We’ve explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it.
Another way to provide a service on an Internet port is to let the daemon
inetd do the listening.
inetd is a program that
runs all the time and waits (using
select) for messages on a
specified set of ports. When it receives a message, it accepts the
connection (if the socket style calls for connections) and then forks a
child process to run the corresponding server program. You specify the
ports and their programs in the file /etc/inetd.conf.