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· TELNET is a
general protocol, meant to support logging in from almost any type of terminal
to almost any type of computer.
· It allows a user
at one site to establish a TCP connection to a login server or terminal server
at another site.
· A TELNET server
generally listens on TCP Port 23.
How it works
· A user is logged
in to the local system, and invokes a TELNET program (the TELNET client) by
typing
·
telnet
xxx.xxx.xxx
where
xxx.xxx.xxx is either a host name or an IP address.
· The TELNET
client is started on the local machine (if it isn't already running). That client
establishes a TCP connection with the TELNET server on the destination system.
· Once the
connection has been established, the client program accepts keystrokes from the
user and relays them, generally one character at a time, to the TELNET
server.
· The server on
the destination machine accepts the characters sent to it by the client, and passes
them to a terminal server.
· A "terminal server" is just
some facility provided by the operating system for entering keystrokes from a
user's keyboard.
o The terminal
server treats the remote user as it would any other user logged in to the
system, including relaying commands to other applications.
o The terminal
server passes outputs back to the TELNET server, which relays
them to the
client, which displays them on the user's screen.
· In general, a
TELNET server is implemented as a master server with some number of slave
servers. The master server listens for service requests from clients. When it
hears one, it spawns a slave server to handle that specific request, while the
master goes back to listening for more requests.
· The only thing
that makes TELNET hard to implement is the heterogeneity of the
terminals and
operating systems that must be supported. Not all of them use the same control
characters for the same purposes.
· To accomodate
this heterogeneity, TELNET defines a Network Virtual Terminal (NVT). Any user
TELNETting in to a remote site is deemed to be on an NVT, regardless of the actual
terminal type being used.
· It is the
responsibility of the client program to translate user keystrokes from the
actual terminal type into NVT format, and of the server program to translate
NVT characters into the format needed by the destination host. For data sent
back from the destination host, the translation is the reverse.
· NVT format
defines all characters to be 8 bits (one byte) long. At startup, 7 bit US ASCII
is used for data; bytes with the high order bit = 1 are command sequences.
· The 128 7-bit
long US ASCII characters are divided into 95 printable characters and 33 control
codes. NVT maps the 95 printable characters into their defined values – decimal
65 = "A", decimal 97 = "a", etc.
· The
33 control codes are defined for NVT as:
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