Learn Kubernetes Basics

Getting started

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Installing kubeadm

This page shows how to install the kubeadm toolbox. For information how to create a cluster with kubeadm once you have performed this installation process, see the Using kubeadm to Create a Cluster page.

Before you begin

Verify the MAC address and product_uuid are unique for every node

It is very likely that hardware devices will have unique addresses, although some virtual machines may have identical values. Kubernetes uses these values to uniquely identify the nodes in the cluster. If these values are not unique to each node, the installation process may fail.

Check network adapters

If you have more than one network adapter, and your Kubernetes components are not reachable on the default route, we recommend you add IP route(s) so Kubernetes cluster addresses go via the appropriate adapter.

Ensure iptables tooling does not use the nftables backend

In Linux, nftables is available as a modern replacement for the kernel’s iptables subsystem. The iptables tooling can act as a compatibility layer, behaving like iptables but actually configuring nftables. This nftables backend is not compatible with the current kubeadm packages: it causes duplicated firewall rules and breaks kube-proxy.

If your system’s iptables tooling uses the nftables backend, you will need to switch the iptables tooling to ‘legacy’ mode to avoid these problems. This is the case on at least Debian 10 (Buster), Ubuntu 19.04, Fedora 29 and newer releases of these distributions by default. RHEL 8 does not support switching to legacy mode, and is therefore incompatible with current kubeadm packages.

update-alternatives --set iptables /usr/sbin/iptables-legacy
update-alternatives --set ip6tables /usr/sbin/ip6tables-legacy
update-alternatives --set arptables /usr/sbin/arptables-legacy
update-alternatives --set ebtables /usr/sbin/ebtables-legacy

update-alternatives --set iptables /usr/sbin/iptables-legacy

Check required ports

Master node(s)

Protocol Direction Port Range Purpose Used By
TCP Inbound 6443* Kubernetes API server All
TCP Inbound 2379-2380 etcd server client API kube-apiserver, etcd
TCP Inbound 10250 Kubelet API Self, Control plane
TCP Inbound 10251 kube-scheduler Self
TCP Inbound 10252 kube-controller-manager Self

Worker node(s)

Protocol Direction Port Range Purpose Used By
TCP Inbound 10250 Kubelet API Self, Control plane
TCP Inbound 30000-32767 NodePort Services** All

** Default port range for NodePort Services.

Any port numbers marked with * are overridable, so you will need to ensure any custom ports you provide are also open.

Although etcd ports are included in master nodes, you can also host your own etcd cluster externally or on custom ports.

The pod network plugin you use (see below) may also require certain ports to be open. Since this differs with each pod network plugin, please see the documentation for the plugins about what port(s) those need.

Installing runtime

Since v1.6.0, Kubernetes has enabled the use of CRI, Container Runtime Interface, by default.

Since v1.14.0, kubeadm will try to automatically detect the container runtime on Linux nodes by scanning through a list of well known domain sockets. The detectable runtimes and the socket paths, that are used, can be found in the table below.

Runtime Domain Socket
Docker /var/run/docker.sock
containerd /run/containerd/containerd.sock
CRI-O /var/run/crio/crio.sock

If both Docker and containerd are detected together, Docker takes precedence. This is needed, because Docker 18.09 ships with containerd and both are detectable. If any other two or more runtimes are detected, kubeadm will exit with an appropriate error message.

On non-Linux nodes the container runtime used by default is Docker.

If the container runtime of choice is Docker, it is used through the built-in dockershim CRI implementation inside of the kubelet.

Other CRI-based runtimes include:

Refer to the CRI installation instructions for more information.

Installing kubeadm, kubelet and kubectl

You will install these packages on all of your machines:

kubeadm will not install or manage kubelet or kubectl for you, so you will need to ensure they match the version of the Kubernetes control plane you want kubeadm to install for you. If you do not, there is a risk of a version skew occurring that can lead to unexpected, buggy behaviour. However, one minor version skew between the kubelet and the control plane is supported, but the kubelet version may never exceed the API server version. For example, kubelets running 1.7.0 should be fully compatible with a 1.8.0 API server, but not vice versa.

For information about installing kubectl, see Install and set up kubectl.

Warning: These instructions exclude all Kubernetes packages from any system upgrades. This is because kubeadm and Kubernetes require special attention to upgrade.

For more information on version skews, see:

apt-get update && apt-get install -y apt-transport-https curl
curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key add -
cat <<EOF >/etc/apt/sources.list.d/kubernetes.list
deb https://apt.kubernetes.io/ kubernetes-xenial main
EOF
apt-get update
apt-get install -y kubelet kubeadm kubectl
apt-mark hold kubelet kubeadm kubectl

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

# Set SELinux in permissive mode (effectively disabling it)
setenforce 0
sed -i 's/^SELINUX=enforcing$/SELINUX=permissive/' /etc/selinux/config

yum install -y kubelet kubeadm kubectl --disableexcludes=kubernetes

systemctl enable --now kubelet

Note:

  • Setting SELinux in permissive mode by running setenforce 0 and sed ... effectively disables it. This is required to allow containers to access the host filesystem, which is needed by pod networks for example. You have to do this until SELinux support is improved in the kubelet.

  • Some users on RHEL/CentOS 7 have reported issues with traffic being routed incorrectly due to iptables being bypassed. You should ensure net.bridge.bridge-nf-call-iptables is set to 1 in your sysctl config, e.g.

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

  • Make sure that the br_netfilter module is loaded before this step. This can be done by running lsmod | grep br_netfilter. To load it explicitly call modprobe br_netfilter.

Install CNI plugins (required for most pod network):

CNI_VERSION="v0.6.0"
mkdir -p /opt/cni/bin
curl -L "https://github.com/containernetworking/plugins/releases/download/${CNI_VERSION}/cni-plugins-amd64-${CNI_VERSION}.tgz" | tar -C /opt/cni/bin -xz

Install crictl (required for kubeadm / Kubelet Container Runtime Interface (CRI))

CRICTL_VERSION="v1.11.1"
mkdir -p /opt/bin
curl -L "https://github.com/kubernetes-incubator/cri-tools/releases/download/${CRICTL_VERSION}/crictl-${CRICTL_VERSION}-linux-amd64.tar.gz" | tar -C /opt/bin -xz

Install kubeadm, kubelet, kubectl and add a kubelet systemd service:

RELEASE="$(curl -sSL https://dl.k8s.io/release/stable.txt)"

mkdir -p /opt/bin
cd /opt/bin
curl -L --remote-name-all https://storage.googleapis.com/kubernetes-release/release/${RELEASE}/bin/linux/amd64/{kubeadm,kubelet,kubectl}
chmod +x {kubeadm,kubelet,kubectl}

curl -sSL "https://raw.githubusercontent.com/kubernetes/kubernetes/${RELEASE}/build/debs/kubelet.service" | sed "s:/usr/bin:/opt/bin:g" > /etc/systemd/system/kubelet.service
mkdir -p /etc/systemd/system/kubelet.service.d
curl -sSL "https://raw.githubusercontent.com/kubernetes/kubernetes/${RELEASE}/build/debs/10-kubeadm.conf" | sed "s:/usr/bin:/opt/bin:g" > /etc/systemd/system/kubelet.service.d/10-kubeadm.conf

Enable and start kubelet:

systemctl enable --now kubelet

The kubelet is now restarting every few seconds, as it waits in a crashloop for kubeadm to tell it what to do.

Configure cgroup driver used by kubelet on Master Node

When using Docker, kubeadm will automatically detect the cgroup driver for the kubelet and set it in the /var/lib/kubelet/kubeadm-flags.env file during runtime.

If you are using a different CRI, you have to modify the file /etc/default/kubelet with your cgroup-driver value, like so:

KUBELET_EXTRA_ARGS=--cgroup-driver=<value>

This file will be used by kubeadm init and kubeadm join to source extra user defined arguments for the kubelet.

Please mind, that you only have to do that if the cgroup driver of your CRI is not cgroupfs, because that is the default value in the kubelet already.

Restarting the kubelet is required:

systemctl daemon-reload
systemctl restart kubelet

Troubleshooting

If you are running into difficulties with kubeadm, please consult our troubleshooting docs.

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