What indicates that a port is closed during a stealth scan

Question

In a stealth scan, the attacker avoids a full TCP handshake. What specific response signals that a port is closed?

Answer 1

​In a stealth scan, such as Nmap's SYN scan (-sS), the attacker sends a SYN packet to the target port without completing the TCP handshake. The response received indicates the port's status:​

• Open Port: If the port is open, the target responds with a SYN-ACK packet, indicating it is ready to establish a connection.​

• Closed Port: If the port is closed, the target responds with a RST (reset) packet. This response indicates that the port is accessible but there is no application listening on it.​

• Filtered Port: If no response is received, it suggests that the port is filtered, possibly due to a firewall blocking the probe.

This method allows the attacker to determine the status of ports without establishing a full connection, making it less detectable by intrusion detection systems.