kubeadm init

This command initializes a Kubernetes control plane node.

Run this command in order to set up the Kubernetes control plane

Synopsis

Run this command in order to set up the Kubernetes control plane

The "init" command executes the following phases:

preflight                     Run pre-flight checks
certs                         Certificate generation
  /ca                           Generate the self-signed Kubernetes CA to provision identities for other Kubernetes components
  /apiserver                    Generate the certificate for serving the Kubernetes API
  /apiserver-kubelet-client     Generate the certificate for the API server to connect to kubelet
  /front-proxy-ca               Generate the self-signed CA to provision identities for front proxy
  /front-proxy-client           Generate the certificate for the front proxy client
  /etcd-ca                      Generate the self-signed CA to provision identities for etcd
  /etcd-server                  Generate the certificate for serving etcd
  /etcd-peer                    Generate the certificate for etcd nodes to communicate with each other
  /etcd-healthcheck-client      Generate the certificate for liveness probes to healthcheck etcd
  /apiserver-etcd-client        Generate the certificate the apiserver uses to access etcd
  /sa                           Generate a private key for signing service account tokens along with its public key
kubeconfig                    Generate all kubeconfig files necessary to establish the control plane and the admin kubeconfig file
  /admin                        Generate a kubeconfig file for the admin to use and for kubeadm itself
  /super-admin                  Generate a kubeconfig file for the super-admin
  /kubelet                      Generate a kubeconfig file for the kubelet to use *only* for cluster bootstrapping purposes
  /controller-manager           Generate a kubeconfig file for the controller manager to use
  /scheduler                    Generate a kubeconfig file for the scheduler to use
etcd                          Generate static Pod manifest file for local etcd
  /local                        Generate the static Pod manifest file for a local, single-node local etcd instance
control-plane                 Generate all static Pod manifest files necessary to establish the control plane
  /apiserver                    Generates the kube-apiserver static Pod manifest
  /controller-manager           Generates the kube-controller-manager static Pod manifest
  /scheduler                    Generates the kube-scheduler static Pod manifest
kubelet-start                 Write kubelet settings and (re)start the kubelet
upload-config                 Upload the kubeadm and kubelet configuration to a ConfigMap
  /kubeadm                      Upload the kubeadm ClusterConfiguration to a ConfigMap
  /kubelet                      Upload the kubelet component config to a ConfigMap
upload-certs                  Upload certificates to kubeadm-certs
mark-control-plane            Mark a node as a control-plane
bootstrap-token               Generates bootstrap tokens used to join a node to a cluster
kubelet-finalize              Updates settings relevant to the kubelet after TLS bootstrap
  /enable-client-cert-rotation  Enable kubelet client certificate rotation
addon                         Install required addons for passing conformance tests
  /coredns                      Install the CoreDNS addon to a Kubernetes cluster
  /kube-proxy                   Install the kube-proxy addon to a Kubernetes cluster
show-join-command             Show the join command for control-plane and worker node

kubeadm init [flags]

Options

Options inherited from parent commands

Init workflow

kubeadm init bootstraps a Kubernetes control plane node by executing the following steps:

1. Runs a series of pre-flight checks to validate the system state before making changes. Some checks only trigger warnings, others are considered errors and will exit kubeadm until the problem is corrected or the user specifies --ignore-preflight-errors=<list-of-errors>.

2. Generates a self-signed CA to set up identities for each component in the cluster. The user can provide their own CA cert and/or key by dropping it in the cert directory configured via --cert-dir (/etc/kubernetes/pki by default). The API server certs will have additional SAN entries for any --apiserver-cert-extra-sans arguments, lowercased if necessary.

3. Writes kubeconfig files in /etc/kubernetes/ for the kubelet, the controller-manager, and the scheduler to connect to the API server, each with its own identity. Also additional kubeconfig files are written, for kubeadm as administrative entity (admin.conf) and for a super admin user that can bypass RBAC (super-admin.conf).

4. Generates static Pod manifests for the API server, controller-manager and scheduler. In case an external etcd is not provided, an additional static Pod manifest is generated for etcd.

   Static Pod manifests are written to /etc/kubernetes/manifests; the kubelet watches this directory for Pods to create on startup.

   Once control plane Pods are up and running, the kubeadm init sequence can continue.

5. Apply labels and taints to the control plane node so that no additional workloads will run there.

6. Generates the token that additional nodes can use to register themselves with a control plane in the future. Optionally, the user can provide a token via --token, as described in the kubeadm token documents.

7. Makes all the necessary configurations for allowing node joining with the Bootstrap Tokens and TLS Bootstrap mechanism:

   • Write a ConfigMap for making available all the information required for joining, and set up related RBAC access rules.

   • Let Bootstrap Tokens access the CSR signing API.

   • Configure auto-approval for new CSR requests.

   See kubeadm join for additional information.

8. Installs a DNS server (CoreDNS) and the kube-proxy addon components via the API server. In Kubernetes version 1.11 and later CoreDNS is the default DNS server. Please note that although the DNS server is deployed, it will not be scheduled until CNI is installed.

   Warning:

   kube-dns usage with kubeadm is deprecated as of v1.18 and is removed in v1.21.

Using init phases with kubeadm

kubeadm allows you to create a control plane node in phases using the kubeadm init phase command.

To view the ordered list of phases and sub-phases you can call kubeadm init --help. The list will be located at the top of the help screen and each phase will have a description next to it. Note that by calling kubeadm init all of the phases and sub-phases will be executed in this exact order.

Some phases have unique flags, so if you want to have a look at the list of available options add --help, for example:

sudo kubeadm init phase control-plane controller-manager --help

You can also use --help to see the list of sub-phases for a certain parent phase:

sudo kubeadm init phase control-plane --help

kubeadm init also exposes a flag called --skip-phases that can be used to skip certain phases. The flag accepts a list of phase names and the names can be taken from the above ordered list.

An example:

sudo kubeadm init phase control-plane all --config=configfile.yaml
sudo kubeadm init phase etcd local --config=configfile.yaml
# you can now modify the control plane and etcd manifest files
sudo kubeadm init --skip-phases=control-plane,etcd --config=configfile.yaml

What this example would do is write the manifest files for the control plane and etcd in /etc/kubernetes/manifests based on the configuration in configfile.yaml. This allows you to modify the files and then skip these phases using --skip-phases. By calling the last command you will create a control plane node with the custom manifest files.

Alternatively, you can use the skipPhases field under InitConfiguration.

Using kubeadm init with a configuration file

It's possible to configure kubeadm init with a configuration file instead of command line flags, and some more advanced features may only be available as configuration file options. This file is passed using the --config flag and it must contain a ClusterConfiguration structure and optionally more structures separated by ---\n. Mixing --config with others flags may not be allowed in some cases.

The default configuration can be printed out using the kubeadm config print command.

If your configuration is not using the latest version it is recommended that you migrate using the kubeadm config migrate command.

For more information on the fields and usage of the configuration you can navigate to our API reference page.

Using kubeadm init with feature gates

kubeadm supports a set of feature gates that are unique to kubeadm and can only be applied during cluster creation with kubeadm init. These features can control the behavior of the cluster. Feature gates are removed after a feature graduates to GA.

To pass a feature gate you can either use the --feature-gates flag for kubeadm init, or you can add items into the featureGates field when you pass a configuration file using --config.

Passing feature gates for core Kubernetes components directly to kubeadm is not supported. Instead, it is possible to pass them by Customizing components with the kubeadm API.

List of feature gates:

Feature gate descriptions:

ControlPlaneKubeletLocalMode

With this feature gate enabled, when joining a new control plane node, kubeadm will configure the kubelet to connect to the local kube-apiserver. This ensures that there will not be a violation of the version skew policy during rolling upgrades.

EtcdLearnerMode

With this feature gate enabled, when joining a new control plane node, a new etcd member will be created as a learner and promoted to a voting member only after the etcd data are fully aligned.

NodeLocalCRISocket

With this feature gate enabled, kubeadm will read/write the CRI socket for each node from/to the file /var/lib/kubelet/instance-config.yaml instead of reading/writing it from/to the annotation kubeadm.alpha.kubernetes.io/cri-socket on the Node object. The new file is applied as an instance configuration patch, before any other user managed patches are applied when the --patches flag is used. It contains a single field containerRuntimeEndpoint from the KubeletConfiguration file format. If the feature gate is enabled during upgrade, but the file /var/lib/kubelet/instance-config.yaml does not exist yet, kubeadm will attempt to read the CRI socket value from the file /var/lib/kubelet/kubeadm-flags.env.

WaitForAllControlPlaneComponents

With this feature gate enabled, kubeadm will wait for all control plane components (kube-apiserver, kube-controller-manager, kube-scheduler) on a control plane node to report status 200 on their /livez or /healthz endpoints. These checks are performed on https://ADDRESS:PORT/ENDPOINT.

• PORT is taken from --secure-port of a component.
• ADDRESS is --advertise-address for kube-apiserver and --bind-address for the kube-controller-manager and kube-scheduler.
• ENDPOINT is only /healthz for kube-controller-manager until it supports /livez as well.

If you specify custom ADDRESS or PORT in the kubeadm configuration they will be respected. Without the feature gate enabled, kubeadm will only wait for the kube-apiserver on a control plane node to become ready. The wait process starts right after the kubelet on the host is started by kubeadm. You are advised to enable this feature gate in case you wish to observe a ready state from all control plane components during the kubeadm init or kubeadm join command execution.

List of deprecated feature gates:

Feature gate descriptions:

PublicKeysECDSA

Can be used to create a cluster that uses ECDSA certificates instead of the default RSA algorithm. Renewal of existing ECDSA certificates is also supported using kubeadm certs renew, but you cannot switch between the RSA and ECDSA algorithms on the fly or during upgrades. Kubernetes versions before v1.31 had a bug where keys in generated kubeconfig files were set use RSA, even when you had enabled the PublicKeysECDSA feature gate. This feature gate is deprecated in favor of the encryptionAlgorithm functionality available in kubeadm v1beta4.

RootlessControlPlane

Setting this flag configures the kubeadm deployed control plane component static Pod containers for kube-apiserver, kube-controller-manager, kube-scheduler and etcd to run as non-root users. If the flag is not set, those components run as root. You can change the value of this feature gate before you upgrade to a newer version of Kubernetes.

List of removed feature gates:

Feature gate descriptions:

IPv6DualStack

This flag helps to configure components dual stack when the feature is in progress. For more details on Kubernetes dual-stack support see Dual-stack support with kubeadm.

UnversionedKubeletConfigMap

This flag controls the name of the ConfigMap where kubeadm stores kubelet configuration data. With this flag not specified or set to true, the ConfigMap is named kubelet-config. If you set this flag to false, the name of the ConfigMap includes the major and minor version for Kubernetes (for example: kubelet-config-1.32). Kubeadm ensures that RBAC rules for reading and writing that ConfigMap are appropriate for the value you set. When kubeadm writes this ConfigMap (during kubeadm init or kubeadm upgrade apply), kubeadm respects the value of UnversionedKubeletConfigMap. When reading that ConfigMap (during kubeadm join, kubeadm reset, kubeadm upgrade...), kubeadm attempts to use unversioned ConfigMap name first. If that does not succeed, kubeadm falls back to using the legacy (versioned) name for that ConfigMap.

UpgradeAddonsBeforeControlPlane

This feature gate has been removed. It was introduced in v1.28 as a deprecated feature and then removed in v1.31. For documentation on older versions, please switch to the corresponding website version.

Adding kube-proxy parameters

For information about kube-proxy parameters in the kubeadm configuration see:

• kube-proxy reference

For information about enabling IPVS mode with kubeadm see:

• IPVS

Passing custom flags to control plane components

For information about passing flags to control plane components see:

• control-plane-flags

Running kubeadm without an Internet connection

For running kubeadm without an Internet connection you have to pre-pull the required control plane images.

You can list and pull the images using the kubeadm config images sub-command:

kubeadm config images list
kubeadm config images pull

You can pass --config to the above commands with a kubeadm configuration file to control the kubernetesVersion and imageRepository fields.

All default registry.k8s.io images that kubeadm requires support multiple architectures.

Using custom images

By default, kubeadm pulls images from registry.k8s.io. If the requested Kubernetes version is a CI label (such as ci/latest) gcr.io/k8s-staging-ci-images is used.

You can override this behavior by using kubeadm with a configuration file. Allowed customization are:

• To provide kubernetesVersion which affects the version of the images.
• To provide an alternative imageRepository to be used instead of registry.k8s.io.
• To provide a specific imageRepository and imageTag for etcd or CoreDNS.

Image paths between the default registry.k8s.io and a custom repository specified using imageRepository may differ for backwards compatibility reasons. For example, one image might have a subpath at registry.k8s.io/subpath/image, but be defaulted to my.customrepository.io/image when using a custom repository.

To ensure you push the images to your custom repository in paths that kubeadm can consume, you must:

• Pull images from the defaults paths at registry.k8s.io using kubeadm config images {list|pull}.
• Push images to the paths from kubeadm config images list --config=config.yaml, where config.yaml contains the custom imageRepository, and/or imageTag for etcd and CoreDNS.
• Pass the same config.yaml to kubeadm init.

Custom sandbox (pause) images

To set a custom image for these you need to configure this in your container runtime to use the image. Consult the documentation for your container runtime to find out how to change this setting; for selected container runtimes, you can also find advice within the Container Runtimes topic.

Uploading control plane certificates to the cluster

By adding the flag --upload-certs to kubeadm init you can temporary upload the control plane certificates to a Secret in the cluster. Please note that this Secret will expire automatically after 2 hours. The certificates are encrypted using a 32byte key that can be specified using --certificate-key. The same key can be used to download the certificates when additional control plane nodes are joining, by passing --control-plane and --certificate-key to kubeadm join.

The following phase command can be used to re-upload the certificates after expiration:

kubeadm init phase upload-certs --upload-certs --config=SOME_YAML_FILE

If a predefined certificate key is not passed to kubeadm init and kubeadm init phase upload-certs a new key will be generated automatically.

The following command can be used to generate a new key on demand:

kubeadm certs certificate-key

Certificate management with kubeadm

For detailed information on certificate management with kubeadm see Certificate Management with kubeadm. The document includes information about using external CA, custom certificates and certificate renewal.

Managing the kubeadm drop-in file for the kubelet

The kubeadm package ships with a configuration file for running the kubelet by systemd. Note that the kubeadm CLI never touches this drop-in file. This drop-in file is part of the kubeadm DEB/RPM package.

For further information, see Managing the kubeadm drop-in file for systemd.

Use kubeadm with CRI runtimes

By default, kubeadm attempts to detect your container runtime. For more details on this detection, see the kubeadm CRI installation guide.

Setting the node name

By default, kubeadm assigns a node name based on a machine's host address. You can override this setting with the --node-name flag. The flag passes the appropriate --hostname-override value to the kubelet.

Be aware that overriding the hostname can interfere with cloud providers.

Automating kubeadm

Rather than copying the token you obtained from kubeadm init to each node, as in the basic kubeadm tutorial, you can parallelize the token distribution for easier automation. To implement this automation, you must know the IP address that the control plane node will have after it is started, or use a DNS name or an address of a load balancer.

1. Generate a token. This token must have the form <6 character string>.<16 character string>. More formally, it must match the regex: [a-z0-9]{6}\.[a-z0-9]{16}.

   kubeadm can generate a token for you:

   kubeadm token generate
   

2. Start both the control plane node and the worker nodes concurrently with this token. As they come up they should find each other and form the cluster. The same --token argument can be used on both kubeadm init and kubeadm join.

3. Similar can be done for --certificate-key when joining additional control plane nodes. The key can be generated using:

   kubeadm certs certificate-key
   

Once the cluster is up, you can use the /etc/kubernetes/admin.conf file from a control plane node to talk to the cluster with administrator credentials or Generating kubeconfig files for additional users.

Note that this style of bootstrap has some relaxed security guarantees because it does not allow the root CA hash to be validated with --discovery-token-ca-cert-hash (since it's not generated when the nodes are provisioned). For details, see the kubeadm join.

What's next

• kubeadm init phase to understand more about kubeadm init phases
• kubeadm join to bootstrap a Kubernetes worker node and join it to the cluster
• kubeadm upgrade to upgrade a Kubernetes cluster to a newer version
• kubeadm reset to revert any changes made to this host by kubeadm init or kubeadm join