Thanks for your answer.
Actually I tried to use the port forwarding but I did not succeed. I did the following:
imagine the real DNS server is 220.127.116.11
the external address of zeroshell is 18.104.22.168
the internal address of zeroshell is 192.168.2.1
within the DNS page of zero shell I activated the port forwarding as:
in the clients (192.168.2.2) DNS configuration I put 22.214.171.124
It did not work. I co not understand why.
Where am I wrong??