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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<meta name="generator" content="HTML Tidy, see www.w3.org">
<title>ntpdate - set the date and time via NTP</title>
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<h3><tt>ntpdate</tt> - set the date and time via NTP</h3>
<img align="left" src="pic/rabbit.gif" alt="gif"><a href=
"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Alice's
Adventures in Wonderland</i>, Lewis Carroll</a>
<p>I told you it was eyeball and wristwatch.<br clear="left">
</p>
<hr>
<p>Disclaimer: The functionality of this program is now available
in the <tt>ntpd</tt> program. See the <tt>-q</tt> command line
option in the <a href="ntpd.htm"><tt>ntpd</tt> - Network Time
Protocol (NTP) daemon</a> page. After a suitable period of
mourning, the <tt>ntpdate</tt> program is to be retired from this
distribution</p>
<h4>Synopsis</h4>
<tt>ntpdate [ -bBdoqsuv ] [ -a <i>key</i> ] [ -e <i>authdelay</i> ]
[ -k <i>keyfile</i> ] [ -o <i>version</i> ] [ -p <i>samples</i> ] [
-t <i>timeout</i> ] <i>server</i> [ ... ]</tt>
<h4>Description</h4>
<tt>ntpdate</tt> sets the local date and time by polling the
Network Time Protocol (NTP) server(s) given as the <i>server</i>
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 <tt>ntpdate</tt> depends on
the number of servers, the number of polls each time it is run and
the interval between runs.
<p><tt>ntpdate</tt> 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 <tt>ntpd</tt>. It is
also possible to run <tt>ntpdate</tt> from a <tt>cron</tt> script.
However, it is important to note that <tt>ntpdate</tt> with
contrived <tt>cron</tt> scripts is no substitute for the NTP
daemon, which uses sophisticated algorithms to maximize accuracy
and reliability while minimizing resource use. Finally, since <tt>
ntpdate</tt> does not discipline the host clock frequency as does
<tt>ntpd</tt>, the accuracy using <tt>ntpdate</tt> is limited.</p>
<p>Time adjustments are made by <tt>ntpdate</tt> in one of two
ways. If <tt>ntpdate</tt> determines the clock is in error more
than 0.5 second it will simply step the time by calling the system
<tt>settimeofday()</tt> routine. If the error is less than 0.5
seconds, it will slew the time by calling the system <tt>
adjtime()</tt> routine. The latter technique is less disruptive and
more accurate when the error is small, and works quite well when
<tt>ntpdate</tt> is run by <tt>cron</tt> every hour or two.</p>
<p><tt>ntpdate</tt> will decline to set the date if an NTP server
daemon (e.g., <tt>ntpd</tt>) is running on the same host. When
running <tt>ntpdate</tt> on a regular basis from <tt>cron</tt> 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.</p>
<p>If NetInfo support is compiled into <tt>ntpdate</tt>, then the
<tt>server</tt> argument is optional if <tt>ntpdate</tt> can find a
time server in the NetInfo configuration for <tt>ntpd</tt>.</p>
<h4>Command Line Options</h4>
<dl>
<dt><tt>-a <i>key</i></tt></dt>
<dd>Enable the authentication function and specify the key
identifier to be used for authentication as the argument <i>
key</i><tt>ntpdate</tt>. The keys and key identifiers must match in
both the client and server key files. The default is to disable the
authentication function.</dd>
<dt><tt>-B</tt></dt>
<dd>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.</dd>
<dt><tt>-b</tt></dt>
<dd>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.</dd>
<dt><tt>-d</tt></dt>
<dd>Enable the debugging mode, in which <tt>ntpdate</tt> will go
through all the steps, but not adjust the local clock. Information
useful for general debugging will also be printed.</dd>
<dt><tt>-e <i>authdelay</i></tt></dt>
<dd>Specify the processing delay to perform an authentication
function as the value <i>authdelay</i>, in seconds and fraction
(see <tt>ntpd</tt> 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.</dd>
<dt><tt>-k <i>keyfile</i></tt></dt>
<dd>Specify the path for the authentication key file as the string
<i>keyfile</i>. The default is <tt>/etc/ntp.keys</tt>. This file
should be in the format described in <tt>ntpd</tt>.</dd>
<dt><tt>-o <i>version</i></tt></dt>
<dd>Specify the NTP version for outgoint packets as the integer <i>
version</i>, which can be 1 or 2. The default is 3. This allows
<tt>ntpdate</tt> to be used with older NTP versions.</dd>
<dt><tt>-p <i>samples</i></tt></dt>
<dd>Specify the number of samples to be acquired from each server
as the integer <i>samples</i>, with values from 1 to 8 inclusive.
The default is 4.</dd>
<dt><i><tt>-q</tt></i></dt>
<dd>Query only - don't set the clock.</dd>
<dt><tt>-s</tt></dt>
<dd>Divert logging output from the standard output (default) to the
system <tt>syslog</tt> facility. This is designed primarily for
convenience of <tt>cron</tt> scripts.</dd>
<dt><tt>-t <i>timeout</i></tt></dt>
<dd>Specify the maximum time waiting for a server response as the
value <i>timeout</i>, 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.</dd>
<dt><tt>-u</tt></dt>
<dd>Direct <tt>ntpdate</tt> 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 <tt>-d</tt> option
always uses unprivileged ports.</dd>
<dt><tt>-<i>v</i></tt></dt>
<dd>Be verbose. This option will cause <tt>ntpdate</tt>'s version
identification string to be logged.</dd>
</dl>
<h4>Files</h4>
<tt>/etc/ntp.keys</tt> - encryption keys used by <tt>ntpdate</tt>.
<h4>Bugs</h4>
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
<tt>tick</tt> and <tt>tickadj</tt>.
<hr>
<a href="index.htm"><img align="left" src="pic/home.gif" alt=
"gif"></a>
<address><a href="mailto:mills@udel.edu">David L. Mills
<mills@udel.edu></a></address>
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