NetBSD/usr.sbin/ntp/ntpdate/ntpdate.8

.\"	$NetBSD: ntpdate.8,v 1.6 2002/02/02 01:25:25 wiz Exp $
.\" Converted from HTML to mandoc by ntp-html2mdoc.pl
.\"
.Dd March 29, 2000
.Dt NTPDATE 8
.Os
.Sh NAME
.Nm ntpdate
.Nd set the date and time via NTP
.Sh SYNOPSIS
.Nm
.Op Fl bBdoqsuv
.Op Fl a Ar key
.Op Fl e Ar authdelay
.Op Fl k Ar keyfile
.Op Fl o Ar version
.Op Fl p Ar samples
.Op Fl t Ar timeout
.Op Ar server ...
.Sh DESCRIPTION
.Nm
sets the local date and time by polling the Network Time
Protocol (NTP) server(s) given as the
.Ar server
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
.Nm
depends on the number of servers, the number of polls each time it is run
and the interval between runs.
.Pp
.Nm
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
.Pa ntpd .
It is also possible to run
.Nm
from a
.Pa cron
script. However, it is important to note that
.Nm
with contrived
.Pa cron
scripts is no substitute for the NTP daemon,
which uses sophisticated algorithms to maximize accuracy and reliability
while minimizing resource use. Finally, since
.Nm
does not
discipline the host clock frequency as does
.Pa ntpd ,
the accuracy
using
.Nm
is limited.
.Pp
Time adjustments are made by
.Nm
in one of two ways. If
.Nm
determines the clock is in error more than 0.5 second
it will simply step the time by calling the system
.Xr settimeofday 2
routine. If the error is less than 0.5 seconds, it will slew the time by
calling the system
.Xr adjtime 2
routine. The latter technique is
less disruptive and more accurate when the error is small, and works quite
well when
.Nm
is run by
.Pa cron
every hour or two.
.Pp
.Nm
will decline to set the date if an NTP server daemon
(e.g.,
.Pa ntpd
) is running on the same host. When running
.Nm
on a regular basis from
.Pa cron
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.
.Pp
If NetInfo support is compiled into
.Nm "" ,
then the
.Pa server
argument is optional if
.Nm
can find a time
server in the NetInfo configuration for
.Pa ntpd
.
.Sh COMMAND LINE OPTIONS
.Bl -tag -width indent
.It Fl a Ar key
Enable the authentication function and specify the key identifier to be
used for authentication as the argument
.Ar key
.Nm "" .
The
keys and key identifiers must match in both the client and server key files.
The default is to disable the authentication function.
.It Fl B
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.
.It Fl b
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.
.It Fl d
Enable the debugging mode, in which
.Nm
will go through all
the steps, but not adjust the local clock. Information useful for general
debugging will also be printed.
.It Fl e Ar authdelay
Specify the processing delay to perform an authentication function as the
value
.Ar authdelay
, in seconds and fraction (see
.Pa ntpd
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.
.It Fl k Ar keyfile
Specify the path for the authentication key file as the string
.Ar keyfile
.
The default is
.Pa /etc/ntp.keys .
This file should be in the format
described in
.Pa ntpd
.
.It Fl o Ar version
Specify the NTP version for outgoint packets as the integer
.Ar version
,
which can be 1 or 2. The default is 3. This allows
.Nm
to
be used with older NTP versions.
.It Fl p Ar samples
Specify the number of samples to be acquired from each server as the integer
.Ar samples
, with values from 1 to 8 inclusive. The default is 4.
.It Fl q
Query only - don't set the clock.
.It Fl s
Divert logging output from the standard output (default) to the system
.Pa syslog
facility. This is designed primarily for convenience of
.Pa cron
scripts.
.It Fl t Ar timeout
Specify the maximum time waiting for a server response as the value
.Ar timeout
,
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.
.It Fl u
Direct
.Nm
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
.Fl d
option always uses unprivileged ports.
.It Fl v
Be verbose. This option will cause
.Nm
's version identification
string to be logged.
.El
.Sh FILES
.Bl -tag -width /etc/ntp.keys -compact
.It Pa /etc/ntp.keys
encryption keys used by
.Nm "" .
.El
.Sh AUTHORS
David L. Mills (mills@udel.edu)
.Sh BUGS
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
.Pa tick
and
.Pa tickadj .