the difference between tun and tap is that one is routed on layer 3 and one is bridged on layer 2 - normally layer 3 configuration (tun device) is more simple and sufficient. layer 2 bridging is only useful if you need some protocols like ipx supported. I guess you dont get the dhcp reply from your openvpn. Because of the bridging you are in your local network and get the dhcp reply from your router and this one listens on 192.168.1.1 and forwards the dns requests to your provider like for clients in the lan.