ntp/doc/ntpdate.8

.\"
.\" $FreeBSD$
.\"
.Dd May 17, 2006
.Dt NTPDATE 8
.Os
.Sh NAME
.Nm ntpdate
.Nd set the date and time via NTP
.Sh SYNOPSIS
.Nm
.Op Fl 46bBdoqsuv
.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 ...
.Sh DESCRIPTION
.Em Note :
The functionality of this program is now available
in the
.Xr ntpd 8
program.
See the
.Fl q
command line
option in the
.Xr ntpd 8
page.
After a suitable period of
mourning, the
.Nm
utility is to be retired from this
distribution.
.Pp
The
.Nm
utility 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
The following options are available:
.Bl -tag -width indent
.It Fl 4
Force DNS resolution of following host names on the command line to the
IPv4 namespace.
.It Fl 6
Force DNS resolution of following host names on the command line to the
IPv6 namespace.
.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 - do not 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 Ar 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
.Pp
The
.Nm
utility 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
.Xr cron 8
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
The
.Nm
utility 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
Note that in contexts where a host name is expected, a
.Fl 4
qualifier preceding the host name forces DNS resolution to the
IPv4 namespace, while a
.Fl 6
qualifier forces DNS resolution to the IPv6 namespace.
.Pp
If NetInfo support is compiled into
.Nm ,
then the
.Cm server
argument is optional if
.Nm
can find a
time server in the NetInfo configuration for
.Xr ntpd 8 .
.Sh FILES
.Bl -tag -width /etc/ntp.keys -compact
.It Pa /etc/ntp.keys
contains the encryption keys used by
.Nm .
.El
.Sh SEE ALSO
.Xr ntpd 8
.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 kern.clockrate.tick
and
.Va kern.clockrate.tickadj .