How to renew certificate manually in Kubernetes

Certificate renewal for client certificates becomes usual process for a long running Kubernetes clusters. This blog is an implementation of the process to perform manual cert upgrade. There might be multiple business reasons to perform manual cert rotation due to stringent business requirements. For the Kubernetes clusters installed with kubeadm, all the client certificates generated by kubeadm expire after 1 year. As a standard procedure, manual renewal is not usually required as cert rotation gets performed automatically when we perform Kubeadm upgrade.

For scenarios to perform manual certificate renewal, during the last month or week of expiry period, we can use Kubeadm commands to verify the certs and renew them for the cluster. The operations team can connect to control plane node and access the kubeadm utility to manage the certificates.

Kubeadm Cert management

Kubeadm is equipped with cert management and renewal commands.

[root@kmaster ~]# kubeadm certs
Commands related to handling kubernetes certificates
 
Usage:
  kubeadm certs [command]
 
Available Commands:
  certificate-key  Generate certificate keys
  check-expiration Check certificates expiration for a Kubernetes cluster
  generate-csr     Generate keys and certificate signing requests
  renew            Renew certificates for a Kubernetes cluster

Kubeadm command for renew

 
Usage:
  kubeadm certs renew [flags]
  kubeadm certs renew [command]
 
Available Commands:
  admin.conf               Renew the certificate embedded in the kubeconfig file for the admin to use and for kubeadm itself
  all                      Renew all available certificates
  apiserver                Renew the certificate for serving the Kubernetes API
  apiserver-etcd-client    Renew the certificate the apiserver uses to access etcd
  apiserver-kubelet-client Renew the certificate for the API server to connect to kubelet
  controller-manager.conf  Renew the certificate embedded in the kubeconfig file for the controller manager to use
  etcd-healthcheck-client  Renew the certificate for liveness probes to healthcheck etcd
  etcd-peer                Renew the certificate for etcd nodes to communicate with each other
  etcd-server              Renew the certificate for serving etcd
  front-proxy-client       Renew the certificate for the front proxy client
  scheduler.conf           Renew the certificate embedded in the kubeconfig file for the scheduler manager to use

Config and certs backup

For precautionary measure, it’s advised to perform backup for Kubernetes configs and certificates.

# Backup certs
mkdir -p $HOME/k8scluster-old-certs/pki
/bin/cp -p /etc/kubernetes/pki/*.* $HOME/k8scluster-old-certs/pki
ls -l $HOME/k8scluster-old-certs/pki/
 
#Backup configs
/bin/cp -p /etc/kubernetes/*.conf $HOME/k8scluster-old-certs
ls -ltr $HOME/k8scluster-old-certs
 
#Backup local config
mkdir -p $HOME/k8scluster-old-certs/.kube
/bin/cp -p ~/.kube/config $HOME/k8scluster-old-certs/.kube/.
ls -l $HOME/k8scluster-old-certs/.kube/.

Certificate Expiry Check

We can check the cert expiry period with follow command :

 
[root@kmaster ~]# kubeadm certs check-expiration
[check-expiration] Reading configuration from the cluster...
 
CERTIFICATE                EXPIRES                  RESIDUAL TIME   CERTIFICATE AUTHORITY   EXTERNALLY MANAGED
admin.conf                 Nov 19, 2022 14:53 UTC   344d                                    no
apiserver                  Nov 19, 2022 14:53 UTC   344d            ca                      no
apiserver-etcd-client      Nov 19, 2022 14:53 UTC   344d            etcd-ca                 no
apiserver-kubelet-client   Nov 19, 2022 14:53 UTC   344d            ca                      no
controller-manager.conf    Nov 19, 2022 14:53 UTC   344d                                    no
etcd-healthcheck-client    Nov 19, 2022 14:53 UTC   344d            etcd-ca                 no
etcd-peer                  Nov 19, 2022 14:53 UTC   344d            etcd-ca                 no
etcd-server                Nov 19, 2022 14:53 UTC   344d            etcd-ca                 no
front-proxy-client         Nov 19, 2022 14:53 UTC   344d            front-proxy-ca          no
scheduler.conf             Nov 19, 2022 14:53 UTC   344d                                    no
 
CERTIFICATE AUTHORITY   EXPIRES                  RESIDUAL TIME   EXTERNALLY MANAGED
ca                      Nov 17, 2031 14:53 UTC   9y              no
etcd-ca                 Nov 17, 2031 14:53 UTC   9y              no
front-proxy-ca          Nov 17, 2031 14:53 UTC   9y              no

The above command shows the expiration/residual time for the client certificates. These certificates are located at /etc/kubernetes/pki on the control plane nodes of Kubernetes cluster.

NOTE

kubelet.conf is not included in the list above because kubeadm configures kubelet for automatic certificate renewal with rotatable certificates under /var/lib/kubelet/pki. Certs renew uses the existing certificates as the authoritative source for attributes (Common Name, Organization, SAN, etc.) instead of the kubeadm-config ConfigMap. It is strongly recommended to keep them both in sync.

Implementation of manual certificate renewal

We can renew the certificates manually at any time with the kubeadm certs renew command. This command performs the renewal using CA certificate and key stored in /etc/kubernetes/pki.

For an HA Kubernetes cluster, kubeadm certs renew command needs to be executed on all the control-plane nodes.

Renew single certificate

 
[root@kmaster kubernetes]# kubeadm certs renew admin.conf
[renew] Reading configuration from the cluster...
 
certificate embedded in the kubeconfig file for the admin to use and for kubeadm itself renewed

Result of above step

 
[root@kmaster kubernetes]# kubeadm certs check-expiration
[check-expiration] Reading configuration from the cluster...
 
CERTIFICATE                EXPIRES                  RESIDUAL TIME   CERTIFICATE AUTHORITY   EXTERNALLY MANAGED
admin.conf                 Dec 10, 2022 05:53 UTC   364d                                    no
.......

Renew all certificates

 
[root@kmaster kubernetes]# kubeadm certs renew all
[renew] Reading configuration from the cluster...
 
certificate embedded in the kubeconfig file for the admin to use and for kubeadm itself renewed
certificate for serving the Kubernetes API renewed
certificate the apiserver uses to access etcd renewed
certificate for the API server to connect to kubelet renewed
certificate embedded in the kubeconfig file for the controller manager to use renewed
certificate for liveness probes to healthcheck etcd renewed
certificate for etcd nodes to communicate with each other renewed
certificate for serving etcd renewed
certificate for the front proxy client renewed
certificate embedded in the kubeconfig file for the scheduler manager to use renewed
 
Done renewing certificates. You must restart the kube-apiserver, kube-controller-manager, kube-scheduler and etcd, so that they can use the new certificates.

Result of above step

 
[root@kmaster kubernetes]# kubeadm certs check-expiration
[check-expiration] Reading configuration from the cluster...
 
CERTIFICATE                EXPIRES                  RESIDUAL TIME   CERTIFICATE AUTHORITY   EXTERNALLY MANAGED
admin.conf                 Dec 10, 2022 05:56 UTC   364d                                    no
apiserver                  Dec 10, 2022 05:56 UTC   364d            ca                      no
apiserver-etcd-client      Dec 10, 2022 05:56 UTC   364d            etcd-ca                 no
apiserver-kubelet-client   Dec 10, 2022 05:56 UTC   364d            ca                      no
controller-manager.conf    Dec 10, 2022 05:56 UTC   364d                                    no
etcd-healthcheck-client    Dec 10, 2022 05:56 UTC   364d            etcd-ca                 no
etcd-peer                  Dec 10, 2022 05:56 UTC   364d            etcd-ca                 no
etcd-server                Dec 10, 2022 05:56 UTC   364d            etcd-ca                 no
front-proxy-client         Dec 10, 2022 05:56 UTC   364d            front-proxy-ca          no
scheduler.conf             Dec 10, 2022 05:56 UTC   364d                                    no
 
CERTIFICATE AUTHORITY   EXPIRES                  RESIDUAL TIME   EXTERNALLY MANAGED
ca                      Nov 17, 2031 14:53 UTC   9y              no
etcd-ca                 Nov 17, 2031 14:53 UTC   9y              no
front-proxy-ca          Nov 17, 2031 14:53 UTC   9y              no

Post implementation steps

After running the kubeadm renew command we should restart the control plane Pods. Dynamic certificate reload is currently not supported for all components and certificates. Since,Static Pods are managed by the local kubelet and not by the API Server, thus kubectl cannot be used to delete and restart them.To restart a static Pod we can temporarily remove its manifest file from /etc/kubernetes/manifests/ and wait for 20 seconds.The kubelet will terminate the Pod if it’s no longer in the manifest directory.We can then move the file back and after another fileCheckFrequency period, the kubelet will recreate the Pod and the certificate renewal for the component can complete.

Managing Kubelet certificate renewal

Kubeadm configures a kubelet with automatic rotation of client certificates by using the /var/lib/kubelet/pki/kubelet-client-current.pem symlink specified in /etc/kubernetes/kubelet.conf. In case of failure in rotation process, we might see errors such as x509: certificate has expired or is not yet valid in kube-apiserver logs. To manage the manual renewal of kubelet certificates we can follow Kubernetes doc.