Reference Documentation

Design docs, concept definitions, and references for APIs and CLIs.

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kubeadm Setup Tool

This document provides information on how to use kubeadm’s advanced options.

Running kubeadm init bootstraps a Kubernetes cluster. This consists of the following steps:

  1. kubeadm 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 --skip-preflight-checks.

  2. kubeadm generates a token that additional nodes can use to register themselves with the master in future. Optionally, the user can provide a token.

  3. kubeadm generates a self-signed CA to provision identities for each component (including nodes) in the cluster. It also generates client certificates to be used by various components. If the user has provided their own CA by dropping it in the cert directory (configured via --cert-dir, by default /etc/kubernetes/pki), this step is skipped.

  4. Outputting a kubeconfig file for the kubelet to use to connect to the API server, as well as an additional kubeconfig file for administration.

  5. kubeadm generates Kubernetes Static Pod manifests for the API server, controller manager and scheduler. It places them in /etc/kubernetes/manifests. The kubelet watches this directory for Pods to create on startup. These are the core components of Kubernetes. Once they are up and running kubeadm can set up and manage any additional components.

  6. kubeadm “taints” the master node so that only control plane components will run there. It also sets up the RBAC authorization system and writes a special ConfigMap that is used to bootstrap trust with the kubelets.

  7. kubeadm installs add-on components via the API server. Right now this is the internal DNS server and the kube-proxy DaemonSet.

Running kubeadm join on each node in the cluster consists of the following steps:

  1. kubeadm downloads root CA information from the API server. It uses the token to verify the authenticity of that data.

  2. kubeadm creates a local key pair. It prepares a certificate signing request (CSR) and sends that off to the API server for signing. The bootstrap token is used to authenticate. The API server is configured to sign this automatically.

  3. kubeadm configures the local kubelet to connect to the API server


Fields that support multiple values do so either with comma separation, or by specifying the flag multiple times.

The kubeadm command line interface is currently in beta. We are aiming to not break any scripted use of the main kubeadm init and kubeadm join. The single exception here is the format of the kubeadm config file as detailed below. That format is still considered alpha and may change.

kubeadm init

It is usually sufficient to run kubeadm init without any flags, but in some cases you might like to override the default behaviour. Here we specify all the flags that can be used to customise the Kubernetes installation.

This is the address the API Server will advertise to other members of the cluster. This is also the address used to construct the suggested kubeadm join line at the end of the init process. If not set (or set to then IP for the default interface will be used.

This address is also added to the certifcate that the API Server uses.

The port that the API server will bind on. This defaults to 6443.

Additional hostnames or IP addresses that should be added to the Subject Alternate Name section for the certificate that the API Server will use. If you expose the API Server through a load balancer and public DNS you could specify this with,,

The path where to save and store the certificates. The default is “/etc/kubernetes/pki”.

A kubeadm specific config file. This can be used to specify an extended set of options including passing arbitrary command line flags to the control plane components.

Note: When providing configuration values using both a configuration file and flags, the file will take precedence. For example, if a file exists with:

kind: MasterConfiguration
token: 1234

and the user ran kubeadm init --config file.yaml --token 5678, the chosen token value will be 1234.

The v1.6 version of kubeadm only supports building clusters that are at least v1.6.0. There are many reasons for this including kubeadm’s use of RBAC, the Bootstrap Token system, and enhancements to the Certificates API. With this flag you can try any future version of Kubernetes. Check releases page for a full list of available versions.

For certain networking solutions the Kubernetes master can also play a role in allocating network ranges (CIDRs) to each node. This includes many cloud providers and flannel. You can specify a subnet range that will be broken down and handed out to each node with the --pod-network-cidr flag. This should be a minimum of a /16 so controller-manager is able to assign /24 subnets to each node in the cluster. If you are using flannel with this manifest you should use --pod-network-cidr= Most CNI based networking solutions do not require this flag.

You can use the --service-cidr flag to override the subnet Kubernetes uses to assign pods IP addresses. If you do, you will also need to update the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file to reflect this change else DNS will not function correctly.

By default, kubeadm init deploys a cluster that assigns services with DNS names <service_name>.<namespace>.svc.cluster.local. You can use the --service-dns-domain to change the DNS name suffix. Again, you will need to update the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file accordingly else DNS will not function correctly.

Note: This flag has an effect (it’s needed for the kube-dns Deployment manifest and the API Server’s serving certificate) but not as you might expect, since you will have to modify the arguments to the kubelets in the cluster for it to work fully. Specifying DNS parameters using this flag only is not enough. Rewriting the kubelet’s CLI arguments is out of scope for kubeadm as it should be agnostic to how you run the kubelet. However, making all kubelets in the cluster pick up information dynamically via the API is in scope and is a planned feature for upcoming releases.

By default, kubeadm runs a series of preflight checks to validate the system before making any changes. Advanced users can use this flag to bypass these if necessary.

By default, kubeadm init automatically generates the token used to initialise each new node. If you would like to manually specify this token, you can use the --token flag. The token must be of the format [a-z0-9]{6}\.[a-z0-9]{16}. A compatible random token can be generated kubeadm token generate. Tokens can be managed through the API after the cluster is created. See the section on managing tokens below.

This sets an expiration time for the token. This is specified as a duration from the current time. After this time the token will no longer be valid and will be removed. A value of 0 specifies that the token never expires. 0 is the default. See the section on managing tokens below.

kubeadm join

When joining a kubeadm initialized cluster, we need to establish bidirectional trust. This is split into discovery (having the Node trust the Kubernetes master) and TLS bootstrap (having the Kubernetes master trust the Node).

There are 2 main schemes for discovery. The first is to use a shared token along with the IP address of the API server. The second is to provide a file (a subset of the standard kubeconfig file). This file can be a local file or downloaded via an HTTPS URL. The forms are kubeadm join --discovery-token abcdef.1234567890abcdef, kubeadm join --discovery-file path/to/file.conf or kubeadm join --discovery-file https://url/file.conf. Only one form can be used. If the discovery information is loaded from a URL, HTTPS must be used and the host installed CA bundle is used to verify the connection.

The TLS bootstrap mechanism is also driven via a shared token. This is used to temporarily authenticate with the Kubernetes master to submit a certificate signing request (CSR) for a locally created key pair. By default kubeadm will set up the Kubernetes master to automatically approve these signing requests. This token is passed in with the --tls-bootstrap-token abcdef.1234567890abcdef flag.

Often times the same token is use for both parts. In this case, the --token flag can be used instead of specifying the each token individually.

Here’s an example on how to use it:

kubeadm join --token=abcdef.1234567890abcdef

Specific options:

Extended options a specified in the kubeadm specific config file.

By default, kubeadm runs a series of preflight checks to validate the system before making any changes. Advanced users can use this flag to bypass these if necessary.

A local file path or HTTPS URL. The file specified must be a kubeconfig file with nothing but an unnamed cluster entry. This is used to find both the location of the API server to join along with a root CA bundle to use when talking to that server.

This might look something like this:

apiVersion: v1
- cluster:
    certificate-authority-data: <really long certificate data>
  name: ""
contexts: []
current-context: ""
kind: Config
preferences: {}
users: []

The discovery token is used along with the address of the API server (as an unnamed argument) to download and verify information about the cluster. The most critical part of the cluster information is the root CA bundle used to verify the identity of the server during subsequent TLS connections.

The token used to authenticate to the API server for the purposes of TLS bootstrapping.

Often times the same token is used for both --discovery-token and --tls-bootstrap-token. This option specifies the same token for both. Other flags override this flag if present.

Using kubeadm with a configuration file

WARNING: While kubeadm command line interface is in beta, the config file is still considered alpha and may change in future versions.

It’s possible to configure kubeadm 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 to the --config option on both kubeadm init and kubeadm join.

Sample Master Configuration

kind: MasterConfiguration
  advertiseAddress: <address|string>
  bindPort: <int>
  - <endpoint1|string>
  - <endpoint2|string>
  caFile: <path|string>
  certFile: <path|string>
  keyFile: <path|string>
  dnsDomain: <string>
  serviceSubnet: <cidr>
  podSubnet: <cidr>
kubernetesVersion: <string>
cloudProvider: <string>
- <authorizationMode1|string>
- <authorizationMode2|string>
token: <string>
tokenTTL: <time duration>
selfHosted: <bool>
  <argument>: <value|string>
  <argument>: <value|string>
  <argument>: <value|string>
  <argument>: <value|string>
  <argument>: <value|string>
  <argument>: <value|string>
  - <name1|string>
  - <name2|string>
certificatesDir: <string>

In addition, if authorizationMode is set to ABAC, you should write the config to /etc/kubernetes/abac_policy.json. However, if authorizationMode is set to Webhook, you should write the config to /etc/kubernetes/webhook_authz.conf.

Sample Node Configuration

kind: NodeConfiguration
caCertPath: <path|string>
discoveryFile: <path|string>
discoveryToken: <string>

# Currently only the first server is used as a target for the cluster
# bootstrap flow.
  - <address|string>
  - <address|string>

tlsBootstrapToken: <string>

Managing Tokens

You can use the kubeadm tool to manage tokens on a running cluster. It will automatically grab the default admin credentials on a master from a kubeadm created cluster (/etc/kubernetes/admin.conf). You can specify an alternate kubeconfig file for credentials with the --kubeconfig to the following commands.

In addition, you can use the kubeadm token generate command to locally creates a new token. This token is of the correct form for specifying with the --token argument to kubeadm init.

For the gory details on how the tokens are implemented (including managing them outside of kubeadm) see the Bootstrap Token docs.

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 master 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 master 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 master node at /etc/kubernetes/admin.conf and use that to talk to the cluster.

Environment variables

There are some environment variables that modify the way that kubeadm works. Most users will have no need to set these. These environment variables are a short-term solution, eventually they will be integrated in the kubeadm configuration file.

Variable Default Description
KUBE_KUBERNETES_DIR /etc/kubernetes Where most configuration files are written to and read from
KUBE_HYPERKUBE_IMAGE   If set, use a single hyperkube image with this name. If not set, individual images per server component will be used.
KUBE_ETCD_IMAGE<arch>:3.0.17 The etcd container image to use.
KUBE_REPO_PREFIX The image prefix for all images that are used.

If KUBE_KUBERNETES_DIR is specified, you may need to rewrite the arguments of the kubelet. (e.g. –kubeconfig, –pod-manifest-path)

If KUBE_REPO_PREFIX is specified, you may need to set the kubelet flag --pod-infra-container-image which specifies which pause image to use. Defaults to${ARCH}:3.0 where ${ARCH} can be one of amd64, arm, arm64, ppc64le or s390x.

cat > /etc/systemd/system/kubelet.service.d/20-pod-infra-image.conf <<EOF
systemctl daemon-reload
systemctl restart kubelet

If you want to use kubeadm with an http proxy, you may need to configure it to support http_proxy, https_proxy, or no_proxy.

For example, if your kube master node IP address is and you have a proxy which supports both http/https on port 8080, you can use the following command:

export PROXY_PORT=8080
export PROXY_IP=
export http_proxy=http://$PROXY_IP:$PROXY_PORT
export HTTP_PROXY=$http_proxy
export https_proxy=$http_proxy
export HTTPS_PROXY=$http_proxy
export no_proxy="localhost,,localaddress,,,"

Remember to change proxy_ip and add a kube master node IP address to no_proxy.

Use Kubeadm with other CRI runtimes

Since Kubernetes 1.6 release, Kubernetes container runtimes have been transferred to using CRI by default. Currently, the build-in container runtime is Docker which is enabled by build-in dockershim in kubelet.

Using other CRI based runtimes with kubeadm is very simple, and currently supported runtimes are:

After you have successfully installed kubeadm and kubelet, please follow these two steps:

  1. Install runtime shim on every node. You will need to follow the installation document in the runtime shim project listing above.

  2. Configure kubelet to use 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
Environment="KUBELET_EXTRA_ARGS=--container-runtime=remote --container-runtime-endpoint=$RUNTIME_ENDPOINT --feature-gates=AllAlpha=true"
  $ systemctl daemon-reload

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

Using custom certificates

By default kubeadm will generate all the certificates needed for a cluster to run. You can override this behaviour 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 both exist, kubeadm will skip the generation step and those files will be validated and used for the prescribed use-case.

This means you can, for example, prepopulate /etc/kubernetes/pki/ca.crt and /etc/kubernetes/pki/ca.key with an existing CA, which then will be used for signing the rest of the certs.

Releases and release notes

If you already have kubeadm installed and want to upgrade, run apt-get update && apt-get upgrade or yum update to get the latest version of kubeadm.

Refer to the for more information.


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