OS fingerprinting is a technique wherein a remote machine sends various types of commands to a target device and analyzes the responses to attempt to identify the target devices' operating system and version. Knowing which operating system a device is running makes it possible to use exploits specific to that operating system.

Nmap detects operating systems based on a series of port scans, ICMP pings, and numerous other tests, and then runs a set of follow-up tests based on the results to further define the OS version running.

In the following screenshot, you can see the test results verbiage from the GUI version of Nmap (Zenmap) as it completes an OS detection scan, as well as its best estimate of the operating system and version:

A Wireshark capture of the OS detection activity described earlier included as an example of one of the OS fingerprinting scripts that are run, a bogus HTTP request to the target device (172.20.0.1) for /nice%20ports%2C/Tri%6Eity.txt%2ebak to see exactly what kind of error response was generated, which is used to help pinpoint the OS version:

The exact format of the HTML response from the preceding request could be used to identify the OS and/or web server version, as seen in the following Wireshark packet details screenshot:

Analyzing packet captures of these kinds of OS fingerprinting requests and responses will make it much easier to spot similar activities from malicious entities.