NAME
ntpdate
- set the date and time via NTP
SYNOPSIS
ntpdate
[-bBdoqsuv]
[-a key]
[-e authdelay]
[-k keyfile]
[-o version]
[-p samples]
[-t timeout]
[server ...]
DESCRIPTION
ntpdate
sets the local date and time by polling the Network Time
Protocol (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 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(2)
routine. If the error is less than 0.5 seconds, it will slew the time by
calling the system
adjtime(2)
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.
If NetInfo support is compiled into
,
then the
server
argument 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
.
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. During
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 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.
- -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 outgoing 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 inclusive. 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 second, a value suitable for polling across a LAN.
- -u
-
Direct
ntpdate
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
-d
option always uses unprivileged ports.
- -v
-
Be verbose. This option will cause
ntpdate
string to be logged.
FILES
/etc/ntp.keys
-
encryption keys used by
.
AUTHORS
David L. Mills (mills@udel.edu)
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
.