Attackers frequently use ACK scans to circumvent a firewall or other filtering tools. During a NULL scan, all packet flags are enabled. The most recent versions of Nessus Server and Client are compatible with Windows, Mac OS X, FreeBSD, and the vast majority of Linux variants.
<h3>What is ack scan ?</h3>
ACK scans are used to identify hosts or ports that have been blocked or are resistant to other types of scanning. An attacker uses TCP ACK segments to learn about firewall or ACL configuration.
Attackers probe our router or send unsolicited SYN, ACK, and FIN requests to specific UDP/TCP ports.
TCP ACK Scan sends an ACK message to the target port to determine whether or not it is filtered.
On unfiltered ports, a RST reply packet will be sent for both open and closed ports. Filtered ports will either generate no response or generate an ICMP reply packet with an unreachable destination.
The TCP ACK scanning technique attempts to determine whether a port is filtered by using packets with the ACK flag set.
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