An adversary uses TCP SYN packets as a means towards host discovery. Typical RFC 793 behavior specifies that when a TCP port is open, a host must respond to an incoming SYN \'synchronize\' packet by completing stage two of the 'three-way handshake' - by sending an SYN/ACK in response. When a port is closed, RFC 793 behavior is to respond with a RST \'reset\' packet. This behavior can be used to 'ping' a target to see if it is alive by sending a TCP SYN packet to a port and then looking for a RST or an ACK packet in response.
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