VirtualBox with a promiscuous floozy...

Everything was working just fine on my laptop machine, my VirtualBox VMs could see each other, Jumpstart and JET were just fine, and all of the VMs could see the outside world. When I moved my configs into my ridiculously over-configured desktop machine (see blahg entries from about 18 months ago), networking stopped functioning. Weird. In fact, my Windows host machine stopped communicating to the outside world as well. Ouch.

VirtualBox uses a "TAP" interface driver to bridge or NAT the VM networks through the WIndows host. On the Windows side, in order for this "TAP" adapter to talk to the outside world, it must use the "Network Bridge" or "MAC Miniport Adapter". If you select the TAP adapter, CTRL-click your network adapter that talks to the outside world, right mouse click and select "Add to Bridge", the two adapters are linked together into a single logical interface. Packets from one side can magically appear on the other side. In fact, you should be able to "snoop" from a Solaris virtual machine and see packets flying around on your external network. The final Windows XP Network Connections window should look something like this:

Yeah, that's how it is supposed to work. Mine didn't. I checked the Firewall settings, and even disabled it temporarily to make sure that it wasn't the problem. I Googled (and yes, that is a verb), and found nothing apropos. After several hours of head scratching, reading blogs, reading through the Microsoft technical docs, and banging my head against the keyboard, I stumbled on the answer. My adapters were not in "promiscuous mode". Promiscuous mode allows adapters to pass through all of the packets that they see, rather than just the packets that are addressed to this host. That sounds like what I need.

The magic mojo to make my bridged adapters go into promiscuous mode was fairly painless. Click on [Start] -> [Run] and type "cmd" to get a shell window on the XP host. In the shell window, I was able to run the netsh commands necessary to make the adapters go promiscuous, and to check that the changes were in place. This changes registry entries, so you should be able to just jump through these hoops once, and the settings should remain through reboots:

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

E:\\Documents and Settings\\Bill Walker>netsh bridge show adapter

----------------------------------------------------------------------
 ID AdapterFriendlyName         ForceCompatibilityMode
----------------------------------------------------------------------
----------------------------------------------------------------------


E:\\>netsh bridge show adapter

----------------------------------------------------------------------
 ID AdapterFriendlyName         ForceCompatibilityMode
----------------------------------------------------------------------
  1 VirtualBox Host Interface 1 unknown
  2 Local Area Connection 2     unknown
----------------------------------------------------------------------


E:\\>netsh bridge set adapter 1 forcecompatmode=enable


E:\\>netsh bridge set adapter 2 forcecompatmode=enable


E:\\>netsh bridge show adapter

----------------------------------------------------------------------
 ID AdapterFriendlyName         ForceCompatibilityMode
----------------------------------------------------------------------
  1 VirtualBox Host Interface 1 enabled
  2 Local Area Connection 2     enabled
----------------------------------------------------------------------

Voila! My Windows network is back to a sane mode. My VPN connections to the office are alive and well. My VirtualBox VMs can now see the outside world. I now have my Frankenstein super-desktop (TBs of disk, 8G of memory, 4x Opteron cores) back in a happy state. As always, your mileage may vary, objects in mirror may be closer than they appear, no warranty expressed or implied, but I hope this helps someone save hours of head banging on fragile keyboards at some point.


bill.


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