Configuring NAT Port Redirection rules is the typical way to allow the internal servers to be accessible from the Internet. However, once the port opens, it is exposed to the Internet and can be scanned by the malware.Port knocking is a technology that can add an extra layer of protection to the internal servers. Its basic idea is that only open ports are at risk of being attacked, so it allows all ports to be closed at the beginning. Do not open them, and then set a password based on the port combination. Only those who know the password can open the ports and connect.