Set Kubelet parameters via a config file

A subset of the Kubelet’s configuration parameters may be set via an on-disk config file, as a substitute for command-line flags.

Providing parameters via a config file is the recommended approach because it simplifies node deployment and configuration management.

Create the config file

The subset of the Kubelet’s configuration that can be configured via a file is defined by the KubeletConfiguration struct.

The configuration file must be a JSON or YAML representation of the parameters in this struct. Make sure the Kubelet has read permissions on the file.

Here is an example of what this file might look like:

  1. apiVersion: kubelet.config.k8s.io/v1beta1
  2. kind: KubeletConfiguration
  3. address: "192.168.0.8"
  4. port: 20250
  5. serializeImagePulls: false
  6. evictionHard:
  7.     memory.available:  "200Mi"

In the example, the Kubelet is configured to serve on IP address 192.168.0.8 and port 20250, pull images in parallel, and evict Pods when available memory drops below 200Mi. Since only one of the four evictionHard thresholds is configured, other evictionHard thresholds are reset to 0 from their built-in defaults. All other Kubelet configuration values are left at their built-in defaults, unless overridden by flags. Command line flags which target the same value as a config file will override that value.

Note: In the example, by changing the default value of only one parameter for evictionHard, the default values of other parameters will not be inherited and will be set to zero. In order to provide custom values, you should provide all the threshold values respectively.

Start a Kubelet process configured via the config file

Note: If you use kubeadm to initialize your cluster, use the kubelet-config while creating your cluster with kubeadm init. See configuring kubelet using kubeadm for details.

Start the Kubelet with the --config flag set to the path of the Kubelet’s config file. The Kubelet will then load its config from this file.

Note that command line flags which target the same value as a config file will override that value. This helps ensure backwards compatibility with the command-line API.

Note that relative file paths in the Kubelet config file are resolved relative to the location of the Kubelet config file, whereas relative paths in command line flags are resolved relative to the Kubelet’s current working directory.

Note that some default values differ between command-line flags and the Kubelet config file. If --config is provided and the values are not specified via the command line, the defaults for the KubeletConfiguration version apply. In the above example, this version is kubelet.config.k8s.io/v1beta1.

Drop-in directory for Kubelet configuration files

As of Kubernetes v1.28.0, the Kubelet has been extended to support a drop-in configuration directory. The location of it can be specified with --config-dir flag, and it defaults to "", or disabled, by default.

You can only set --config-dir if you set the environment variable KUBELET_CONFIG_DROPIN_DIR_ALPHA for the kubelet process (the value of that variable does not matter). For Kubernetes v1.28, the kubelet returns an error if you specify --config-dir without that variable set, and startup fails. You cannot specify the drop-in configuration directory using the kubelet configuration file; only the CLI argument --config-dir can set it.

One can use the Kubelet configuration directory in a similar way to the Kubelet config file.

Note: The suffix of a valid Kubelet drop-in configuration file must be .conf. For instance: 99-kubelet-address.conf

For instance, you may want a baseline Kubelet configuration for all nodes, but you may want to customize the address field. This can be done as follows:

Main Kubelet configuration file contents:

  1. apiVersion: kubelet.config.k8s.io/v1beta1
  2. kind: KubeletConfiguration
  3. port: 20250
  4. serializeImagePulls: false
  5. evictionHard:
  6.     memory.available:  "200Mi"

Contents of a file in --config-dir directory:

  1. apiVersion: kubelet.config.k8s.io/v1beta1
  2. kind: KubeletConfiguration
  3. address: "192.168.0.8"

On startup, the Kubelet merges configuration from:

  • Command line arguments (lowest precedence).
  • the Kubelet configuration
  • Drop-in configuration files, according to sort order.
  • Feature gates specified over the command line (highest precedence).

This produces the same outcome as if you used the single configuration file used in the earlier example.

What’s next