Probing target systems to discover open TCP/UDP ports using techniques like SYN stealth; basis for service fingerprinting.

• A. Packet sniffing
• B. Traffic shaping
• C. Vulnerability scanning
• D. Port scanning

Answer: (D)

Probing target systems to discover open TCP/UDP ports is what port scanning is all about. A SYN stealth scan, a classic method within port scanning, sends a TCP SYN packet and watches the reply. If the port is open, a SYN-ACK is returned; if it's closed, a RST; if it's filtered, there may be no reply or an ICMP message. Because the scan doesn’t complete the full TCP handshake, it can be quieter and faster, helping map which ports are listening without fully establishing connections. This information is exactly what service fingerprinting builds on: knowing which ports are open reveals which services are likely running, and those services can be further probed to identify versions, configurations, and potential weaknesses.

Packet sniffing, by contrast, is about passively capturing and analyzing traffic that’s already flowing on the network, not actively discovering which ports are open. Traffic shaping is about controlling bandwidth and prioritizing traffic, not discovery. Vulnerability scanning looks for exploitable weaknesses across a system and often uses port discovery as a step, but it isn’t defined by the act of probing for open ports or fingerprinting services.