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Much like individual people, operating systems have unique fingerprints that help identify them. The hard part is knowing how and where to look for these unique details and determine what each means.
There are two types of fingerprinting: Active and Passive...
How it works
Active fingerprinting uses specially crafted packets where passive fingerprinting uses sniffing techniques to capture packets coming form the target system.
Analysis
Actively sending probes to the target host and compare the responses to a database of known responses. Passively sniffing what ever responses available and analyzing the data for details of the OS used.
Chance of detection
Active OS fingerprinting has a high chance of detection because it introduces traffic into the network where passive OS fingerprinting has a low chance of detection because sniffing does not introduce traffic to the network.
Tools like Nmap has a great capability to do automated OS fingerprinting using a build in database.
There are two types of fingerprinting: Active and Passive...
How it works
Active fingerprinting uses specially crafted packets where passive fingerprinting uses sniffing techniques to capture packets coming form the target system.
Analysis
Actively sending probes to the target host and compare the responses to a database of known responses. Passively sniffing what ever responses available and analyzing the data for details of the OS used.
Chance of detection
Active OS fingerprinting has a high chance of detection because it introduces traffic into the network where passive OS fingerprinting has a low chance of detection because sniffing does not introduce traffic to the network.
Tools like Nmap has a great capability to do automated OS fingerprinting using a build in database.