ntpdate(1) -- Linux man page
NAMEntpdate - 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
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() 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., 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.
COMMAND LINE OPTIONS
- 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.
- 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.
- 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.
- Divert logging output from the standard output (default) to the system syslog facility. This is designed primarily for convenience of cron scripts.
- Direct 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
- user_name ntpdate process drops root privileges and changes user ID to user_name and group ID to the primary group of server_user.
- always uses unprivileged ports.
- Be verbose. This option will cause ntpdate 's version identification string to be logged.
BUGSThe 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 .
SEE ALSOPrimary source of documentation: /usr/share/doc/ntp-*/ntpdate.php
AUTHORDavid L. Mills <firstname.lastname@example.org>