281 lines
6.2 KiB
Groff
281 lines
6.2 KiB
Groff
.\"
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.\" $FreeBSD$
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.\"
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.Dd May 17, 2006
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.Dt NTPDATE 8
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.Os
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.Sh NAME
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.Nm ntpdate
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.Nd set the date and time via NTP
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.Sh SYNOPSIS
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.Nm
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.Op Fl 46bBdoqsuv
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.Op Fl a Ar key
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.Op Fl e Ar authdelay
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.Op Fl k Ar keyfile
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.Op Fl o Ar version
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.Op Fl p Ar samples
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.Op Fl t Ar timeout
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.Ar server ...
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.Sh DESCRIPTION
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.Pp
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.Em Note :
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The functionality of this program is now available
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in the
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.Xr ntpd 8
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program.
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See the
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.Fl q
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command line
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option in the
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.Xr ntpd 8
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page.
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After a suitable period of
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mourning, the
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.Nm
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utility is to be retired from this
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distribution.
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.Pp
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The
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.Nm
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utility sets the local date and time by polling the
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Network Time Protocol (NTP) server(s) given as the
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.Ar server
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arguments to determine the correct time.
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It must be run as root on
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the local host.
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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.
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Note
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that the accuracy and reliability of
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.Nm
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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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.Pp
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The following options are available:
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.Bl -tag -width indent
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.It Fl 4
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Force DNS resolution of following host names on the command line to the
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IPv4 namespace.
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.It Fl 6
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Force DNS resolution of following host names on the command line to the
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IPv6 namespace.
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.It Fl a Ar key
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Enable the authentication function and specify the key
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identifier to be used for authentication as the argument
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.Ar key .
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The keys and key identifiers must match
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in both the client and server key files.
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The default is to disable
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the authentication function.
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.It Fl B
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Force the time to always be slewed using the
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.Xr adjtime 2
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system
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call, even if the measured offset is greater than +-128 ms.
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The
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default is to step the time using
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.Xr settimeofday 2
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if the offset is
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greater than +-128 ms.
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Note that, if the offset is much greater
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than +-128 ms in this case, it can take a long time (hours) to
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slew the clock to the correct value.
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During this time, the host
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should not be used to synchronize clients.
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.It Fl b
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Force the time to be stepped using the
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.Xr settimeofday 2
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system
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call, rather than slewed (default) using the
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.Xr adjtime 2
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system call.
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This option should be used when called from a startup file at boot
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time.
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.It Fl d
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Enable the debugging mode, in which
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.Nm
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will go
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through all the steps, but not adjust the local clock.
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Information
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useful for general debugging will also be printed.
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.It Fl e Ar authdelay
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Specify the processing delay to perform an authentication
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function as the value
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.Ar authdelay ,
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in seconds and fraction
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(see
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.Xr ntpd 8
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for details).
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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.
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.It Fl k Ar keyfile
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Specify the path for the authentication key file as the string
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.Ar keyfile .
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The default is
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.Pa /etc/ntp.keys .
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This file
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should be in the format described in
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.Xr ntpd 8 .
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.It Fl o Ar version
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Specify the NTP version for outgoing packets as the integer
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.Ar version ,
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which can be 1 or 2.
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The default is 3.
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This allows
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.Nm
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to be used with older NTP versions.
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.It Fl p Ar samples
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Specify the number of samples to be acquired from each server
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as the integer
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.Ar samples ,
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with values from 1 to 8 inclusive.
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The default is 4.
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.It Fl q
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Query only - do not set the clock.
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.It Fl s
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Divert logging output from the standard output (default) to the
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system
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.Xr syslog 3
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facility.
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This is designed primarily for
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convenience of
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.Xr cron 8
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scripts.
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.It Fl t Ar timeout
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Specify the maximum time waiting for a server response as the
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value
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.Ar timeout ,
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in seconds and fraction.
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The value is
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rounded to a multiple of 0.2 seconds.
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The default is 1 second, a
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value suitable for polling across a LAN.
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.It Fl u
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Direct
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.Nm
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to use an unprivileged port for outgoing
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packets.
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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.
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Note that the
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.Fl d
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option
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always uses unprivileged ports.
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.It Fl v
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Be verbose.
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This option will cause
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.Nm Ns 's
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version
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identification string to be logged.
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.El
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.Pp
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The
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.Nm
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utility 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.
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This is useful in some cases to set the
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clock initially before starting the NTP daemon
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.Xr ntpd 8 .
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It is
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also possible to run
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.Nm
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from a
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.Xr cron 8
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script.
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However, it is important to note that
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.Nm
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with
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contrived
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.Xr cron 8
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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.
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Finally, since
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.Nm
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does not discipline the host clock frequency as
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does
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.Xr ntpd 8 ,
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the accuracy using
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.Nm
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is
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limited.
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.Pp
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Time adjustments are made by
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.Nm
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in one of two
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ways.
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If
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.Nm
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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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.Xr settimeofday 2
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routine.
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If the error is less than 0.5
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seconds, it will slew the time by calling the system
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.Xr adjtime 2
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routine.
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The latter technique is less disruptive
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and more accurate when the error is small, and works quite well
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when
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.Nm
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is run by
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.Xr cron 8
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every hour or
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two.
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.Pp
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The
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.Nm
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utility will decline to set the date if an NTP server
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daemon (e.g.,
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.Xr ntpd 8 )
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is running on the same host.
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When
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running
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.Nm
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on a regular basis from
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.Xr cron 8
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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.
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.Pp
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Note that in contexts where a host name is expected, a
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.Fl 4
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qualifier preceding the host name forces DNS resolution to the
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IPv4 namespace, while a
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.Fl 6
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qualifier forces DNS resolution to the IPv6 namespace.
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.Pp
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If NetInfo support is compiled into
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.Nm ,
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then the
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.Cm server
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argument is optional if
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.Nm
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can find a
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time server in the NetInfo configuration for
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.Xr ntpd 8 .
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.Sh FILES
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.Bl -tag -width /etc/ntp.keys -compact
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.It Pa /etc/ntp.keys
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contains the encryption keys used by
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.Nm .
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.El
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.Sh SEE ALSO
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.Xr ntpd 8
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.Sh BUGS
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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.
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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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.Va kern.clockrate.tick
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and
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.Va kern.clockrate.tickadj .
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