Using remote host computers via Telnet
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Understanding Telnet
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.
