Port Knocking is a clever and interesting method of allowing remote firewall manipulation whilst leaving all ports closed to all IPs. When I attempt to initiate a TCP connection to a remote host I send a packet with a ‘SYN’ flag, indicating my intention, along with other information such as a source port, destination port, source IP and destination IP. The target machine has the option of responding by accepting, responding by rejecting, or simply ignoring the packet alltogether, known under iptables and most other firewalls as ACCEPT, REJECT or DROP.
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Tags: accept, connection, drop, firewall, IP, iptables, port, port knocking, protocol, reject, source ip, syn, target, tcp