Network interface on DisplayLink does not function with Virtualbox bridge mode
 

I have been trying to use a Virtualbox Win10 guest on my Ubuntu 20.04 installation.
Virtualbox guests can be configured to NAT through the host or BRIDGE the host network interface.
When I tried to bridge with the DisplayLink network interface, the guest would get an DHCP address from the network server, but then not be pingable from anywhere.
With the network cable plugged directly into the laptop, the bridged Virtualbox guest works perfectly.

NAT mode works perfectly with either interface, but BRIDGED mode is required to expose the Win10 guest on the LAN.

I have a similar story. Host: Linux Mint 20. Guest: Debian 10. The latest packages everywhere.
VidtualBox is the latest 6.1.14.
The guest VM has both NAT and BRIDGE interfaces. NAT interface works well.
The problem is: no IP from DHCP on BRIDGE interfaces.

The DHCP server does see DHCP DISCOVER requests from VM
and replies with the DHCP OFFER, but VM does NOT see it, DisplayLink D6000 blocks them.

The tcpdump provides the following details:
1) Debian Linux VM sends DHCP DISCOVER without BROADCAST flag set.
2) DHCP server replies with OFFER without BROADCAST flag set.
3) D6000 drops (do not forward) DHCP OFFER to Laptop/VM. DHCP OFFER IS NOT available on laptop and VM tcpdumps.

NOTE: D6000 do forward DHCP OFFER in case if DHCP server replies on stage 2) above with BROADCAST flag set. All works well in this case (even if BROADCAST flag was NOT set in DHCP DISCOVER).

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We also have tried to start Win10 VM instead of Debian.
It looks like Win10 tries to request DHCP IP using both cases with and without BROADCAST flag set.
If the BROADCAST flag set -> DHCP server replies with BROADCAST flag set too and D6000 bypass the packet into laptop. It works on Windows. :D


IMHO it is a D6000 issue and the fix in firmware is necessary.
D6000 should bypass all DHCP OFFER messages with and without the BROADCAST flag set.

Tnx!