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website/content/en/docs/reference/setup-tools/kubeadm/kubeadm-init.md

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{{% capture overview %}} This command initializes a Kubernetes control-plane node. {{% /capture %}}

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{{< include "generated/kubeadm_init.md" >}}

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.

  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 docs.

  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 info.

  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. To install kube-dns instead of CoreDNS, the DNS addon has to configured in the kubeadm ClusterConfiguration. For more information about the configuration see the section Using kubeadm init with a configuration file bellow. 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 create a control-plane node in phases. In 1.13 the kubeadm init phase command has graduated to GA from it’s previous alpha state under kubeadm alpha phase.

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 expose 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. {{< /caution >}}

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 in the --config option.

In Kubernetes 1.11 and later, the default configuration can be printed out using the kubeadm config print command.

It is recommended that you migrate your old v1alpha3 configuration to v1beta1 using the kubeadm config migrate command, because v1alpha3 will be removed in Kubernetes 1.15.

For more details on each field in the v1beta1 configuration you can navigate to our API reference pages.

Adding kube-proxy parameters

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

For information about enabling IPVS mode with kubeadm see:

Passing custom flags to control plane components

For information about passing flags to control plane components see:

Using custom images

By default, kubeadm pulls images from k8s.gcr.io, unless the requested Kubernetes version is a CI version. In this case, 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.

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, kubeadm skips the generation step and existing files are used for the prescribed use case. 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 certs.

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 configuration for how the kubelet should be run. Note that the kubeadm CLI command never touches this drop-in file. This drop-in file belongs to the kubeadm deb/rpm package.

This is what it looks like:

[Service]
Environment="KUBELET_KUBECONFIG_ARGS=--bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf --kubeconfig=/etc/kubernetes/kubelet.conf"
Environment="KUBELET_SYSTEM_PODS_ARGS=--pod-manifest-path=/etc/kubernetes/manifests --allow-privileged=true"
Environment="KUBELET_NETWORK_ARGS=--network-plugin=cni --cni-conf-dir=/etc/cni/net.d --cni-bin-dir=/opt/cni/bin"
Environment="KUBELET_DNS_ARGS=--cluster-dns=10.96.0.10 --cluster-domain=cluster.local"
Environment="KUBELET_AUTHZ_ARGS=--authorization-mode=Webhook --client-ca-file=/etc/kubernetes/pki/ca.crt"
Environment="KUBELET_CADVISOR_ARGS="
Environment="KUBELET_CERTIFICATE_ARGS=--rotate-certificates=true --cert-dir=/var/lib/kubelet/pki"
ExecStart=/usr/bin/kubelet $KUBELET_KUBECONFIG_ARGS $KUBELET_SYSTEM_PODS_ARGS $KUBELET_NETWORK_ARGS $KUBELET_DNS_ARGS $KUBELET_AUTHZ_ARGS $KUBELET_CADVISOR_ARGS $KUBELET_CERTIFICATE_ARGS $KUBELET_EXTRA_ARGS

Here's a breakdown of what/why:

  • --bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf path to a kubeconfig file that is used to get client certificates for kubelet during node join. On success, a kubeconfig file is written to the path specified by --kubeconfig.
  • --kubeconfig=/etc/kubernetes/kubelet.conf points to the kubeconfig file that tells the kubelet where the API server is. This file also has the kubelet's credentials.
  • --pod-manifest-path=/etc/kubernetes/manifests specifies from where to read static Pod manifests used for starting the control plane.
  • --allow-privileged=true allows this kubelet to run privileged Pods.
  • --network-plugin=cni uses CNI networking.
  • --cni-conf-dir=/etc/cni/net.d specifies where to look for the CNI spec file(s).
  • --cni-bin-dir=/opt/cni/bin specifies where to look for the actual CNI binaries.
  • --cluster-dns=10.96.0.10 use this cluster-internal DNS server for nameserver entries in Pods' /etc/resolv.conf.
  • --cluster-domain=cluster.local uses this cluster-internal DNS domain for search entries in Pods' /etc/resolv.conf.
  • --client-ca-file=/etc/kubernetes/pki/ca.crt authenticates requests to the Kubelet API using this CA certificate.
  • --authorization-mode=Webhook authorizes requests to the Kubelet API by POST-ing a SubjectAccessReview to the API server.
  • --rotate-certificates auto rotate the kubelet client certificates by requesting new certificates from the kube-apiserver when the certificate expiration approaches.
  • --cert-dirthe directory where the TLS certs are located.

Use kubeadm with CRI runtimes

Since v1.6.0, Kubernetes has enabled the use of CRI, Container Runtime Interface, by default. The container runtime used by default is Docker, which is enabled through the built-in dockershim CRI implementation inside of the kubelet.

Other CRI-based runtimes include:

Refer to the CRI installation instructions for more information.

After you have successfully installed kubeadm and kubelet, execute these two additional steps:

  1. Install the runtime shim on every node, following the installation document in the runtime shim project listing above.

  2. Configure kubelet to use the remote CRI runtime. Please remember to change RUNTIME_ENDPOINT to your own value like /var/run/{your_runtime}.sock:

cat > /etc/systemd/system/kubelet.service.d/20-cri.conf <<EOF
[Service]
Environment="KUBELET_EXTRA_ARGS=--container-runtime=remote --container-runtime-endpoint=$RUNTIME_ENDPOINT"
EOF
systemctl daemon-reload

Now kubelet is ready to use the specified CRI runtime, and you can continue with the kubeadm init and kubeadm join workflow to deploy Kubernetes cluster.

You may also want to set --cri-socket to kubeadm init and kubeadm reset when using an external CRI implementation.

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-nameflag. The flag passes the appropriate --hostname-override to the kubelet.

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

Self-hosting the Kubernetes control plane

As of 1.8, you can experimentally create a self-hosted Kubernetes control plane. This means that key components such as the API server, controller manager, and scheduler run as DaemonSet pods configured via the Kubernetes API instead of static pods configured in the kubelet via static files.

To create a self-hosted cluster see the kubeadm alpha selfhosting command.

Caveats

  1. Self-hosting in 1.8 and later has some important limitations. In particular, a self-hosted cluster cannot recover from a reboot of the control-plane node without manual intervention.

  2. A self-hosted cluster is not upgradeable using kubeadm upgrade.

  3. By default, self-hosted control plane Pods rely on credentials loaded from hostPath volumes. Except for initial creation, these credentials are not managed by kubeadm.

  4. The self-hosted portion of the control plane does not include etcd, which still runs as a static Pod.

Process

The self-hosting bootstrap process is documented in the kubeadm design document.

In summary, kubeadm alpha selfhosting works as follows:

  1. Waits for this bootstrap static control plane to be running and healthy. This is identical to the kubeadm init process without self-hosting.

  2. Uses the static control plane Pod manifests to construct a set of DaemonSet manifests that will run the self-hosted control plane. It also modifies these manifests where necessary, for example adding new volumes for secrets.

  3. Creates DaemonSets in the kube-system namespace and waits for the resulting Pods to be running.

  4. Once self-hosted Pods are operational, their associated static Pods are deleted and kubeadm moves on to install the next component. This triggers kubelet to stop those static Pods.

  5. When the original static control plane stops, the new self-hosted control plane is able to bind to listening ports and become active.

Running kubeadm without an internet connection

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

In Kubernetes 1.11 and later, you can list and pull the images using the kubeadm config images sub-command:

kubeadm config images list
kubeadm config images pull

In Kubernetes 1.12 and later, the k8s.gcr.io/kube-*, k8s.gcr.io/etcd and k8s.gcr.io/pause images don't require an -${ARCH} suffix.

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.

  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.

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.

{{% /capture %}}

{{% capture whatsnext %}}

  • 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 {{% /capture %}}