.: Advanced Linux Networking :.

Normally, a server runs and ties itself to a specific port (a resource identified by protocol type and a number between 1 and 65,535). Incoming requests are directed to specific ports associated with particular server types. For instance, Simple Mail Transfer Protocol (SMTP) mail servers conventionally use TCP port 25, and Hypertext Transfer Protocol (HTTP) servers (Web servers) normally use TCP port 80.

The inetd program is one of two common super servers for Linux. These are servers that function as intermediaries; instead of running the target server itself, the computer runs the super server, which links itself to the ports used by all of the servers it needs to handle. Then, when a connection is made to one of those ports, the super server launches the target server and lets it handle the data transfer. This has two advantages over running a server directly. First, the memory load is reduced; a super server consumes little RAM compared to the servers it handles, particularly if it handles many servers. Second, the super server can filter incoming requests based on various criteria, which can improve your system's security. The drawback is that it takes time for the super server to launch the target server, so the time to connect to a server may be increased, although usually only by a second or two. As a general rule, it's good to use super servers for small or seldom-used servers, whereas big and frequently used servers should be run directly.