k8s worker cluster setup
k3s has different ways to form a cluster than kubeadm join.
https://docs.k3s.io/quick-start
k3s specifies the init command in /etc/systemd/system/k3s.service. (k3s-agent.service if installed as agent) usually it is k3s server.
you need to change it to k3s agent in order to join the master node, or pass additional environment variables K3S_URL=https://<node_ip>:6443 and K3S_TOKEN=<node-token> while running k3s installation script.
the node token is at /var/lib/rancher/k3s/server/node-token
the agent node still needs to configure registry mirrors at /etc/rancher/k3s/registries.yaml for successfully pulling images
k3sup can automatically install k3s cluster using ssh connection
multi server cluster setup: