Configuring Ntp Server Software On Linux

NTP, or Network Time Protocol, was invented over 25 years ago as a means of synchronising time critical processes over the Internet. It is one of the oldest Internet protocols still in use today. NTP allows network time clients to synchronise to an accurate time reference. Originally developed for the Linux operating system, recently there have been a number of ports to other operating systems, such as Microsoft Windows.

This article provides straightforward installation and configuration information for the Linux version of the NTP distribution. It attempts to describe a simple NTP installation and goes through the configuration procedure required to provide a network time resource on a Linux machine.

The NTP distribution is a free, downloadable application licensed under the GNU public license. It can be downloaded from the Network Time Protocol website. Alternatively, many Linux operating system distributions, such as Redhat and Debian, install the daemon by default or at least provide it as a pre-configured package. As well a providing the NTP application, the distribution also provides a number of very useful tools for configuring and debugging installations. In fact the tools can be used to interrogate any NTP server, including internet based ones, to provide synchronisation status information.

NTP is configured using a configuration file called 'ntp.conf'. The file contains a list of commands indicating which time references should be used for synchronisation purposes and also specifies authentication and security options.

To synchronise to an Internet based or local NTP server, you need to use the 'server' command along with the IP address or domain name of the reference. External hardware reference clocks can also be specified. Indeed a number of such reference clock drivers are compiled into the distribution by default. Many GPS receivers and radio time code receivers can be directly configured to be used as hardware reference clocks. These hardware clocks can provide an extremely accurate external source of time. Particularly GPS clocks, they can often provide accuracies to within nanoseconds of UTC. Multiple hardware and Internet reference clocks can be specified to provide redundancy in case of failure. In this case, NTP will choose the most stable clock from the list and utilise it as the preferred time source.

The NTP daemon is itself is controlled by a number of scripts to start, stop and restart the service. The scripts are simply named 'ntpd start', 'ntpd stop' and 'ntpd restart'. Additionally, a very useful debugging tool is provided which can query any NTP server to find out its current synchronisation state and to see which reference clocks it is currently using. The 'ntpq' utility also provides stability information on each of the utilised reference clocks.

There are a number of security commands that can be utilised in the configuration file to control access to the NTP installation and also to provide authentication.

NTP can be easily configured so that only a specified subset of computers on a network are granted access. This is achieved using the 'restrict' command, which is used in conjunction with the IP address or domain name of the computer to be allowed access. Multiple 'restrict' commands can be specified in the configuration file to restrict access to a range of computers.

Authentication is a security feature that allows network time clients to authenticate a server that they receive time from. A matching set of encrypted keywords available to both the client and server are used to confirm the identity of the time reference. Shared keywords are specified in a file called 'ntp.keys' on both the server and client machines. A trusted subset of keywords are then specified in the configuration file 'ntp.conf'.

To summarise, NTP is free to download, if its not already installed on your computer, and is easy to install and configure. Essentially, it is very east to set up a local time reference within your organisation to maintain network time synchronisation.