17.4 Security Implications of Network Services

Network servers are the portals through which the outside world accesses the information stored on your computer. Every server must:

• Determine what information or action the client requests.

• Decide whether or not the client is entitled to the information, optionally authenticating the person (or program) on the other side of the network that is requesting service.

• Transfer the requested information or perform the desired service.

By their design, many servers must run with root privileges. A bug or an intentional back door built into a server can therefore compromise the security of an entire computer, opening the system to any user of the network who is aware of the flaw. Even a relatively innocuous program can be the downfall of an entire computer. Flaws may remain in programs distributed by vendors for many years, only to be uncovered some time in the future.

Furthermore, many UNIX network servers rely on IP numbers or hostnames to authenticate incoming network connections. This approach is fundamentally flawed, as neither the IP protocol nor DNS were designed to be resistant to attack. There have been many reports of computers that have fallen victim to successful IP spoofing attacks or DNS compromise.

Given these factors, you may wish to adopt one or more of the following strategies to protect your servers and data:

• Use encryption to protect your data. If it is stolen, the data will do your attacker no good. Furthermore, making alterations in your data that you will not notice will be difficult, if not impossible.

• Avoid using passwords and host-based authentication. Instead, rely on tokens, one-time passwords, or cryptographically secure communications.

• Use a firewall to isolate your internal network from the outside world.

• Disconnect your internal network from the outside world. You can still relay electronic mail between the two networks using UUCP or some other mechanism. Set up separate network workstations to allow people to access the WWW or other Internet services.

• Create a second internal network for the most confidential information.

• Disable all services that you are not sure you need, and put wrappers around the rest to log connections and restrict connectivity.