A common Transmission Control Protocol (TCP) port scan sends a TCP SYN (synchronize) packet to a specific port of a server as part of the TCP three-way handshake. If the server responds with a SYN/ACK (synchronize/acknowledge) packet, the scanner knows the port is open. However, instead of completing the three-way handshake, the scanner can send an RST (reset) packet to reset the connection and then repeat the process with a different port.

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