;;; DNS Redirect for External DNS Old to External DNS New Servers
chain=dstnat dst-address=209.63.0.2 protocol=tcp dst-port=53
action=dst-nat to-addresses=216.194.124.21 to-ports=53
chain=dstnat dst-address=209.63.0.2 protocol=udp dst-port=53
action=dst-nat to-addresses=216.194.124.21 to-ports=53
Where 209.62.0.2 is the old DNS that your clients might have in their routers/computers and 216.194.124.21 is the DNS you want them to be using (without them having to manually change it). Note that this NATs the TCP and UDP ports 53 for just one address.