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kubeadm init

This command initializes a Kubernetes control-plane node.

• What's next

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
kubelet-start              Write kubelet settings and (re)start the kubelet
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
  /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
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
etcd                       Generate static Pod manifest file for local etcd
  /local                     Generate the static Pod manifest file for a local, single-node local etcd instance
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
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

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 (or using an existing one if provided) to set up identities for each component in the cluster. If the user has provided their own CA cert and/or key by dropping it in the cert directory configured via --cert-dir (/etc/kubernetes/pki by default) this step is skipped as described in the Using custom certificates document. The APIServer 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 use to connect to the API server, each with its own identity, as well as an additional kubeconfig file for administration named 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.

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

2. 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 docs.

3. 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 info.

1. 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. To install kube-dns instead of CoreDNS, the DNS addon has to be configured in the kubeadm ClusterConfiguration. For more information about the configuration see the section Using kubeadm init with a configuration file below. Please note that although the DNS server is deployed, it will not be scheduled until CNI is installed.

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.

Using kubeadm init with a configuration file

Caution: The config file is still considered beta and may change in future versions.

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 with the --config option.

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

It is recommended that you migrate your old v1beta1 configuration to v1beta2 using the kubeadm config migrate command.

For more details on each field in the v1beta2 configuration you can navigate to our API reference pages.

Adding kube-proxy parameters

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

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

Using custom images

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

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

• To provide an alternative imageRepository to be used instead of k8s.gcr.io.
• To set useHyperKubeImage to true to use the HyperKube image.
• To provide a specific imageRepository and imageTag for etcd or DNS add-on.

Please note that the configuration field kubernetesVersion or the command line flag --kubernetes-version affect the version of the images.

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 --certificate-key=SOME_VALUE --config=SOME_YAML_FILE

If the flag --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 alpha certs certificate-key

Using custom certificates

By default, kubeadm generates all the certificates needed for a cluster to run. You can override this behavior by providing your own certificates.

To do so, you must place them in whatever directory is specified by the --cert-dir flag or CertificatesDir configuration file key. By default this is /etc/kubernetes/pki.

If a given certificate and private key pair exists before running kubeadm init, kubeadm will not overwrite them. This means you can, for example, copy an existing CA into /etc/kubernetes/pki/ca.crt and /etc/kubernetes/pki/ca.key, and kubeadm will use this CA for signing the rest of the certificates.

External CA mode

It is also possible to provide just the ca.crt file and not the ca.key file (this is only available for the root CA file, not other cert pairs). If all other certificates and kubeconfig files are in place, kubeadm recognizes this condition and activates the “External CA” mode. kubeadm will proceed without the CA key on disk.

Instead, run the controller-manager standalone with --controllers=csrsigner and point to the CA certificate and key.

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.

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

All images that kubeadm requires such as k8s.gcr.io/kube-*, k8s.gcr.io/etcd and k8s.gcr.io/pause support multiple architectures.

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 alpha certs certificate-key

Once the cluster is up, you can grab the admin credentials from the control-plane node at /etc/kubernetes/admin.conf and use that to talk to the cluster.

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

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