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13.2.2. 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
  2. When you find a satisfactory server, run the ntpdate command followed with on or more server addresses:
    ~]# ntpdate server_address...
    For instance:
    ~]# ntpdate
    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 13.2.1, “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 7, Controlling Access to Services.


    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:
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:


    To speed the initial synchronization up, add the iburst directive at the end of each server line:
    server iburst
    server iburst
    server iburst
  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 -6 ::1
  4. Save all changes, exit the editor, and restart the NTP daemon:
    ~]# service ntpd restart
  5. Make sure that ntpd daemon is started at boot time:
    ~]# chkconfig ntpd on

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