Edit This Page

Run this on any machine you wish to join an existing cluster


When joining a kubeadm initialized cluster, we need to establish bidirectional trust. This is split into discovery (having the Node trust the Kubernetes Control Plane) and TLS bootstrap (having the Kubernetes Control Plane 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. Also, in that case the host installed CA bundle is used to verify the connection.

If you use a shared token for discovery, you should also pass the –discovery-token-ca-cert-hash flag to validate the public key of the root certificate authority (CA) presented by the Kubernetes Control Plane. The value of this flag is specified as “:”, where the supported hash type is “sha256”. The hash is calculated over the bytes of the Subject Public Key Info (SPKI) object (as in RFC7469). This value is available in the output of “kubeadm init” or can be calculated using standard tools. The –discovery-token-ca-cert-hash flag may be repeated multiple times to allow more than one public key.

If you cannot know the CA public key hash ahead of time, you can pass the –discovery-token-unsafe-skip-ca-verification flag to disable this verification. This weakens the kubeadm security model since other nodes can potentially impersonate the Kubernetes Control Plane.

The TLS bootstrap mechanism is also driven via a shared token. This is used to temporarily authenticate with the Kubernetes Control Plane to submit a certificate signing request (CSR) for a locally created key pair. By default, kubeadm will set up the Kubernetes Control Plane 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 used for both parts. In this case, the –token flag can be used instead of specifying each token individually.

The “join [api-server-endpoint]” command executes the following phases:

preflight              Run join pre-flight checks
control-plane-prepare  Prepare the machine for serving a control plane
  /download-certs        [EXPERIMENTAL] Download certificates shared among control-plane nodes from the kubeadm-certs Secret
  /certs                 Generate the certificates for the new control plane components
  /kubeconfig            Generate the kubeconfig for the new control plane components
  /control-plane         Generate the manifests for the new control plane components
kubelet-start          Write kubelet settings, certificates and (re)start the kubelet
control-plane-join     Join a machine as a control plane instance
  /etcd                  Add a new local etcd member
  /update-status         Register the new control-plane node into the ClusterStatus maintained in the kubeadm-config ConfigMap
  /mark-control-plane    Mark a node as a control-plane
kubeadm join [api-server-endpoint] [flags]


      --apiserver-advertise-address string            If the node should host a new control plane instance, the IP address the API Server will advertise it's listening on. If not set the default network interface will be used.
      --apiserver-bind-port int32                     If the node should host a new control plane instance, the port for the API Server to bind to. (default 6443)
      --certificate-key string                        Use this key to decrypt the certificate secrets uploaded by init.
      --config string                                 Path to kubeadm config file.
      --control-plane                                 Create a new control plane instance on this node
      --cri-socket string                             Path to the CRI socket to connect. If empty kubeadm will try to auto-detect this value; use this option only if you have more than one CRI installed or if you have non-standard CRI socket.
      --discovery-file string                         For file-based discovery, a file or URL from which to load cluster information.
      --discovery-token string                        For token-based discovery, the token used to validate cluster information fetched from the API server.
      --discovery-token-ca-cert-hash strings          For token-based discovery, validate that the root CA public key matches this hash (format: "<type>:<value>").
      --discovery-token-unsafe-skip-ca-verification   For token-based discovery, allow joining without --discovery-token-ca-cert-hash pinning.
      --experimental-control-plane                    Create a new control plane instance on this node
  -h, --help                                          help for join
      --ignore-preflight-errors strings               A list of checks whose errors will be shown as warnings. Example: 'IsPrivilegedUser,Swap'. Value 'all' ignores errors from all checks.
      --node-name string                              Specify the node name.
      --skip-phases strings                           List of phases to be skipped
      --tls-bootstrap-token string                    Specify the token used to temporarily authenticate with the Kubernetes Control Plane while joining the node.
      --token string                                  Use this token for both discovery-token and tls-bootstrap-token when those values are not provided.

Options inherited from parent commands

      --rootfs string   [EXPERIMENTAL] The path to the 'real' host root filesystem.