1. A SYN flood is a type of DoS attack. A SYN packet notifies a server of a new connection. The server then allocates some memory in order to handle the incoming connection, sends back an acknowledgement, then waits for the client to complete the connection and start sending data. By spoofing large numbers of SYN requests, an attacker can fill up memory on the server, which will sit their waiting for more data that never will arrive. Once memory has filled up, the server will be unable to accept connections from legitimate clients. This effectively disables the server. Key point: SYN floods exploit a flaw in the core of the TCP/IP technology itself. There is no complete defense against this attack. There are, however, partial defenses. Servers can be configured to reserve more memory and decrease the amount of time they wait for connections to complete. Likewise, routers and firewalls can filter out some of the spoofed SYN packets. Finally, there are techniques (such as "SYN cookies") that can play tricks with the protocol in order to help distinguish good SYNs from bad ones. From Hacking-Lexicon