Using Multus CNI in K3S

By default, K3S will run with flannel as the CNI and use custom directories to store CNI plugin binaries and config files(You can inspect the kubelet args K3S uses via journalctl -u k3s|grep cni-conf-dir). So you need to configure that properly When deploying Multus CNI.

For example given the official Multus manifests in https://github.com/intel/multus-cni/blob/36f2fd64e0965e639a0f1d17ab754f0130951aba/images/multus-daemonset.yml, the following changes are needed:

volumes:
  - name: cni
    hostPath:
      path: /var/lib/rancher/k3s/agent/etc/cni/net.d
  - name: cnibin
    hostPath:
      path: /var/lib/rancher/k3s/data/<replace-with-your-hash>/bin

containers:
  - name: kube-multus
    image: nfvpe/multus:v3.4.1
    command: ["/entrypoint.sh"]
    args:
      - "--multus-conf-file=auto"
      - "--cni-version=0.3.1"
      # Add the following arg
      - "--multus-kubeconfig-file-host=/var/lib/rancher/k3s/agent/etc/cni/net.d/multus.d/multus.kubeconfig"

Once Multus CNI is deployed properly it works as normal in K3S.