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ntpdate(1) ntpdate(1)
NAME
ntpdate - set the date and time via NTP
Disclaimer: The functionality of this program is now available in the
ntpd program. See the -q command line option in the ntpd - Network
Time Protocol (NTP) daemon page. After a suitable period of mourning,
the ntpdate program is to be retired from this distribution
SYNOPSIS
ntpdate [ -bBdoqsuv ] [ -a key ] [ -e authdelay ] [ -k keyfile ] [
-o version ] [ -p samples ] [ -t timeout ] [ -U user_name ] server
[ ... ]
DESCRIPTION
ntpdate sets the local date and time by polling the Network Time Pro-
tocol (NTP) server(s) given as the 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 start-
ing 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 disci-
pline the host clock frequency as does ntpd , the accuracy using ntp-
date 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 ntp-
date 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.
If NetInfo support is compiled into ntpdate , then the server argu-
ment is optional if ntpdate can find a time server in the NetInfo
configuration for ntpd .
COMMAND LINE OPTIONS
-a key Enable the authentication function and specify the key
identifier to be used for authentication as the argument key
ntpdate . The keys and key identifiers must match in both the
client and server key files. The default is to disable the
authentication function.
-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. Dur-
ing this time. the host should not be used to synchronize
clients.
-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.
-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.
-e authdelay Specify the processing delay to perform an authenti-
cation function as the value authdelay , in seconds and frac-
tion (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.
-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 .
-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.
-p samples Specify the number of samples to be acquired from each
server as the integer samples , with values from 1 to 8 inclu-
sive. The default is 4.
-q Query only - don't set the clock.
-s Divert logging output from the standard output (default) to
the system syslog facility. This is designed primarily for
convenience of cron scripts.
-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 sec-
ond, a value suitable for polling across a LAN.
-u Direct ntpdate to use an unprivileged port or outgoing pack-
ets. This is most useful when behind a firewall that blocks
incoming traffic to privileged ports, and you want to synchro-
nise with hosts beyond the firewall. Note that the
-U user_name ntpdate process drops root privileges and changes
user ID to user_name and group ID to the primary group of
server_user.
-d always uses unprivileged ports.
-v Be verbose. This option will cause ntpdate 's version identi-
fication string to be logged.
FILES
/etc/ntp/keys - encryption keys used by ntpdate .
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 tick-
adj .
SEE ALSO
Primary source of documentation: /usr/share/doc/ntp-*/ntpdate.html
AUTHOR
David L. Mills
ntp 4.1.1b-r5 ntpdate(1)
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