A SYN flood DoS attack is a resource-consumption attack. It works by sending a large number of TCP SYN requests to the remote port associated with the service that is the target of the attack. For each initial SYN packet that is received by the target service, it will then send out a SYN+ACK packet and hold the connection open to wait for the final ACK packet from the initiating client. By overloading the target with these half-open requests, an attacker can render a service unresponsive.