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Reconfiguring a kubeadm cluster

kubeadm does not support automated ways of reconfiguring components that were deployed on managed nodes. One way of automating this would be by using a custom operator.

To modify the components configuration you must manually edit associated cluster objects and files on disk.

This guide shows the correct sequence of steps that need to be performed to achieve kubeadm cluster reconfiguration.

Before you begin

  • You need a cluster that was deployed using kubeadm
  • Have administrator credentials (/etc/kubernetes/admin.conf) and network connectivity to a running kube-apiserver in the cluster from a host that has kubectl installed
  • Have a text editor installed on all hosts

Reconfiguring the cluster

kubeadm writes a set of cluster wide component configuration options in ConfigMaps and other objects. These objects must be manually edited. The command kubectl edit can be used for that.

The kubectl edit command will open a text editor where you can edit and save the object directly.

You can use the environment variables KUBECONFIG and KUBE_EDITOR to specify the location of the kubectl consumed kubeconfig file and preferred text editor.

For example:

KUBECONFIG=/etc/kubernetes/admin.conf KUBE_EDITOR=nano kubectl edit <parameters>

Applying cluster configuration changes

Updating the ClusterConfiguration

During cluster creation and upgrade, kubeadm writes its ClusterConfiguration in a ConfigMap called kubeadm-config in the kube-system namespace.

To change a particular option in the ClusterConfiguration you can edit the ConfigMap with this command:

kubectl edit cm -n kube-system kubeadm-config

The configuration is located under the data.ClusterConfiguration key.

Reflecting ClusterConfiguration changes on control plane nodes

kubeadm manages the control plane components as static Pod manifests located in the directory /etc/kubernetes/manifests. Any changes to the ClusterConfiguration under the apiServer, controllerManager, scheduler or etcd keys must be reflected in the associated files in the manifests directory on a control plane node.

Such changes may include:

  • extraArgs - requires updating the list of flags passed to a component container
  • extraMounts - requires updated the volume mounts for a component container
  • *SANs - requires writing new certificates with updated Subject Alternative Names.

Before proceeding with these changes, make sure you have backed up the directory /etc/kubernetes/.

To write new certificates you can use:

kubeadm init phase certs <component-name> --config <config-file>

To write new manifest files in /etc/kubernetes/manifests you can use:

kubeadm init phase control-plane <component-name> --config <config-file>

The <config-file> contents must match the updated ClusterConfiguration. The <component-name> value must be the name of the component.

Applying kubelet configuration changes

Updating the KubeletConfiguration

During cluster creation and upgrade, kubeadm writes its KubeletConfiguration in a ConfigMap called kubelet-config in the kube-system namespace.

You can edit the ConfigMap with this command:

kubectl edit cm -n kube-system kubelet-config

The configuration is located under the data.kubelet key.

Reflecting the kubelet changes

To reflect the change on kubeadm nodes you must do the following:

  • Log in to a kubeadm node
  • Run kubeadm upgrade node phase kubelet-config to download the latest kubelet-config ConfigMap contents into the local file /var/lib/kubelet/config.conf
  • Edit the file /var/lib/kubelet/kubeadm-flags.env to apply additional configuration with flags
  • Restart the kubelet service with systemctl restart kubelet

Applying kube-proxy configuration changes

Updating the KubeProxyConfiguration

During cluster creation and upgrade, kubeadm writes its KubeProxyConfiguration in a ConfigMap in the kube-system namespace called kube-proxy.

This ConfigMap is used by the kube-proxy DaemonSet in the kube-system namespace.

To change a particular option in the KubeProxyConfiguration, you can edit the ConfigMap with this command:

kubectl edit cm -n kube-system kube-proxy

The configuration is located under the data.config.conf key.

Reflecting the kube-proxy changes

Once the kube-proxy ConfigMap is updated, you can restart all kube-proxy Pods:

Obtain the Pod names:

kubectl get po -n kube-system | grep kube-proxy

Delete a Pod with:

kubectl delete po -n kube-system <pod-name>

New Pods that use the updated ConfigMap will be created.

Applying CoreDNS configuration changes

Updating the CoreDNS Deployment and Service

kubeadm deploys CoreDNS as a Deployment called coredns and with a Service kube-dns, both in the kube-system namespace.

To update any of the CoreDNS settings, you can edit the Deployment and Service objects:

kubectl edit deployment -n kube-system coredns
kubectl edit service -n kube-system kube-dns

Reflecting the CoreDNS changes

Once the CoreDNS changes are applied you can delete the CoreDNS Pods:

Obtain the Pod names:

kubectl get po -n kube-system | grep coredns

Delete a Pod with:

kubectl delete po -n kube-system <pod-name>

New Pods with the updated CoreDNS configuration will be created.

Persisting the reconfiguration

During the execution of kubeadm upgrade on a managed node, kubeadm might overwrite configuration that was applied after the cluster was created (reconfiguration).

Persisting Node object reconfiguration

kubeadm writes Labels, Taints, CRI socket and other information on the Node object for a particular Kubernetes node. To change any of the contents of this Node object you can use:

kubectl edit no <node-name>

During kubeadm upgrade the contents of such a Node might get overwritten. If you would like to persist your modifications to the Node object after upgrade, you can prepare a kubectl patch and apply it to the Node object:

kubectl patch no <node-name> --patch-file <patch-file>

Persisting control plane component reconfiguration

The main source of control plane configuration is the ClusterConfiguration object stored in the cluster. To extend the static Pod manifests configuration, patches can be used.

These patch files must remain as files on the control plane nodes to ensure that they can be used by the kubeadm upgrade ... --patches <directory>.

If reconfiguration is done to the ClusterConfiguration and static Pod manifests on disk, the set of node specific patches must be updated accordingly.

Persisting kubelet reconfiguration

Any changes to the KubeletConfiguration stored in /var/lib/kubelet/config.conf will be overwritten on kubeadm upgrade by downloading the contents of the cluster wide kubelet-config ConfigMap. To persist kubelet node specific configuration either the file /var/lib/kubelet/config.conf has to be updated manually post-upgrade or the file /var/lib/kubelet/kubeadm-flags.env can include flags. The kubelet flags override the associated KubeletConfiguration options, but note that some of the flags are deprecated.

A kubelet restart will be required after changing /var/lib/kubelet/config.conf or /var/lib/kubelet/kubeadm-flags.env.

What's next

Last modified April 05, 2022 at 7:28 PM PST: kubeadm: add task page for cluster reconf (931581eb88)