Local and wide area computer networks have changed the landscape of computing forever. Almost gone are the days when each computer was separate and distinct. Today, networks allow people across a room or across the globe to exchange electronic messages, share resources such as printers and disk drives, or even use each other's computers. Networks have become such an indispensable part of so many people's lives that one can hardly imagine using modern computers without them.
But networks have also brought with them their share of security problems, precisely because of their power to let users easily share information and resources. Networks allow people you have never met to reach out and touch you?and erase all of your files in the process. They have enabled individuals to launch sophisticated electronic attacks against both major institutions and desktop computers in home offices. Indeed, for every opportunity that networks have created, they have similarly created a corresponding risk.
This chapter describes local and wide area networks, and shows how they fit into the Unix security picture. In Chapter 12, we'll describe how network services can be effectively secured so that you can take advantage of the opportunities while reducing exposure to the risks.