Deploy Kubernetes With Kubeadm on CentOS 7

Overview

This article is meant to help you get a Kubernetes cluster up and running with kubeadm in no time. This guide will be deploying two servers, one master and one worker, however you can deploy as many servers as you would like.

What is kubeadm?

Kubeadm is a tool developed by Kubernetes which allows you to get a minimum viable cluster up and running by following best practices. It will only bootstrap your cluster, not provision machines. Things such as addons, the Kubernetes dashboard, monitoring solutions and so on are not something kubeadm will do for you.

Prerequisites

There are a few requirements for the servers we will deploy. One or more machines running a deb/rpm-compatible OS. We will be using CentOS.

• 2 GB or more of RAM per machine
• 2 CPUs or more on the master

Full network connectivity among all machines in the cluster

The two servers deployed in this guide are the following:
– 1 CPU 2GB RAM with CentOS 7 (Worker node)
– 2 CPU 4GB RAM with CentOS 7 (Master node)

With this amount of RAM on both servers, Kubernetes will have plenty of room to breathe.

Configuring the worker and master

Here are the steps we will have to take on both the master and worker node:

• Yum update & packages
• Install docker
• Disable selinux
• Disable swap
• Disable Firewall
• Update IPTables
• Install kubelet/kubeadm/kubectl

Installing Docker

We’ll be using version 1.14 of Kubernetes in this tutorial. For this version, Kubernetes recommends running Docker version 18.06.2. Be sure to check the recommended Docker version for your version of Kuberenetes

We will be adding the Docker repository to yum and specifically installing 18.06.2. Once Docker is installed, we’ll need to configure the docker daemon to the settings recommended by Kubernetes.

###Add yum-utils, if not installed already
yum install yum-utils

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

###Install Docker CE.
yum update && yum install docker-ce-18.06.2.ce

###Create /etc/docker directory.
mkdir /etc/docker

###Setup daemon.
cat > /etc/docker/daemon.json <<EOF
{
  "exec-opts": ["native.cgroupdriver=systemd"],
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "100m"
  },
  "storage-driver": "overlay2",
  "storage-opts": [
    "overlay2.override_kernel_check=true"
  ]
}
EOF

mkdir -p /etc/systemd/system/docker.service.d

###Restart Docker
systemctl daemon-reload
systemctl enable docker.service
systemctl restart docker

Disable SELinux

Since we are using CentOS we need to disable SELinux. This is necessary to allow containers access to the host filesystem.

setenforce 0
sed -i 's/^SELINUX=enforcing$/SELINUX=disable/' /etc/selinux/config

Disable Swap

Swap needs to be disabled to allow kubelet to work properly.

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

Disable Firewall

Kubernetes uses IPTables to handle inbound and outbound traffic – so to avoid any issues we disable firewalld.

systemctl disable firewalld
systemctl stop firewalld

Update IPTables

Kubernetes recommends that we ensure net.bridge.bridge-nf-call-iptables is set to 1. This is due to issues where REHL/CentOS 7 has had issues with traffic being rerouted incorrectly due to bypassing iptables.

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 kubelet/kubeadm/kubectl

We will need to add the kubernetes repo to yum. Once we do that we just need to run the install command and enable kubelet.

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
exclude=kube*
EOF

yum install -y kubelet kubeadm kubectl --disableexcludes=kubernetes
systemctl enable --now kubelet

Now we have fully configured both our master and worker node. We can now initialize our master node and join our worker nodes to the master!

Note If you wanted to add more worker nodes the process above would have to be done on all of those nodes as well.

Master Node setup

We want to initialize our master node by running the following command. You’ll want to substitute your master node’s IP address in the command below. Additionally, we’ll pass in the pod-network-cidr to make it easier for us later when we install the Flannel network overlay.

kubeadm init --apiserver-advertise-address=YOUR_IP_HERE --pod-network-cidr=10.244.0.0/16

This may take a while to complete but once it is completed you will see something similar at the end of the output like the following.

kubeadm join YOUR_IP:6443 --token 4if8c2.pbqh82zxcg8rswui /
--discovery-token-ca-cert-hash sha256:a0b2bb2b31bf7b06bb5058540f02724240fc9447b0e457e049e59d2ce19fcba2

This command is what your worker nodes need to execute to join the cluster, so take note of it.

Next up is Flannel. Flannel is what allows pod to pod communication. There are various other types of network overlays that you can install but for simplicity this guide will use Flannel.

Copy the kube/config file over to your $Home so you can execute kubectl commands.

mkdir $HOME/.kube
cp /etc/kubernetes/admin.conf $HOME/.kube/config

One final step on the master node is to install Flannel. Run the following command.

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

With this config copied over you will be able to run kubectl get cs and get a response.

NAME                 STATUS    MESSAGE             ERROR
scheduler            Healthy   ok
controller-manager   Healthy   ok
etcd-0               Healthy   {"health":"true"}

Your master node is set and ready to go. Onto the worker node!

Worker Node

At this point there is no extra work that is necessary on the worker node. All we need to do is run the kubeadm join command that we got from our kubeadm init output.

If by some chance you misplaced the kubeadm join command you can generate another one on the master node by running
kubeadm token create --print-join-command

Once you run the kubeadm join command, if you run kubectl get nodes on master you will see a similar output to the following.

NAME          STATUS   ROLES    AGE    VERSION
k8-master   Ready    master   107m   v1.14.2
k8-worker   Ready    <none>   45m    v1.14.2

Wrapping up

Just like that you have bootstrapped a Kubernetes cluster using kubeadm. You could also do this with private networks. Vultr, as well as other cloud providers, allow for private networks. Also, if you want to execute kubectl commands from your local machine against your cluster, you can accomplish this by having kubectl installed locally and pull down the .kube/config file from the cluster to your local machine in $HOME/.kube/config.

Hopefully this guide helps you traverse kubeadm and gets you playing with kubernetes in no time!

Useful links:

• Kubeadm
• Flannel