The Network Time Protocol (NTP) is a protocol used to help synchronize your Linux system's clock with an accurate time source. There are that allow the general public to synchronize with them. They are divided into two types:- Stratum 1: NTP sites using an atomic clock for timing.
- Stratum 2: NTP sites with slightly less accurate time sources.
It is good practice to have at least one server on your network be the local time server for all your other devices. This makes the correlation of system events on different systems much easier. It also reduces Internet bandwidth usage due to NTP traffic and reduces the need to manage firewall rules for each NTP client on your network. Sometimes, not all your servers will have Internet access; in such cases you'll need a central server that all can access.
For a list of available Stratum 1 and 2 servers consult http://www.ntp.org/
Download and Install The NTP Package
Most RedHat and Fedora Linux software products are available in the RPM format. Downloading and installing RPMs isn't hard. If you need a refresher, Chapter 6, "Installing Linux Software", has all the details.
When searching for the file, remember that the NTP RPM's filename usually starts with the word ntp followed by a version number as in ntp-4.1.2-5.i386.rpm.
The /etc/ntp.conf File
The /etc/ntp.conf file is the main configuration file for Linux NTP in which you place the IP addresses of the stratum 1 and stratum 2 servers you want to use. Here are the steps to create a configuration file using a pair of sample Internet-based NTP servers:
1) First we specify the servers you're interested in:
server otherntp.server.org # A stratum 1 server at server.org
server ntp.research.gov # A stratum 2 server at research.gov
2) Restrict the type of access you allow these servers. In this example the servers are not allowed to modify the run-time configuration or query your Linux NTP server.
restrict otherntp.server.org mask 255.255.255.255 nomodify notrap noquery
restrict ntp.research.gov mask 255.255.255.255 nomodify notrap noquery
The mask 255.255.255.255 statement is really a subnet mask limiting access to the single IP address of the remote NTP servers.
3) If this server is also going to provide time for other computers, such as PCs, other Linux servers and networking devices, then you'll have to define the networks from which this server will accept NTP synchronization requests. You do so with a modified restrict statement removing the noquery keyword to allow the network to query your NTP server. The syntax is:
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
In this case the mask statement has been expanded to include all 255 possible IP addresses on the local network.
4) We also want to make sure that localhost (the universal IP address used to refer to a Linux server itself) has full access without any restricting keywords:
restrict 127.0.0.1
5) Save the file and restart NTP for these settings to take effect. You can now configure other Linux hosts on your network to synchronize with this new master NTP server in a similar fashion.
How To Get NTP Started
You have to restart the NTP process every time you make a change to the configuration file for the changes to take effect on the running process.
To get NTP configured to start at boot, use the line:
[root@bigboy tmp]# chkconfig ntpd on
To start, stop and restart NTP after booting, follow these examples:
[root@bigboy tmp]# service ntpd start
[root@bigboy tmp]# service ntpd stop
[root@bigboy tmp]# service ntpd restart
Testing And Troubleshooting NTP
After configuring and starting NTP, you should test it to make sure it is working. Here are some guidelines you can follow to get NTP working correctly.
Verifying NTP is Running
To test whether the NTP process is running use the command
[root@bigboy tmp]# pgrep ntpd
You should get a response of plain old process ID numbers.
Sumber :
http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch24_:_The_NTP_Server
For a list of available Stratum 1 and 2 servers consult http://www.ntp.org/
Download and Install The NTP Package
Most RedHat and Fedora Linux software products are available in the RPM format. Downloading and installing RPMs isn't hard. If you need a refresher, Chapter 6, "Installing Linux Software", has all the details.
When searching for the file, remember that the NTP RPM's filename usually starts with the word ntp followed by a version number as in ntp-4.1.2-5.i386.rpm.
The /etc/ntp.conf File
The /etc/ntp.conf file is the main configuration file for Linux NTP in which you place the IP addresses of the stratum 1 and stratum 2 servers you want to use. Here are the steps to create a configuration file using a pair of sample Internet-based NTP servers:
1) First we specify the servers you're interested in:
server otherntp.server.org # A stratum 1 server at server.org
server ntp.research.gov # A stratum 2 server at research.gov
2) Restrict the type of access you allow these servers. In this example the servers are not allowed to modify the run-time configuration or query your Linux NTP server.
restrict otherntp.server.org mask 255.255.255.255 nomodify notrap noquery
restrict ntp.research.gov mask 255.255.255.255 nomodify notrap noquery
The mask 255.255.255.255 statement is really a subnet mask limiting access to the single IP address of the remote NTP servers.
3) If this server is also going to provide time for other computers, such as PCs, other Linux servers and networking devices, then you'll have to define the networks from which this server will accept NTP synchronization requests. You do so with a modified restrict statement removing the noquery keyword to allow the network to query your NTP server. The syntax is:
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
In this case the mask statement has been expanded to include all 255 possible IP addresses on the local network.
4) We also want to make sure that localhost (the universal IP address used to refer to a Linux server itself) has full access without any restricting keywords:
restrict 127.0.0.1
5) Save the file and restart NTP for these settings to take effect. You can now configure other Linux hosts on your network to synchronize with this new master NTP server in a similar fashion.
How To Get NTP Started
You have to restart the NTP process every time you make a change to the configuration file for the changes to take effect on the running process.
To get NTP configured to start at boot, use the line:
[root@bigboy tmp]# chkconfig ntpd on
To start, stop and restart NTP after booting, follow these examples:
[root@bigboy tmp]# service ntpd start
[root@bigboy tmp]# service ntpd stop
[root@bigboy tmp]# service ntpd restart
Testing And Troubleshooting NTP
After configuring and starting NTP, you should test it to make sure it is working. Here are some guidelines you can follow to get NTP working correctly.
Verifying NTP is Running
To test whether the NTP process is running use the command
[root@bigboy tmp]# pgrep ntpd
You should get a response of plain old process ID numbers.
Sumber :
http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch24_:_The_NTP_Server
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