Virtualbox Bridging

Edit: This is now pretty much unneeded, the new version of VirtualBox seems to handle this all nicely on its own.

As I mentioned in my last post, One of the useful advantages of the network boot setup is that I can use it to quickly install virtual machines.

Now a few things:

  • My Desktop is a lot more powerful than my server, so I run the virtual machines on it.
  • I use virtualbox rather than vmware.
  • All the network boot stuff is on my server not my desktop (obviously)

So in order to allow this, virtualbox needed to be setup to bridge to my existing adapter, this was quite straight forward, pretty much exactly as the manual said.

sudo apt-get install bridge-utils

Edit /etc/network/interfaces, and add

auto br0
iface br0 inet dhcp
    bridge_ports eth0

Now the next suggestion was to setup a tap0 device and tell virtualbox to use that, or to use a dynamic configuration.

The dynamic configuration sounded better as it meant I didn’t need to remember to add a new tap device for each vm.

The suggested dynamic configuration suggests using kdesu/gksudo and a script in the home dir of the user that will setup and cleaup the tap device (this means inputting your password every tiem you start/stop the VM and requiring a separate script for each user that wants to have a vm with bridging) this seemed rather annoying so I came up with an alternative.



# Make sure we are root
if [ $(whoami) != root ]; then
        exit 1;

# Create an new TAP interface for the user and remember its name.
interface=`VBoxTunctl -b -u ${SUDO_USER}`
# If for some reason the interface could not be created, return 1 to
# tell this to VirtualBox.
if [ -z "$interface" ]; then
        exit 1
# Write the name of the interface to the standard output.
echo ${interface}

# Bring up the interface.
/sbin/ifconfig ${interface} up
# And add it to the bridge.
/usr/sbin/brctl addif br0 ${interface}



# Make sure we are root
if [ $(whoami) != root ]; then
        exit 1;

# Remove the interface from the bridge.  The second script parameter is
# the interface name.
/usr/sbin/brctl delif br0 $2
# And use VBoxTunctl to remove the interface.
VBoxTunctl -d $2

Now these scripts run with sudo as any user will setup the tap device for that user (thats what ${SUDO_USER} is for)

This still requires a password for starting/stopping the VMs tho, so we use

sudo visudo

or if you prefer nano

sudo EDITOR=nano visudo

and add

# Allow virtualbox users to setup/cleanup tap devices
%vboxusers        ALL=NOPASSWD:/usr/bin/setuptap,/usr/bin/cleanuptap


  • configure virtualbox to attach the network device to a “host interface”
  • leave the Interface name blank (setuptap creates the next available one)
  • Setup Application: sudo /usr/bin/setuptap
  • Terminate Application: sudo /usr/bin/cleanuptap

And virtualbox will be able to create/destroy the tap device as needed.

However. there is still one problem, DHCP will not work for these VMs without a little help, so we need to:

sudo apt-get install dhcp3-relay

and answer the questions asked. (DHCP Server IP, and Interface to listen on (br0))

Virtualbox unfortunatly seems to need a little push to actually network boot, so I also use an etherboot iso to actually boot from the network along with the “PCnet-FAST III” adapter type.

and thats all there is to it, you can now network boot and dhcp from virtual machines not hosted on the server.