Sensible KVM Networking

I wanted a networking setup I could use on my MacBook Air and on my Hetzner server (although I haven't tested it on Hetzner yet). I also wanted to learn how QEMU/KVM is really working without the libvirt abstractions.

It tunrs out that you can have a perfectly good KVM setup without libvirt at all. In fact if you follow these instructions you can disable it completely if you like (although you can also leave it running because the naming convention here doesn't conflict with it):

You'll use this command exaclty as it is but change the networking configuration.

I've got eth1 here rather than eth0 because the wireless card is actually the physical interface.

You now have a bridge with its own IP address, 192.168.100.254. You could have chosen a different IP if you preferred.

QUESTION: Is it possible to choose an IP on the same network as eth1 to have the guests bridged directly onto the host network? ANSWER: No! The guest can ping the bridge and host but not the internet

You'll also need to remove the -S option which stops the PC and remove the -vnc 127.0.0.1:0 if you want it to load in a window rather than having to connect via VNC.

You'd replace this with eth0 on a normal server, I'm actually using my wireless card!

Each guest then need a configuration like this in /etc/network/interfaces. The first guest gets the address 192.168.100.1, second gets 192.168.100.2 etc:

Then you need to run this on each guest if you performed the configuration after booting:

Networking should now work. You can ping the guest from the host and from the guests you can ping any address on the internet, the host or any computer on the bridge including other guests.

Finally you need to edit /etc/resolv.conf on the guest so it can resolve domain names. Change it so that it uses the same IP address as the same file on the host. In my case:

At this point you could actually start libvirt again since we've used a different naming convention so the networks won't conflict. There's really no point in doing so though.

WARNING: Just don't load the same image via libvirt and on the command line at the same time! I'm not sure what that would do!

Making it Permanant

If you want to make it permanant you can use the kvm-manager.sh script from http://www.linux-kvm.org/page/Simple_shell_script_to_manage_your_virtual_machine_with_bridged_networking as a basis. You'll need to install socat though:

sudo apt-get install socat

The script shutsdown virtual machines with a qemu quit command, equivalent to pulling the power out and clearly not very friendly. My approach is to change the way the virtual machine deals with the keys ctrl+alt+del being sent so that they poweroff the guest rather than restart it. On the guest edit this:

$ sudo vi /etc/event.d/control-alt-delete

So that the command executed is /sbin/poweroff. Then change the kvm-manager.sh script so that it sends the ctrl+alt+del keys, waits 10 seconds (hopefully long enough for the machine to shut down) and then pulls the power. Here's the relevant section:

stop_vm() {
        echo "stop virtual machine"

        get_vm_pid_to "stop vm"
        # check if monitor file there
        if [ ! -e ${FILE_MONITOR} ]
        then
                echo "${FILE_MONITOR} not found, can not stop vm"
                exit 1
        fi
        # if the process is still running
        # send command quit to its monitor, and wait
        if [ -d /proc/${VM_PID} ]
        then
                send_cmd "sendkey ctrl-alt-delete"
                sleep 10
        fi
        # check if the process is still running
        if [ -d /proc/${VM_PID} ]
        then
                sleep 1
        fi
        # if the process is still running
        # send command quit to its monitor, and wait
        if [ -d /proc/${VM_PID} ]
        then
                send_cmd "quit"
                sleep 1
        fi
        if [ ! -d /proc/${VM_PID} ]
        then
                # yes, done
                rm ${FILE_PID}
                rm ${FILE_MONITOR}
                echo "vm stopped successfully"
        else
                # no, something wrong there...
                echo "failed to stop vm"
                exit 1
        fi
}

An alternative approach is to use advanced power management options to make the machine power down but this seemed more complicated than the above approach.

You could also do some work to make this script work as an init script to load all your VMs.