What is Telnet? A Basic Protocol for Remote Connection

Before SSH became the standard for secure connections, “Telnet” was a crucial tool for accessing remote systems, particularly in the era before the year 2000. Although Telnet is no longer recommended for use in systems requiring high security, a basic understanding of Telnet remains beneficial for learning network systems and debugging systems that do not require data encryption.

What is Telnet?

Telnet (short for Teletype Network) is a protocol used to connect to a remote computer over a network, allowing users to access the system and use the command line as if they were physically at the machine.

Telnet operates over TCP/IP by establishing a connection through port 23 without encryption. This means that data transmitted can be easily intercepted by malicious parties.

Key Features of Telnet:

Use the standard port, which is Port 23.
Unencrypted Data (Plain Text)
For use in applications that do not require security, such as port or service testing.

History of Telnet

Developed in 1969 by the ARPANET group
One of the first protocols that enabled remote access to systems.
Replaced with SSH for security reasons.

Basic Usage of Telnet

Installing Telnet

On Windows:

Enable-WindowsOptionalFeature -Online -FeatureName TelnetClient

On Linux:

sudo apt install telnet

On macOS:

brew install telnet

Example usage command:

telnet example.com 80
telnet localhost 23

Using Telnet for Port Testing

Telnet can be used to check whether the port of a destination service is open, for example:

telnet smtp.gmail.com 587

If the connection is successful, it means the port is enabled.

Risks and Limitations of Telnet

No encryption: Data is transmitted in plain text.
It can be easily intercepted.
There is no strong identity verification.
Should not be used in production systems with important data.

Telnet vs SSH

Topic	Telnet	SSH
Encryption	None.	Strong Encryption
Standard port	23	22
Safety	Low	High
Suitable for	Test Internal System	Production System

Disabling Telnet on the Server

For security reasons, Telnet should be disabled and only SSH should be enabled.

sudo systemctl disable telnet
sudo systemctl stop telnet

Summary:

Telnet is one of the fundamental protocols that once served as the foundation for remote communication. Although it is no longer recommended for use in production systems, Telnet remains useful in education, network testing, and understanding TCP/IP connections, which form the basis of the modern internet.