Therefore, this result proves the thesis that the system is responding to ACK indiscriminately. In addition, we can check the TCP segments with ACK in the Wireshark and the corresponding responses with RST. Thus, we can deduce that we do not have a stateful firewall between source and destination. We can see that the answer was that the ports were not filtered. Therefore, this scan is helpful to verify if we have correctly configured the firewall.Įxample of a scan using ACK to a host at address 192.168.0.1 and ports 20 to 25: sudo nmap -sA -p 20-25 192.168.0.1 nmap scan tcp ack If the target responds with RST, we assume we do not have a stateful firewall rule before the analyzed service. Instead, we want to check if there is an RST response to our query. That way, we do not want to know whether the doors are open. First, let us talk about the objective of our scan, which is to get information if the firewall is stateful. We now present a different scan in terms of its functioning and its purpose. The command below uses the -sF option to indicate that a segment with the FIN flag should be sent.

Thus, if a target does not respond to a specific port, it will be considered open or filtered.Īt first, Nmap has no way of knowing if the port is open or if there is some security mechanism preventing the response. Thus, when receiving a FIN flag for a port that is not open, the system responds with a RST.

This scan sends TCP segments with the FIN flag. We can see in Wireshark that the TCP segments flag field sent to the target is null.Īdditionally, in this type of scan, the target’s lack of response Indicates that the port is open or filtered by a firewall.

Therefore, NMAP sets the field which refers to the flags to zero. In this scanning mode, we will use a null TCP flag header. The first type of scan we will look at is TCP NULL.

Advanced nmap