190 lines
4.8 KiB
Groff
190 lines
4.8 KiB
Groff
''' $Header
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'''
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.de Sh
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.br
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.ne 5
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..
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.de Sp
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.IP "\\$1" \\$2
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..
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'''
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''' Set up \*(-- to give an unbreakable dash;
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''' string Tr holds user defined translation string.
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''' Greek uppercase omega is used as a dummy character.
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'''
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.tr \(*W-|\(bv\*(Tr
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.ie n \{\
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.ds -- \(*W-
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.if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
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.if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\" diablo 12 pitch
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'br\}
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.el\{\
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.ds L" ``
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.ds L' `
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'br\}
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.TH NTPDATE 8 LOCAL
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.SH NAME
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ntpdate - set the date and time via NTP
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.SH SYNOPSIS
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.B ntpdate
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[
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.B -bds
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] [
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.B -o
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.I version
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] [
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.B -a
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.I key#
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] [
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.B -e
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.I authdelay
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] [
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.B -k
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.I keyfile
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] [
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.B -p
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.I samples
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] [
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.B -t
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.I timeout
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]
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server ...
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.SH DESCRIPTION
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.I Ntpdate
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sets the local date and time by polling the Network Time Protocol
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server(s) on the host(s) given as arguments to determine
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the correct time. It must be run as root on the local host. A number
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of samples are obtained from each of the servers specified and the
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standard NTP clock filter and selection algorithms are applied to select
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the best of these. Typically,
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.I ntpdate
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can be inserted in the
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.I /etc/rc.local
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startup up script to set the time of day at boot time and/or can be run
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from time\-to\-time via
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.IR cron (8).
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Note that
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.IR ntpdate 's
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reliability and precision will improve dramatically with greater numbers
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of servers. While a single server may be used, better performance and
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greater resistance to insanity on the part of any one server
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will be obtained by providing at least three or four servers, if not more.
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.PP
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Time adjustments are made by
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.I ntpdate
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in one of two ways. If
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.I ntpdate
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determines your clock is off by more than 0.5 seconds it will simply
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step the time by calling
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.IR settimeofday (2).
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If the error is less than 0.5 seconds, however, it will by default slew
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the clock's time via a call to
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.IR adjtime (2)
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with the offset. The latter technique is less disruptive and more
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accurate when the offset is small, and works quite well when
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.I ntpdate
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is run by
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.I cron (8)
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every hour or two. The adjustment made in the latter
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case is actually 50% larger than the measured offset since this will
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tend to keep a badly drifting clock more accurate (at some expense to
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stability, though this tradeoff is usually advantageous). At boot time,
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however, it is usually better to always step the time. This can be forced
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in all cases by specifying the
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.B -b
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switch on the command line. The
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.B -s
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switch tells
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.I ntpdate
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to log its actions via the
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.IR syslog (3)
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facility rather than to the standard output, a useful option when
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running the program from
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.IR cron (8).
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.PP
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The
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.B -d
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flag may be used to determine what
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.I ntpdate
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will do without it actually doing it. Information useful for general
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debugging will also be printed. By default
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.I ntpdate
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claims to be an NTP version 3 implementation in its outgoing packets. As
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some older software will decline to respond to version 3 queries, the
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.B -o
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.I version
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switch can be used to force the program to poll as a version 2 or 1
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implementation instead.
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.PP
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The number of samples
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.I ntpdate
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acquires from each server can be set to between 1 and 8 inclusive
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using the
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.B -p
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switch. The default is 4. The time it will spend waiting for a
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response can be set using the
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.B -t
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switch, and will be rounded to a multiple of 0.2 seconds. The default
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is 1 second, a value suitable for polling across a LAN.
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.PP
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.I Ntpdate
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will authenticate its transactions if need be. The
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.B -a
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switch specifies that all packets should be authenticated using the
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key number indicated. The
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.B -k
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switch allows the name of the file from which the keys may be read
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to be modified from the default of
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.I /etc/ntp.keys.
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This file should be in the format described in
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.IR xntpd (8).
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The
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.B -e
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option allows the specification of an authentication processing delay,
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in seconds (see
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.IR xntpd (8)
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for details). This number is usually small enough to be negligible for
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.IR ntpdate 's
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purposes, though specifying a value may improve timekeeping on very slow
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CPU's.
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.PP
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.I Ntpdate
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will decline to set the date if an NTP server daemon (e.g.
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.IR xntpd (8))
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is running on the same host. When running
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.I ntpdate
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on a regular basis from
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.IR cron (8)
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as 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 clock.
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.SH FILES
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.nf
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/etc/ntp.keys\0\0contains the encryption keys used by \fIntpdate\fP.
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.fi
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.SH SEE ALSO
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xntpd(8)
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.SH HISTORY
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Written by Dennis Ferguson at the University of Toronto
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.SH BUGS
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The technique used for improving accuracy by compensating for clock
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oscillator errors sucks, but doing better would require the program
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to save state from previous runs.
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