Introduction to TELNET – GeeksforGeeks

Prerequisite: Network protocol types

TELNET stands for Teletype Network. It is a type of protocol that allows a computer to connect to the local computer. It is used as a standard TCP/IP protocol for virtual terminal service which is provided by ISO. The computer that initiates the connection is known as the local computer.

The computer to which you are connecting, that is, it accepts the connection known as

the remote computer.

During telnet operation, whatever is being done on the remote computer will be displayed by the local computer. Telnet works on a client/server principle. The local computer uses a telnet client program, and the remote computers use a telnet server program.

Registration

The registration process can be classified into two parts:

Local logon

  1. Remote logon

1. Local logon: Every time a user logs on to their local system, it is known as local logon.

The procedure of local login keystrokes

  • is accepted by the terminal driver when the user types in the terminal
  • .

  • Terminal Driver passes these characters to the operating system
  • .

  • Now, OS validates the character combination and opens the required application

.

2. Remote login: Remote logon is a process where users can log in to a remote site, i.e. a computer and use the services that are available on the remote computer. With the help of remote logon, a user can understand the result of transferring the result of processing from the remote computer to the local computer.

The

remote logon procedure

  • When the user types something on
  • the local computer, the local operating system accepts the character. The local

  • computer does not interpret the characters, it will send
  • them to the TELNET client.

  • The TELNET client transforms these characters into a universal character set called Network Virtual Terminal (NVT) characters and passes them to the local TCP/IP protocol stack.
  • Commands or text that are in the form of NVT, travel over the Internet and will reach the TCP/IP stack on the remote computer.
  • The characters are delivered to the operating system and then passed to the TELNET server.
  • The TELNET server then changes those characters to characters that can be understood by a remote computer.
  • The remote operating system receives characters from a pseudo-terminal driver, which is a piece of software that pretends that the characters come from a terminal.
  • The operating system then passes the character to the appropriate application program.

Network

Virtual Terminal

(NVT) is a

virtual terminal on TELNET that has a fundamental structure that is shared by many different types of real terminals. NVT (Network Virtual Terminal) was created to make communication between different types of terminals with different operating systems viable.

Telnet

commands

Telnet commands are identified by a prefix character, Interpret As Command (IAC) with the code 255. IAC is followed by command and option codes.

The

basic format of the command is as shown

in the following figure: The following are some of the

important TELNET commands

:

Here are some common options used with telnet:

Advantages of Telnet

  1. It provides remote access to someone’s computer system
  2. . Telnet

  3. allows the user greater access with fewer problems in data transmission
  4. .

  5. Telnet saves a lot of time.
  6. The older system

  7. can be connected to a newer system with telnet that has different operating systems.

Disadvantages of Telnet

As it is

  1. somehow complex, it becomes difficult for beginners in understanding
  2. .

  3. The data is sent here in plain text form, that’s why it’s not as secure
  4. .

  5. Some capabilities are disabled due to lack of proper interconnection of remote and local devices. Modes

of operation

Most telnet implementations work in one of the following three modes:

Default mode Character mode

  1. Line

mode 1. Default mode: If no other modes are invoked, this mode is used. The echo is made in this mode by the customer. In this mode, the user types a character and the client echoes the character on the screen, but does not send it until the entire line is completed.

2. Character mode: Each character written in this mode is sent by the client to the server. A server in this type of mode typically echoes the characters to be displayed on the client screen.

3. Line mode: Editing lines like echo, character erasing, etc. is done from the client side. The client will send the entire line to the server.

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