Configure Certificate Rotation for the Kubelet

This page shows how to enable and configure certificate rotation for the kubelet.

FEATURE STATE: Kubernetes v1.19 [stable]

Before you begin

• Kubernetes version 1.8.0 or later is required

Overview

The kubelet uses certificates for authenticating to the Kubernetes API. By default, these certificates are issued with one year expiration so that they do not need to be renewed too frequently.

Kubernetes contains kubelet certificate rotation, that will automatically generate a new key and request a new certificate from the Kubernetes API as the current certificate approaches expiration. Once the new certificate is available, it will be used for authenticating connections to the Kubernetes API.

Enabling client certificate rotation

The kubelet process accepts an argument --rotate-certificates that controls if the kubelet will automatically request a new certificate as the expiration of the certificate currently in use approaches.

The kube-controller-manager process accepts an argument --cluster-signing-duration (--experimental-cluster-signing-duration prior to 1.19) that controls how long certificates will be issued for.

Understanding the certificate rotation configuration

When a kubelet starts up, if it is configured to bootstrap (using the --bootstrap-kubeconfig flag), it will use its initial certificate to connect to the Kubernetes API and issue a certificate signing request. You can view the status of certificate signing requests using:

kubectl get csr

Initially a certificate signing request from the kubelet on a node will have a status of Pending. If the certificate signing requests meets specific criteria, it will be auto approved by the controller manager, then it will have a status of Approved. Next, the controller manager will sign a certificate, issued for the duration specified by the --cluster-signing-duration parameter, and the signed certificate will be attached to the certificate signing request.

The kubelet will retrieve the signed certificate from the Kubernetes API and write that to disk, in the location specified by --cert-dir. Then the kubelet will use the new certificate to connect to the Kubernetes API.

As the expiration of the signed certificate approaches, the kubelet will automatically issue a new certificate signing request, using the Kubernetes API. This can happen at any point between 30% and 10% of the time remaining on the certificate. Again, the controller manager will automatically approve the certificate request and attach a signed certificate to the certificate signing request. The kubelet will retrieve the new signed certificate from the Kubernetes API and write that to disk. Then it will update the connections it has to the Kubernetes API to reconnect using the new certificate.