[vbox-dev] 'bridged adapter' issue.

Klaus Espenlaub klaus.espenlaub at oracle.com
Thu Aug 6 14:53:48 UTC 2015


Hi Maarten,

On 06.08.2015 15:18, Maarten Hoes wrote:
> Hi,
>
>
> Well I finally figured it out. On my ISP's wifi modem/router, there is 
> a setting called 'DHCP Binding'. This can be used to make sure that a 
> certain MAC always gets the same IP address assigned through DHCP. 
> (essentially creating a DHCP scope of 1 single IP and 1 single MAC).
>
> When I enable this for a bridged host, reaching the default gateway 
> becomes impossible. I verified the MAC both in the VM settings as in 
> the guest os, and made sure that they match the 'DHCP Binding' 
> settings on the modem/router. As soon as I remove the entry from the 
> modem/router and restart the guest, networking resumes as expected.
>
> I had this setting enabled for the 'old' VM's, but not for the newly 
> created ones; which explains the difference in behavior.
>
> Now what I dont understand is why such a setting should/does break 
> bridged to wifi networking - especially when at the same time NAT does 
> work as expected with that setting enabled.
Taking a deep breath... let's start with the most surprising piece of 
information for most people. Bridged to wifi actually isn't deserving 
the name "bridging" at all. Bridging happens at the Ethernet level, and 
with wifi that's not possible as 99% of the wifi cards out there simply 
don't support promisc mode (which VirtualBox uses for sending/receiving 
packets with the guest's MAC address).

What "bridged to wifi" implements is a funky routing scheme, packet 
filtering/suppression and some packet content rewriting. It entirely 
shares the host's MAC address (and uses the guest's IP address for 
actually filtering the traffic it needs to receive, so the host's IP 
stack must not see the packets targeted at the guest and vice versa), 
and convinces the wifi router's DHCP server to give out a lease for each 
VM (which I guess is where your solution exploded) even though the MAC 
address "on the wire" is always the host's. The final step is to 
translate the host's MAC address to the guest's and when the packets are 
passed to/from the VM.

Klaus
>
> - Maarten.
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