TELNET (TErminaL NETwork)

Definition

TELNET (TErminaL NETwork) is a remote login application protocol that enables a user to log in to a remote computer over a network and use its resources as if working directly on that computer.

TELNET follows the client-server paradigm, where the TELNET client runs on the user's computer and the TELNET server runs on the remote host.

TELNET uses TCP as its transport protocol and the default port number 23.

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Why TELNET is Needed?

Suppose a student wants to use a Java compiler installed on a university server. Instead of installing the compiler on every personal computer, the student can:

• Connect to the university server using TELNET.
• Log in with a username and password.
• Execute the Java compiler directly on the remote server.

Thus, TELNET provides remote access to applications and resources available on another computer.

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Need for Remote Login

Creating separate client-server applications for every software service (such as Java compiler, C compiler, database software, etc.) is impractical.

Instead, TELNET provides a generic remote login service, allowing users to access any application available on the remote machine after logging in.

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TELNET Architecture

            INTERNET

+----------------+                     +----------------+
|  TELNET Client |-------------------->| TELNET Server  |
+----------------+                     +----------------+
        |                                      |
   Local Computer                       Remote Computer

• The TELNET client accepts user commands.
• The TELNET server receives the commands.
• The server executes the commands on the remote operating system.
• The output is sent back to the client.

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Local Login vs Remote Login

1. Local Login

In local login, the user works directly on the local computer.

Working

1. User types commands.
2. Terminal driver receives the keystrokes.
3. Operating system interprets the commands.
4. Requested application executes locally.

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2. Remote Login (TELNET)

In remote login, the user accesses another computer over the Internet.

Working

1. User enters commands.
2. Local terminal driver receives the keystrokes.
3. TELNET client converts the characters into Network Virtual Terminal (NVT) format.
4. Data is sent through TCP over the Internet.
5. TELNET server receives the NVT characters.
6. TELNET server converts them into the remote system's character format.
7. A pseudo-terminal driver passes them to the remote operating system.
8. The requested application executes.
9. The output follows the reverse path back to the user.

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Network Virtual Terminal (NVT)

One challenge in remote login is that different computers may use different character sets and operating systems.

To solve this problem, TELNET uses a Network Virtual Terminal (NVT).

NVT

• A standard universal character representation used by TELNET.
• Acts as a common language between different computer systems.
• Ensures communication between heterogeneous systems.

Working

At the sender:

Local Character Set
        ↓
TELNET Client
        ↓
NVT Format

At the receiver:

NVT Format
        ↓
TELNET Server
        ↓
Remote Character Set

Thus, computers with different architectures can communicate successfully.

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Pseudo-Terminal Driver

The remote operating system normally expects input from a physical keyboard.

However, in TELNET:

• Input comes from the network.
• Therefore, a Pseudo-Terminal Driver is used.

Function

• Pretends to be a real terminal.
• Receives data from the TELNET server.
• Passes the commands to the operating system exactly as if they came from a local keyboard.

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Features of TELNET

• Provides remote login facility.
• Uses client-server architecture.
• Uses TCP for reliable communication.
• Supports command-line access to remote computers.
• Allows execution of remote programs.
• Uses Network Virtual Terminal (NVT) for interoperability.

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Advantages of TELNET

• Easy remote administration of computers.
• Provides access to remote applications.
• Platform independent through NVT.
• Reliable communication using TCP.
• Useful for testing and debugging network services.

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Limitations of TELNET

The major drawback of TELNET is lack of security.

• Usernames are transmitted in plain text.
• Passwords are transmitted in plain text.
• Commands are transmitted in plain text.
• Anyone intercepting the communication can read the data.

Because of these security issues, TELNET has largely been replaced by Secure Shell (SSH) for remote login.

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TELNET vs SSH

Feature	TELNET	SSH
Full Form	TErminaL NETwork	Secure Shell
Security	No encryption	Encrypted communication
Password Transmission	Plain text	Encrypted
Default Port	23	22
Authentication	Basic	Secure authentication
Data Privacy	Low	High
Current Usage	Testing, diagnostics, legacy systems	Secure remote administration

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Applications of TELNET

• Remote login to servers.
• Remote system administration (legacy systems).
• Network troubleshooting.
• Testing server applications.
• Debugging network services.
• Accessing remote command-line interfaces.

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Summary

• TELNET (TErminaL NETwork) is a remote login protocol that enables users to access and control a remote computer over a network.
• It follows the client-server model, uses TCP as the transport protocol, and communicates through port 23.
• TELNET converts local characters into Network Virtual Terminal (NVT) format, allowing communication between heterogeneous systems, and uses a pseudo-terminal driver on the remote host to interface with the operating system.
• Since all communication, including usernames and passwords, is sent in plain text, TELNET is considered insecure and has been largely replaced by SSH (Secure Shell) for secure remote access.