The purpose of the Telnet protocol is to provide a fairly general, bi-directional, eight-bit byte-oriented communications facility. Its primary goal is to allow a standard method of interfacing terminal devices and terminal-oriented processes to each other.

Telnet not only allows the user to log in to a remote host, it allows that user to execute commands on that host. Thus, an individual in Los Angeles can Telnet to a machine in New York and begin running programs on the New York machine just as though the user were actually in New York.

For those of you who are unfamiliar with Telnet, it operates much like the interface of a bulletin board system (BBS). Telnet is an excellent application for providing a terminal-based front end to databases. For example, better than 80 percent of all university library catalogs can be accessed via Telnet.

Even though GUI applications have taken the world by storm, Telnet--which is essentially a text-based application--is still incredibly popular. There are many reasons for this. First, Telnet allows you to perform a variety of functions (retrieving mail, for example) at a minimal cost in network resources. Second, implementing secure Telnet is a pretty simple task. There are several programs to implement this, the most popular of which is Secure Shell.

To use Telnet, the user issues whatever command necessary to start his or her Telnet client, followed the name (or numeric IP address) of the target host. In UNIX, this is done as follows:

#telnet internic.net

This command launches a Telnet session, contacts internic.net, and requests a connection. That connection will either be honored or denied, depending on the configuration at the target host. In UNIX, the Telnet command has long been a native one. That is, Telnet has been included with basic UNIX distributions for well over a decade. However, not all operating systems have a native Telnet client. Table 6.3 shows Telnet clients for various operating systems. The following table lists Telnet clients for various operating systems.

Threats posed via Telnet are numerous. If not properly implemented, Telnet can cause some serious damage.

File Transfer Protocol

File Transfer Protocol is the standard method of transferring files from one system to another. Its purpose is set forth in RFC 0765 as follows:

The objectives of FTP are: 

1. to promote sharing of files (computer programs and/or data).
2. to encourage indirect or implicit (via programs) use of remote computers.
3. to shield a user from variations in file storage systems among Hosts.
4. to transfer data reliably and efficiently.

FTP, though usable directly by a user at a terminal, is designed mainly for use by programs. For over two decades, researchers have investigated a wide variety of file-transfer methods. The development of FTP has undergone many changes in that time.

The following table enlists popular FTP clients for various platforms.

How Does FTP Work?

FTP file transfers occur in a client/server environment. The requesting machine starts one of the clients named in the table. This generates a request that is forwarded to the targeted file server (usually a host on another network). Typically, the request is sent by inetd to port 21. For a connection to be established, the targeted file server must be running an FTP server or FTP daemon.

FTP is a very useful service. Novice users are advised to use graphical clients like Cute-ftp for FTP purposes. Network administrators require to have careful thought before implementation of FTP service on their network. Even fools will admit that if you allow anonymous login FTP for your network, even novice hackers will be able to create havoc.