What is the proper response for a NULL scan if the port is open?

What is the proper response for a NULL scan if the port is open?

• A. ACK response
• B. No response (F)
• C. RST-ACK
• D. SYN-ACK

Answer: (B)

In a NULL scan the probe packet carries no TCP flags. Because there is no connection attempt being made, an open port typically does not produce any reply—the TCP stack has nothing to respond with for a packet that has no flags. That silence is how the scanner identifies an open port. If the port were closed, the host would usually respond with a reset (RST) to terminate the attempt. The other responses listed come from different scan methods or states and do not occur when probing an open port with a NULL scan.

In a NULL scan the probe packet carries no TCP flags. Because there is no connection attempt being made, an open port typically does not produce any reply—the TCP stack has nothing to respond with for a packet that has no flags. That silence is how the scanner identifies an open port. If the port were closed, the host would usually respond with a reset (RST) to terminate the attempt. The other responses listed come from different scan methods or states and do not occur when probing an open port with a NULL scan.