I think may be a little bug when FreeNas is configured to use DHCP.
When DHCP is active, the client(freeNAS itself) seems don't receive/request the DNS server list from DHCP server.
In fact it is not able to resolve names in Ping and Traceroute utility.
This is true even the field DNS Server in "General Setup" page are filled with the right DNS server (the same from DHCP request !).
To make freeNAS able to resolve names, I was forced do disable DHCP feature and assign a static IP to freeNAS.
In this case it correctly use the 2 DNS server I configure in "General Setup" page