K8s Renew Certs

#### Reference: https://raynix.info/archives/4296

# in the master node, run as root
kubeadm certs renew all
[renew] Reading configuration from the cluster...
[renew] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -o yaml'
[renew] Error reading configuration from the Cluster. Falling back to default configuration

certificate embedded in the kubeconfig file for the admin to use and for kubeadm itself 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.
# it's easier to restart the kubelet service which runs those static pods 
systemctl restart kubelet.service
...

# copy the updated kubeconfig from the master node
ssh ubuntu@kmaster -- sudo cat /etc/kubernetes/admin.conf > /tmp/admin.conf

# delete current cluster, user, context from local kubeconfig
# by default, the kubeadm cluster is named kubernetes
k config delete-context kubernetes
k config delete-cluster kubernetes
k config delete-user kubernetes-admin

# merge the new admin.conf with current kubeconfig
KUBECONFIG=~/.kube/config:/tmp/admin.conf k config view --flatten > /tmp/config

# test the new kubeconfig
k --kubeconfig=/tmp/config get nodes
NAME       STATUS   ROLES                  AGE    VERSION
kmaster1   Ready    control-plane,master   376d   v1.21.0
...

# replace the current kubeconfig
mv /tmp/config ~/.kube/config