Linux in a Windows World

Keeping Clocks Synchronized with NTP

One of the frustrations of using a computer network is that the individual computer clocks can report different times. This fact can be a minor annoyance when, say, you seem to have jumped back in time by two minutes when switching computers. It can become more serious when programs that rely on time stamps see inconsistent times on files. A make utility, for instance, might fail to compile a file if it believes that file changed before the last compilation, when in fact it changed after the last compilation but the time stamps are off. Even more serious temporal problems result with tools such as Kerberos, which embed timestamps in some of their packets as a security measure. Depending on your Kerberos settings, logins can fail completely if two computers’ clocks are inconsistent.

These problems can all be overcome using a protocol designed to keep computers’ clocks synchronized. The most popular protocol to do this job is known as the Network Time Protocol, which is headquartered at http://www.ntp.org. Before proceeding with the details of NTP configuration, you should understand its basic operating principles; this will help you decide how to implement NTP. As with most protocols, you must then configure both servers and clients. The Linux NTP server, though, does double duty as both a server and a client. On the Windows side, you can use either a full NTP implementation or a built-in Windows command.

Principles of NTP

NTP can do more than simply ...