Testing Open Ports in Linux

Testing open ports is a fundamental task in network troubleshooting and security assessment. While Telnet has been a traditional tool for this purpose, there are several alternatives available in Linux that offer similar functionality with added features and capabilities. I will share few ways and how they can be used to test open ports effectively.

1. Netcat (nc):

Netcat is a versatile networking utility that can be used for reading from and writing to network connections using TCP or UDP. Its -z option allows you to scan for open ports without sending any data, making it a useful tool for testing port availability.

Example usage:

nc -zv host_ip port

Replace host_ip with the IP address or hostname of the target system and port with the port number you want to test.

Output When Port Connection Working

Output When Port Connection Not Working

2. Nmap:

Nmap is a powerful network scanning tool that can be used to discover hosts and services on a computer network. It can also be used to check for open ports on a remote system. Nmap's flexibility and extensive feature set make it a preferred choice for network administrators and security professionals.

Example usage:

nmap -p port host_ip

Replace port with the port number you want to test and host_ip with the IP address or hostname of the target system.

Output When Port Connection Working

Output When Port Connection Not Working

3. Telnet Client:

Telnet's client can also be used to test open ports by attempting to establish a connection to a specific host and port. While not as versatile as Netcat or Nmap, Telnet's simplicity makes it a straightforward option for basic port testing.

Example usage:

telnet host_ip port

Replace host_ip with the IP address or hostname of the target system and port with the port number you want to test.

Output When Port Connection Working

Output When Port Connection Not Working

4. Curl:

Curl is primarily used for making HTTP requests, but it can also be used to test TCP ports by sending a simple request to the target system. While not as commonly used for port testing, Curl's availability on most Linux distributions makes it a convenient option when other tools are not available. Example usage:

curl telnet://host_ip:port

Replace host_ip with the IP address or hostname of the target system and port with the port number you want to test.

Output When Port Connection Working

if working no output. No need to telnet to be installed.