TCP (Transmission Control Protocol) uses a three-way handshake (SYN, SYN-ACK, ACK) toestablish connections, as per RFC 793. When a client sends a SYN packet to a port:
Open Port:The server responds with SYN-ACK.
Closed Port (no firewall):The server sends an RST (Reset) packet, often with ACK, to terminate the attempt immediately.
However, when afirewallis present, its configuration dictates the response. Modern firewalls typically operate instealth mode, using a "drop" rule for closed ports rather than a "reject" rule:
Drop:Silently discards the packet without replying, resulting inno response. The client experiences a timeout (e.g., 30 seconds), as no feedback is provided.
Reject:Sends an RST or ICMP "Port Unreachable," but this is less common for security reasons, as it confirms the firewall’s presence.
For a closed TCP port behind a firewall, "no response" (drop) is the standard behavior in secure configurations, minimizing information leakage to attackers. This aligns with CNSP’s focus on firewall best practices to obscure network topology during port scanning (e.g., with Nmap).
Why other options are incorrect:
A. A FIN and an ACK packet:FIN-ACK is used to close anestablishedTCP connection gracefully (e.g., after data transfer), not to respond to an initial SYN on a closed port.
B. RST and an ACK packet:RST-ACK is the host’s response to a closed port without a firewall. A firewall’s drop rule overrides this by silently discarding the packet.
C. A SYN and an ACK packet:SYN-ACK indicates an open port accepting a connection, the opposite of a closed port scenario.
Real-World Context:Tools like Nmap interpret "no response" as "filtered" (firewall likely present) vs. "closed" (RST received), aiding in firewall detection.References:CNSP Official Study Guide (Firewall Operations and TCP/IP); RFC 793 (TCP).
