Build a Kubernetes Cluster with kubeadm: Step‑by‑Step Guide

Kubernetes is an open-source platform for automatically deploying, scaling, and managing containerized applications. The kubeadm tool helps initialize and configure a Kubernetes cluster. This article provides a detailed step-by-step guide to build a cluster with kubeadm on your machines.

Preparation

Before starting, prepare at least two machines (one master node and at least one worker node) that meet the following requirements:

Ubuntu 18.04+ (recommended), CentOS 7+ or Debian 9+ Linux distribution

At least 2 GB RAM per machine

Network configuration that allows inter-node communication

Master node with at least 2 CPU cores

Swap disabled (required by Kubernetes)

The examples use Ubuntu.

Environment Setup

Perform the following steps on all nodes.

Update the OS

<code>sudo apt-get update && sudo apt-get upgrade -y</code>

Install apt-transport-https

<code>sudo apt-get install -y apt-transport-https curl</code>

Disable Swap

<code>sudo swapoff -a</code>

Also comment out the swap line in /etc/fstab to prevent it from re-enabling after reboot.

Disable SELinux (CentOS only)

<code>setenforce 0
sed -i 's/^SELINUX=enforcing$/SELINUX=permissive/' /etc/selinux/config</code>

Adjust kernel parameters

<code>cat <<EOF | sudo tee /etc/sysctl.d/k8s.conf
net.bridge.bridge-nf-call-iptables=1
net.bridge.bridge-nf-call-ip6tables=1
net.ipv4.ip_forward=1
EOF

sudo sysctl --system</code>

Install Docker

<code>sudo apt-get install -y docker.io
sudo systemctl enable --now docker</code>

Ensure Docker is installed and enabled on all nodes.

Install kubeadm, kubelet and kubectl

Add the Kubernetes apt repository

<code>curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -
cat <<EOF | sudo tee /etc/apt/sources.list.d/kubernetes.list
deb https://apt.kubernetes.io/ kubernetes-xenial main
EOF</code>

Install the packages

<code>sudo apt-get update
sudo apt-get install -y kubelet kubeadm kubectl
sudo apt-mark hold kubelet kubeadm kubectl</code>

kubelet runs on every node to manage containers, kubeadm initializes the cluster, and kubectl is the command-line tool for interacting with the cluster.

Cluster Initialization

Initialize the master

<code>sudo kubeadm init --pod-network-cidr=10.244.0.0/16</code>

The --pod-network-cidr must match the network plugin; this example uses Flannel.

Configure kubectl for the user

<code>mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config</code>

Install a pod network (Flannel)

<code>kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml</code>

Join Worker Nodes

After the master initialization finishes, the console outputs a kubeadm join command similar to:

<code>kubeadm join <MASTER_IP>:<PORT> --token <TOKEN> --discovery-token-ca-cert-hash sha256:<HASH></code>

Run this command on each worker node.

Validate the Cluster

Check node status

<code>kubectl get nodes</code>

All nodes should show the Ready status.

Run a test pod

<code>kubectl run hello-kubernetes --image=k8s.gcr.io/echoserver:1.4 --port=8080</code>

General Maintenance

Regularly update packages on all nodes

Monitor node and pod health

Periodically back up cluster state and data using kubectl or other tools

Conclusion

You now have a Kubernetes cluster built with kubeadm . From here you can start deploying applications and learn how to operate and maintain the cluster.