ntpdate - set the date and time via NTP
ntpdate [-46bBdqsuv] [-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 (unless
the option -q is used). 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, if the -u flag was not specified, 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.
Note that in contexts where a host name is expected, a -4
qualifier preceding the host name forces DNS resolution to the IPv4
namespace, while a -6 qualifier forces DNS resolution to the IPv6
namespace.
- -4
- Force DNS resolution of following host names on the command line to the
IPv4 namespace.
- -6
- Force DNS resolution of following host names on the command line to the
IPv6 namespace.
- -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.
- -B
- Force the time to always be slewed using the adjtime() system call, even
if the measured offset is greater than +-500 ms. The default is to step
the time using settimeofday() if the offset is greater than +-500 ms. Note
that, if the offset is much greater than +-500 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 and using an unprivileged port. 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, 2, 3 or 4. The default is 4. 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 2 seconds, 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's version identification string
to be logged.
ntpdate's exit status is zero if it found a server and
updates the clock, and nonzero otherwise.
- /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.
David L. Mills (mills@udel.edu)
This manpage converted from html to roff by Fabrizio Polacco
<fpolacco@debian.org>