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.TH ntpdate 8 
.SH NAME
ntpdate \- set the date and time via NTP
.SH SYNOPSIS
.B ntpdate 
.RB [\| \-bBdoqsuv \|] 
.RB [\| \-a 
.IR key \|] 
.RB [\| \-e 
.IR authdelay \|] 
.RB [\| \-k 
.IR keyfile \|]
.RB [\| \-o
.IR version \|]
.RB [\| \-p
.IR samples \|]
.RB [\| \-U
.IR username
.RB [\| \-T
.IR chroot_dir \|] \|]
.RB [\| \-t
.IR timeout \|]
.IR server
.RB [\| ... \|]
.SH DESCRIPTION
.B ntpdate 
sets the local date and time by polling the Network Time
Protocol (NTP) server(s) given as the
.I 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
ntpdate depends on the number of servers, the number of polls each
time it is run and the interval between runs.

ntpdate 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 ntpd. It is also possible to run ntpdate from
a cron script. However, it is important to note that ntpdate 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 ntpdate does not discipline
the host clock frequency as does ntpd, the accuracy using ntpdate is
limited.

Time adjustments are made by ntpdate in one of two ways. If ntpdate
determines the clock is in error more than 0.5 second it will simply
step the time by calling the system settimeofday() routine. If the
error is less than 0.5 seconds, it will slew the time by calling the
system adjtime() routine. The latter technique is less disruptive and
more accurate when the error is small, and works quite well when
ntpdate is run by cron every hour or two.

ntpdate will decline to set the date if an NTP server daemon (e.g.,
ntpd) is running on the same host. When running ntpdate on a regular
basis from 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.
.SH OPTIONS
.TP
.BI \-a \ key
Enable the authentication function and specify the key
identifier to be used for authentication as the argument
keyntpdate. The keys and key identifiers must match in both the
client and server key files. The default is to disable the
authentication function.
.TP
.B \-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.
.TP
.B \-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.
.TP
.B \-d
Enable the debugging mode, in which ntpdate will go through all
the steps, but not adjust the local clock. Information useful
for general debugging will also be printed.
.TP
.BI \-e \ authdelay
Specify the processing delay to perform an authentication
function as the value authdelay, in seconds and fraction (see
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.
.TP
.BI \-k \ keyfile
Specify the path for the authentication key file as the string
keyfile. The default is /etc/ntp.keys. This file should be in
the format described in ntpd.
.TP
.BI \-o \ version
Specify the NTP version for outgoint packets as the integer
version, which can be 1 or 2. The default is 3. This allows
ntpdate to be used with older NTP versions.
.TP
.BI \-p \ samples
Specify the number of samples to be acquired from each server
as the integer samples, with values from 1 to 8 inclusive. The
default is 4.
.TP
.B \-q
Query only - don't set the clock.
.TP
.B \-s
Divert logging output from the standard output (default) to the
system syslog facility. This is designed primarily for
convenience of cron scripts.
.TP
.BI \-t \ timeout
Specify the maximum time waiting for a server response as the
value 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.
.TP
.B \-u
Direct ntpdate 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 -d option always uses
unprivileged ports.
.TP
.B \-v
Be verbose. This option will cause ntpdate's version
identification string to be logged.
.TP
.BI \-T \ chroot_dir
Chroot the ntpdate process into
.IR chroot_dir .
To use this option you have to copy all the files that ntpdate process needs into the
chroot directory.  This option takes effect only if the process also drops
root privileges (see \-U option).
Empty
.I chroot_dir
value disables chroot jailing.
By default, ntpdate chroots to
.IR /var/empty .
.TP
.BI \-U \ username
ntpdate process drops root privileges and changes user ID to
.I user_name
and group ID to the primary group of
.IR server_user .
Empty
.I user_name
value disables dropping privileges.
By default, ntpdate switches to pseudouser
.IR ntpd .
.SH FILES
.TP
.I /etc/ntp.keys 
\- encryption keys used by ntpdate.
.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 tick and
tickadj.
.SH AUTHOR
David L. Mills (mills@udel.edu)
.br
This manpage converted from html to roff by 
Fabrizio Polacco <fpolacco@debian.org>