virtualbox bridged interface won't connect to internet
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I'm trying to connect an ubuntu 12.04  to my local network + internet, i would like it to be directly reachable from the local network, NAT interface don't do it so i tried setting up a bridge (in the virtualbox GUI). But then i can access any other host on the LAN from the guest, but i can't access internet, the DHCP seems to work fine since my guest OS gets an IP in the correct range and with the correct mask. However i can't even ping the router which connect me to the internet (the same machine as the DHCP server).
here's my configuration:
host machine: linux mint debian edition X86-64
guests: win7 64 and ubuntu server 12.04 x86-94 (both have the same issue)
router-gateway-dhcp: livebox from orange ISP
host network interface: wifi usb dongle with chipset RTL8191SU (which works fine for my host)

I know the bridged mode isn't supported by all wireless adapter, but isn't it weird that i can access local network but not the internet?
maybe the problem comes from the gateway itself?
any advice would be very appreciated