Técnica útil para detectar qué sistema operativo se usa en un cierto equipo remoto. Consiste en analizar los paquetes TCP que envía buscando características signulares peculiares de un cierto sistema.

Analyzing packet headers for certain unusual characteristics or combinations of characteristics that are exhibited by particular operating systems or applications. [NIST-SP800-94:2007]

TCP fingerprinting is the user of odd packet header combinations to determine a remote operating system.

http://www.sans.org/security-resources/glossary-of-terms/