A newbie intro to TELNET

By: Kn¿ght

The Telnet protocol has been implemented on a variety of systems. Each
is different, so specific commands depend on your version. However, all
versions function similarly, so there are a few general guidelines to
follow.

The one common element across the disparate environments of the Internet
is the TCP/IP software protocol suite, the basis of communications.

Telnet, the terminal-handler portion of the TCP/IP protocol suite, is
the cornerstone of this striking communications technology. Telnet
handles the remote login to another Internet host, so it is useful to
know something about the way it works.

Telnet operates in a client/server environment in which one host (the
computer you are using, running Client (User) Telnet) negotiates opening
a session on another computer (the remote host, running Server Telnet).
During the behind-the-scenes negotiation process, the two computers
agree on the parameters governing the session. One of the first things
they settle is the terminal type to be used -- in general, a
line-by-line network virtual terminal, for simplicity's sake. Virtual
terminal, in this context, refers to a set of terminal characteristics
and sequences that both sides of a network connection agree to use to
transmit data from terminals across the network, regardless of the
terminal used.

Finding Telnet Commands

Try typing "help" or "?" at the Telnet prompt to get a list of the
commands available in your Telnet software.

Using Local versus Remote Commands

Once you have established a remote session, all commands you type will
be sent to the Server Telnet on the remote host for execution.

If you want a Telnet command issued in the remote environment to be
acted on locally by your client Telnet, on most systems you would
normally precede the command with an escape sequence (a predetermined
character or combination of characters that signal your Telnet software
to execute the command that follows locally). For example, in NCSA
Telnet for pc-compatible microcomputers, the F10 key is the escape
character that alerts Telnet to execute locally the next command you
type (to turn local echo on or off, or to toggle capture on or off,
etc.).

The Telnet escape sequence by itself followed by [cr] returns you
temporarily to your local operating environment. On UNIX systems, the
escape sequence is usually the control key (CNTL) and left bracket ([)
pressed simultaneously.

Logging On

TELNET [host]
or
TELNET [cr] followed by OPEN [host] at the prompt.

The basic command set is simple. You also need to know either the
machine domain name or the machine Internet address (a series of
numbers). The numbers will always work; the names will work if they are
in a software table available to your version of Telnet.

IBM systems that use TN3270 may require you to type a carriage return,
"DIAL VTAM," or just "VTAM" in response to the first prompt from the
remote system.

Logging Off

LOGOFF or LOGOUT (also try QUIT, END, EXIT, STOP, etc.)

CLOSE, prefixed by the escape sequence.

ABORT, prefixed by the escape sequence--use as a last resort!

To exit the remote system, first try that system's logoff command. To
determine what the appropriate logoff command is, check the menus, help,
and welcome screens when you first log on. Oftentimes, the logoff
information is listed there but not always easy to retrieve later.

Logging off the remote system may return you to your primary operating
environment (all the way out of Telnet), or you may be left in Telnet.
If so, type "quit".

But some information systems have no graceful exit for remote users. In
that case, you have two options --- CLOSE or ABORT.

CLOSE should be your next choice after LOGOFF. If you are unable to
CLOSE the connection normally (e.g., if your remote session is hung),
try the Telnet ABORT command to drop your connection locally.

ABORT will return control to you in your local environment, but it may
not properly terminate your session on the remote machine. Since this
can leave the port on the remote machine busy for an indefinite period
even though you are no longer using it, ABORT should be used only as a
last resort.

In either case, you can also try escaping back to your local environment
and then issuing the termination commands. If one method doesn't work,
try the other.

Other commands may allow you to control your communications environment.
Investigate the help systems both in your local Telnet and on the remote
system at the outset.

Using the BREAK Key

Don't be hasty with the Break key. Too many Breaks may cause your
Telnet session to be dropped!

There is no standard BREAK key across versions of Telnet and in remote
information systems. Telnet is based on the concept of a network
virtual terminal, in which the control functions (breaks, etc.) are
communicated with characters regardless of terminal type (rather than
line conditions, used in the terminal server environment). Your local
Telnet receives your break and sends out a character sequence which is
reinterpreted on the other end, hopefully as the break you intended.

Your Break may not always be understood by the remote system, so you
should try HELP or ? when you begin (at the Telnet prompt) to determine
what your version of Telnet uses as BREAK.

Tips: In UNIX, CNTL-C may work for BREAK. In the Mac environment, BREAK
may be a click down menu option or a character combination. In NCSA
Telnet (a popular PC version), BREAK is F10 followed by a lower case
letter "b".

Using the Backspace Key

The backspace character may not be recognized by the remote system.
Investigate in your local Telnet how to set an erasing backspace. Type
? at the Telnet prompt, or SET ? for a list of possibilities.

Adjusting the Settings to your Needs

Most Telnet programs have the ability to SET or TOGGLE many of these
settings on and off. Erasable backspace, local echo, carriage return
interpretation ([cr] or [cr][lf] -- i.e., carriage return or carriage
return with line feed), and the escape character you use to return to
the local environment are things that you can usually SET or TOGGLE at
the Telnet prompt. Type ? and use Telnet's internal help system to
change a setting.

Using Function Keys

Remember that special function keys are local implementations and have
no significance in a remote session. Function keys such as INSERT,
DELETE, ERASE END-OF-FIELD, PF, and PA keys may not be recognized in the
remote environment. Even though function keys and control key
combinations may have significance on the remote system, they may vary
from those on your local system.

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