Network Time Protocol (NTP) is an Internet service that sets the clocks on your system with great precision. Synchronizing time among different machines is important in many ways. From a security point of view, examining the precise times noted on the log files of different machines may help in analyzing patterns of break-ins. Having synchronized clocks is also a requirement for preventing attackers from recording an interaction and then repeating it (a playback attack); if time stamps are encoded in the interaction, they will be incorrect the second time the transaction is replayed. Kerberos authentication, for example, which we discuss in Chapter 10, depends on time synchronization. From a practical point of view, synchronized clocks are also required to successfully use NFS.
You do not have to use NTP across the Internet; it will synchronize clocks to each other within your site, if that's all you want. The reason that people use NTP from the Internet is that a number of hosts with extremely accurate clocks - radio clocks that receive the time signal from the United States master atomic clocks or from the atomic clocks in the Global Positioning System (GPS) satellites - provide NTP service to make certain that your clocks are not only synchronous with each other but also correct. Without an external time service, you might find that all your computers have exactly the same wrong time. Accepting an external service makes you vulnerable to spoofing, but because NTP won't move the clocks very far very fast, a spoofed external clock is unlikely to make you vulnerable to a playback attack, although it could succeed in annoying you by running all your clocks slow or fast. Radio clocks suitable for use as NTP time sources are not terribly expensive, however, and if you are using NTP to synchronize clocks for an authentication protocol like Kerberos, you should buy your own and provide all time service internally, instead of using an external reference.