A place to keep my thoughts

'Bridged Network for Qemu in Arch Using Systemd'

The problem

I want to run a windows virtual machine inside my arch linux development laptop, in order to be able to run Microsoft’s Visual Studio, which I used to enjoy using massively in the past, particularly in combination with Jetbrains' Resharper. Recently I have gravitated more towards a lighter stack, preferring F# over C#. The tooling required to do the same kind of work in F# is much less than in C# (depending on use case of course), and I have always favoured an approach that lets me code using a text editor.

Enough rambling, so for this particular scenario I need VisualStudio, and by extension Windows. I am going to go with Windows Server 2012 r2, as i find the server Windows versions to be generally more amenable to running in a virtualised environment. I also don’t care much for news or music in a vm, so running Windows Server doesn’t limit me in any way.

The options for virtualization

Considering the options, it basically boils down to Hardware-assisted virtualization versus Full virtualization (Vmware, Parallels, etc). I have long been on a Mac as my main box, and have often felt the slowdown incurred by Full virtualization solutions, so being on a shiny Arch Linux box, it was an easy decision to go with Hardware-assisted (hvm).

The choice then is between xen and kvm, but I think the preparation for the xen environment is a lot more intrusive than the kvm option. I also found a recent performance comparison that essentially showed no major performance difference between the technologies. Bring on the flame wars in the comments ;-) So kvm it is going to be for me.


Installing the required packages couldn’t be easier given arch’s excellent package manager. sudo pacman -S qemu brings all the required bits onto my machine, and looking through the installation instructions in the arch wiki shows that I already have all the required kernel modules on my machine. So far (I’m 2 months in) the experience on arch with regards to package quality and packages being up to date has been absolutely brilliant, so I’m still in my honey moon with this distro.


Here be dragons :-)

Considering the options for the network it becomes evident that there two ways about it:

  • The fully integrated solution, that makes it really easy on the user, by effectively creating vlans and bridges in the kvm / qemu stack, and hiding that completely from the user. This solution doesn’t allow to have an optimised interface in the guest, and as a result of that is slower than the other solutions.
  • The bridged networking version, which pushes most of the management complexity onto the user, but provides the benefit of a virtio interface, that maps the io directly to the host, and is much faster as a result.

Thinking how hard can it be I decide to take a stab at the bridged networking option. Now this is a bit of a pain on a laptop, if you want the guest os (windows in my case) to be able to call out to the network. This is a solid requirement in my case, for being able to get windows updates, etc.

The reason it is difficult is that wireless interfaces in linux don’t allow bridging on the kernel network stack (promiscuous mode), which means that they can’t be slaved to the bridge.

The alternative is to create a bridge device and forward the guest’s traffic via network address translation (NAT), to the wifi interface, using the linux firewall (iptables).

Having NetworkManager as my main mechanism for connecting to wifi (I find the gui way a bit easier than arch’s preferred netctl), I need to come up with a way of making the bridge start on system startup, and when I start the vm for the interface to attach to the bridge. The attaching bit is pretty much handled for me in recent versions of kvm, where I can give it a flag with a bridge interface, and it will handle the rest.

So I decide to buy into the brave new world that is systemd, and think I’ll be able to use this knowledge in the future, as that behemoth seems to not stop for anything, and given the engineering power behind it, I would be surprised to see it go away any time soon.

Create the bridge, and start on system startup

To create the bridge device I add the file


For it’s network configuration I add:


In order to enable the network part of systemd I need to enable the systemd-networkd daemon. systemctl enable systemd-networkd. Simple enough.

Allow qemu to connect to the bridge

In order for qemu to be allowed to use the bridge I add the file /etc/qemu/bridge.conf, containing only the line: allow qbr0.

Add dnsmasq to provide ip’s to guests

In order for the guests to be able to receive ip’s I add dnsmasq to serve ips on the bridge device.

I add the configuration file: /etc/dnsmasq-qbr0.conf (apologies for top level folder, this is all still a bit rough around the edges).

Note the assignment of the fixed ip, this is so that I can reliably rdp to the same instance without having to go down the dns route.

I also add a service to systemd, to make this start up on system start (adding dependencies to the network service to have started before): /etc/systemd/system/dnsmasq-qbr0.service

Adding a start script for the kvm instance

Finally I modify a script from the ubuntu forums by the awesome okky-htf, shout out, couldn’t have done it without you!

This is what I run to start the vm:

I have set up rdp in the guest, hence the graphics are disabled at the bottom. For the initial installation plus boot the start up order needs to be changed, and the drives for the cd iso’s need to be enabled.

I use this script to rdp into the instance, and am very happy with the performance and stability of the set up.

This is all, I hope someone finds this useful, I’m mainly leaving it here for myself, to be able to repeat this in the future. Comments (particularly alternatives for the networknig) are very welcome.