Advanced port scan technique involving an incomplete TCP connection. The purpose of using a half-open connection is to know if a given server has a given port open without generating a log in the server with your IP address.

It involves sending a SYN packet to the desired server, as in a normal TCP connection. If we get a SYN+ACK back, we know the port is open. A RST indicates the absence of a listening process on that port. A RST should be sent back to the server right away, to clear the connection.