Trasparent bridge beetween WAN and LAN + DHCP service

[antechno]

Hi Guys,
one question. Is it possible to bridge the LAN and WAN interface together and filtering the LAN traffic that go out through the WAN?

Switch–----->LAN----WAN------>Router

Shall I set this parameters?
net.link.bridge.pfil_member and net.link.bridge.pfil_bridge under System > Advanced on the System Tunables tab? and how?

I have set LAN and WAN interface with IP address field to "None" and the OPT1(bridge interface) with an IP address (192.168.1.x/24) but i can't have access to webGUI anymore. why?

last question: is it possible to set the DHCP service on OPT1 interface in order to assign fixed IP address to computer that reside in LAN segment? The DHCP service on the router has been switched off.

thanks a lot