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

TCP’s three-way handshake, per RFC 793, establishes a connection:

Client → Server: SYN (Synchronize) packet (e.g., port 80).

Server → Client: SYN-ACK (Synchronize-Acknowledge) packet if the port is open and listening.

Client → Server: ACK (Acknowledge) completes the connection.

Scenario: An open TCP port (e.g., 80 for HTTP) with no firewall. When a client sends a SYN to an open

port (e.g., via telnet 192.168.1.1 80), the server responds with a SYN-ACK packet, indicating

willingness to connect. No firewall means no filtering alters this standard response.

Packet Details:

SYN-ACK: Sets SYN and ACK flags in the TCP header, with a sequence number and acknowledgment

number.

Example: Client SYN (Seq=100), Server SYN-ACK (Seq=200, Ack=101).

Security Implications: Open ports responding with SYN-ACK are easily detected (e.g., Nmap “open”

state), inviting exploits if unneeded (e.g., Telnet on 23). CNSP likely stresses port minimization and

monitoring.

Why other options are incorrect:

A . A FIN and an ACK packet: FIN-ACK closes an established connection, not a response to a new SYN.

B . A SYN packet: SYN initiates a connection from the client, not a server response.

D . A RST and an ACK packet: RST-ACK rejects a connection (e.g., closed port), not an open one.

Real-World Context: SYN-ACK from SSH (22/TCP) confirms a server’s presence during reconnaissance.

Reference: CNSP Official Documentation (TCP/IP Fundamentals); RFC 793 (TCP).