ntpdate - set the date and time via NTP
ntpdate [-bBdoqsuv] [-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 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.
-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 +-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 CPUs. -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 - dont 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 ntpdates version identification string to be logged.
ntpdates exit status is zero if it found a server and could update 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 (email@example.com)
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