Nat, bridging and external adaptor issues

Discussions related to using VirtualBox on Mac OS X hosts.
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BitflipDetected
Posts: 1
Joined: 10. Nov 2021, 02:00

Nat, bridging and external adaptor issues

Post by BitflipDetected »

VirtualBox 6.1.28 on OSX Big Sur

I'm testing a device whose traffic I'd like to dump and I don't have a switch which supports promiscuous mode.

I decided to create an Ubuntu VM with the following setup: a VirtualBox network adaptor(NAT) <> software BRIDGE <> VirtualBox network adaptor (Bridge to physical device).

Within Ubuntu I create a bridge between the two interfaces, with netplan.

The Ubuntu VM works perfectly, and can ping Google DNS.
The other device however cannot ping Google DNS.
I checked its networking configuration, and it receives and IP address in the same range as the Ubuntu VM has on the bridge.

But traffic just doesn't flow.

Now, a reading through documentation I figured out that this has to do with Promiscuous mode, so I decided to look at that, and came up with the following:
From the point of the bridged adaptor:

Promiscuous mode Deny:
Ubuntu VM: works perfectly, can contact the world
Device: gets IP from DHCP, but cannot ping, not even Ubuntu VM

Promiscuous mode Allow VMs:
Ubuntu VM: works perfectly, can contact the world
Device: gets IP from DHCP, but cannot ping, not even Ubuntu VM

Promiscuous mode Allow All:
Ubuntu VM: Fails to contact the world, 99% packets lost
Device: gets IP from DHCP, can ping Ubuntu VM, but not further (which I think relates to the fact that the Ubuntu VM itself doesn't know where to send traffic)


I think that there's something wrong on the Ubuntu VM that it doesn't know over which interface to send the traffic (?) when in full Promiscuous mode, BUT I am not sure if that mode is actually needed and that I'm doing something else wrong.

Any thoughts?
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