TCP’sthree-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:Anopen 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 aSYN-ACKpacket, 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.References:CNSP Official Documentation (TCP/IP Fundamentals); RFC 793 (TCP).
