Hosts attached to a networkparticularly the worldwide Internetare exposed to a wider range of security threats than are unconnected hosts. Network security reduces the risks of connecting to a network. But by nature, network access and computer security work at cross-purposes. A network is a data highway designed to increase access to computer systems, while security is designed to control access to those systems. Providing network security is a balancing act between open access and security.
The highway analogy is very appropriate. Like a highway, the network provides equal access for allwelcome visitors as well as unwelcome intruders. At home, you provide security for your possessions by locking your house, not by blocking the streets. Likewise, network security requires adequate security on individual host computers. Simply securing the network with a firewall is not enough.
In very small towns where people know each other, doors are often left unlocked. But in big cities, doors have deadbolts and chains. The Internet has grown from a small town of a few thousand users into a big city of millions of users. Just as the anonymity of a big city turns neighbors into strangers, the growth of the Internet has reduced the level of trust between network neighbors. The ever-increasing need for computer security is an unfortunate side effect. Growth, however, is not all bad. In the same way that a big city offers more choices and more services, the expanded network provides increased services. For most of us, security consciousness is a small price to pay for network access.
Network break-ins have increased as the network has grown and become more impersonal, but it is easy to exaggerate the extent of these security breaches. Overreacting to the threat of break-ins may hinder the way you use the network. Don't make the cure worse than the disease. The best advice about network security is to use common sense. RFC 1244, now replaced by RFC 2196, stated this principle very well:
Common sense is the most appropriate tool that can be used to establish your security policy. Elaborate security schemes and mechanisms are impressive, and they do have their place, yet there is little point in investing money and time on an elaborate implementation scheme if the simple controls are forgotten.
This chapter emphasizes the simple controls that can be used to increase your network's security. A reasonable approach to security, based on the level of security required by your system, is the most cost-effectiveboth in terms of actual expense and in terms of productivity.