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.