Topics: Red Hat / Linux, System Admin

Configuring NTP on CentOS 6

Configuring NTP on CentOS 6 (and similar versions) involves a number of steps - especially if you want to have it configured right and secure. Here's a quick guide how to do it:

First of all you have to determine the IP addresses of the NTP servers you are going to use. You may have to contact your network administrator to find out. Ensure that you get at least two time server IP addresses to use.

Then, install and verify the NTP packages:

# yum -y install ntp ntpdate
# yum -q ntp ntpdate
Edit file /etc/ntp.conf and ensure that option "broadcastclient" is commented out (which it is by default with a new installation).

Enable ntp and ntpdate at system boot time:
# chkconfig ntpd on
# chkconfig ntpdate on
Ensure that file /etc/ntp/step-tickers is empty. This will make sure that if ntpdate is run, that it will use one of the time servers configured in /etc/ntp.conf.
# cp /dev/null /etc/ntp/step-tickers
Add two time servers to /etc/ntp.conf, or use any of the pre-configured time servers in this file. Comment out the pre-configured servers, if you are using your own time servers.
#server iburst
#server iburst
#server iburst
#server iburst
Do not copy the example above. Use the IP addresses for each time server that you've received from your network administrator instead.

Enable NTP slewing (for slow time stepping if the time on the server is off, instead of suddenly making big time jump changes), by adding "-x" to OPTIONS in /etc/sysconfig/ntpd. Also add "SYNC_HWCLOCK=yes" in /etc/sysconfig/ntpdate to synchronize the hardware clock with any time changes.

Stop the NTP service, if it is running:
# service ntpd stop
Start the ntpdate service (this will synchronize the system clock and the hardware clock):
# service ntpdate start
Now, start the time service:
# service ntpd start
Wait a few minutes for the server to synchronize its time with the time servers. This may take anywhere between a few and 15 minutes. Then check the status of the time synchronization:
# ntpq -p
# ntpstat
The asterisk in front of the time server name in the "ntpq -p" output indicates that the client has reached time synchronization with that particular time server.


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