UNIX Network Support

Common UNIX Network Applications
The majority of network applications are not strictly required for the network to operate, but they provide user services that are central to the network's implementation; without them, the network serves no real purpose. Many of these applications require no special configuration. Once the UNIX system is configured properly and the network is set up (including the setup of the Internet super daemon inetd), a number of network applications can be used immediately; other network applications require some administration. Among the most common network applications, we will briefly discuss three:

1. telnet
The network terminal protocol, which provides remote login over the network
2. ftp
The file transfer protocol, which is used for file transfers over the network
3. finger
Provides information about remote users
These applications are instrumental in daily UNIX administration. Because of their inherent
interactive nature, they are primarily used from the command line, but they can also be a part of
shell scripts and other programs.

All three applications are based on the client/server model. On the client side, the corresponding
application program is supposed to be started (from the command line, script, or any other program) on an as−needed basis. The server side is handled by the corresponding daemons (telnetd, ftpd, and fingerd) that are invoked by the Internet super server (the inetd daemon) once a client request is received at the corresponding port.

None of the three applications require a lot of work to be properly set; they just need to be enabledor disabled on the server side. These actions are provided through the inetd configuration file /etc/inetd.conf. Since the inetd daemon and the /etc/inetd.conf file were covered in Tcp/Ip Network, only the /etc/inetd.conf entries related to these applications are presented in the following example:
$ cat /etc/inetd.conf
.....
.....
# ARPA/Berkeley services
ftp stream tcp nowait root /etc/ftpd ftpd −l
telnet stream tcp nowait root /etc/telnetd telnetd
# finger stream tcp nowait bin /etc/fingerd fingerd
.....
In this example, ftp and telnet are enabled, while finger is disabled (the entry is commented−out
and deactivated).

Telnet
Telnet provides a user interface to a remote system using the TELNET protocol. If telnet is invoked without arguments, it enters command mode, indicated by its own prompt (telnet). In this mode, it accepts and executes the "telnet commands" (these will be listed later). When invoked with arguments, it performs an open command with those arguments.