don't know if this is 'old news' but found it interesting. its from: http://www.theregister.co.uk/2005/03/11/clock_skews_finger_pcs/ "Researchers say they have found a technique which, with development, will enable someone to track hardware on the internet or within applications and identify it as a form of electronic fingerprinting. By measuring tiny deviations, known as clock skews, in a devices hardware experts say that it may be possible to make the identification so unique that it could be used in a court of law, if the skews can be proven to be stable. The passive technique is not OS-dependant but does rely on TCP timestamps in TCP headers, a feature which can be disabled in Linux or BSD. A NAT or firewall will have no effect on the fingerprinted device."