Installing and Administering Internet Services

Chapter 7 283

Conﬁguring the Network Time Protocol (NTP)

Getting Started with NTP

The Network Time Protocol (NTP) is a family of programs used to adjust

the system clock on your computer and keep it synchronized with

external sources of time. All clocks drift including clocks inside your

computers. Computers are very sensitive to time deviations caused by

this drifting. NTP provides accuracy from the microsecond to millisecond

range.

Some of the pervasive computing processes that may be affected by

disparity in time include, debugging, database and transaction

processing, and compiling software using the make utility.

Debugging system problems becomes difﬁcult if the timestamp in the

system logs are not true.

Databases rely heavily on time. Databases and transaction processing

application may get confused if clients and servers have differences in

“correct” time.

Many people use the make utility to manage the compilation of software.

This utility looks at ﬁle timestamps, with one-second granularity, to

decide which .0 ﬁles need to be rebuilt when the underlying source ﬁle

has been changed. The problemis compounded whenﬁles on machines at

various sites in different time zones need to be compiled and built into

the “new” version of the source ﬁles. Also, if some of the directories are

NFS mounted and the server and client have different notions of the

current time, then make can fail to rebuild some derived objects and

produce an executable that is not based on the most up-to-date sources.

Even the one-second granularity of ﬁle stamps means that your client

and server must be synchronized closer to 1000 milliseconds in order to

guarantee that make will compile the correct ﬁles.

Equipment Needed for NTP

You will need the following equipment to effectively use the NTP

programs:

• Internet or your own radio receiver, such as GPS, as a time source

• An ordinary network, like Ethernet, in your building

• A little knowledge about how to conﬁgure NTP and get it working