It took me a little while to put the pieces together to figure out how to take a vm-builder created vm and use briding with it instead of kvm/qemu’s user-mode networking. All the pieces are available on the internet, but there was some emphasis lacking to make it all clear to me.

You’ll need to have a bridge set up on your host. Install the ‘bridge-utils’ package first. Then the relevant section of my /etc/network/interfaces file looks like:

# The primary network interface
auto eth0
iface eth0 inet manual
up ifconfig $IFACE up

auto br0
iface br0 inet static
address 10.0.0.60
netmask 255.255.255.0
gateway 10.0.0.1
bridge_ports eth0
bridge_stp off
bridge_maxwait 0
bridge_fd 0
bridge_hello 0

You could probably use ‘dhcp’ instead of a ‘static’ address on the bridge. The point is that your ipv4 address should be on the bridge, not on the actual interface.

Then create a ‘br-ifup’ script in your vm directory. This is based on /etc/qemu-ifup. This script is passed the name of the interface (tap0) which brings the interface up, and then adds it to your bridge.
#!/bin/sh
#sudo -p "Password for $0:" /sbin/ifconfig $1 172.20.0.1
sudo /sbin/ifconfig $1 up
sudo /usr/sbin/brctl addif br0 $1


Then run kvm with something like this:

sudo kvm -m 128 -smp 1 -drive file=disk0.qcow2 -net nic -net tap,script=br-ifup

‘/etc/kvm-ifup: could not launch network script’ means that the script passed in ‘script=’ could not be found.

‘Could not initialize device ‘tap” means that kvm is unable to create the TAP/TUN interface. Running kvm as root via sudo is the easy solution.

‘warning: could not open /dev/net/tun: no virtual network emulation’ probably means that the ‘tun’ module isn’t loaded. You can load it with ‘sudo modprobe tun’.

The tap interface is removed from the bridge when the guest is shutdown.