What is the response from an open TCP port which is not behind a firewall

Question

If a TCP port is open and not behind a firewall, what should I expect the response to be when I send a SYN packet? Does the target system send a SYN-ACK back to indicate the port is open? Or, in cases where there’s no service listening on the port, will the system respond differently?

Answer 1

When you send a TCP SYN packet to a target system, the response varies based on the state of the destination port:

• Open Port: If an application is actively listening on the port, the target system responds with a SYN-ACK packet, indicating readiness to establish a connection.

• Closed Port: If no application is listening, the system responds with a RST (Reset) packet, signaling that the connection cannot be established.

• Filtered Port: If a firewall or filter is present, it may block the SYN packet, resulting in no response or an ICMP unreachable error, indicating the port's status cannot be determined.