ACK scan is meant to be used for mapping firewall rule sets rather than discovering listening ports on hosts. As we can see below all combinations gave the same result. TCP ACK Service State No Firewall Firewall -sA Listening State: Unfiltered State: Unfiltered nmap target ACK RST nmap target ACK RST Not Listening State: Unfiltered State: Unfiltered nmap target ACK RST nmap target ACK RST
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Source: Wikipedia