INTERNET SERVERS

Linux Internet services are provided by daemons that run continuously or by a daemon that is started automatically by the xinetd daemon when a service request comes in. The /etc/services file lists network services (for example, telnet, ftp, and ssh) and their associated numbers. Any service that uses TCP/IP or UDP/IP has an entry in this file. IANA (Internet Assigned Numbers Authority) maintains a database of all permanent, registered services. The /etc/services file usually lists a small, commonly used subset of services. Visit www.rfc.net/rfc1700.html for more information and a complete list of registered services.

Most of the daemons (the executable files) are stored in /usr/sbin. By convention the names of many daemons end with the letter d to distinguish them from utilities (one common daemon whose name does not end in d is sendmail). The prefix in. or rpc. is often used for daemon names. Look at /usr/sbin/*d to see a list of many of the daemon programs on the local system.

To see how a daemon works, consider what happens when you run ssh. The local system contacts the ssh daemon (sshd) on the remote system to establish a connection. The two systems negotiate the connection according to a fixed protocol. Each system identifies itself to the other, and then they take turns asking each other specific questions and waiting for valid replies. Each network service follows its own protocol.

In addition to the daemons that support the utilities described up to this point, many other daemons support system-level network services that you will not typically interact with.