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Reason: Many missing prompts. (Discuss in Talk:Kubernetes)

Kubernetes (aka. k8s) is an open-source system for automating the deployment, scaling, and management of containerized applications.

A k8s cluster consists of its control-plane components and node components (each representing one or more host machines running a container runtime and `kubelet.service`). There are two options to install kubernetes, "the real one", described here, and a local install with k3s, kind, or minikube.

## Installation

There are many methods to setup a kubernetes cluster. This article will focus on bootstrapping with kubeadm.

### Deployment tools

When bootstrapping a Kubernetes cluster with kubeadm, install kubeadm and kubelet on each node.

When manually creating a Kubernetes cluster install etcdAUR and the package group kubernetes-control-plane (for a control-plane node) and kubernetes-node (for a worker node).

To control a kubernetes cluster, install kubectl on the control-plane hosts and any external host that is supposed to be able to interact with the cluster.

### Container runtime

Both control-plane and regular worker nodes require a container runtime for their `kubelet` instances which is used for hosting containers. Install either containerd or cri-o to meet this dependency.

#### Prerequisites

To setup forwarding IPv4 and letting iptables see bridged traffic, begin by loading the kernel modules `overlay` and `br_netfilter` manually.

To perform this step on subsequent boots, create:

    /etc/modules-load.d/k8s.conf

    overlay
    br_netfilter

Some module parameters are required:

    /etc/sysctl.d/k8s.conf

    net.bridge.bridge-nf-call-iptables  = 1
    net.bridge.bridge-nf-call-ip6tables = 1
    net.ipv4.ip_forward                 = 1

Apply them without rebooting with:

    # sysctl --system

(Optionally) verify that the `br_netfilter`, `overlay` modules are loaded by running the following commands:

    lsmod | grep br_netfilter
    lsmod | grep overlay

(Optionally) verify that the `net.bridge.bridge-nf-call-iptables`, `net.bridge.bridge-nf-call-ip6tables`, and `net.ipv4.ip_forward` system variables are set to 1 in your `sysctl` config by running the following command:

    sysctl net.bridge.bridge-nf-call-iptables net.bridge.bridge-nf-call-ip6tables net.ipv4.ip_forward

Refer to official document[1] for more details.

#### Install containerd

To install a rootless `containerd`, use nerdctl-full-binAUR, which is nerdctl full pkg, bundle with containerd/CNI plugin/RootlessKit:

    containerd-rootless-setuptool.sh install

Remember Arch Linux use systemd as its init system (no matter you systemd-boot or GRUB as bootloader), so you need to choose systemd cgroup driver before deploying the control plane.

Finally enable/start `containerd.service`.

### (Optional) Package Manager

helm is a tool for managing pre-configured Kubernetes resources which may be helpful for getting started.

## Configuration

All nodes in a cluster (control-plane and worker) require a running instance of `kubelet.service`.

Tip: Read the following subsections closely before starting `kubelet.service` or using `kubeadm`.

All provided systemd services accept CLI overrides in environment files:

- `kubelet.service`: `/etc/kubernetes/kubelet.env`
- `kube-apiserver.service`: `/etc/kubernetes/kube-apiserver.env`
- `kube-controller-manager.service`: `/etc/kubernetes/kube-controller-manager.env`
- `kube-proxy.service`: `/etc/kubernetes/kube-proxy.env`
- `kube-scheduler.service`: `/etc/kubernetes/kube-scheduler.env`

This article or section needs expansion.

Reason:

- Example for setup without `kubeadm`, using `kube-apiserver.service`, `kube-controller-manager.service`, `kube-proxy.service` and `kube-scheduler.service`.
- Example for setup with `kubeadm` using configuration files.

(Discuss in Talk:Kubernetes)

### Disable swap

Kubernetes currently does not support having swap enabled on the system.

see KEP-2400: Node system swap support for details

If the swap is on, disable it with the following command and restart

    sudo swapoff -a
    sudo sed -i '/ swap / s/^/#/' /etc/fstab

If you see `/dev/zram0` you should also remove the zram kernel module by:

    swapoff /dev/zram0
    modprobe -r zram

If your zram is managed by systemd, try finding the .swap unit:

    systemctl --type swap

Once found, you can mask it:

    sudo systemctl mask "dev-XYZ.swap"

Then reboot.

### Choose cgroup driver

To use the `systemd` cgroup driver in `/etc/containerd/config.toml` with `runc`, set

    [plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc]
      ...
      [plugins."io.containerd.grpc.v1.cri".containerd.runtimes.runc.options]
        SystemdCgroup = true

If `/etc/containerd/config.toml` does not exist, the default configuration can be generated via[2]

    containerd config default > /etc/containerd/config.toml

Remember to restart `containerd.service` to make the change take effect.

See this[3] official document for a deeper discussion on whether to keep `cgroupfs` driver or use `systemd` cgroup driver.

### Choose container runtime interface (CRI)

A container runtime has to be configured and started, before `kubelet.service` can make use of it.

You will pass flag `--cri-socket` with the container runtime interface endpoint to `kubeadm init` or `kubeadm join` in order to create or join a cluster.

For example, if you choose containerd as CRI runtime, the flag `--cri-socket` will be:

    kubeadm init --cri-socket /run/containerd/containerd.sock

#### Containerd

Before Kubernetes version 1.27.4, when using containerd as container runtime, it is required to provide `kubeadm init` or `kubeadm join` with its CRI endpoint. To do so, specify their flag `--cri-socket` to `/run/containerd/containerd.sock`[4].

    kubeadm join --cri-socket=/run/containerd/containerd.sock

After Kubernetes version 1.27.4, kubeadm will auto detect this CRI for you, flag `--cri-socket` is only needed when you installed multiple CRI.

#### CRI-O

When using CRI-O as container runtime, it is required to provide `kubeadm init` or `kubeadm join` with its CRI endpoint: `--cri-socket='unix:///run/crio/crio.sock'`

Note: CRI-O by default uses `systemd` as its `cgroup_manager` (see `/etc/crio/crio.conf`). This is not compatible with kubelet us default (`cgroupfs`) when using kubelet < v1.22.

Change kubelet us default by appending `--cgroup-driver='systemd'` to the `KUBELET_ARGS` environment variable in `/etc/kubernetes/kubelet.env` upon first start (i.e. before using `kubeadm init`).

Note that the `KUBELET_EXTRA_ARGS` variable, used by older versions is now no longer read by the default `kubelet.service`!

When kubeadm updates from 1.19.x to 1.20.x, then it should be possible to use https://kubernetes.io/docs/reference/setup-tools/kubeadm/kubeadm-init/#config-file as explained on https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/install-kubeadm/#configure-cgroup-driver-used-by-kubelet-on-control-plane-node, as in https://github.com/cri-o/cri-o/pull/4440/files, instead of the above. (TBC, untested.)

After the node has been configured, the CLI flag could (but does not have to) be replaced by a configuration entry for `kubelet`:

    /var/lib/kubelet/config.yaml

    cgroupDriver: 'systemd'

### Choose Cluster Network Parameter

#### Choose a pod CIDR range

The networking setup for the cluster has to be configured for the respective container runtime. This can be done using cni-plugins.

The pod CIDR addresses refer to the IP address range that is assigned to pods within a Kubernetes cluster. When pods are scheduled to run on nodes in the cluster, they are assigned IP addresses from this CIDR range.

The pod CIDR range is specified when deploying a Kubernetes cluster and is confined within the cluster network. It should not overlap with other IP ranges used within the cluster, such as the service CIDR range.

You will pass flag `--pod-network-cidr` with value of the virtual network's CIDR to `kubeadm init` or `kubeadm join` in order to create or join a cluster.

For example:

    kubeadm init --pod-network-cidr='10.85.0.0/16'

will set your kubernetes' pod CIDR range to '10.85.0.0/16'.

#### (Optional) Choose API server advertising address

If your node for control plane is in multiple subnets (for example you may have installed a tailscale tailnet), when initializing the Kubernetes master with kubeadm init, you can specify the IP address that the API server will advertise with the --apiserver-advertise-address flag. This IP address should be accessible to all nodes in your cluster.

#### (Optional) Choose alternative node network proxy provider

Node proxy provider like `kube-proxy` is a network proxy that runs on each node in your cluster, maintaining network rules on nodes to allow network communication to your Pods from network sessions inside or outside of your cluster.

By default kubeadm choose `kube-proxy` as the node proxy that runs on each node in your cluster.

Container Network Interface (CNI) plugins like cilium offer a complete replacement for kube-proxy.

If you want to use cilium's implementation of node network proxy to fully leverage cilium's network policy feature, you should pass flag `--skip-phases=addon/kube-proxy` to `kubeadm init` to skip the install of kube-proxy.

Cilium will install a full replacement during its installation. See this[5] for details.

## Create cluster

Before creating a new kubernetes cluster with `kubeadm` start and enable `kubelet.service`.

Note: `kubelet.service` will fail (but restart) until configuration for it is present.

This article or section needs expansion.

Reason:

- Example for setup without `kubeadm`, using `kube-apiserver.service`, `kube-controller-manager.service`, `kube-proxy.service` and `kube-scheduler.service`.
- Example for setup with `kubeadm` using configuration files.

(Discuss in Talk:Kubernetes)

When creating a new kubernetes cluster with `kubeadm` a control-plane has to be created before further worker nodes can join it.

Tip:

- If the cluster is supposed to be turned into a high availability cluster (a stacked etcd topology) later on `kubeadm init` needs to be provided with `--control-plane-endpoint=<IP or domain>` (it is not possible to do this retroactively!).
- It is possible to use a config file for `kubeadm init` instead of a set of parameters.

### Initialize control-plane

To initialize control-plane, you need pass the following necessary flags to `kubeadm init`

If run successfully, `kubeadm init` will have generated configurations for the `kubelet` and various control-plane components below `/etc/kubernetes/` and `/var/lib/kubelet/`.

Finally, it will output commands ready to be copied and pasted to setup kubectl and make a worker node join the cluster (based on a token, valid for 24 hours).

To use `kubectl` with the freshly created control-plane node, setup the configuration (either as root or as a normal user):

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

### Installing CNI plugins (pod network addon)

Note: You must deploy a Container Network Interface (CNI) based Pod network add-on so that your Pods can communicate with each other. Cluster DNS (CoreDNS) will not start up before a network is installed.

Pod network add-on (CNI plugins) implements the Kubernetes network model[6] differently from simple solutions like flannel to more complicated solutions like calico

An increasingly adopted advanced CNI plugin is cilium, which achieves impressive performance with eBPF[7]. To install `cilium` as CNI plugin, use cilium-cli:

    cilium-cli install

For more details on pod network, see this[8] official document.

## Join cluster

With the token information generated in #Control-plane[broken link: invalid section] it is possible to make another machine join the cluster as worker node with command `kubeadm join`.

Remember you need to specify #Specify container runtime[broken link: invalid section] for working nodes as well by passing flag `<SOCKET>` to command `kubeadm join`.

For example:

    # kubeadm join <api-server-ip>:<port> --token <token> --discovery-token-ca-cert-hash sha256:<hash> --node-name=<name_of_the_node> --cri-socket=<SOCKET>

To generate new bootstrap token,

    kubeadm token create --print-join-command

If you are using Cilium and find the working node remains to be `NotReady`, check the status of working node using:

    kubectl describe node <node-id> --namespace=kube-system

If you found the following condition status:

    Type                  Status       Reason
    ----                  ------       ------
    NetworkUnavailable    False        CiliumIsUp
    Ready                 False        KubeletNotReady container runtime network not ready: NetworkReady=false reason:NetworkPluginNotReady message:Network plugin returns error: cni plugin not initialized

Restart `containerd.service` and `kubelet.service` on the working node

## Tips and tricks

### Tear down a cluster

When it is necessary to start from scratch, use kubectl to tear down a cluster.

    kubectl drain <node name> --delete-local-data --force --ignore-daemonsets

Here `<node name>` is the name of the node that should be drained and reset. Use `kubectl get node -A` to list all nodes.

Then reset the node:

    # kubeadm reset

### Operating from Behind a Proxy

`kubeadm` reads the `https_proxy`, `http_proxy`, and `no_proxy` environment variables. Kubernetes internal networking should be included in the latest one, for example

    export no_proxy="192.168.122.0/24,10.96.0.0/12,192.168.123.0/24"

where the second one is the default service network CIDR.

## Troubleshooting

### `kubelet fail` to start during `kubeadm init` phase

Disable swap on the host, otherwise `kubelet.service` will fail to start.

See #Disable swap for an instruction on how to disable swap.

### Failed to get container stats

If `kubelet.service` emits

    Failed to get system container stats for "/system.slice/kubelet.service": failed to get cgroup stats for "/system.slice/kubelet.service": failed to get container info for "/system.slice/kubelet.service": unknown container "/system.slice/kubelet.service"

it is necessary to add configuration for the kubelet (see relevant upstream ticket).

    /var/lib/kubelet/config.yaml

    systemCgroups: '/systemd/system.slice'
    kubeletCgroups: '/systemd/system.slice'

### Pods cannot communicate when using Flannel CNI and systemd-networkd

See upstream bug report.

systemd-networkd assigns a persistent MAC address to every link. This policy is defined in its shipped configuration file `/usr/lib/systemd/network/99-default.link`. However, Flannel relies on being able to pick its own MAC address. To override systemd-networkd's behaviour for `flannel*` interfaces, create the following configuration file:

    /etc/systemd/network/50-flannel.link

    [Match]
    OriginalName=flannel*

    [Link]
    MACAddressPolicy=none

Then restart `systemd-networkd.service`.

If the cluster is already running, you might need to manually delete the `flannel.1` interface and the `kube-flannel-ds-*` pod on each node, including the master. The pods will be recreated immediately and they themselves will recreate the `flannel.1` interfaces.

Delete the interface `flannel.1`:

    # ip link delete flannel.1

Delete the `kube-flannel-ds-*` pod. Use the following command to delete all `kube-flannel-ds-*` pods on all nodes:

    $ kubectl -n kube-system delete pod -l="app=flannel"

### CoreDNS Pod pending forever, the control plane node remains "NotReady"

When bootstrap the Kubernetes with `kubeadm init` on a single machine, and there is no other machine `kubeadm join` the cluster, the control-plane node is default to be tainted. As a result, no workload will be scheduled on the working machine.

One can confirm the control-plane node is tainted by the following commands:

    kubectl get nodes -o json | jq '.items[].spec.taints

To temporarily allow scheduling on the control-plane node, execute:

    kubectl taint nodes <your-node-name> node-role.kubernetes.io/control-plane:NoSchedule-

Then restart `containerd.service` and `kubelet.service` to apply the updates.

### [kubelet-finalize] malformed header: missing HTTP content-type

You may have forgotten to choose systemd cgroup driver. See this GitHub issue reporting this.

## See also

- Kubernetes Documentation - The upstream documentation
- Kubernetes Cluster with Kubeadm - Upstream documentation on how to setup a Kubernetes cluster using kubeadm
- Kubernetes Glossary - The official glossary explaining all Kubernetes specific terminology
- Kubernetes Addons - A list of third-party addons
- Kubelet Config File - Documentation on the Kubelet configuration file
- Taints and Tolerations - Documentation on node affinities and taints

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- Virtualization

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