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
ntpdate, 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 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. 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 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 ntpdate.
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.