Which of the following responses from a firewall indicates that a port is closed?Select one:a.RSTb.SYN-ACKc.FINd.ACK

1. Break Down the Problem

We need to identify which response from a firewall indicates that a port is closed out of the provided options.

2. Relevant Concepts

In TCP communication:

• RST: A reset packet indicates that no connection could be established; typically used to signal that a port is closed.
• SYN-ACK: This indicates that the port is open and ready to establish a connection.
• FIN: This indicates that a connection is being properly closed, not that a port is closed to new connections.
• ACK: This acknowledges a received packet but doesn’t specify the state of the port.

3. Analysis and Detail

• An RST response signals that the target port is not open and refuses the connection attempt. Thus, it indicates that the port is closed.
• The SYN-ACK response indicates that the port is fully open and willing to communicate.
• A FIN signal does not pertain to the initial port state and typically indicates an ongoing session ending.
• An ACK only indicates that a previously sent packet has been acknowledged.

4. Verify and Summarize

Cross-examining the function of each packet type confirms that the reset packet (RST) clearly indicates a closed port. Therefore, the correct answer is option a.

Final Answer

a. RST indicates that a port is closed.