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: http://www.sai.msu.su/~er/xntp/ntpdate.html
Дата изменения: Unknown Дата индексирования: Sat Dec 22 01:48:07 2007 Кодировка: Поисковые слова: jupiter |
ntpdate
- set the date and time via NTP
ntpdate [ -bBdosu ] [ -a key ] [ -e authdelay ] [
-k keyfile ] [ -o version ] [ -p samples ] [ -t
timeout ] server [ ... ]
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 xntpd
. 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 xntpd
, 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., xntpd
) 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.
-a 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
-b
-d
ntpdate
will go
through all the steps, but not adjust the local clock. Information
useful for general debugging will also be printed.
-e authdelay
xntpd
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
/etc/ntp.keys
. This file
should be in the format described in xntpd
.
-o version
ntpdate
to be used with older NTP versions.
-p samples
-s
syslog
facility. This is designed primarily for
convenience of cron
scripts.
-t timeout
-u
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.
/etc/ntp.keys
- encryption keys used by
ntpdate
.
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
.