Chapter 5 explained all layers of Internet networks except the application layer. Network applications (clients and servers) interact with the operating system by using a transport layer protocol and interface, such as TCP. This chapter covers basic network servers, including the inetd superserver and SSH servers. In addition, you will see some tools that will help you debug the servers.
Unix network servers come in many forms. A server may listen to a port on its own, or it may use a superserver such as inetd. Servers have no common configuration database, and they vary widely in features. Most servers have a configuration file to control their behavior (though with no common format), and most use the operating system's syslog service for message logging.
If you understand the system of TCP and UDP ports described in Section 5.12, you won't run into much trouble with network servers. Let's start out by getting an idea of how network clients talk to network servers.