The command telnet

This command takes 2 arguments:

• the first is the host, that is the IP address or the name of the server to which we want to connect.
• The second is the port, that is the port number of the server to which we want to connect. It is optional if its value is 23. Note: it is also possible to put the name of the protocol instead of the port number.
  The program then finds the correct number by consulting, under Unix, the file "/etc/services" or, under Windows, the file "\windows\system32\drivers\etc\services"

Example of use (to connect to a local web server):

telnet 127.0.0.1 80

Role

The role of this command is:

• to link the keyboard (that is, the standard input) to the communication channel so that all characters entered are sent to the channel.
• to link the screen (that is, the standard output) to the communication channel so that all characters received from the channel are displayed on the screen.

The escape character

It is a key defined by default as "^]" which allows to temporarily switch to command mode.

Note: as indicated by the command "man telnet", it is possible to redefine the escape character to the one desired by adding the switch "-e" followed by the desired character before the usual arguments. Example to define the A as escape character:

telnet -eA 127.0.0.1 80

The chosen character can no longer be sent directly to the communication channel; then we must switch to command mode by typing the chosen character and then enter the command "send escape".

How to close the connection

To close the connection on the client side, simply switch to command mode by typing the escape character and then enter the command "close".

Use

In addition to the standard original use, as a client of the server (telnetd) that allows to deport (without encryption and therefore without the currently required security) access to a command line interface via the network, this application will allow to "connect" a human to a server application for a protocol using a text dialogue. We can thus cite, for example, the SMTP, FTP, POP3, IDENT, HTTP protocols, ...