This OS fingerprinting probe tests the target system's assignment of TCP sequence numbers. One common way to test TCP Sequence Number generation is to send a probe packet to an open port on the target and then compare the how the Sequence Number generated by the target relates to the Acknowledgement Number in the probe packet. Different operating systems assign Sequence Numbers differently, so a fingerprint of the operating system can be obtained by categorizing the relationship between the acknowledgement number and sequence number as follows: 1) the Sequence Number generated by the target is Zero, 2) the Sequence Number generated by the target is the same as the acknowledgement number in the probe, 3) the Sequence Number generated by the target is the acknowledgement number plus one, or 4) the Sequence Number is any other non-zero number.

Prerequisites

The ability to monitor and interact with network communications.Access to at least one host, and the privileges to interface with the network interface card.

Resources Required

A tool capable of sending and receiving packets from a remote system.

Related Weaknesses

References

REF-33

Hacking Exposed: Network Security Secrets & Solutions
Stuart McClure, Joel Scambray, George Kurtz.

REF-128

RFC793 - Transmission Control Protocol
Defense Advanced Research Projects Agency Information Processing Techniques Office, Information Sciences Institute University of Southern California.
http://www.faqs.org/rfcs/rfc793.html

REF-212

Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning
Gordon "Fyodor" Lyon.

Submission

Modifications