Kubernetes is a tool for orchestrating and managing containerized applications at scale on on-premise server or across hybrid cloud environments. Kubeadm is a tool provided with Kubernetes to help users install a production ready Kubernetes cluster with best practices enforcement. This tutorial will demonstrate how you can perform an installation Kubernetes Cluster on Ubuntu 20.04 with kubeadm. Kubeadm is a command line tool that provides kubeadm init and kubeadm join as best-practice “fast paths” for creating Kubernetes clusters.

There are two server types used in deployment of Kubernetes clusters:

  • Master: A Kubernetes Master is where control API calls for the pods, replications controllers, services, nodes and other components of a Kubernetes cluster are executed.
  • Node: A Node is a system that provides the run-time environments for the containers. A set of container pods can span multiple nodes.

The minimum requirements for the viable setup are:

  • Memory: 2 GiB or more of RAM per machine
  • CPUs: At least 2 CPUs on the control plane machine.
  • Internet connectivity for pulling containers required (Private registry can also be used)
  • Full network connectivity between machines in the cluster – This is private or public

My Lab setup contain three servers. One control plane machine and two nodes to be used for running containerized workloads. You can add more nodes to suit your desired use case and load, for example using three control plane nodes for HA.

Server TypeServer HostnameSpecs
Masterk8s-master01.computingforgeeks.com4GB Ram, 2vcpus
Workerk8s-worker01.computingforgeeks.com4GB Ram, 2vcpus
Workerk8s-worker02.computingforgeeks.com4GB Ram, 2vcpus

1. Install Kubernetes Servers

Provision the servers to be used in the deployment of Kubernetes on Ubuntu 20.04. The setup process will vary depending on the virtualization or cloud environment you’re using.

Once the servers are ready, update them.

sudo apt update && sudo apt -y full-upgrade
[ -f /var/run/reboot-required ] && sudo reboot -f

2. Install kubelet, kubeadm and kubectl

Once the servers are rebooted, add Kubernetes repository for Ubuntu 20.04 to all the servers.

Specify the version of Kubernetes to be installed.

VER=1.30

Then import GPG key and configure APT repository.

curl -fsSL https://pkgs.k8s.io/core:/stable:/v${VER}/deb/Release.key | sudo gpg --dearmor -o /etc/apt/keyrings/kubernetes-apt-keyring.gpg
echo "deb [signed-by=/etc/apt/keyrings/kubernetes-apt-keyring.gpg] https://pkgs.k8s.io/core:/stable:/v${VER}/deb/ /" | sudo tee /etc/apt/sources.list.d/kubernetes.list

Then install required packages.

sudo apt update
sudo apt -y install vim git curl wget kubelet kubeadm kubectl
sudo apt-mark hold kubelet kubeadm kubectl

Confirm installation by checking the version of kubectl.

$ kubectl version --client && kubeadm version
Client Version: v1.29.6+k3s2
Kustomize Version: v5.0.4-0.20230601165947-6ce0bf390ce3
kubeadm version: &version.Info{Major:"1", Minor:"30", GitVersion:"v1.30.3", GitCommit:"6fc0a69044f1ac4c13841ec4391224a2df241460", GitTreeState:"clean", BuildDate:"2024-07-16T23:53:15Z", GoVersion:"go1.22.5", Compiler:"gc", Platform:"linux/amd64"}

3. Disable Swap

Turn off swap.

sudo sed -i '/ swap / s/^\(.*\)$/#\1/g' /etc/fstab

Now disable Linux swap space permanently in /etc/fstab. Search for a swap line and add # (hashtag) sign in front of the line.

$ sudo vim /etc/fstab
#/swap.img	none	swap	sw	0	0

Confirm setting is correct

sudo swapoff -a
sudo mount -a
free -h

Enable kernel modules and configure sysctl.

# Enable kernel modules
sudo modprobe overlay
sudo modprobe br_netfilter

# Add some settings to sysctl
sudo tee /etc/sysctl.d/kubernetes.conf<<EOF
net.bridge.bridge-nf-call-ip6tables = 1
net.bridge.bridge-nf-call-iptables = 1
net.ipv4.ip_forward = 1
EOF

# Reload sysctl
sudo sysctl --system

4. Install Container runtime

To run containers in Pods, Kubernetes uses a container runtime. Supported container runtimes are:

  • Docker
  • CRI-O
  • Containerd

NOTE: You have to choose one runtime at a time.

Option 1: Install and Use Docker CE runtime:

# Add repo and Install packages
sudo apt update
sudo apt install -y curl gnupg2 software-properties-common apt-transport-https ca-certificates
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /etc/apt/trusted.gpg.d/docker-archive-keyring.gpg
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
sudo apt update
sudo apt install -y containerd.io docker-ce docker-ce-cli

# Create required directories
sudo mkdir -p /etc/systemd/system/docker.service.d

# Create daemon json config file
sudo tee /etc/docker/daemon.json <<EOF
{
  "exec-opts": ["native.cgroupdriver=systemd"],
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "100m"
  },
  "storage-driver": "overlay2"
}
EOF

# Start and enable Services
sudo systemctl daemon-reload 
sudo systemctl restart docker
sudo systemctl enable docker

For Docker Engine you need a shim interface. You can install Mirantis cri-dockerd as covered in the guide below.

Mirantis cri-dockerd CRI socket file path is /run/cri-dockerd.sock. This is what will be used when configuring Kubernetes cluster.

Option 2: Install and Use CRI-O:

Switch to root user

sudo su -

The run the commands below.

# Ensure you load modules
sudo modprobe overlay
sudo modprobe br_netfilter
# Set up required sysctl params
sudo tee /etc/sysctl.d/kubernetes.conf<<EOF
net.bridge.bridge-nf-call-ip6tables = 1
net.bridge.bridge-nf-call-iptables = 1
net.ipv4.ip_forward = 1
EOF
# Reload sysctl
sudo sysctl --system
# Add Cri-o repo
CRIO_VERSION=v1.30
curl -fsSL https://pkgs.k8s.io/addons:/cri-o:/stable:/$CRIO_VERSION/deb/Release.key|gpg --dearmor -o /etc/apt/keyrings/cri-o-apt-keyring.gpg
echo "deb [signed-by=/etc/apt/keyrings/cri-o-apt-keyring.gpg] https://pkgs.k8s.io/addons:/cri-o:/stable:/$CRIO_VERSION/deb/ /" |sudo tee /etc/apt/sources.list.d/cri-o.list
# Install CRI-O
sudo apt update
sudo apt install cri-o -y
# Update CRI-O CIDR subnet
sudo sed -i 's/10.85.0.0/172.24.0.0/g' /etc/cni/net.d/*
# Start and enable Service
sudo systemctl daemon-reload
sudo systemctl restart crio
sudo systemctl enable crio
sudo systemctl status crio

Option 3: Install and Use Containerd:

# Configure persistent loading of modules
sudo tee /etc/modules-load.d/containerd.conf <<EOF
overlay
br_netfilter
EOF

# Load at runtime
sudo modprobe overlay
sudo modprobe br_netfilter

# Ensure sysctl params are set
sudo tee /etc/sysctl.d/kubernetes.conf<<EOF
net.bridge.bridge-nf-call-ip6tables = 1
net.bridge.bridge-nf-call-iptables = 1
net.ipv4.ip_forward = 1
EOF

# Reload configs
sudo sysctl --system

# Install required packages
sudo apt install -y curl gnupg2 software-properties-common apt-transport-https ca-certificates

# Add Docker repo
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /etc/apt/trusted.gpg.d/docker-archive-keyring.gpg
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

# Install containerd
sudo apt update
sudo apt install -y containerd.io

# Configure containerd and start service
sudo mkdir -p /etc/containerd
sudo containerd config default|sudo tee /etc/containerd/config.toml

# restart containerd
sudo systemctl restart containerd
sudo systemctl enable containerd
systemctl status  containerd

To use the systemd cgroup driver, set plugins.cri.systemd_cgroup = true in /etc/containerd/config.toml. When using kubeadm, manually configure the cgroup driver for kubelet

5. Initialize master node

Login to the server to be used as master and make sure that the br_netfilter module is loaded:

$ lsmod | grep br_netfilter
br_netfilter           22256  0 
bridge                151336  2 br_netfilter,ebtable_broute

Enable kubelet service.

sudo systemctl enable kubelet

We now want to initialize the machine that will run the control plane components which includes etcd (the cluster database) and the API Server.

Pull container images:

$ sudo kubeadm config images pull
[config/images] Pulled registry.k8s.io/kube-apiserver:v1.30.3
[config/images] Pulled registry.k8s.io/kube-controller-manager:v1.30.3
[config/images] Pulled registry.k8s.io/kube-scheduler:v1.30.3
[config/images] Pulled registry.k8s.io/kube-proxy:v1.30.3
[config/images] Pulled registry.k8s.io/coredns/coredns:v1.11.1
[config/images] Pulled registry.k8s.io/pause:3.9
[config/images] Pulled registry.k8s.io/etcd:3.5.12-0

If you have multiple CRI sockets, please use --cri-socket to select one:

# CRI-O
sudo kubeadm config images pull --cri-socket unix:///var/run/crio/crio.sock

# Containerd
sudo kubeadm config images pull --cri-socket unix:///run/containerd/containerd.sock

# Docker
sudo kubeadm config images pull --cri-socket unix:///run/cri-dockerd.sock

These are the basic kubeadm init options that are used to bootstrap cluster.

--control-plane-endpoint :  set the shared endpoint for all control-plane nodes. Can be DNS/IP
--pod-network-cidr : Used to set a Pod network add-on CIDR
--cri-socket : Use if have more than one container runtime to set runtime socket path
--apiserver-advertise-address : Set advertise address for this particular control-plane node's API server

Bootstrap without shared endpoint

To bootstrap a cluster without using DNS endpoint, run:

### With CRI-O###
sudo sysctl -p
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --cri-socket unix:///var/run/crio/crio.sock

### With Containerd ###
sudo sysctl -p
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --cri-socket unix:///run/containerd/containerd.sock

### With Docker CE ###
sudo sysctl -p
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --cri-socket unix:///run/cri-dockerd.sock

Bootstrap with shared endpoint (DNS name for control plane API)

Set cluster endpoint DNS name or add record to /etc/hosts file.

$ sudo vim /etc/hosts
172.29.20.5 k8s-cluster.computingforgeeks.com

Create cluster:

sudo sysctl -p
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --upload-certs \
  --control-plane-endpoint=k8s-cluster.computingforgeeks.com

Note: If 172.24.0.0/16 is already in use within your network you must select a different pod network CIDR, replacing 172.24.0.0/16 in the above command.

Container runtime sockets:

RuntimePath to Unix domain socket
Dockerunix:///run/cri-dockerd.sock
containerdunix:///run/containerd/containerd.sock
CRI-Ounix:///var/run/crio/crio.sock

You can optionally pass Socket file for runtime and advertise address depending on your setup.

# CRI-O
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --cri-socket unix:///var/run/crio/crio.sock \
  --upload-certs \
  --control-plane-endpoint=k8s-cluster.computingforgeeks.com

# Containerd
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --cri-socket unix:///run/containerd/containerd.sock \
  --upload-certs \
  --control-plane-endpoint=k8s-cluster.computingforgeeks.com

# Docker
# Must do https://computingforgeeks.com/install-mirantis-cri-dockerd-as-docker-engine-shim-for-kubernetes/
sudo kubeadm init \
  --pod-network-cidr=172.24.0.0/16 \
  --cri-socket unix:///run/cri-dockerd.sock  \
  --upload-certs \
  --control-plane-endpoint=k8s-cluster.computingforgeeks.com

Here is the output of my initialization command.

....
[init] Using Kubernetes version: v1.30.3
[preflight] Running pre-flight checks
	[WARNING Firewalld]: firewalld is active, please ensure ports [6443 10250] are open or your cluster may not function correctly
[preflight] Pulling images required for setting up a Kubernetes cluster
[preflight] This might take a minute or two, depending on the speed of your internet connection
[preflight] You can also perform this action in beforehand using 'kubeadm config images pull'
[kubelet-start] Writing kubelet environment file with flags to file "/var/lib/kubelet/kubeadm-flags.env"
[kubelet-start] Writing kubelet configuration to file "/var/lib/kubelet/config.yaml"
[kubelet-start] Starting the kubelet
[certs] Using certificateDir folder "/etc/kubernetes/pki"
[certs] Using existing ca certificate authority
[certs] Using existing apiserver certificate and key on disk
[certs] Using existing apiserver-kubelet-client certificate and key on disk
[certs] Using existing front-proxy-ca certificate authority
[certs] Using existing front-proxy-client certificate and key on disk
[certs] Using existing etcd/ca certificate authority
[certs] Using existing etcd/server certificate and key on disk
[certs] Using existing etcd/peer certificate and key on disk
[certs] Using existing etcd/healthcheck-client certificate and key on disk
[certs] Using existing apiserver-etcd-client certificate and key on disk
[certs] Using the existing "sa" key
[kubeconfig] Using kubeconfig folder "/etc/kubernetes"
[kubeconfig] Using existing kubeconfig file: "/etc/kubernetes/admin.conf"
[kubeconfig] Using existing kubeconfig file: "/etc/kubernetes/kubelet.conf"
[kubeconfig] Using existing kubeconfig file: "/etc/kubernetes/controller-manager.conf"
[kubeconfig] Using existing kubeconfig file: "/etc/kubernetes/scheduler.conf"
[control-plane] Using manifest folder "/etc/kubernetes/manifests"
[control-plane] Creating static Pod manifest for "kube-apiserver"
[control-plane] Creating static Pod manifest for "kube-controller-manager"
W0611 22:34:23.276374    4726 manifests.go:225] the default kube-apiserver authorization-mode is "Node,RBAC"; using "Node,RBAC"
[control-plane] Creating static Pod manifest for "kube-scheduler"
W0611 22:34:23.278380    4726 manifests.go:225] the default kube-apiserver authorization-mode is "Node,RBAC"; using "Node,RBAC"
[etcd] Creating static Pod manifest for local etcd in "/etc/kubernetes/manifests"
[wait-control-plane] Waiting for the kubelet to boot up the control plane as static Pods from directory "/etc/kubernetes/manifests". This can take up to 4m0s
[apiclient] All control plane components are healthy after 8.008181 seconds
[upload-config] Storing the configuration used in ConfigMap "kubeadm-config" in the "kube-system" Namespace
[kubelet] Creating a ConfigMap "kubelet-config-1.21" in namespace kube-system with the configuration for the kubelets in the cluster
[upload-certs] Skipping phase. Please see --upload-certs
[mark-control-plane] Marking the node k8s-master01.computingforgeeks.com as control-plane by adding the label "node-role.kubernetes.io/master=''"
[mark-control-plane] Marking the node k8s-master01.computingforgeeks.com as control-plane by adding the taints [node-role.kubernetes.io/master:NoSchedule]
[bootstrap-token] Using token: zoy8cq.6v349sx9ass8dzyj
[bootstrap-token] Configuring bootstrap tokens, cluster-info ConfigMap, RBAC Roles
[bootstrap-token] configured RBAC rules to allow Node Bootstrap tokens to get nodes
[bootstrap-token] configured RBAC rules to allow Node Bootstrap tokens to post CSRs in order for nodes to get long term certificate credentials
[bootstrap-token] configured RBAC rules to allow the csrapprover controller automatically approve CSRs from a Node Bootstrap Token
[bootstrap-token] configured RBAC rules to allow certificate rotation for all node client certificates in the cluster
[bootstrap-token] Creating the "cluster-info" ConfigMap in the "kube-public" namespace
[kubelet-finalize] Updating "/etc/kubernetes/kubelet.conf" to point to a rotatable kubelet client certificate and key
[addons] Applied essential addon: CoreDNS
[addons] Applied essential addon: kube-proxy

Your Kubernetes control-plane has initialized successfully!

To start using your cluster, you need to run the following as a regular user:

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

You should now deploy a pod network to the cluster.
Run "kubectl apply -f [podnetwork].yaml" with one of the options listed at:
  https://kubernetes.io/docs/concepts/cluster-administration/addons/

You can now join any number of control-plane nodes by copying certificate authorities
and service account keys on each node and then running the following as root:

  kubeadm join k8s-cluster.computingforgeeks.com:6443 --token sr4l2l.2kvot0pfalh5o4ik \
    --discovery-token-ca-cert-hash sha256:c692fb047e15883b575bd6710779dc2c5af8073f7cab460abd181fd3ddb29a18 \
    --control-plane 

Then you can join any number of worker nodes by running the following on each as root:

kubeadm join k8s-cluster.computingforgeeks.com:6443 --token sr4l2l.2kvot0pfalh5o4ik \
    --discovery-token-ca-cert-hash sha256:c692fb047e15883b575bd6710779dc2c5af8073f7cab460abd181fd3ddb29a18

Configure kubectl using commands in the output:

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

Check cluster status:

$ kubectl cluster-info
Kubernetes master is running at https://k8s-cluster.computingforgeeks.com:6443
KubeDNS is running at https://k8s-cluster.computingforgeeks.com:6443/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy

To further debug and diagnose cluster problems, use 'kubectl cluster-info dump'.

Additional Master nodes can be added using the command in installation output:

kubeadm join k8s-cluster.computingforgeeks.com:6443 --token sr4l2l.2kvot0pfalh5o4ik \
    --discovery-token-ca-cert-hash sha256:c692fb047e15883b575bd6710779dc2c5af8073f7cab460abd181fd3ddb29a18 \
    --control-plane

6. Install network plugin on Master

In this guide we’ll use Calico. You can choose any other supported network plugins.

Download operator and custom resource files. See releases page for latest version.

VER=$(curl -s https://api.github.com/repos/projectcalico/calico/releases/latest|grep tag_name|cut -d '"' -f 4|sed 's/v//')
curl -O https://raw.githubusercontent.com/projectcalico/calico/v$VER/manifests/tigera-operator.yaml
curl -O https://raw.githubusercontent.com/projectcalico/calico/v$VER/manifests/custom-resources.yaml

First, install the operator on your cluster.

$ kubectl create -f tigera-operator.yaml
namespace/tigera-operator created
customresourcedefinition.apiextensions.k8s.io/bgpconfigurations.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/bgppeers.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/blockaffinities.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/caliconodestatuses.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/clusterinformations.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/felixconfigurations.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/globalnetworkpolicies.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/globalnetworksets.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/hostendpoints.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/ipamblocks.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/ipamconfigs.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/ipamhandles.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/ippools.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/ipreservations.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/kubecontrollersconfigurations.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/networkpolicies.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/networksets.crd.projectcalico.org created
customresourcedefinition.apiextensions.k8s.io/apiservers.operator.tigera.io created
customresourcedefinition.apiextensions.k8s.io/imagesets.operator.tigera.io created
customresourcedefinition.apiextensions.k8s.io/installations.operator.tigera.io created
customresourcedefinition.apiextensions.k8s.io/tigerastatuses.operator.tigera.io created
serviceaccount/tigera-operator created
clusterrole.rbac.authorization.k8s.io/tigera-operator created
clusterrolebinding.rbac.authorization.k8s.io/tigera-operator created
deployment.apps/tigera-operator created

If you wish to customize the Calico install, customize the downloaded custom-resources.yaml. For example we are updating CIDR.

sed -ie 's/192.168.0.0/172.24.0.0/g' custom-resources.yaml

Use the manifest locally then install Calico:

$ kubectl create -f custom-resources.yaml
installation.operator.tigera.io/default created
apiserver.operator.tigera.io/default created

Confirm that all of the pods are running:

$ kubectl get pods --all-namespaces -w
NAMESPACE         NAME                               READY   STATUS    RESTARTS   AGE
kube-system       coredns-5d78c9869d-5fmlg           1/1     Running   0          9m43s
kube-system       coredns-5d78c9869d-gtlwr           1/1     Running   0          9m43s
kube-system       etcd-focal                         1/1     Running   0          10m
kube-system       kube-apiserver-focal               1/1     Running   0          9m58s
kube-system       kube-controller-manager-focal      1/1     Running   0          9m59s
kube-system       kube-proxy-pgvnf                   1/1     Running   0          9m43s
kube-system       kube-scheduler-focal               1/1     Running   0          9m57s
tigera-operator   tigera-operator-549d4f9bdb-gfdcf   1/1     Running   0          91s

For Single node cluster allow Pods to run on master nodes:

kubectl taint nodes --all node-role.kubernetes.io/master-
kubectl taint nodes --all  node-role.kubernetes.io/control-plane-

Confirm master node is ready:

# CRI-O
$ kubectl get nodes -o wide
NAME     STATUS   ROLES                  AGE   VERSION   INTERNAL-IP      EXTERNAL-IP   OS-IMAGE             KERNEL-VERSION     CONTAINER-RUNTIME
ubuntu   Ready    control-plane,master   38s   v1.30.3   143.198.114.46   <none>        Ubuntu 20.04.3 LTS   5.4.0-88-generic   cri-o://1.30.3

# Containerd
$ kubectl get nodes -o wide
NAME     STATUS   ROLES                  AGE   VERSION   INTERNAL-IP      EXTERNAL-IP   OS-IMAGE             KERNEL-VERSION     CONTAINER-RUNTIME
ubuntu   Ready    control-plane,master   15m   v1.30.3   143.198.114.46   <none>        Ubuntu 20.04.3 LTS   5.4.0-88-generic   containerd://1.7.19

# Docker
$ kubectl get nodes -o wide
NAME           STATUS   ROLES    AGE   VERSION   INTERNAL-IP      EXTERNAL-IP   OS-IMAGE           KERNEL-VERSION     CONTAINER-RUNTIME
k8s-master01   Ready    master   64m   v1.30.3   135.181.28.113   <none>        Ubuntu 20.04 LTS   5.4.0-37-generic   docker://27.1.1

7. Add worker nodes to the cluster

With the control plane ready you can add worker nodes to the cluster for running scheduled workloads.

If endpoint address is not in DNS, add record to