Saturday, September 21, 2013

Install NTP on CentOS

Install NTP
sudo yum install ntp

Start NT
sudo /etc/init.d/ntpd start

Configure NTP servers
Select NTP server which close to your location from area list.
http://support.ntp.org/bin/view/Servers/NTPPoolServers
sudo vi /etc/ntp.conf

Replace default NTP servers.
server 0.centos.pool.ntp.org
server 1.centos.pool.ntp.org
server 2.centos.pool.ntp.org
to
server 0.north-america.pool.ntp.org
server 1.north-america.pool.ntp.org
server 2.north-america.pool.ntp.org
server 3.north-america.pool.ntp.org

Restart NTP
sudo /etc/init.d/ntp stop sudo /etc/init.d/ntp start

Check sync status.
ntpstat
unsynchronised
 time server re-starting
  polling server every 64 s

If just started NTP, it won't start synchronize, it will take 10 or more minutes to start syncronizing.
If successfully synchorised, you will see messages like this.
synchronised to NTP server (180.235.254.234) at stratum 3
  time correct to within 30 ms
  polling server every 64 s

Configure automatic run. at booting.
sudo /sbin/chkconfig ntpd on
sudo /sbin/chkconfig --list
ntpd ntpd 0:off 1:off 2:on 3:on 4:on 5:on 6:off

Note. NTP uses UDP port 123 to synchronize.
If you have filtered this port, you have to open port 123.
To open this port you can open with the following iptables arguments.
-I INPUT -p udp --dport 123 -j ACCEPT
-I OUTPUT -p udp --sport 123 -j ACCEPT

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