When designing a firewall, what is the recommended approach for opening and closing ports?

answer

Close all ports; open ports $20,21,53,80,$ and $443.$

Open all ports; close ports that show improper traffic or attacks in progress.

Close all ports.

Close all ports; open only ports required by applications inside the DMZ$.$

Open all ports; close ports that expose common network attacks.

Identifying inherent vulnerabilities