2001-08-29 14:35:15 +00:00
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<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>
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<img align="left" src="pic/rabbit.gif" alt="gif"><a href=
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"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Alice's
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Adventures in Wonderland</i>, Lewis Carroll</a>
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<p>I told you it was eyeball and wristwatch.<br clear="left">
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</p>
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<hr>
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<p>Disclaimer: The functionality of this program is now available
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in the <tt>ntpd</tt> program. See the <tt>-q</tt> command line
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option in the <a href="ntpd.htm"><tt>ntpd</tt> - Network Time
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Protocol (NTP) daemon</a> page. After a suitable period of
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mourning, the <tt>ntpdate</tt> program is to be retired from this
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distribution</p>
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<h4>Synopsis</h4>
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<tt>ntpdate [ -bBdoqsuv ] [ -a <i>key</i> ] [ -e <i>authdelay</i> ]
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[ -k <i>keyfile</i> ] [ -o <i>version</i> ] [ -p <i>samples</i> ] [
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-t <i>timeout</i> ] <i>server</i> [ ... ]</tt>
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<h4>Description</h4>
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<tt>ntpdate</tt> sets the local date and time by polling the
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Network Time Protocol (NTP) server(s) given as the <i>server</i>
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arguments to determine the correct time. It must be run as root on
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the local host. A number of samples are obtained from each of the
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servers specified and a subset of the NTP clock filter and
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selection algorithms are applied to select the best of these. Note
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that the accuracy and reliability of <tt>ntpdate</tt> depends on
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the number of servers, the number of polls each time it is run and
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the interval between runs.
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<p><tt>ntpdate</tt> can be run manually as necessary to set the
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host clock, or it can be run from the host startup script to set
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the clock at boot time. This is useful in some cases to set the
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clock initially before starting the NTP daemon <tt>ntpd</tt>. It is
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also possible to run <tt>ntpdate</tt> from a <tt>cron</tt> script.
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However, it is important to note that <tt>ntpdate</tt> with
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contrived <tt>cron</tt> scripts is no substitute for the NTP
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daemon, which uses sophisticated algorithms to maximize accuracy
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and reliability while minimizing resource use. Finally, since <tt>
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ntpdate</tt> does not discipline the host clock frequency as does
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<tt>ntpd</tt>, the accuracy using <tt>ntpdate</tt> is limited.</p>
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<p>Time adjustments are made by <tt>ntpdate</tt> in one of two
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ways. If <tt>ntpdate</tt> determines the clock is in error more
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than 0.5 second it will simply step the time by calling the system
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<tt>settimeofday()</tt> routine. If the error is less than 0.5
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seconds, it will slew the time by calling the system <tt>
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adjtime()</tt> routine. The latter technique is less disruptive and
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more accurate when the error is small, and works quite well when
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<tt>ntpdate</tt> is run by <tt>cron</tt> every hour or two.</p>
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<p><tt>ntpdate</tt> will decline to set the date if an NTP server
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daemon (e.g., <tt>ntpd</tt>) is running on the same host. When
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running <tt>ntpdate</tt> on a regular basis from <tt>cron</tt> as
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an alternative to running a daemon, doing so once every hour or two
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will result in precise enough timekeeping to avoid stepping the
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clock.</p>
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<p>If NetInfo support is compiled into <tt>ntpdate</tt>, then the
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<tt>server</tt> argument is optional if <tt>ntpdate</tt> can find a
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time server in the NetInfo configuration for <tt>ntpd</tt>.</p>
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<h4>Command Line Options</h4>
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<dl>
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<dt><tt>-a <i>key</i></tt></dt>
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<dd>Enable the authentication function and specify the key
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identifier to be used for authentication as the argument <i>
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key</i><tt>ntpdate</tt>. The keys and key identifiers must match in
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both the client and server key files. The default is to disable the
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authentication function.</dd>
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<dt><tt>-B</tt></dt>
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<dd>Force the time to always be slewed using the adjtime() system
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call, even if the measured offset is greater than +-128 ms. The
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default is to step the time using settimeofday() if the offset is
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greater than +-128 ms. Note that, if the offset is much greater
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than +-128 ms in this case, that it can take a long time (hours) to
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slew the clock to the correct value. During this time. the host
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should not be used to synchronize clients.</dd>
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<dt><tt>-b</tt></dt>
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<dd>Force the time to be stepped using the settimeofday() system
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call, rather than slewed (default) using the adjtime() system call.
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This option should be used when called from a startup file at boot
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time.</dd>
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<dt><tt>-d</tt></dt>
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<dd>Enable the debugging mode, in which <tt>ntpdate</tt> will go
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through all the steps, but not adjust the local clock. Information
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useful for general debugging will also be printed.</dd>
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<dt><tt>-e <i>authdelay</i></tt></dt>
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<dd>Specify the processing delay to perform an authentication
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function as the value <i>authdelay</i>, in seconds and fraction
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(see <tt>ntpd</tt> for details). This number is usually small
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enough to be negligible for most purposes, though specifying a
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value may improve timekeeping on very slow CPU's.</dd>
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<dt><tt>-k <i>keyfile</i></tt></dt>
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<dd>Specify the path for the authentication key file as the string
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<i>keyfile</i>. The default is <tt>/etc/ntp.keys</tt>. This file
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should be in the format described in <tt>ntpd</tt>.</dd>
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<dt><tt>-o <i>version</i></tt></dt>
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<dd>Specify the NTP version for outgoint packets as the integer <i>
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version</i>, which can be 1 or 2. The default is 3. This allows
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<tt>ntpdate</tt> to be used with older NTP versions.</dd>
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<dt><tt>-p <i>samples</i></tt></dt>
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<dd>Specify the number of samples to be acquired from each server
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as the integer <i>samples</i>, with values from 1 to 8 inclusive.
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The default is 4.</dd>
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<dt><i><tt>-q</tt></i></dt>
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<dd>Query only - don't set the clock.</dd>
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<dt><tt>-s</tt></dt>
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<dd>Divert logging output from the standard output (default) to the
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system <tt>syslog</tt> facility. This is designed primarily for
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convenience of <tt>cron</tt> scripts.</dd>
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<dt><tt>-t <i>timeout</i></tt></dt>
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<dd>Specify the maximum time waiting for a server response as the
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value <i>timeout</i>, in seconds and fraction. The value is is
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rounded to a multiple of 0.2 seconds. The default is 1 second, a
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value suitable for polling across a LAN.</dd>
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<dt><tt>-u</tt></dt>
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<dd>Direct <tt>ntpdate</tt> to use an unprivileged port or outgoing
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packets. This is most useful when behind a firewall that blocks
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incoming traffic to privileged ports, and you want to synchronise
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with hosts beyond the firewall. Note that the <tt>-d</tt> option
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always uses unprivileged ports.</dd>
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<dt><tt>-<i>v</i></tt></dt>
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<dd>Be verbose. This option will cause <tt>ntpdate</tt>'s version
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identification string to be logged.</dd>
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</dl>
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<h4>Files</h4>
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<tt>/etc/ntp.keys</tt> - encryption keys used by <tt>ntpdate</tt>.
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<h4>Bugs</h4>
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The slew adjustment is actually 50% larger than the measured
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offset, since this (it is argued) will tend to keep a badly
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drifting clock more accurate. This is probably not a good idea and
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may cause a troubling hunt for some values of the kernel variables
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<tt>tick</tt> and <tt>tickadj</tt>.
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<hr>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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"gif"></a>
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<address><a href="mailto:mills@udel.edu">David L. Mills
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<mills@udel.edu></a></address>
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</body>
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</html>
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