Kubernetes

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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 etcd^AUR 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 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-bin^AUR, which is nerdctl full pkg, bundle with containerd/CNI plugin/RootlessKit:

containerd-rootless-setuptool.sh install

Remeber 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 exists, 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) Chosse API server advertising address

If your node for control plane is in multiple subnetes (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] offcial 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.

Remeber 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    Fasle        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 forgetted to choose systemd cgroup driver. See this GitHub issue reporting this.