k3k/docs/advanced-usage.md

Advanced Usage

This document provides advanced usage information for k3k, including detailed use cases and explanations of the Cluster resource fields for customization.

Customizing the Cluster Resource

The Cluster resource provides a variety of fields for customizing the behavior of your virtual clusters. You can check the CRD documentation for the full specs.

Note: Most of these customization options can also be configured using the k3kcli tool. Refer to the k3kcli documentation for more details.

This example creates a "shared" mode K3k cluster with:

• 3 servers
• K3s version v1.31.3-k3s1
• Custom network configuration
• Deployment on specific nodes with the nodeSelector
• kube-api exposed using an ingress
• Custom K3s serverArgs
• ETCD data persisted using a PVC

apiVersion: k3k.io/v1beta1
kind: Cluster
metadata:
  name: my-virtual-cluster
  namespace: my-namespace
spec:
  mode: shared
  version: v1.31.3-k3s1
  servers: 3
  tlsSANs:
    - my-cluster.example.com
  nodeSelector:
    disktype: ssd
  expose:
    ingress:
      ingressClassName: nginx
      annotations:
        nginx.ingress.kubernetes.io/ssl-passthrough: "true"
        nginx.ingress.kubernetes.io/backend-protocol: "HTTPS"
        nginx.ingress.kubernetes.io/ssl-redirect: "HTTPS"
  clusterCIDR: 10.42.0.0/16
  serviceCIDR: 10.43.0.0/16
  clusterDNS: 10.43.0.10
  serverArgs:
  - --tls-san=my-cluster.example.com
  persistence:
    type: dynamic
    storageClassName: local-path

mode

The mode field specifies the cluster provisioning mode, which can be either shared or virtual. The default mode is shared.

• shared mode: In this mode, the virtual cluster shares the host cluster's resources and networking. This mode is suitable for lightweight workloads and development environments where isolation is not a primary concern.
• virtual mode: In this mode, the virtual cluster runs as a separate K3s cluster within the host cluster. This mode provides stronger isolation and is suitable for production workloads or when dedicated resources are required.

version

The version field specifies the Kubernetes version to be used by the virtual nodes. If not specified, K3k will use the same K3s version as the host cluster. For example, if the host cluster is running Kubernetes v1.31.3, K3k will use the corresponding K3s version (e.g., v1.31.3-k3s1).

servers

The servers field specifies the number of K3s server nodes to deploy for the virtual cluster. The default value is 1.

agents

The agents field specifies the number of K3s agent nodes to deploy for the virtual cluster. The default value is 0.

Note: In shared mode, this field is ignored, as the Virtual Kubelet acts as the agent, and there are no K3s worker nodes.

nodeSelector

The nodeSelector field allows you to specify a node selector that will be applied to all server/agent pods. In shared mode, the node selector will also be applied to the workloads.

expose

The expose field contains options for exposing the API server of the virtual cluster. By default, the API server is only exposed as a ClusterIP, which is relatively secure but difficult to access from outside the cluster.

You can use the expose field to enable exposure via NodePort, LoadBalancer, or Ingress.

TLS passthrough is required

The K3s API server authenticates clients with mTLS certificates, so the TLS connection must reach the API server untouched. An ingress controller that terminates TLS will break both kubectl and agent authentication. Any controller used with expose.ingress must therefore support TLS passthrough.

expose.ingress also requires at least one DNS name in spec.tlsSANs: the SANs are used as the Ingress hosts, and the Ingress API does not accept IP addresses there. IPs in tlsSANs are ignored when building the Ingress, and a cluster with no DNS SAN stays in the Pending phase with a ValidationFailed condition rather than generating an invalid Ingress.

Nginx

Supported directly through expose.ingress. The ingress controller has to be started with the --enable-ssl-passthrough flag, and the annotations enabling passthrough must be set on the Ingress:

spec:
  tlsSANs:
    - my-cluster.example.com
  expose:
    ingress:
      ingressClassName: nginx
      annotations:
        nginx.ingress.kubernetes.io/ssl-passthrough: "true"
        nginx.ingress.kubernetes.io/backend-protocol: "HTTPS"
        nginx.ingress.kubernetes.io/ssl-redirect: "HTTPS"

Traefik

Traefik cannot perform layer 4 TLS passthrough with a standard Ingress, so expose.ingress does not work with it. This matters on K3s and RKE2, where Traefik is the default ingress controller.

Use expose.loadBalancer or expose.nodePort instead, or leave expose unset and create a Traefik IngressRouteTCP pointing at the cluster's ClusterIP service:

apiVersion: traefik.io/v1alpha1
kind: IngressRouteTCP
metadata:
  name: my-virtual-cluster
  namespace: my-namespace
spec:
  entryPoints:
    - websecure
  routes:
    - match: HostSNI(`my-cluster.example.com`)
      services:
        - name: k3k-my-virtual-cluster-service # k3k-<cluster-name>-service
          port: 443
  tls:
    passthrough: true

Add my-cluster.example.com to spec.tlsSANs so the API server certificate covers it, and generate the kubeconfig with the matching endpoint:

k3kcli kubeconfig generate --namespace my-namespace --name my-virtual-cluster \
  --kubeconfig-server https://my-cluster.example.com

Limitation in hcp mode: when the routing resource is managed outside of K3k, K3k does not know the external endpoint. In hcp mode it owns the default/kubernetes Endpoints inside the virtual cluster so that pods on external worker nodes can reach the API server, and without an expose configuration it can only point them at the host cluster's ClusterIP, which external nodes cannot route to. For hcp clusters with external workers, use expose.nodePort or expose.loadBalancer.

clusterCIDR

The clusterCIDR field specifies the CIDR range for the pods of the cluster. The default value is 10.42.0.0/16 in shared mode, and 10.52.0.0/16 in virtual mode.

serviceCIDR

The serviceCIDR field specifies the CIDR range for the services in the cluster. The default value is 10.43.0.0/16 in shared mode, and 10.53.0.0/16 in virtual mode.

Note: In shared mode, the serviceCIDR should match the host cluster's serviceCIDR to prevent conflicts and in virtual mode both serviceCIDR and clusterCIDR should be different than the host cluster.

clusterDNS

The clusterDNS field specifies the IP address for the CoreDNS service. It needs to be in the range provided by serviceCIDR. The default value is 10.43.0.10.

serverArgs

The serverArgs field allows you to specify additional arguments to be passed to the K3s server pods.

Using the cli

You can check the k3kcli documentation for the full specs.

No storage provider:

• Ephemeral Storage:

  k3kcli cluster create --persistence-type ephemeral my-cluster
  

Important Notes:

• Using --persistence-type ephemeral will result in data loss if the nodes are restarted.

• It is highly recommended to use --persistence-type dynamic with a configured storage class.