Cert Rotation

Checking Certificate Expiration

# For kubeadm provisioned clusters
kubeadm alpha certs check-expiration

# For all clusters
openssl x509 -noout -dates -in /etc/kubernetes/pki/apiserver.crt

Updating Expiration Dates

Depending on the type of cluster, there are several methods to update the expiration dates of certificates (choose any one):

Method 1: Automatically rotate certificates with kubeadm when upgrading the cluster

kubeadm upgrade apply --certificate-renewal v1.15.0

Method 2: Manually generate and replace certificates using kubeadm

# Step 1): Backup old certs and kubeconfigs
mkdir /etc/kubernetes.bak
cp -r /etc/kubernetes/pki/ /etc/kubernetes.bak
cp /etc/kubernetes/*.conf /etc/kubernetes.bak

# Step 2): Renew all certs
kubeadm alpha certs renew all --config kubeadm.yaml

# Step 3): Renew all kubeconfigs
kubeadm alpha kubeconfig user --client-name=admin
kubeadm alpha kubeconfig user --org system:masters --client-name kubernetes-admin  > /etc/kubernetes/admin.conf
kubeadm alpha kubeconfig user --client-name system:kube-controller-manager > /etc/kubernetes/controller-manager.conf
kubeadm alpha kubeconfig user --org system:nodes --client-name system:node:$(hostname) > /etc/kubernetes/kubelet.conf
kubeadm alpha kubeconfig user --client-name system:kube-scheduler > /etc/kubernetes/scheduler.conf

# Another way to renew kubeconfigs
# kubeadm init phase kubeconfig all --config kubeadm.yaml

# Step 4): Copy certs/kubeconfigs and restart Kubernetes services

Method 3: For non-kubeadm clusters

For non-kubeadm clusters, please refer to Configuring CA and Creating TLS Certificates for regenerating certificates and then restart all Kubernetes services.

kubelet Automatic Certificate Rotation

Starting from v1.8.0, kubelet supports certificate rotation. When a certificate expires, it can automatically generate a new key and apply for a new certificate from the Kubernetes API.

To enable certificate rotation, use the following:

# Step 1): Config kube-controller-manager
kube-controller-manager --experimental-cluster-signing-duration=87600h \
                --feature-gates=RotateKubeletClientCertificate=true \
                ...

# Step 2): Config RBAC
# Refer https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/#approval

# Step 3): Config Kubelet
kubelet --feature-gates=RotateKubeletClientCertificate=true \
                --cert-dir=/var/lib/kubelet/pki \
                --rotate-certificates \
                --rotate-server-certificates \
                ...

Revoking Certificates

Kubernetes currently does not support Certificate Revocation List (CRL) for revoking certificates. Therefore, the only way to revoke a certificate currently is to regenerate all certificates with a new CA and then restart all services.

To avoid this issue, it is recommended to configure client authentication using OIDC, such as implementing it with the dex project.

Note: Etcd supports certificate revocation with CRL, the implementation reference can be found here.

Appendix: Glossary

CA (Certificate Authority): The root certificate issuing agency that issues certificates (i.e., verifies certificates are legitimate).
- A CA holds a private key (ca.key) and a certificate (ca.crt, which includes the public key). For a self-signed CA, ca.crt needs to be distributed to all clients.
- ca.crt is automatically mounted into Pods at /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
key (Public key or Private key): The public or private cryptographic key.
csr (Certificate Signing Request): A request sent to a certificate authority to obtain a signed certificate, which usually includes the public key (while keeping the private key secure).
crt/cer (Certificate): The issued certificate, usually in PEM format (also supports DER format).

References

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