Describe a SYN flood.
A SYN flood, or half-open TCP attack, happens when the attacker sends a large number of TCP SYN segments to the victim server. Each SYN begins a TCP session opening process on the server. The server sets aside RAM and other resources for the connection. The server then sends back a SYN/ACK segment. The attacker never completes the connection opening by sending a final ACK. As the attacker sends more SYN segments, the victim host keeps setting aside resources until it crashes or refuses to provide any more connections, even to legitimate users.