|online dns? Eh? All dns is online [internet] do that doesn't make much sense.|
Assuming the adsl router has multiple ports you use a crossover cable from adsl lan port to dlink lan [not wan] port. You disable dhcp on the dlink and assign it a static ip like x.x.x.254 so you can admin it from the lan.
adsl router set to provide dhcp.
Connect the buffalo to the adsl router as set to be dhcp client and you are off and running.