22.15. Configure ntpdate Servers
The purpose of the
ntpdateservice is to set the clock during system boot. This can be used to ensure that the services started after
ntpdatewill have the correct time and will not observe a jump in the clock. The use of
ntpdateand the list of step-tickers is considered deprecated and so Red Hat Enterprise Linux 6 uses the
-goption to the
ntpdcommand by default and not
ntpdate. However, the
-goption only enables
ntpdto ignore the offset limit of 1000s and attempt to synchronize the time. It does not guarantee the time will be correct when other programs or services are started. Therefore the
ntpdateservice can be useful when
ntpdis disabled or if there are services which need to be started with the correct time and not observe a jump in the clock.
To check if the
ntpdateservice is enabled to run at system start, issue the following command:
chkconfig --list ntpdatentpdate 0:off 1:off 2:on 3:on 4:on 5:on 6:off
To enable the service to run at system start, issue the following command as
chkconfig ntpdate on
ntpdateservers, using a text editor running as root, edit
/etc/ntp/step-tickersto include one or more host names as follows:
clock1.example.com clock2.example.comThe number of servers listed is not very important as
ntpdatewill only use this to obtain the date information once when the system is starting. If you have an internal time server then use that host name for the first line. An additional host on the second line as a backup is sensible. The selection of backup servers and whether the second host is internal or external depends on your risk assessment. For example, what is the chance of any problem affecting the first server also affecting the second server? Would connectivity to an external server be more likely to be available than connectivity to internal servers in the event of a network failure disrupting access to the first server?
ntpdateservice has a file that must contain a list of
NTPservers to be used on system start. It is recommend to have at last four servers listed to reduce the chance of a “false ticker” (incorrect time source) influencing the quality of the time offset calculation. However, publicly accessible time sources are rarely incorrect.