Port Scanning Assumptions
All forms of port scanning rely on the assumption that the targeted host is compliant with RFC 793 - Transmission Control Protocol. Although it is the case most of the time, there is still a chance a host might send back strange packets or even generate false positives when the TCP/IP stack of the host is non-RFC-compliant or has been altered. This is especially true for less common scan techniques that are OS-dependent (FIN scanning, for example). The TCP/IP stack fingerprinting method also relies on these kind of different network responses from a specific stimulus to guess the type of the operating system the host is running.
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“What a man believes may be ascertained, not from his creed, but from the assumptions on which he habitually acts.”
—George Bernard Shaw (1856–1950)