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As the result of an ACK scan, you receive an ICMP destination unreachable message. This tells that the port is:
Open
Closed
Unfiltered
Filtered
This indicates that a firewall is filtering the port. Edit on 24 Feb 2018: is the answer correct? If a stateful firewall is behind the port(that means the port is filtered), no response will be sent to attacker. This is the CEH courseware. Another reference is "www.hackingloopos.com/ack-flag-scanning-attack". Please check. Edit: It is correct. Filtered port returns ICMP error message or no reply at all.
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