Dual-stack support with kubeadm

FEATURE STATE: Kubernetes v1.21 [beta]

Your Kubernetes cluster can run in dual-stack networking mode, which means that cluster networking lets you use either address family. In a dual-stack cluster, the control plane can assign both an IPv4 address and an IPv6 address to a single Pod or a Service.

Before you begin

You need to have installed the kubeadm tool, following the steps from Installing kubeadm.

For each server that you want to use as a node, make sure it allows IPv6 forwarding. On Linux, you can set this by running run sysctl -w net.ipv6.conf.all.forwarding=1 as the root user on each server.

You need to have an IPv4 and and IPv6 address range to use. Cluster operators typically use private address ranges for IPv4. For IPv6, a cluster operator typically chooses a global unicast address block from within 2000::/3, using a range that is assigned to the operator.
You don’t have to route the cluster’s IP address ranges to the public internet.

The size of the IP address allocations should be suitable for the number of Pods and Services that you are planning to run.

Note: If you are upgrading an existing cluster then, by default, the kubeadm upgrade command changes the feature gate IPv6DualStack to true if that is not already enabled.
However, kubeadm does not support making modifications to the pod IP address range (“cluster CIDR”) nor to the cluster’s Service address range (“Service CIDR”).

Create a dual-stack cluster

To create a dual-stack cluster with kubeadm init you can pass command line arguments similar to the following example:

  1. # These address ranges are examples
  2. kubeadm init --pod-network-cidr=10.244.0.0/16,2001:db8:42:0::/56 --service-cidr=10.96.0.0/16,2001:db8:42:1::/112

To make things clearer, here is an example kubeadm configuration file kubeadm-config.yaml for the primary dual-stack control plane node.

  1. ---
  2. apiVersion: kubeadm.k8s.io/v1beta2
  3. kind: ClusterConfiguration
  4. featureGates:
  5.   IPv6DualStack: true
  6. networking:
  7.   podSubnet: 10.244.0.0/16,2001:db8:42:0::/56
  8.   serviceSubnet: 10.96.0.0/16,2001:db8:42:1::/112
  9. ---
  10. apiVersion: kubeadm.k8s.io/v1beta2
  11. kind: InitConfiguration
  12. localAPIEndpoint:
  13.   advertiseAddress: "10.100.0.1"
  14.   bindPort: 6443
  15. nodeRegistration:
  16.   kubeletExtraArgs:
  17.     node-ip: 10.100.0.2,fd00:1:2:3::2

advertiseAddress in InitConfiguration specifies the IP address that the API Server will advertise it is listening on. The value of advertiseAddress equals the --apiserver-advertise-address flag of kubeadm init

Run kubeadm to initiate the dual-stack control plane node:

  1. kubeadm init --config=kubeadm-config.yaml

Currently, the kube-controller-manager flags --node-cidr-mask-size-ipv4|--node-cidr-mask-size-ipv6 are being left with default values. See enable IPv4/IPv6 dual stack.

Note: The --apiserver-advertise-address flag does not support dual-stack.

Join a node to dual-stack cluster

Before joining a node, make sure that the node has IPv6 routable network interface and allows IPv6 forwarding.

Here is an example kubeadm configuration file kubeadm-config.yaml for joining a worker node to the cluster.

  1. apiVersion: kubeadm.k8s.io/v1beta2
  2. kind: JoinConfiguration
  3. discovery:
  4.   bootstrapToken:
  5.     apiServerEndpoint: 10.100.0.1:6443
  6. nodeRegistration:
  7.   kubeletExtraArgs:
  8.     node-ip: 10.100.0.3,fd00:1:2:3::3

Also, here is an example kubeadm configuration file kubeadm-config.yaml for joining another control plane node to the cluster.

  1. apiVersion: kubeadm.k8s.io/v1beta2
  2. kind: JoinConfiguration
  3. controlPlane:
  4.   localAPIEndpoint:
  5.     advertiseAddress: "10.100.0.2"
  6.     bindPort: 6443
  7. discovery:
  8.   bootstrapToken:
  9.     apiServerEndpoint: 10.100.0.1:6443
  10. nodeRegistration:
  11.   kubeletExtraArgs:
  12.     node-ip: 10.100.0.4,fd00:1:2:3::4

advertiseAddress in JoinConfiguration.controlPlane specifies the IP address that the API Server will advertise it is listening on. The value of advertiseAddress equals the --apiserver-advertise-address flag of kubeadm join.

  1. kubeadm join --config=kubeadm-config.yaml ...

Create a single-stack cluster

Note: Enabling the dual-stack feature doesn’t mean that you need to use dual-stack addressing.
You can deploy a single-stack cluster that has the dual-stack networking feature enabled.

In 1.21 the IPv6DualStack feature is Beta and the feature gate is defaulted to true. To disable the feature you must configure the feature gate to false. Note that once the feature is GA, the feature gate will be removed.

  1. kubeadm init --feature-gates IPv6DualStack=false

To make things more clear, here is an example kubeadm configuration file kubeadm-config.yaml for the single-stack control plane node.

  1. apiVersion: kubeadm.k8s.io/v1beta2
  2. kind: ClusterConfiguration
  3. featureGates:
  4.   IPv6DualStack: false
  5. networking:
  6.   podSubnet: 10.244.0.0/16
  7.   serviceSubnet: 10.96.0.0/16

What’s next