Firewall

Firewalls are security devices or software that monitor and control incoming and outgoing network traffic based on predetermined security rules. They act as a barrier between a trusted internal network and untrusted external networks, such as the internet.

Drop incoming traffic with TCP Reset. This is useful to prevent connections to a specific port, e.g. when someone uses 'nmap -sS' to scan for open ports.

iptables -I INPUT -p tcp --dport <port> -j REJECT --reject-with tcp-reset