What type of port scanning is referred to as "half open" scanning?

The term "half open" scanning refers to TCP SYN scanning, which is a method used in port scanning to identify open ports on a target system. This technique utilizes the TCP three-way handshake process but only completes the first step, which is the SYN (synchronize) packet sent by the scanner to initiate a connection.

When a SYN packet is sent to a port, the expected response can differ based on the port's state: if the port is open, the target typically responds with a SYN-ACK (synchronize-acknowledge) packet, indicating it is ready to establish a connection. If the port is closed, the response is typically an RST (reset) packet. However, since the scanner does not complete the handshake by sending the final ACK (acknowledge) packet, this scanning method is referred to as "half open."

This approach can be advantageous in stealthily scanning a network, as it often does not create a full connection that could be logged by the target system. It can help an attacker avoid detection, making it a popular choice in security assessments and penetration testing.

In contrast, options related to UDP scanning and TCP FIN scanning do not operate by leveraging the SYN/ACK mechanism inherent in TCP connections,