NtpDate (Linux command) to set Date, Time via NTP


	[ -bBdoqsuv ] 

	[ -a key ] 

	[ -e authdelay ] 

	[ -k keyfile ] 

	[ -o version ] 

	[ -p samples ] 

	[ -t timeout ] server [ ... ]

Description of NtpDate for servers

NtpDate configures the local date and time by polling the NTP server(s) given as the server arguments to determine the accurate and correct time. 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. Best results are obtained with Stratum 0 server placed on LAN. This server provides atomic accurate time from satellites around the globe.

NtpDate can be run manually as necessary to set the server's clock, or it can be run from the host startup script to set the clock at boot time (which is very easy to implement). This is useful in some cases to set the clock initially before starting the NTP daemon ntpd. It is also possible to run ntpdate at server 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 on host.

NtpDate servers selection

NtpDate server selection

Time corrections and adjustments are made by NtpDate in one of two ways. If NtpDate servers determine the ntp 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. When running NTP Daemon without NtpDate servers, the time synchronisation is constant and without disruptions (if using a stratum-0 server)

NtpDate fails to set the date if a real NTP server daemon is running on the same server. When running NtpDate on a regular basis from cron on servers 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.

Command Line Options

Option NtpDate functionality
-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 +-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 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 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.
-v Be verbose. This option will cause ntpdate’s version identification string to be logged.

Encryption keys used by ntpdate to synchronise with ntpdate servers: /etc/ntp.keys