Telnet, short for telecommunications network, is part of the TCP/IP suite of protocols, which is used to access a remote host on the Internet. Telnet was developed to provide a means for two machines to directly access each other without having to use a dedicated port. Although a dedicated port is an easy method of access, it is also processing intensive. When connected to another machine, the user’s computer has to translate terminal codes between the two. Although less of an issue today, in the early days of TCP/IP such translations imposed a heavy load on the CPUs. Telnet solved this problem by placing all of the terminal codes within the Telnet protocol itself. This enabled Telnet to determine and set terminal parameters automatically. When Telnet establishes a connection, the machines on each end agree on the parameters governing information exchange, thereby reducing the burden on their CPUs.
When connected via TCP, the user’s machine behaves as if it were a directly attached terminal. The remote host need not run UNIX, but must support the Telnet protocol. Finding the host only requires that the user know its name, after which a user ID and password may be required for access. Although Telnet client software is available with a user-friendly graphical user interface, once access is granted to the remote host, menus and text-based input must be relied upon for navigation and command execution. The Telnet command supports both character-at-a-time and line-by-line data entry.
The hosts that support Telnet may contain such things as software archives, library catalogs, specialized databases, games and other programs. They may also provide access to other Internet resources such as e-mail, FTP, and Gopher sites. Some of the more than 10,000 public Telnet sites even support Internet Relay Chat (IRC), an interactive method of communicating with other online users. Lists of publicly available Telnet sites can be obtained by searching the Web using the keywords telnet sites.
Another common use of remote host access via Telnet is for program initialization. For example, a Web site developer may want to use the Perl language for processing HTML form input. Once the Perl program is written, it must be placed in a cgi-bin directory at the Web server (i.e., the “host”). This can be done via the File Transfer Protocol (FTP). But before the program can start processing forms data, its permissions must be set. The way to do this is by accessing the host directly via Telnet, getting into the cgi-bin directory, and issuing a command such as:
Essentially, chmod changes the permissions for a file, allowing filename.pl to read (r) and execute (x) for everyone (a+). Filename.pl is the name assigned for the particular program, and .pl is the extension indicating that it is a Perl program.
Starting a Session
A Telnet session is started with either the name or the IP address of the remote host. Within a Web browser, this information is entered as:
The plain-text name of the remote host is usable only if the system can translate the name into an IP address using the Domain Name System (DNS). In this case, the Telnet session is with the Dartmouth College Library Online System, which contains such items as the World Factbook, the MLA Bibliography, the full text of 33 Shakespeare plays, the King James version of the Bible, and a portion of the MEDLINE database.
When the connection is established, the remote host may request a user ID and password. If the host is configured for public access, users can usually gain entry by typing guest if the system asks for a password. Any commands the user issues must be those of the remote host. Hosts that are configured for Telnet access usually have menus to aid navigation and provide help on the commands that are supported.
A Web browser can be configured to automatically open a Telnet client (Figure 114) when a Telnet host name or IP address is typed on the command line normally used to enter the addresses for Web sites.
Terminal Emulation Mode
Several software vendors offer emulation programs that are based on Telnet. One of these emulation programs is TN3270, which lets workstations and PCs communicate with a mainframe host via TCP/IP’s Telnet terminal emulation mode. By using a terminal emulation mode that is native to
Figure 114
A Web browser, such as Netscape Navigator, can be configured to automatically start a Telnet client, such as InterSoft International’s NetTerm.
TCP/IP, TN3270 users can communicate with a mainframe across a TCP/IP network without requiring a boundary router to convert the protocols. This approach is inexpensive and easy to implement, since Telnet software is widely installed and available from a variety of vendors.
TN3270 has some limitations, however. For example, it does not offer the reliability and integrity of native SNA (Systems Network Architecture). For example, TN3270 cannot support the SNA traffic prioritization schemes often required in mission-critical SNA applications. And because it uses TCP/IP, TN3270 does not offer the data integrity of methods that use SNA on an end-to-end basis. A newer version of the standard called TN3270 Extended (TN3270E) offers additional SNA capabilities, such as 3270 printing, but it also does not support APPC (Advanced Program to Program Communications), which is at the heart of IBM’s next-generation SNA applications.
Finallyy some TN3270 configurations require a dedicated terminal server to convert 3270 data streams into TCP/IP and back again. Users can avoid this problem by implementing TCP/IP on the mainframe or front-end communications server as well as on the desktop machines.
Last Word
Although it originated as a means of interconnecting UNIX machines via TCP, Telnet has become a general method of logging into remote ma-chines—whether or not they are running UNIX. A number of Windows programs and Web browsers are available that simplify the use of Telnet with a graphical user interface.
