alienvault openvas vulnerability scan shows non existing hosts in the reports

Dear all,

When performing a vulnerability scan that scans hosts through our fortigate firewall it shows non existing hosts in te reports that do not appear when doing the same scan without traffic going through a fortigate firewall.

This is wat alienvault support had to say about it:

Looks like this issue is related with a firewall that is answering in all the ips This is how the plugin works:  Special IP packets are forged and sent with a little delay in between to the  target IP. The responses are searched for a timestamps. If found, the timestamps are reported. 

How can I configure the fortigate D600 firewall so it does not interfere with these scans?

Best regards,

Ronald Verlaan