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3. Using crun-vm as a Kubernetes runtime

It is possible to use crun-vm as a Kubernetes runtime, allowing you to run VMs as regular pods.

Preparation

To enable crun-vm on a Kubernetes cluster, follow these steps:

Ensure that the cluster is using the CRI-O container runtime. Refer to the Kubernetes docs on container runtimes.
Install crun-vm on all cluster nodes where pods may be scheduled. Refer to the installation instructions.
Append the following to /etc/crio/crio.conf (adjust the runtime_path if necessary):
```
[crio.runtime.runtimes.crun-vm]
runtime_path = "/usr/local/bin/crun-vm"
```

Create a RuntimeClass that references crun-vm:

apiVersion: node.k8s.io/v1
kind: RuntimeClass
metadata:
  name: crun-vm
handler: crun-vm

Using the runtime

Under examples/minikube you can find a script that sets up a local minikube Kubernetes cluster with crun-vm available as a runtime. You can use it to easily try out the examples below.

From then on, you can run VM images packaged in container images by creating pods that use this RuntimeClass:

apiVersion: v1
kind: Pod
metadata:
  name: my-vm
spec:
  containers:
    - name: my-vm
      image: quay.io/crun-vm/example-http-server:latest
      ports:
        - containerPort: 80
  runtimeClassName: crun-vm

Logging

The VM's console output is logged:

$ kubectl logs my-vm

SSH'ing into the pod/VM

Assuming the VM supports cloud-init or Ignition, you can also SSH into it using kubectl exec:

$ kubectl exec my-vm -- --as fedora whoami
fedora

$ kubectl exec -it my-vm -- --as fedora bash
[fedora@my-vm ~]$

Port forwarding

The pod/VM defined above actually exposes an HTTP server on port 80. To talk to it, we must first forward a local port to the pod/VM:

$ kubectl port-forward my-vm 8000:80
Forwarding from 127.0.0.1:8000 -> 80
Forwarding from [::1]:8000 -> 80

With this command running, navigate to localhost:8000 on your browser, or run the following on a second terminal:

$ curl localhost:8000
<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Directory listing for /</title>
</head>
<body>
[...]

cloud-init and Ignition

When using crun-vm as a Kubernetes runtime, paths given to --cloud-init and --ignition are interpreted in the context of the container/VM, instead of the host. This means that config files can be retrieved from mounted volumes. For instance, you could store your cloud-init config in a ConfigMap:

apiVersion: v1
kind: ConfigMap
metadata:
  name: my-cloud-init-config
data:
  meta-data: ""
  user-data: |
    #cloud-config
    runcmd:
      - echo 'Hello, world!' > /home/fedora/hello-world

And pass it to your VMs like so:

apiVersion: v1
kind: Pod
metadata:
  name: my-other-vm
spec:
  containers:
    - name: my-other-vm
      image: quay.io/containerdisks/fedora:40
      args:
        - --cloud-init=/etc/cloud-init
      volumeMounts:
        - name: cloud-init-vol
          mountPath: /etc/cloud-init
  volumes:
    - name: cloud-init-vol
      configMap:
        name: my-cloud-init-config
  runtimeClassName: crun-vm

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