186 lines
6.4 KiB
HTML
186 lines
6.4 KiB
HTML
<HTML>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="GENERATOR" CONTENT="Mozilla/4.01 [en] (Win95; I) [Netscape]">
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<TITLE>ntpdate - set the date and time via NTP
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</TITLE>
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</HEAD>
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<BODY>
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<H3>
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<TT>ntpdate</TT> - set the date and time via NTP</H3>
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<HR>
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<H4>
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Synopsis</H4>
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<TT>ntpdate [ -bBdoqsuv ] [ -a <I>key</I> ] [ -e <I>authdelay</I> ] [ -k
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<I>keyfile</I> ] [ -o <I>version</I> ] [ -p <I>samples</I> ] [ -t <I>timeout</I>
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] <I>server</I> [ ... ]</TT>
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<H4>
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Description</H4>
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<TT>ntpdate</TT> sets the local date and time by polling the Network Time
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Protocol (NTP) server(s) given as the <I>server</I> arguments to determine
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the correct time. It must be run as root on the local host. A number of
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samples are obtained from each of the servers specified and a subset of
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the NTP clock filter and selection algorithms are applied to select the
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best of these. Note that the accuracy and reliability of <TT>ntpdate</TT>
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depends on the number of servers, the number of polls each time it is run
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and the interval between runs.
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<P><TT>ntpdate</TT> can be run manually as necessary to set the host clock,
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or it can be run from the host startup script to set the clock at boot
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time. This is useful in some cases to set the clock initially before starting
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the NTP daemon <TT>ntpd</TT>. It is also possible to run <TT>ntpdate</TT>
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from a <TT>cron</TT> script. However, it is important to note that <TT>ntpdate</TT>
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with contrived <TT>cron</TT> scripts is no substitute for the NTP daemon,
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which uses sophisticated algorithms to maximize accuracy and reliability
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while minimizing resource use. Finally, since <TT>ntpdate</TT> does not
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discipline the host clock frequency as does <TT>ntpd</TT>, the accuracy
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using <TT>ntpdate</TT> is limited.
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<P>Time adjustments are made by <TT>ntpdate</TT> in one of two ways. If
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<TT>ntpdate</TT> determines the clock is in error more than 0.5 second
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it will simply step the time by calling the system <TT>settimeofday()</TT>
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routine. If the error is less than 0.5 seconds, it will slew the time by
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calling the system <TT>adjtime()</TT> routine. The latter technique is
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less disruptive and more accurate when the error is small, and works quite
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well when <TT>ntpdate</TT> is run by <TT>cron</TT> every hour or two.
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<P><TT>ntpdate</TT> will decline to set the date if an NTP server daemon
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(e.g., <TT>ntpd</TT>) is running on the same host. When running <TT>ntpdate</TT>
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on a regular basis from <TT>cron</TT> as an alternative to running a daemon,
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doing so once every hour or two will result in precise enough timekeeping
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to avoid stepping the clock.
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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 time
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server in the NetInfo configuration for <TT>ntpd</TT>.
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<H4>
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Command Line Options</H4>
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<DL>
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<DT>
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<TT>-a <I>key</I></TT></DT>
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<DD>
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Enable the authentication function and specify the key identifier to be
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used for authentication as the argument <I>key</I><TT>ntpdate</TT>. The
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keys and key identifiers must match in both the client and server key files.
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The default is to disable the authentication function.</DD>
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<DT>
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<TT>-B</TT></DT>
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<DD>
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Force the time to always be slewed using the adjtime() system call, even
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if the measured offset is greater than +-128 ms. The default is to step
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the time using settimeofday() if the offset is greater than +-128 ms. Note
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that, if the offset is much greater than +-128 ms in this case, that it
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can take a long time (hours) to slew the clock to the correct value. During
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this time. the host should not be used to synchronize clients.</DD>
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<DT>
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<TT>-b</TT></DT>
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<DD>
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Force the time to be stepped using the settimeofday() system call, rather
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than slewed (default) using the adjtime() system call. This option should
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be used when called from a startup file at boot time.</DD>
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<DT>
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<TT>-d</TT></DT>
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<DD>
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Enable the debugging mode, in which <TT>ntpdate</TT> will go through all
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the steps, but not adjust the local clock. Information useful for general
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debugging will also be printed.</DD>
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<DT>
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<TT>-e <I>authdelay</I></TT></DT>
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<DD>
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Specify the processing delay to perform an authentication function as the
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value <I>authdelay</I>, in seconds and fraction (see <TT>ntpd</TT> for
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details). This number is usually small enough to be negligible for most
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purposes, though specifying a value may improve timekeeping on very slow
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CPU's.</DD>
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<DT>
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<TT>-k <I>keyfile</I></TT></DT>
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<DD>
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Specify the path for the authentication key file as the string <I>keyfile</I>.
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The default is <TT>/etc/ntp.keys</TT>. This file should be in the format
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described in <TT>ntpd</TT>.</DD>
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<DT>
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<TT>-o <I>version</I></TT></DT>
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<DD>
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Specify the NTP version for outgoint packets as the integer <I>version</I>,
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which can be 1 or 2. The default is 3. This allows <TT>ntpdate</TT> to
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be used with older NTP versions.</DD>
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<DT>
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<TT>-p <I>samples</I></TT></DT>
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<DD>
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Specify the number of samples to be acquired from each server as the integer
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<I>samples</I>, with values from 1 to 8 inclusive. The default is 4.</DD>
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<DT>
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<I><TT>-q</TT></I></DT>
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<DD>
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Query only - don't set the clock.</DD>
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<DT>
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<TT>-s</TT></DT>
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<DD>
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Divert logging output from the standard output (default) to the system
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<TT>syslog</TT> facility. This is designed primarily for convenience of
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<TT>cron</TT> scripts.</DD>
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<DT>
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<TT>-t <I>timeout</I></TT></DT>
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<DD>
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Specify the maximum time waiting for a server response as the value <I>timeout</I>,
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in seconds and fraction. The value is is rounded to a multiple of 0.2 seconds.
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The default is 1 second, a value suitable for polling across a LAN.</DD>
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<DT>
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<TT>-u</TT></DT>
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<DD>
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Direct <TT>ntpdate</TT> to use an unprivileged port or outgoing packets.
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This is most useful when behind a firewall that blocks incoming traffic
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to privileged ports, and you want to synchronise with hosts beyond the
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firewall. Note that the <TT>-d</TT> option always uses unprivileged ports.</DD>
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<DT>
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<TT>-<I>v</I></TT></DT>
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<DD>
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Be verbose. This option will cause <TT>ntpdate</TT>'s version identification
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string to be logged.</DD>
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</DL>
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<H4>
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Files</H4>
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<TT>/etc/ntp.keys</TT> - encryption keys used by <TT>ntpdate</TT>.
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<H4>
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Bugs</H4>
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The slew adjustment is actually 50% larger than the measured offset, since
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this (it is argued) will tend to keep a badly drifting clock more accurate.
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This is probably not a good idea and may cause a troubling hunt for some
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values of the kernel variables <TT>tick</TT> and <TT>tickadj</TT>.
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<HR>
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<ADDRESS>
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David L. Mills (mills@udel.edu)</ADDRESS>
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</BODY>
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</HTML>
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