VMs lose network adapter when display settings change

[Repwoc]

I first noticed this when I upgraded VBox from 6.1.14 to 6.1.16 and I had display problems. I've since downgraded back to 6.1.14 but the problem persists. I'm not sure if the problem was there before I upgraded and I just didn't notice it or if upgrading was the cause and has persisted when I downgraded.

Windows 7 host, Windows server 2008 R2 guest (+ others but trying to get this one to work).

On all my VMs when I open the settings app there is a message at the bottom of the window "Invalid settings detected". When I hover over the icon a test box appears:

"The virtual machine is currently using less than 48 MB of display memory ..."
and
"The virtual machine is configured to use a graphics controller other than the recommended one (VBoxSVGA). Please consider switching ..."

In Settings/Display, Video Memory is set to 30 MB and the graphics controller is VBoxVGA. However, if I change one or both of these then the VM won't start with the message:

"Could not start the machine <name> because the following physical network interfaces were not found:
NVIDIA nForce 10/100/1000 Mbps Ethernet
(Adapter 1)

You can either change the machine's network settings or stop the machine."

The network adapter is still there and other VMs can use it without any issue. After stopping the VM, if I switch to a different snapshot then the lost network card miraculously is found and the VM starts.

Why does changing the display settings make the VM lose the network interface?

[scottgus1]

if I switch to a different snapshot

Snapshots also include hardware changes. There could be something messed up in the offending snapshot's hardware configuration. Couldn't begin to say what, though.

The warning about the video card is only pertinent if you are using 3D acceleration in the guest. Some changes were made to 3D-accelerated video to plug a security hole. If you want 3D while running 6.1.anything, you must use VboxSVGA (note the S) for Windows and VMSVGA for Linux & others. The old-school VboxVGA (no S) is still available for older OS's and compatibility, but no longer has 3D acceleration.

[Repwoc]

I should say that this isn't just happening on one VM. I've tried a couple and they both did the same thing.

In the snapshot that won't boot because it has lost the network interface, if I change the network settings to NAT then it boots just fine. If I switch it back to Bridged (like all my VMs have been for years) then it loses the network interface again.

How should I troubleshoot the lost network interface issue?

[Repwoc]

So it turns out that changing any setting, not only the display settings, causes the VM to lose the network interface. If I subsequently restore to a different snapshot and then restore back to the original snapshot, the settings are also restored and the VM boots again. Eg If I change the Drag 'n' Drop setting to Disabled, the VM subsequently refuses to boot and gives the network interface not found error. If I then switch to a different snapshot and back again, the settings are restored (drag 'n' drop bidirectional) and the VM boots.

When I change the settings the .vbox xml file changes (obviously) but the virtual disk files (.vhd in this VM) are unchanged. If I restore the .vbox file to a pre-settings change copy, however, the VM still won't boot. So something else changes when I change the settings - another file somewhere or a registry setting - but I don't know what. I'm about to start investigating using Process Monitor, but if anyone knows what else is changed when settings are updated I'd appreciate the heads-up.

[Repwoc]

So is it only the .vbox xml file that is updated when VM settings are changed?

[fth0]

In the VM's folder, the .vbox (and .vbox-prev) files represent the virtual PC, while the .vdi file represents the virtual hard disk. The Snapshots subfolder contains differencing virtual hard disk images. Additionally, there is the VirtualBox global configuration folder named .VirtualBox (note the '.') in your user folder, which contains all global settings and logs.

You have a strange problem, and I don't have a specific idea ATM. If you like, you can post a zipped copy of the global configuration folder and the VM's folder, but without all virtual hard disk image files, and I'll take a look.

[Repwoc]

Thanks for your reply @fth0 and thanks for your offer of help.

Would you like me to post the VMs folder in its working state, or after I have changed a setting when it won't boot, or both?

[fth0]

Perhaps you can do the following: Reboot your host, then perform a sequence of actions that shows the problem. Before each step, write down the wall clock time and what you do, and copy both the .vbox and .vbox-prev files if their timestamp has changed since the previous step. The other files will automatically bring their (short term) history with them.

[Repwoc]

Yes I can do that. There is only one step needed - if I change any setting then the VM refuses to boot and displays the network interface not found error message. I'll do it after work this evening.