Easy KVM Debian CloudInit

kvm

Debian provides cloud images that are suitable for homelab or other libvirt/KVM oriented use. They have 2 problems though:

  • They essentially require cloud-init for initial connectivity
  • The networking is a little bit tricky

In my ideal homelab world, on a single machine, I want to create a debian cloud host with a hostname of debian-cloud01, and then be able to address it by that hostname. Thankfully recent versions of libvirt and Debian make this possible.

Step 1: Configure the libvrt network #

The <network> libvirt configuration supports the domain option, which, when combined with systemd-networkd and specifically systemd-resolved, will automatically wire up DNS for you. Here's an example excerpt that you can add to the default libvirt network:

<domain name='kvm.internal' localOnly='yes' register='yes'/>

Once this is done, hosts created on this network will be accessible via the <hostname>.kvm.internal address.

Step 2: Virt-install Debian #

Virt install -- and the corresponding debian cloud-init -- contain 2 other tricks that give us what we need. First, ensure that you are using the generic image, the cloud images lack enough stuff for this to work. Second, have the ssh key that you want to use somewhere accessible. I pull my keys for this purpose directly from github (https://github.com/bryanalves.keys). Third, setting the system.serial sysinfo command lets you inject a minimal cloud-init configuration to set the hostname. With all of this, you can have a (long) one-liner command to instantiate debian cloud images with local connectivity and no extra file dependencies, no need to go through the step of creating extra files for cloud-init to load.

Here is a sample command:

virt-install \
  --import \
  --name debian-test01 \
  --memory 1024  \
  --vcpus 2  \
  --cpu host  \
  --disk=size=20,backing_store=/var/lib/libvirt/images/base/debian-12-generic-amd64-20240717-1811.qcow2  \
  --os-variant=debian12  \
  --noautoconsole  \
  --cloud-init clouduser-ssh-key=<(curl https://github.com/bryanalves.keys | head -1),root-password-generate=on,disable=on  \
  --sysinfo system.serial="ds=nocloud;h=debian-test01"

After running this command, you should be able to ping debian-test01.kvm.internal, and you should be able to ssh debian@debian-test01.kvm.internal, after which you can use whatever second-stage configuration you'd like.

This makes spinning up throwaway VMs as easy as spinning up throwaway containers