freebsd-dev/usr.sbin/ntp/doc/ntpdate.8

.\"
.\" $FreeBSD$
.\"
.Dd January 6, 2000
.Dt NTPDATE 8
.Os
.Sh NAME
.Nm ntpdate
.Nd set the date and time via NTP
.Sh SYNOPSIS
.Nm
.Op Fl bBdqsuv
.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
.Ar server
.Op Ar ...
.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
.Xr ntpd 8 .
It is also possible to run
.Nm
from a
.Xr cron 8
script.
However, it is important to note that
.Nm
with contrived 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
.Xr ntpd 8 ,
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
.Xr cron 8
every hour or two.
.Pp
.Nm
will decline to set the date if an NTP server daemon
(e.g.,
.Xr ntpd 8 )
is running on the same host.
When running
.Nm
on a regular basis from
.Xr cron 8
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 Ns ,
then the server argument is optional if
.Nm
can find a time server in the NetInfo configuration for
.Xr ntpd 8 .
.Pp
The following options are available:
.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 .
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
.Xr adjtime 2
system call,
even if the measured offset is greater than +-128 ms.
The default is to step the time using
.Xr settimeofday 2
if the offset is greater than +-128 ms.
Note that,
if the offset is much greater than +-128 ms in this case,
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
.Xr settimeofday 2
system call,
rather than slewed (default) using the
.Xr adjtime 2
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
.Xr ntpd 8
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
.Xr ntpd 8 .
.It Fl o Ar version
Specify the NTP version for outgoing 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
.Xr syslog 3
facility.
This is designed primarily for convenience of
.Xr cron 8
scripts.
.It Fl t timeout
Specify the maximum time waiting for a server response
as the value
.Ar timeout ,
in seconds and fraction.
The value 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 for 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 Ns 's
version identification string to be logged.
.El
.Sh FILES
.Bl -tag -width /etc/ntp.keys -compact
.It Pa /etc/ntp.keys
contains the encryption keys used by
.Nm Ns .
.El
.Sh SEE ALSO
.Xr ntpd 8
.Sh HISTORY
Written by
.An Dennis Ferguson
at the University of Toronto
.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
.Va tick
and
.Va tickadj .