How to Install and Setup Kubernetes Cluster with Flannel CNI in CentOS 7

Kubernetes also written as k8s, is open source solution for management and orchestration of containerized applications. This article assumes that you have a minimally installed CentOS 7 machine as master node and two worker nodes with internet access. Let proceed for installation. Execute following commands on all nodes (master and worker).

Update all the systems.

yum update -y

Enable epel repository, install utilities and dependencies.

yum install epel-release yum-utils bash-completion net-tools device-mapper-persistent-data lvm2 -y

Disable Selinux.

sed -i 's/SELINUX=enforcing/SELINUX=disabled/g' /etc/selinux/config

Disable swap.

free -m && swapoff -a && free -m
sed -i.bak -r 's/(.+ swap .+)/#\1/' /etc/fstab && cat /etc/fstab

Reboot the machine to disable Selinux and bring up the latest kernel.

reboot

Add and enable Kubernetes official repository.

cat <<EOF > /etc/yum.repos.d/kubernetes.repo
[kubernetes]
name=Kubernetes
baseurl=https://packages.cloud.google.com/yum/repos/kubernetes-el7-x86_64
enabled=1
gpgcheck=1
repo_gpgcheck=1
gpgkey=https://packages.cloud.google.com/yum/doc/yum-key.gpg https://packages.cloud.google.com/yum/doc/rpm-package-key.gpg
EOF

Add host entries or DNS records for name resolution.

cat <<EOF >> /etc/hosts
10.14.7.51 k8s-master.induslevel.com k8s-master
10.14.7.53 k8s-node01.induslevel.com k8s-node01
10.14.7.54 k8s-node02.induslevel.com k8s-node02
EOF

cat /etc/hosts

Add firewalld rules on master and worker nodes.

firewall-cmd --permanent --add-port=8285/tcp
firewall-cmd --permanent --add-port=6443/tcp
firewall-cmd --permanent --add-port=2379-2380/tcp
firewall-cmd --permanent --add-port=10250/tcp
firewall-cmd --permanent --add-port=10251/tcp
firewall-cmd --permanent --add-port=10252/tcp
firewall-cmd --permanent --add-port=10255/tcp
firewall-cmd --reload && firewall-cmd --list-all

For networking to work, we need to tweak sysctl parameters for routing.

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

Install required packages on all nodes (masters and nodes).

yum install -y kubelet kubeadm kubectl

We will be using docker as container engine. Enable docker repository.

yum-config-manager --add-repo  https://download.docker.com/linux/centos/docker-ce.repo

Install docker on all nodes.

yum install docker -y

Enable and start docker services.

systemctl start docker && systemctl enable docker && systemctl status docker

start kublet service and enable it for startup on system booting on all nodes and master.

systemctl enable kubelet && systemctl start kubelet && systemctl status kubelet

Initialize the cluster on master.

kubeadm init --apiserver-advertise-address 10.14.7.51 --pod-network-cidr=172.16.0.0/16

Copy the last lines of the output which will be used for connecting worker nodes with the master.

[init] Using Kubernetes version: v1.22.4
[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'
[certs] Using certificateDir folder "/etc/kubernetes/pki"
[certs] Generating "ca" certificate and key
[certs] Generating "apiserver" certificate and key
[certs] apiserver serving cert is signed for DNS names [kubernetes kubernetes.default kubernetes.default.svc kubernetes.default.svc.cluster.local rwc-k8s-master1.i2cinc.com] and IPs [10.96.0.1 10.14.7.51]
[certs] Generating "apiserver-kubelet-client" certificate and key
[certs] Generating "front-proxy-ca" certificate and key
[certs] Generating "front-proxy-client" certificate and key
[certs] Generating "etcd/ca" certificate and key
[certs] Generating "etcd/server" certificate and key
[certs] etcd/server serving cert is signed for DNS names [localhost rwc-k8s-master1.i2cinc.com] and IPs [10.14.7.51 127.0.0.1 ::1]
[certs] Generating "etcd/peer" certificate and key
[certs] etcd/peer serving cert is signed for DNS names [localhost rwc-k8s-master1.i2cinc.com] and IPs [10.14.7.51 127.0.0.1 ::1]
[certs] Generating "etcd/healthcheck-client" certificate and key
[certs] Generating "apiserver-etcd-client" certificate and key
[certs] Generating "sa" key and public key
[kubeconfig] Using kubeconfig folder "/etc/kubernetes"
[kubeconfig] Writing "admin.conf" kubeconfig file
[kubeconfig] Writing "kubelet.conf" kubeconfig file
[kubeconfig] Writing "controller-manager.conf" kubeconfig file
[kubeconfig] Writing "scheduler.conf" kubeconfig file
[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
[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"
[control-plane] Creating static Pod manifest for "kube-scheduler"
[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 7.002213 seconds
[upload-config] Storing the configuration used in ConfigMap "kubeadm-config" in the "kube-system" Namespace
[kubelet] Creating a ConfigMap "kubelet-config-1.22" 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 rwc-k8s-master1.i2cinc.com as control-plane by adding the labels: [node-role.kubernetes.io/master(deprecated) node-role.kubernetes.io/control-plane node.kubernetes.io/exclude-from-external-load-balancers]
[mark-control-plane] Marking the node rwc-k8s-master1.i2cinc.com as control-plane by adding the taints [node-role.kubernetes.io/master:NoSchedule]
[bootstrap-token] Using token: k6ft3u.5ybf6kpmxjmr8wwo
[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

Alternatively, if you are the root user, you can run:

  export KUBECONFIG=/etc/kubernetes/admin.conf

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/

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

kubeadm join 10.14.7.51:6443 --token k6ft3u.5ybf6kpmxjmr8wwo \
        --discovery-token-ca-cert-hash sha256:1939449bb8aec7dae9cf877127c99abb07b5d12515c617603969dcc0ebb795f2

Create .kube directory to manage the kubernetes with non-root user.

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

Install Flannel CNI for intercommunication of pods on master.

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

Connect nodes with master. Execute the command on worker nodes.

kubeadm join 10.14.7.51:6443 --token k6ft3u.5ybf6kpmxjmr8wwo \
        --discovery-token-ca-cert-hash sha256:1939449bb8aec7dae9cf877127c99abb07b5d12515c617603969dcc0ebb795f2

Check the status of nodes connectivity using command on master node. Cluster will be configured once node status changes to ready state.

kubectl get nodes -o wide
kubectl get pods --all-namespaces

Now we are going to deploy a nginx container deployment to test out setup.

mkdir -p /root/nginx/ && cd /root/nginx/
cat > /root/nginx/nginx-deployment.yaml<<"EOF"
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.0
        ports:
        - containerPort: 80
EOF

Now create deployment using kubectl command on master node.

kubectl create -f /root/nginx/nginx-deployment.yaml

Check the status of deployment. Once deployment is complete, we will be creating a service to expose the port of this container.

kubectl get deployments
kubectl describe deployment nginx-deployment

Create the new service to expose the nginx port for outside world.

cat > /root/nginx/nginx-service.yaml<<"EOF"
apiVersion: v1
kind: Service
metadata:
  name: nginx-service
  labels:
    run: nginx-service
spec:
  type: NodePort
  ports:
  - port: 80
    protocol: TCP
  selector:
    app: nginx
EOF

Create service using this yaml file.

kubectl create -f /root/nginx/nginx-service.yaml

Check the status using following commands.

kubectl get service
kubectl describe service nginx-service

After successful service setup, check the status of service using curl command.

NodePort=$(kubectl describe service nginx-service |grep 'NodePort:'|awk '{print $3}'|awk -F/ '{print $1}') && curl k8s-node01:$NodePort
NodePort=$(kubectl describe service nginx-service |grep 'NodePort:'|awk '{print $3}'|awk -F/ '{print $1}') && curl k8s-node02:$NodePort