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 a 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 managing your
kubelet configurations centrally.
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
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
--service-cidr parameter to kubeadm:
kubeadm init --service-cidr 10.96.0.0/12
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 the kubelet’s ComponentConfig.
The ComponentConfig 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:
For more details on the ComponentConfig have a look at this section.
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 path to the DNS resolution file, as specified by the
configuration flag, may differ among operating systems, or depending on whether you are using
systemd-resolved. If this path is wrong, DNS resolution will fail on the Node whose kubelet
is configured incorrectly.
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 detects 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
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
API object is passed with a configuration file like so
kubeadm ... --config some-config-file.yaml.
kubeadm config print-default --api-objects KubeletConfiguration you can
see all the default values for this structure.
Also have a look at the API reference for the kubelet ComponentConfig for more information on the individual fields.
When you call
kubeadm init, the kubelet configuration is marshalled to disk
/var/lib/kubelet/config.yaml, and also uploaded to a ConfigMap in the cluster. The ConfigMap
.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
propagate cluster-level configuration to each kubelet.
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:
KUBELET_KUBEADM_ARGS="--flag1=value1 --flag2=value2 ..."
In addition to the flags used when starting the kubelet, the file also contains dynamic
parameters such as the cgroup driver and whether to use a different CRI runtime socket
After marshalling these two files to disk, kubeadm attempts to run the following two commands, if you are using systemd:
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 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 runs the following two commands to load the new configuration into the kubelet:
systemctl daemon-reload && systemctl restart 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 this file is written, the kubelet
has finished performing the TLS Bootstrap.
The configuration file installed by the kubeadm DEB or RPM package is written to
/etc/systemd/system/kubelet.service.d/10-kubeadm.conf and is used by systemd.
# This is a file that "kubeadm init" and "kubeadm join" generates at runtime, populating
the KUBELET_KUBEADM_ARGS variable dynamically
# This is a file that the user can use for overrides of the kubelet args as a last resort. Preferably,
#the user should use the .NodeRegistration.KubeletExtraArgs object in the configuration files instead.
# KUBELET_EXTRA_ARGS should be sourced from this file.
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.
but it is only used if
/etc/kubernetes/kubelet.conf does not exist.
KUBELET_KUBEADM_ARGS is sourced from
KUBELET_EXTRA_ARGS is sourced from
/etc/default/kubelet (for DEBs), or
/etc/systconfig/kubelet (for RPMs).
is last in the flag chain and has the highest priority in the event of conflicting settings.
The DEB and RPM packages shipped with the Kubernetes releases are:
/usr/bin/kubeadm CLI tool and [The kubelet drop-in file(#the-kubelet-drop-in-file-for-systemd) for the kubelet.
|Installs the official CNI binaries into the
/usr/bin/crictl binary from https://github.com/kubernetes-incubator/cri-tools.
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