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How to Install Kubernetes Cluster on CentOS 7

How to Install Kubernetes Cluster on CentOS 7

Kubernetes (k8s) is an open-source, cloud-native, container orchestration and management platform. It’s the go-to way to automate the deployment, scaling, and maintenance of containerised applications across different nodes. From service discovery to auto-restarts, and from resource allocation tracking to compute utilisation and scaling; a well-configured k8s cluster can manage a lot on its own.

Got Ubuntu on your VMs? Learn How to install Kubernetes and deploy a cluster with Docker on Ubuntu 18.04

What is a Kubernetes Cluster?

A Kubernetes cluster consists of a Master and at least one to several worker node(s). The Master is the virtual machine (VM) that administers all activities on your cluster. A node is a VM that serves as a worker machine in your k8s cluster to host running applications. We strongly recommend you only use VMs aka Cloud Servers to run Kubernetes, not system containers aka VPS, as these can cause issues with k8s.

Kubernetes Cluster Diagram on CentOS VMs

A node is comprised of the Kubelet, a container runtime, and the kube-proxy. The k8s installation’s three core modules: Kubelet, kubeadm, and kubectl are agents that control the node and communicate with the Kubernetes Master. Once they have been installed and other configurations done, you will be able to create your first k8s cluster. You can manage this cluster from the command line on your kubemaster node.

Every Kubernetes instance runs on top of a container runtime, which is software responsible for managing container operations. Containers in this case are not virtualised servers but rather a solution that packages code and dependencies to run a single application (service) in an isolated (containerised) environment, essentially disassociating applications from the host machine. The most popular and recommended one is Docker, and it’s the one we will use for the purpose of this guide. However, if you want to install a different underlying container runtime, you can harness the power of the Container Runtime Interface and use basically any runtime you want.

Kubernetes groups containers into pods, its most basic operational unit, which are basically just groups of containers running on the same node. Pods are connected over a network and share storage resources.

In order to connect your nodes or VMs and make them private, make sure to choose a hosting company who provides a Virtual Local Area Network (VLAN) with their VMs. We offer a VLAN add-on to our Cloud Servers for R200 per month.

• Multiple CentOS 7 VMs (Cloud Servers) to house the Master and worker nodes.
• Docker or any other container runtime.
• User with sudo or root privileges on every server.

How to install Kubernetes on CentOS 7

Step 1. Install Docker on all CentOS 7 VMs

Update the package database

sudo yum check-update

Install the dependencies

sudo yum install -y yum-utils device-mapper-persistent-data lvm2

Add and enable official Docker Repository to CentOS 7

sudo yum-config-manager --add-repo

Install the latest Docker version on CentOS 7

sudo yum install docker-ce

A successful installation output will be concluded with a Complete!

You may be prompted to accept the GPG key, this is to verify that the fingerprint matches. The format will look as follows. If correct, accept it.

060A 61C5 1B55 8A7F 742B 77AA C52F EB6B 621E 9F35

Step 4: Manage Docker Service

Now Docker is installed, but the service is not yet running. Start and enable Docker using the commands

sudo systemctl start docker
sudo systemctl enable docker

To confirm that Docker is active and running use

sudo systemctl status docker
terminal output

Step 2. Set up the Kubernetes Repository

Since the Kubernetes packages aren’t present in the official CentOS 7 repositories, we will need to add a new repository file. Use the following command to create the file and open it for editing:

sudo vi /etc/yum.repos.d/kubernetes.repo

Once the file is open, press I key to enter insert mode, and paste the following contents:


Once pasted, press escape to exit insert mode. Then enter :x to save the file and exit.

terminal output

Step 3. Install Kubelet on CentOS 7

The first core module that we need to install on every node is Kubelet. Use the following command to do so:

sudo yum install -y kubelet

Once you enter the command, you should see a lot of logs being printed. A successful installation will be indicated by the Complete! keyword at the end. See below:

terminal output

Step 4. Install kubeadm and kubectl on CentOS 7

kubeadm, the next core module, will also have to be installed on every machine. Use the following command:

sudo yum install -y kubeadm

Successful installation should result in the following output:
(Note that kubeadm automatically installs kubectl as a dependency)

terminal output

Step 5. Set hostnames

On your Master node, update your hostname using the following command:

sudo hostnamectl set-hostname master-node
sudo exec bash


sudo hostnamectl set-hostname W-node1
sudo exec bash

Now open the /etc/hosts file and edit the hostnames for your worker nodes:

sudo cat <<EOF>> /etc/hosts master-node node1 W-node1 node2 W-node2

Step 6. Disable SElinux

To allow containers to be able to access the file system, we need to enable the “permissive” mode of SElinux. Use the following commands:
(Note: For these commands to take effect, you will have to reboot)

sudo setenforce 0
sudo sed -i --follow-symlinks 's/SELINUX=enforcing/SELINUX=disabled/g' /etc/sysconfig/selinux

Step 7. Add firewall rules

To allow seamless communication between pods, containers, and VMs, we need to add rules to our firewall on the Master node. Use the following commands:

sudo firewall-cmd --permanent --add-port=6443/tcp
sudo firewall-cmd --permanent --add-port=2379-2380/tcp
sudo firewall-cmd --permanent --add-port=10250/tcp
sudo firewall-cmd --permanent --add-port=10251/tcp
sudo firewall-cmd --permanent --add-port=10252/tcp
sudo firewall-cmd --permanent --add-port=10255/tcp
sudo firewall-cmd –reload

All your firewall rule commands should output success like below:

terminal output

You will also need to run the following commands on each worker node:

sudo firewall-cmd --permanent --add-port=10251/tcp
sudo firewall-cmd --permanent --add-port=10255/tcp
sudo firewall-cmd –reload

Step 8. Update iptables config

We need to update the net.bridge.bridge-nf-call-iptables parameter in our sysctl file to ensure proper processing of packets across all machines. Use the following commands:

cat <<EOF > /etc/sysctl.d/k8s.conf
net.bridge.bridge-nf-call-ip6tables = 1
net.bridge.bridge-nf-call-iptables = 1
sudo sysctl --system

You should get the following output:

terminal output

Step 9. Disable swap

For Kubelet to work, we also need to disable swap on all of our VMs:

sudo sed -i '/swap/d' /etc/fstab
sudo swapoff -a

This concludes our installation and configuration of Kubernetes on CentOS 7. We will now share the steps for deploying a k8s cluster.

Deploying a Kubernetes Cluster on CentOS 7

Step 1. kubeadm initialization

To launch a new Kubernetes cluster instance, you need to initialize kubeadm. Use the following command:

sudo kubeadm init

This command may take several minutes to execute. Upon success, you should get logs similar to those in this screenshot:

terminal output

You will also get an auto-generated command at the end of the output. Copy the text following the line Then you can join any number of worker nodes by running the following on each as root: as highlighted in the above screenshot and save it somewhere safe. We will use this to add worker nodes to our cluster.

Note: If you forgot to copy the command, or have misplaced it, don’t worry. You can retrieve it again by entering the following command:

sudo kubeadm token create --print-join-command

Step 2. Create required directories and start managing Kubernetes cluster

In order to start managing your cluster, you need to create a directory and assume ownership. Run the following commands 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

Step 3. Set up Pod network for the Cluster

Pods within a cluster are connected via the pod network. At this point, it’s not working. This can be verified by entering the following two commands:

sudo kubectl get nodes
sudo kubectl get pods --all-namespaces

terminal output
As you can see, the status of masternode is NotReady. The CoreDNS service is also not running. To fix this, run the following commands:

sudo export kubever=$(kubectl version | base64 | tr -d '\n')
sudo kubectl apply -f$kubever

You should get the following output:

terminal output

And now if you verify the statuses of your node and CoreDNS service, you should get Ready and Running like seen below:

terminal output

Step 4. Add nodes to your cluster

As a final step, you need to add worker nodes to your cluster. We will use the kubeadm join auto-generated token in Step 1. here. Run your own version of the following command on all of the worker node VMs:

sudo kubeadm join --token 848gwg.mpe76povky8qeqvu --discovery-token-ca-cert-hash sha256:f0a16f51dcc077da9e41f01bdcbc465343668f36d55f41250c570a2be8321eac

On successful addition, you should get the following output:

terminal output

Running the following command on the master-node should show your newly added node.

sudo kubectl get nodes
terminal output

To set the role for your worker node, use the following command:

sudo kubectl label node w-node1
terminal output

Now you’re all set up.

Happy Hosting!

The Author

Maab S.

Maab is an experienced software engineer who specializes in explaining technical topics to a wider audience.

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