Kubeadm/dual stack support in 1.21

content/en/docs/concepts/services-networking/dual-stack.md

@@ -242,3 +242,5 @@ Ensure your {{< glossary_tooltip text="CNI" term_id="cni" >}} provider supports
 
 
 * [Validate IPv4/IPv6 dual-stack](/docs/tasks/network/validate-dual-stack) networking
+* [Enable dual-stack networking using kubeadm
+](/docs/setup/production-environment/tools/kubeadm/dual-stack-support/)

content/en/docs/setup/production-environment/tools/kubeadm/dual-stack-support.md

@@ -0,0 +1,154 @@
+---
+title: Dual-stack support with kubeadm
+feature:
+  title: Dual-stack support with kubeadm
+content_type: task
+weight: 110
+min-kubernetes-server-version: 1.21
+---
+
+<!-- overview -->
+
+{{< feature-state for_k8s_version="v1.21" state="beta" >}}
+
+Your Kubernetes cluster can run in [dual-stack](/docs/concepts/services-networking/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 {{< glossary_tooltip text="Pod" term_id="pod" >}} or a {{< glossary_tooltip text="Service" term_id="service" >}}.
+
+<!-- body -->
+
+## {{% heading "prerequisites" %}}
+
+You need to have installed the {{< glossary_tooltip text="kubeadm" term_id="kubeadm" >}} tool, following the steps from [Installing kubeadm](/docs/setup/production-environment/tools/kubeadm/install-kubeadm/).
+
+For each server that you want to use as a {{< glossary_tooltip text="node" term_id="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](/docs/reference/command-line-tools-reference/feature-gates/)
+`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”).
+{{< /note >}}
+
+### 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:
+
+```shell
+# These address ranges are examples
+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](https://pkg.go.dev/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1beta2) `kubeadm-config.yaml` for the primary dual-stack control plane node.
+
+```yaml
+---
+apiVersion: kubeadm.k8s.io/v1beta2
+kind: ClusterConfiguration
+featureGates:
+  IPv6DualStack: true
+networking:
+  podSubnet: 10.244.0.0/16,2001:db8:42:0::/56
+  serviceSubnet: 10.96.0.0/16,2001:db8:42:1::/112
+---
+apiVersion: kubeadm.k8s.io/v1beta2
+kind: InitConfiguration
+localAPIEndpoint:
+  advertiseAddress: "10.100.0.1"
+  bindPort: 6443
+nodeRegistration:
+  kubeletExtraArgs:
+    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:
+
+```shell
+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](/docs/concepts/services-networking/dual-stack#enable-ipv4ipv6-dual-stack).
+
+{{< note >}}
+The `--apiserver-advertise-address` flag does not support dual-stack.
+{{< /note >}}
+
+### 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](https://pkg.go.dev/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1beta2) `kubeadm-config.yaml` for joining a worker node to the cluster.
+
+```yaml
+apiVersion: kubeadm.k8s.io/v1beta2
+kind: JoinConfiguration
+discovery:
+  bootstrapToken:
+    apiServerEndpoint: 10.100.0.1:6443
+nodeRegistration:
+  kubeletExtraArgs:
+    node-ip: 10.100.0.3,fd00:1:2:3::3
+```
+
+Also, here is an example kubeadm [configuration file](https://pkg.go.dev/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1beta2) `kubeadm-config.yaml` for joining another control plane node to the cluster.
+```yaml
+apiVersion: kubeadm.k8s.io/v1beta2
+kind: JoinConfiguration
+controlPlane:
+  localAPIEndpoint:
+    advertiseAddress: "10.100.0.2"
+    bindPort: 6443
+discovery:
+  bootstrapToken:
+    apiServerEndpoint: 10.100.0.1:6443
+nodeRegistration:
+  kubeletExtraArgs:
+    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`.
+
+```shell
+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.
+{{< /note >}}
+
+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.
+
+```shell
+kubeadm init --feature-gates IPv6DualStack=false
+```
+
+To make things more clear, here is an example kubeadm [configuration file](https://pkg.go.dev/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1beta2) `kubeadm-config.yaml` for the single-stack control plane node.
+
+```yaml
+apiVersion: kubeadm.k8s.io/v1beta2
+kind: ClusterConfiguration
+featureGates:
+  IPv6DualStack: false
+networking:
+  podSubnet: 10.244.0.0/16
+  serviceSubnet: 10.96.0.0/16
+```
+
+## {{% heading "whatsnext" %}}
+
+* [Validate IPv4/IPv6 dual-stack](/docs/tasks/network/validate-dual-stack) networking
+* Read about [Dual-stack](/docs/concepts/services-networking/dual-stack/) cluster networking