Understanding FIN Scans

A FIN scan sends TCP segments with the FIN flag set in an attempt to provoke a response (a TCP segment with the RST flag set) and thereby discover an active host or an active port on a host. Attackers might use this approach rather than perform an address sweep with ICMP echo requests or an address scan with SYN segments, because they know that many firewalls typically guard against the latter two approaches but not necessarily against FIN segments. The use of TCP segments with the FIN flag set might evade detection and thereby help the attackers succeed in their reconnaissance efforts.