Remote Login Or Remote desktop connection vie telnet - Cyber security & Technology


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Saturday, June 2, 2018

Remote Login Or Remote desktop connection vie telnet

Remote Login

·      Remote login is a process of logging on to remote site and using the services available on the remote computer

·      With remote login a user can also transfer the result of processing from remote computer to local computer

·      Remote login is implemented by using a protocol called TELNET.


·      Telnet stands for Terminal Network

·      Telnet is a protocol that enables one computer to connect to remote computer. It is a standard TCP/IP protocol for virtual terminal service given by ISO.

·      The user`s Computer, which initiates the connection , is referred to as the local computer.

·      The computer being connected to, which accepts the connection, is referred to as the remote computer.  The remote computer can be physically located in the next room, the next town or in another country.

·      Once the connection is established between the local and remote computer, the local computer emulates the remote computer. The user`s local computer will display what is taking place on the remote computer during telnet session.

·      Telnet operates on a client/server principle. The local computer uses a telnet client program to establish the connection and display data on the local computer`s monitor.

·      The remote computer uses telnet server program to accept the connection and to send responses to requests for information back to the local computer.


1.   When a user types on local computer , the local operating system accepts the character.

2.   The local operating system does not interpret them rather sends these characters to TELNET client.

3.   Telnet client transforms thesre characters to a universal character set called network virtual terminal (NVT) characters and pass them to the local TCP/IP protocol stack.

4.   The commands or text, in NVT form , travel through internet and arrive at the TCP/IP stack at remote  computer.

5.   Here characters are delivered to the operating system and passed to the TELNET server.

6.   The TELNET server changes the characters to the corresponding characters of understandable format by remote computer.

7.   The remote operating system receives the character from a pseudoterminal driver , which is a piece of software that pretends that the characters are coming from a terminal.

8.   The operating system then passes the characters to the appropriate application program.

 NVT Character set:-

·      With the network virtual terminal character set, the client TELNET translates characters that come from the local terminal into NVT form and delivers them to the network.

·      The server TELNET translates data and commands from NVT form into the form acceptable by the remote computer.

·      NVT uses two sets of character set in which the 7 lowest order bits are the same as ASCII and the highest order bit is 0.

·      For control characters, NVT uses an 8-bit character set in which the highest order bit is set to 1.

Telnet commands:-

·      Telnet commands are identified by a prefix character, IAC (interpret as command) having code 255.

·      IAC is followed by the command and option codes.

Command code
Option code

                                      telnet command format

some of the important TELNET commands are listed in the table below:

1.   Offering to enable
2.   Accepting a request to enable.
1.   Rejecting a request to enable.
2.   Offering to disable.
3.   Accepting a request to disables
1.   Approving an offer to enable.
2.   Requesting to enable.

1.   Disapproving an offer to enable.
2.   Approving an offer to disable.
3.   Requesting to disable.

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