Crucial Exams

Certified Ethical Hacker (CEH) Practice Question

During an external assessment you launch a TCP SYN scan with the command nmap -sS -p 21,22,80 203.0.113.10. The results show 21/tcp closed, 22/tcp filtered, and 80/tcp open. According to Nmap's port-state definitions, which situation best explains why only port 22 appears as filtered while the others are not?

  • The SSH daemon on port 22 actively rejects unsolicited SYN packets by returning a TCP RST, so Nmap flags the port as filtered.

  • A firewall is discarding packets to TCP port 22 without sending any response, preventing Nmap from learning whether the port is open or closed.

  • The target host was unreachable during the probe, so Nmap marked all unresponsive ports, including 22, as filtered until it saw an ICMP error.

  • Port 22 is listening but restricted to specific IP addresses; Nmap therefore lists it as filtered even though the service is open.

Certified Ethical Hacker (CEH)
Reconnaissance Techniques
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