A SYN scan or "half-open" scan, on the other hand, are another way an attacker can try to enumerate ports on a system in a stealthy manner. These scans only execute the first two steps of the TCP three-way handshake. The initiating system sends TCP SYN packets as though it were requesting to open a full connection. The target system responds with a SYN-ACK packet. The initiator then sends a TCP RST (reset) packet back to the target,...
thereby closing the connection. The idea here is to prevent the full connection from being established since it may possibly be logged. Most of the scanners also allow SYN scans using UDP packets in much the same manner.
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