install netstat

To use netstat to check active connections and open ports, which can help in enabling the firewall, follow these steps:

1. Install netstat (if not already installed):

• On Debian-based systems like Ubuntu, you can install net-tools, which includes netstat:

sudo apt update
sudo apt install net-tools

2. Check listening ports:

• Use the following command to see all active connections and listening ports:

sudo netstat -tuln

• -t shows TCP connections.
• -u shows UDP connections.
• -l shows only listening ports.
• -n shows numeric addresses instead of resolving names.

3. View specific details:

• To view specific details about the processes listening on the ports:

sudo netstat -tulnp

• -p shows the PID (Process ID) and the name of the program listening on each port.