useful when running the program from
cron(8). Tells ntpdate to use an unprivileged port to
send the packets from. This is useful when you are 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. Runs in verbose mode. Specifies that all packets
should be authenticated using the key number provided.
Specifies an authentication processing delay, in seconds
(see xntpd(8) for details). This number is usually small
enough to be negligible for ntpdate's purposes, though
specifying a value may improve timekeeping on very slow
CPU's. Specifies that authentication keys will be read
from keyfile instead of the default /etc/ntp.keys file.
This file should be in the format described in xntpd(8).
Forces ntpdate to poll as a version implementation. By
default ntpdate claims to be an NTP version 3 implementation
in its outgoing packets. Some older software will
decline to respond to version 3 queries. Acquires a specified
number of samples from each server. The range of
values for samples is from 1 and 8, inclusive. The
default is 4. Waits timeout seconds for a response. Any
value entered will be rounded to a multiple of 0.2 seconds.
The default is 1 second, a value suitable for
polling across a LAN.
The ntpdate command sets the local date and time by
polling the Network Time Protocol server(s) on the host(s)
given as 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 the standard
NTP clock filter and selection algorithms are applied
to select the best of these. The ntpdate command is run
from /sbin/init.d/settime to set the time of day at boot
time, if NTP is configured. (See Network Administration:
Services for information on configuring NTP.) Note that
ntpdate's reliability and precision will improve dramatically
with greater numbers of servers. While a single
server may be used, better performance and integrity will
be obtained by providing at least three or four servers,
if not more.
Time adjustments are made by ntpdate in one of the following
ways: If ntpdate determines your clock is off by more
than 0.5 seconds, it steps the time by calling settimeofday(2). If the error is less than 0.5 seconds, however,
it will by default slew the clock's time by a call to adjtime(2) with the offset.
The latter technique is less disruptive and more accurate
when the offset is small, and works quite well when ntpdate
is run by cron every hour or two. The adjustment made
in the latter case is actually 50% larger than the measured
offset since this will tend to keep a badly drifting
clock more accurate (at some expense to stability, though
this tradeoff is usually advantageous).
Ntpdate will decline to set the date if an NTP server daemon
(for example, xntpd(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.
Note
Because of significant changes in NTP version 3, you
should check all scripts that use the ntpdate command for
correct usage and output.
A common problem is polling a server using the wrong query
version number or wrong authentication key. If either
occurs, ntpdate prints the following error message: 18 Apr
10:20:28 ntpdate(1192]: no server
suitable for synchronization found
At boot time, if NTP is not configured, the ntpdate prints
the following message: WARNING: ntpdate cannot succeed,
please check your
NTP configuration
The following command line sets the date and time after
polling server host1.dec.com as a version 2 implementation:
/usr/sbin/ntpdate -o 2 host1.dec.com The following
command line sets the date and time after polling server
host2.dec.com. All packets are authenticated using
authentication key 1. /usr/sbin/ntpdate -a 1
host2.dec.com
Specifies the command path Contains the encryption keys
used by ntpdate
Commands: ntpq(8), xntpd(8), xntpdc(8)
Files: ntp.conf(4)
ntpdate(8)
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