Next, we will install the packages that we need to set up a Kubernetes control plane on this host. These packages are described in the following list:

• kubelet: The node agent that Kubernetes uses to control the container runtime. This is used to run all the other components of the control plane within Docker containers.
• kubeadm: This utility is responsible for bootstrapping a Kubernetes cluster.
• kubectl: The Kubernetes command-line client, which will allow us to interact with the Kubernetes API server.

First, add the signing key for the apt repository that hosts the Kubernetes packages, as follows:

$ curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -
OK  

Next add the Kubernetes apt repository, as follows:

$ sudo apt-add-repository 'deb http://apt.kubernetes.io/ kubernetes-xenial main'  

Then, resynchronize the package...