How does active fingerprinting reveal OS details

Question

Active fingerprinting sends specific packets to elicit responses. How are these responses analyzed to identify the target operating system?

Answer 1

Active OS fingerprinting is a technique used to determine the operating system of a target machine by sending specially crafted packets and analyzing the responses. This method leverages unique characteristics in the TCP/IP stack implementations of different operating systems.​

How Active Fingerprinting Reveals OS Details?

1. Sending Crafted Packets: Tools like Nmap send packets with specific flags, options, and payloads to the target system. These packets are designed to elicit responses that vary based on the operating system's implementation of network protocols.​

2. Analyzing Responses: The target system responds to these packets in ways that are influenced by its OS. Analysts examine various aspects of these responses, such as:

   • Time To Live (TTL) Values: Different operating systems set the TTL field in IP packets to different initial values.

   • Window Size: The size of the TCP window can indicate the OS's networking stack.

   • TCP Options: The presence and order of TCP options like Maximum Segment Size (MSS) and Window Scale can vary between OSes.

   • Initial Sequence Numbers (ISNs): The algorithm used to generate ISNs can differ across operating systems.​

3. Matching Patterns to Known Signatures: The collected data is compared against a database of known OS fingerprints. Tools like Nmap use these signatures to identify the operating system of the target machine.​

Example

Consider a scenario where a network scanner sends a SYN packet with a specific window size and receives a SYN-ACK response with a particular TTL value. By analyzing these and other factors, the scanner can match the response pattern to a known operating system signature, thereby identifying the target's OS.