After we have established that the target machine is alive, we can then find out which operating system is used by the target machine. This method is commonly known as Operating System (OS) fingerprinting. There are two methods of doing OS fingerprinting: active and passive.
In the active method, the tool sends network packets to the target machine and then analyzes the response it receives to determine the operating system of the target machine. The advantage of this method is that the fingerprinting process is fast. However, the disadvantage is that the target machine may notice our attempt to get its operating system's information.
To overcome the active method's disadvantage, a passive method of OS fingerprinting exists. This method was pioneered by Michal Zalewsky when he released a tool called p0f. The major advantage of passive OS fingerprinting...