Nmap (Network Mapper) is a free and open-source tool used for network discovery and security auditing. It utilizes raw IP packets to identify the hosts available on a network, services they offer, operating systems they use, types of packet filters/firewalls in use, and several other characteristics. The “-O” command in Nmap is used for OS detection.
According to Nmap’s official documentation, the “-O” command enables OS detection. It invokes Nmap’s built-in set of probes and matching system to guess what operating system is running on the systems targeted for scanning. The “-O” command changes Nmap’s behavior by adding many sequences of test packets designed to provoke responses, or lack thereof, which enable Nmap to guess the target’s operating system.
It should be noted that while the “-O” command is very effective and often accurate, it’s not infallible. Inaccuracies could be due to a variety of reasons including, but not limited to: limitations in the OS classifications, the inability to induce the target OS into providing unique responses, the existence of so-called ‘liars’ (machines configured to give false information) and potential interference from firewalls or intrusion detection systems.
A typical usage of the “-O” command would look like the following:
nmap -O target\_ip
The result will provide you with an assumption of what the operating system of the target IP could be.
Despite its great utility, the OS detection functionality of “-O” command works best if the person using it has privilege access. This is because unprivileged users, even those conducting scans of their own systems, may not receive the raw packet responses required to get an accurate OS match. This is why Nmap also offers the “-oS” command, an adjunct to the “-O” command, which writes OS detection output to the specified file.
To qualify the answer, the above information is based on the official Nmap documentation.
“Nmap OS Detection”, Nmap Official Documentation, https://nmap.org/book/osdetect.html.
Finally, it’s important to use Nmap ethically and responsibly. Unauthorized scanning can be illegal in many jurisdictions and also can be disruptive or offensive to the network being scanned.
Sources:
1. Nmap Docs. (n.d.). Chapter 15. Nmap Reference Guide. AlienVault. https://nmap.org/man/fr/man-host-discovery.html.
2. Lyon, G. F. (2008). Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning. Nmap Project.
3. Nmap Docs. (n.d.). OS Detection. AlienVault. https://nmap.org/book/osdetect.html.
