Monday, August 9, 2010

CentOS / Red Hat Configure an NTP Client And Server

Install ntp

The ntp package contains utilities and daemons that will synchronize your computer's time to Coordinated Universal Time (UTC) via the NTP protocol and NTP servers. The ntp packageincludes ntpdate (a program for retrieving the date and time from remote machines via a network) and ntpd (a daemon which continuously adjusts system time). Install the ntp package:
# yum install ntp

How do I configure an NTP Client?

Simply open /etc/ntp.conf file, enter:
# vi /etc/ntp.conf
Make sure the following line exists:
  • : the hostname or IP address of the site NTP server. If your ntp server located at, enter server You can also use public ntp server located at
You can also run ntpd using cron:
# echo '30 * * * * root /usr/sbin/ntpd -q -u ntp:ntp' > /etc/cron.d/ntpd
The above instructs crond to run ntpd and after setting the clock just exit, and the -u option instructs it to run as the ntp user.

Configure an NTP Server

If you have lots of server and desktop system, configure your own NTP server. Your NTP server contacts a central NTP server,provided by your ISP or a public time
server located at, to obtain accurate time data. The server then allows other machines on your network to request the time data. Our sample setup:            ==> CentOS / Fedora / RHEL NTPD Server.              ==> ISP remote NTP server.        ==> NTP clients including desktop systems.
First, install and enable ntpd on
# yum install ntp
# chkconfig ntpd on

Now open /etc/ntp.conf:
# vi /etc/ntp.conf
Make sure the following line exits:
restrict default ignore
Above will deny all access to any machine, server or client. However, you need to specifically authorized policy settings. Set it as follows:
restrict mask nomodify notrap noquery
Replace and mask with actual remote ISP or NTP server IP. Save and close the file.

Configure NTP clients to access your NTP Server

Now, you need to allow legitimate NTP clients to access the Server. For example, allow network to synchronize to this server located at Open /etc/ntp.conf and add policy as follows:
# Hosts on local network are less restricted.
restrict mask nomodify notrap
Update your firewall settings, open /etc/sysconfig/iptables.
# vi /etc/sysconfig/iptables
Add the following line, before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
 -A RH-Firewall-1-INPUT -s -m state --state NEW -p udp --dport 123 -j ACCEPT
Save and close the file. Finally, start ntpd:
# service ntpd start
# service iptables restart
# netstat -tulpn 

NTP to synchronize the system clock


Login as the root user
Type the following command to install ntp
# yum install ntp
Turn on service
# chkconfig ntpd on
Synchronize the system clock with server:
# ntpdate
Start the NTP:
# /etc/init.d/ntpd start
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