[vbox-dev] 'bridged adapter' issue.

Maarten Hoes hoes.maarten at gmail.com
Thu Aug 6 15:04:58 GMT 2015


On Thu, Aug 6, 2015 at 4:53 PM, Klaus Espenlaub <klaus.espenlaub at oracle.com>

> 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

Thank you for taking the time to elaborate/clarify. It's appreciated. I
guess what had me fooled is the fact that the bridged guest's MAC/IP
combo's show up in the DHCP lease list on the wifi modem/router.

Anyway, I guess I now have a bug to close.

Thanks again,

- Maarten.
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