Passive OS fingerprinting involves observing traffic from a remote host and analyzing it to infer details about the operating system without actively sending packets or probes. This is useful in stealthy reconnaissance where avoiding detection is critical.

tcpdump is a packet analyzer that captures traffic in real time. By analyzing certain TCP/IP header fields such as TTL (Time-To-Live), window size, TCP options, and DF (Don't Fragment) flags, attackers can passively deduce the operating system of the target host.

CEH v13 Guide states:

“Passive fingerprinting tools like tcpdump and Wireshark allow the attacker to capture packets and analyze them for OS-specific traits, making it possible to identify the OS without sending packets to the target system.”

Reference – CEH v13 Study Guide:

Module 02: Footprinting and Reconnaissance, Section: “OS Fingerprinting Techniques”, Subsection: “Passive OS Fingerprinting”

Incorrect Options Explained:

A: nmap is primarily an active scanning tool (though it has limited passive capabilities).

C: tracert is used for tracing packet routes, not OS fingerprinting.

D: ping checks host availability and latency, not OS details.

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