Introduction

Telnet is a network protocol that allows users to remotely connect to other computers over TCP/IP networks. It is often used for network testing and troubleshooting. However, Telnet transmits data, including passwords, in plain text, making it insecure for sensitive operations. For secure remote access, SSH (Secure Shell) is the recommended alternative.

This guide will walk you through installing Telnet client and server on Ubuntu/Debian and CentOS/RHEL/AlmaLinux systems, configuring firewall rules, testing network connectivity, and uninstalling Telnet when no longer needed.

Installing Telnet on Ubuntu/Debian Systems

Step 1: Update Your System

Before installing any package, update your system to ensure you have the latest package versions:

sudo apt update && sudo apt upgrade -y

Step 2: Install Telnet Client

The Telnet client allows you to connect to remote Telnet servers. Install it using:

sudo apt install telnet -y

Step 3: Install Telnet Server (If Needed)

If you need to allow Telnet connections to your machine, install the Telnet server:

sudo apt install xinetd telnetd -y

Step 4: Configure Telnet Server

Create a Telnet configuration file for xinetd:

sudo nano /etc/xinetd.d/telnet

Add the following content:

service telnet
{
    disable = no
    flags = REUSE
    socket_type = stream        
    wait = no
    user = root
    server = /usr/sbin/in.telnetd
    log_on_failure += USERID
}

Save and exit (CTRL + X, then Y, then ENTER).

Restart and enable the xinetd service:

sudo systemctl restart xinetd
sudo systemctl enable xinetd

Step 5: Adjust Firewall Settings

If UFW (Uncomplicated Firewall) is enabled, allow Telnet traffic on port 23:

sudo ufw allow 23/tcp

Check firewall status:

sudo ufw status

Step 6: Test Telnet Connection

To test Telnet, connect from the same machine or a remote client:

telnet server_ip_address

You should see a login prompt. Enter your username and password to log in.

Installing Telnet on CentOS/RHEL/AlmaLinux Systems

 

Step 1: Update Your System

Update the system package lists:

sudo yum update -y

Step 2: Install Telnet Client

Install the Telnet client:

sudo yum install telnet -y

Step 3: Install Telnet Server (If Needed)

To allow Telnet access to your server, install the Telnet server package:

sudo yum install telnet-server xinetd -y

Step 4: Configure Telnet Server

Edit the Telnet configuration file:

sudo nano /etc/xinetd.d/telnet

Find the line:

disable = yes

Change it to:

disable = no

Save and close the file.

Start and enable the xinetd service:

sudo systemctl start xinetd
sudo systemctl enable xinetd

Step 5: Adjust Firewall Settings

If using firewalld, open port 23:

sudo firewall-cmd --permanent --add-port=23/tcp
sudo firewall-cmd --reload

Step 6: Test Telnet Connection

To test if Telnet is working, run:

telnet server_ip_address

You should receive a login prompt.

Using Telnet for Network Troubleshooting

 

Testing if a Port is Open

Telnet can check if a remote port is open:

telnet example.com 80

If the connection is successful, the terminal will clear or display a response from the server.

Sending an HTTP Request via Telnet

You can manually send HTTP requests using Telnet:

telnet example.com 80

Type the following and press Enter twice:

GET / HTTP/1.1
Host: example.com

If successful, you will receive an HTTP response.

Security Considerations

⚠️ Telnet is insecure because it transmits data in plain text. Consider the following precautions:

• Use SSH instead of Telnet for secure remote access.
• Only enable Telnet on trusted, internal networks.
• Disable Telnet when not in use to prevent unauthorized access.
• Use firewalls to restrict access to trusted IP addresses only.

Uninstalling Telnet

If you no longer need Telnet, uninstall it for security reasons.

Ubuntu/Debian Systems

sudo apt remove telnet -y
sudo apt remove telnetd xinetd -y

CentOS/RHEL/AlmaLinux Systems

sudo yum remove telnet -y
sudo yum remove telnet-server xinetd -y

Conclusion

You have successfully installed Telnet on your Linux server for network troubleshooting. While Telnet is useful for testing network connectivity and ports, it should be used cautiously due to security risks. Whenever possible, use SSH instead of Telnet for remote administration.

For secure communication and better encryption, disable or remove Telnet when not in use.

Additional Resources

• Telnet Man Page: man telnet
• Comparison: SSH vs. Telnet
• Ubuntu Community Help: Telnet
• Official OpenSSH Documentation: https://www.openssh.com/

By following these best practices, you can ensure a secure and reliable networking environment.