An attacker uses TCP ACK segments to gather information about firewall or
						ACL configuration. The purpose of this type of scan is to discover
						information about filter configurations rather than port state. This type of
						scanning is rarely useful alone, but when combined with SYN scanning, gives
						a more complete picture of the type of firewall rules that are present. When
						a TCP ACK segment is sent to a closed port, or sent out-of-sync to a
						listening port, the RFC 793 expected behavior is for the device to respond
						with a RST. Getting RSTs back in response to a ACK scan gives the attacker
						useful information that can be used to infer the type of firewall present.
						Stateful firewalls will discard out-of-sync ACK packets, leading to no
						response. When this occurs the port is marked as filtered. When RSTs are
						received in response, the ports are marked as unfiltered, as the ACK packets
						solicited the expected behavior from a port. When combined with SYN
						techniques an attacker can gain a more complete picture of which types of
						packets get through to a host and thereby map out its firewall rule-set. ACK
						scanning, when combined with SYN scanning, also allows the attacker analyze
						whether a firewall is stateful or non-stateful. If a SYN solicits a SYN/ACK
						or a RST and an ACK solicits a RST, the port is unfiltered by any firewall
						type. If a SYN solicits a SYN/ACK, but an ACK generates no response, the
						port is statefully filtered. When a SYN generates neither a SYN/ACK or a
						RST, but an ACK generates a RST, the port is statefully filtered. When
						neither SYN nor ACK generates any response, the port is blocked by a
						specific firewall rule, which can occur via any type of firewall.1. Speed: TCP ACK scanning is fast compared to other types of
							scans2. Stealth: TCP ACK scanning is stealthy3. Open Port: Cannot detect open ports4. Closed Port: Cannot detect closed ports5. Filtered Port: Can detect stateful vs non-stateful filters when
							combined with SYN probes6. Unfiltered Port: Can detect unfiltered ports when combined with SYN
							probes