Configuring each kubelet in your cluster using kubeadm

    The lifecycle of the kubeadm CLI tool is decoupled from the kubelet, which is a daemon that runs on each node within the Kubernetes cluster. The kubeadm CLI tool is executed by the user when Kubernetes is initialized or upgraded, whereas the kubelet is always running in the background.

    Since the kubelet is a daemon, it needs to be maintained by some kind of an init system or service manager. When the kubelet is installed using DEBs or RPMs, systemd is configured to manage the kubelet. You can use a different service manager instead, but you need to configure it manually.

    Some kubelet configuration details need to be the same across all kubelets involved in the cluster, while other configuration aspects need to be set on a per-kubelet basis to accommodate the different characteristics of a given machine (such as OS, storage, and networking). You can manage the configuration of your kubelets manually, but kubeadm now provides a KubeletConfiguration API type for .

    The following sections describe patterns to kubelet configuration that are simplified by using kubeadm, rather than managing the kubelet configuration for each Node manually.

    You can provide the kubelet with default values to be used by kubeadm init and kubeadm join commands. Interesting examples include using a different CRI runtime or setting the default subnet used by services.

    If you want your services to use the subnet 10.96.0.0/12 as the default for services, you can pass the --service-cidr parameter to kubeadm:

    Virtual IPs for services are now allocated from this subnet. You also need to set the DNS address used by the kubelet, using the --cluster-dns flag. This setting needs to be the same for every kubelet on every manager and Node in the cluster. The kubelet provides a versioned, structured API object that can configure most parameters in the kubelet and push out this configuration to each running kubelet in the cluster. This object is called KubeletConfiguration. The KubeletConfiguration allows the user to specify flags such as the cluster DNS IP addresses expressed as a list of values to a camelCased key, illustrated by the following example:

    1. apiVersion: kubelet.config.k8s.io/v1beta1
    2. kind: KubeletConfiguration
    3. clusterDNS:

    For more details on the KubeletConfiguration have a look at .

    Some hosts require specific kubelet configurations due to differences in hardware, operating system, networking, or other host-specific parameters. The following list provides a few examples.

    • The Node API object .metadata.name is set to the machine’s hostname by default, unless you are using a cloud provider. You can use the --hostname-override flag to override the default behavior if you need to specify a Node name different from the machine’s hostname.

    • Currently, the kubelet cannot automatically detect the cgroup driver used by the CRI runtime, but the value of --cgroup-driver must match the cgroup driver used by the CRI runtime to ensure the health of the kubelet.

    • Depending on the CRI runtime your cluster uses, you may need to specify different flags to the kubelet. For instance, when using Docker, you need to specify flags such as --network-plugin=cni, but if you are using an external runtime, you need to specify --container-runtime=remote and specify the CRI endpoint using the --container-runtime-endpoint=<path>.

    You can specify these flags by configuring an individual kubelet’s configuration in your service manager, such as systemd.

    It is possible to configure the kubelet that kubeadm will start if a custom KubeletConfiguration API object is passed with a configuration file like so kubeadm ... --config some-config-file.yaml.

    By calling kubeadm config print init-defaults --component-configs KubeletConfiguration you can see all the default values for this structure.

    Also have a look at the reference for the KubeletConfiguration for more information on the individual fields.

    When you call , the kubelet configuration is marshalled to disk at /var/lib/kubelet/config.yaml, and also uploaded to a ConfigMap in the cluster. The ConfigMap is named kubelet-config-1.X, where X is the minor version of the Kubernetes version you are initializing. A kubelet configuration file is also written to /etc/kubernetes/kubelet.conf with the baseline cluster-wide configuration for all kubelets in the cluster. This configuration file points to the client certificates that allow the kubelet to communicate with the API server. This addresses the need to .

    To address the second pattern of providing instance-specific configuration details, kubeadm writes an environment file to /var/lib/kubelet/kubeadm-flags.env, which contains a list of flags to pass to the kubelet when it starts. The flags are presented in the file like this:

    After marshalling these two files to disk, kubeadm attempts to run the following two commands, if you are using systemd:

    1. systemctl daemon-reload && systemctl restart kubelet

    If the reload and restart are successful, the normal kubeadm init workflow continues.

    When you run kubeadm join, kubeadm uses the Bootstrap Token credential to perform a TLS bootstrap, which fetches the credential needed to download the kubelet-config-1.X ConfigMap and writes it to /var/lib/kubelet/config.yaml. The dynamic environment file is generated in exactly the same way as kubeadm init.

    Next, kubeadm runs the following two commands to load the new configuration into the kubelet:

    After the kubelet loads the new configuration, kubeadm writes the /etc/kubernetes/bootstrap-kubelet.conf KubeConfig file, which contains a CA certificate and Bootstrap Token. These are used by the kubelet to perform the TLS Bootstrap and obtain a unique credential, which is stored in /etc/kubernetes/kubelet.conf.

    When the /etc/kubernetes/kubelet.conf file is written, the kubelet has finished performing the TLS Bootstrap. Kubeadm deletes the /etc/kubernetes/bootstrap-kubelet.conf file after completing the TLS Bootstrap.

    kubeadm ships with configuration for how systemd should run the kubelet. Note that the kubeadm CLI command never touches this drop-in file.

    This configuration file installed by the kubeadm or RPM package is written to /etc/systemd/system/kubelet.service.d/10-kubeadm.conf and is used by systemd. It augments the basic or kubelet.service for DEB:

    1. [Service]
    2. Environment="KUBELET_KUBECONFIG_ARGS=--bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf
    3. --kubeconfig=/etc/kubernetes/kubelet.conf"
    4. Environment="KUBELET_CONFIG_ARGS=--config=/var/lib/kubelet/config.yaml"
    5. # This is a file that "kubeadm init" and "kubeadm join" generate at runtime, populating
    7. # This is a file that the user can use for overrides of the kubelet args as a last resort. Preferably,
    8. # the user should use the .NodeRegistration.KubeletExtraArgs object in the configuration files instead.
    9. # KUBELET_EXTRA_ARGS should be sourced from this file.
    10. EnvironmentFile=-/etc/default/kubelet
    11. ExecStart=
    12. ExecStart=/usr/bin/kubelet $KUBELET_KUBECONFIG_ARGS $KUBELET_CONFIG_ARGS $KUBELET_KUBEADM_ARGS $KUBELET_EXTRA_ARGS

    This file specifies the default locations for all of the files managed by kubeadm for the kubelet.

    • The KubeConfig file to use for the TLS Bootstrap is /etc/kubernetes/bootstrap-kubelet.conf, but it is only used if /etc/kubernetes/kubelet.conf does not exist.
    • The KubeConfig file with the unique kubelet identity is /etc/kubernetes/kubelet.conf.
    • The file containing the kubelet’s ComponentConfig is /var/lib/kubelet/config.yaml.
    • The dynamic environment file that contains KUBELET_KUBEADM_ARGS is sourced from /var/lib/kubelet/kubeadm-flags.env.
    • The file that can contain user-specified flag overrides with KUBELET_EXTRA_ARGS is sourced from /etc/default/kubelet (for DEBs), or /etc/sysconfig/kubelet (for RPMs). KUBELET_EXTRA_ARGS is last in the flag chain and has the highest priority in the event of conflicting settings.