CentOS configuring NTP

2.2.2. Network Time Protocol Setup

As opposed to the manual setup described above, you can also synchronize the system clock with a remote server over the Network Time Protocol (NTP). For the one-time synchronization only, use the ntpdate command:

1. Firstly, check whether the selected NTP server is accessible:

   ~]# ntpdate -q server_address

   For example:

   ~]# ntpdate -q 0.rhel.pool.ntp.org

2. When you find a satisfactory server, run the ntpdate command followed by one or more server addresses:

   ~]# ntpdate server_address...

   For instance:

   ~]# ntpdate 0.rhel.pool.ntp.org 1.rhel.pool.ntp.org

   Unless an error message is displayed, the system time should now be set. You can check the current by setting typing date without any additional arguments as shown in Section 2.2.1, “Date and Time Setup”.
3. In most cases, these steps are sufficient. Only if you really need one or more system services to always use the correct time, enable running the ntpdate at boot time:

   ~]# chkconfig ntpdate on

   For more information about system services and their setup, see Chapter 11, Services and Daemons.
   Note

   If the synchronization with the time server at boot time keeps failing, i.e., you find a relevant error message in the /var/log/boot.log system log, try to add the following line to /etc/sysconfig/network:

   NETWORKWAIT=1

However, the more convenient way is to set the ntpd daemon to synchronize the time at boot time automatically:

1. Open the NTP configuration file /etc/ntp.conf in a text editor such as vi or nano, or create a new one if it does not already exist:

   ~]# nano /etc/ntp.conf

2. Now add or edit the list of public NTP servers. If you are using Red Hat Enterprise Linux 6, the file should already contain the following lines, but feel free to change or expand these according to your needs:

   server 0.rhel.pool.ntp.org iburst
   server 1.rhel.pool.ntp.org iburst
   server 2.rhel.pool.ntp.org iburst
   server 3.rhel.pool.ntp.org iburst
   

   The iburst directive at the end of each line is to speed up the initial synchronization. As of Red Hat Enterprise Linux 6.5 it is added by default. If upgrading from a previous minor release, and your /etc/ntp.conf file has been modified, then the upgrade to Red Hat Enterprise Linux 6.5 will create a new file /etc/ntp.conf.rpmnew and will not alter the existing /etc/ntp.conf file.
3. Once you have the list of servers complete, in the same file, set the proper permissions, giving the unrestricted access to localhost only:

   restrict default kod nomodify notrap nopeer noquery
   restrict -6 default kod nomodify notrap nopeer noquery
   restrict 127.0.0.1
   restrict -6 ::1

4. Save all changes, exit the editor, and restart the NTP daemon:

   ~]# service ntpd restart

5. Make sure that ntpd is started at boot time:

   ~]# chkconfig ntpd on