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

reviewers	title	content_type	weight
luxas jbeda	kubeadm join	concept	30

This command initializes a Kubernetes worker node and joins it to the cluster.

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

The join workflow

kubeadm join bootstraps a Kubernetes worker node or a control-plane node and adds it to the cluster. This action consists of the following steps for worker nodes:

1. kubeadm downloads necessary cluster information from the API server. By default, it uses the bootstrap token and the CA key hash to verify the authenticity of that data. The root CA can also be discovered directly via a file or URL.

2. Once the cluster information is known, kubelet can start the TLS bootstrapping process.

   The TLS bootstrap uses the shared token to temporarily authenticate with the Kubernetes API server to submit a certificate signing request (CSR); by default the control plane signs this CSR request automatically.

3. Finally, kubeadm configures the local kubelet to connect to the API server with the definitive identity assigned to the node.

For control-plane nodes additional steps are performed:

1. Downloading certificates shared among control-plane nodes from the cluster (if explicitly requested by the user).

2. Generating control-plane component manifests, certificates and kubeconfig.

3. Adding new local etcd member.

Using join phases with kubeadm

Kubeadm allows you join a node to the cluster in phases using kubeadm join phase.

To view the ordered list of phases and sub-phases you can call kubeadm join --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 join 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:

kubeadm join phase kubelet-start --help

Similar to the kubeadm init phase command, kubeadm join phase allows you to skip a list of phases using the --skip-phases flag.

For example:

sudo kubeadm join --skip-phases=preflight --config=config.yaml

{{< feature-state for_k8s_version="v1.22" state="beta" >}}

Alternatively, you can use the skipPhases field in JoinConfiguration.

Discovering what cluster CA to trust

The kubeadm discovery has several options, each with security tradeoffs. The right method for your environment depends on how you provision nodes and the security expectations you have about your network and node lifecycles.

Token-based discovery with CA pinning

This is the default mode in kubeadm. In this mode, kubeadm downloads the cluster configuration (including root CA) and validates it using the token as well as validating that the root CA public key matches the provided hash and that the API server certificate is valid under the root CA.

The CA key hash has the format sha256:<hex_encoded_hash>. By default, the hash value is printed at the end of the kubeadm init command or in the output from the kubeadm token create --print-join-command command. It is in a standard format (see RFC7469) and can also be calculated by 3rd party tools or provisioning systems. For example, using the OpenSSL CLI:

openssl x509 -pubkey -in /etc/kubernetes/pki/ca.crt | openssl rsa -pubin -outform der 2>/dev/null | openssl dgst -sha256 -hex | sed 's/^.* //'

Example kubeadm join commands:

For worker nodes:

kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-ca-cert-hash sha256:1234..cdef 1.2.3.4:6443

For control-plane nodes:

kubeadm join --discovery-token abcdef.1234567890abcdef --discovery-token-ca-cert-hash sha256:1234..cdef --control-plane 1.2.3.4:6443

You can also call join for a control-plane node with --certificate-key to copy certificates to this node, if the kubeadm init command was called with --upload-certs.

Advantages:

• Allows bootstrapping nodes to securely discover a root of trust for the control-plane node even if other worker nodes or the network are compromised.

• Convenient to execute manually since all of the information required fits into a single kubeadm join command.

Disadvantages:

• The CA hash is not normally known until the control-plane node has been provisioned, which can make it more difficult to build automated provisioning tools that use kubeadm. By generating your CA in beforehand, you may workaround this limitation.

Token-based discovery without CA pinning

This mode relies only on the symmetric token to sign (HMAC-SHA256) the discovery information that establishes the root of trust for the control-plane. To use the mode the joining nodes must skip the hash validation of the CA public key, using --discovery-token-unsafe-skip-ca-verification. You should consider using one of the other modes if possible.

Example kubeadm join command:

kubeadm join --token abcdef.1234567890abcdef --discovery-token-unsafe-skip-ca-verification 1.2.3.4:6443

Advantages:

• Still protects against many network-level attacks.

• The token can be generated ahead of time and shared with the control-plane node and worker nodes, which can then bootstrap in parallel without coordination. This allows it to be used in many provisioning scenarios.

Disadvantages:

• If an attacker is able to steal a bootstrap token via some vulnerability, they can use that token (along with network-level access) to impersonate the control-plane node to other bootstrapping nodes. This may or may not be an appropriate tradeoff in your environment.

File or HTTPS-based discovery

This provides an out-of-band way to establish a root of trust between the control-plane node and bootstrapping nodes. Consider using this mode if you are building automated provisioning using kubeadm. The format of the discovery file is a regular Kubernetes kubeconfig file.

In case the discovery file does not contain credentials, the TLS discovery token will be used.

Example kubeadm join commands:

• kubeadm join --discovery-file path/to/file.conf (local file)

• kubeadm join --discovery-file https://url/file.conf (remote HTTPS URL)

Advantages:

• Allows bootstrapping nodes to securely discover a root of trust for the control-plane node even if the network or other worker nodes are compromised.

Disadvantages:

• Requires that you have some way to carry the discovery information from the control-plane node to the bootstrapping nodes. If the discovery file contains credentials you must keep it secret and transfer it over a secure channel. This might be possible with your cloud provider or provisioning tool.

Use of custom kubelet credentials with kubeadm join

To allow kubeadm join to use predefined kubelet credentials and skip client TLS bootstrap and CSR approval for a new node:

1. From a working control plane node in the cluster that has /etc/kubernetes/pki/ca.key execute kubeadm kubeconfig user --org system:nodes --client-name system:node:$NODE > kubelet.conf. $NODE must be set to the name of the new node.
2. Modify the resulted kubelet.conf manually to adjust the cluster name and the server endpoint, or run kubeadm kubeconfig user --config (it accepts InitConfiguration).

If your cluster does not have the ca.key file, you must sign the embedded certificates in the kubelet.conf externally.

1. Copy the resulting kubelet.conf to /etc/kubernetes/kubelet.conf on the new node.
2. Execute kubeadm join with the flag --ignore-preflight-errors=FileAvailable--etc-kubernetes-kubelet.conf on the new node.

Securing your installation even more

The defaults for kubeadm may not work for everyone. This section documents how to tighten up a kubeadm installation at the cost of some usability.

Turning off auto-approval of node client certificates

By default, there is a CSR auto-approver enabled that basically approves any client certificate request for a kubelet when a Bootstrap Token was used when authenticating. If you don't want the cluster to automatically approve kubelet client certs, you can turn it off by executing this command:

kubectl delete clusterrolebinding kubeadm:node-autoapprove-bootstrap

After that, kubeadm join will block until the admin has manually approved the CSR in flight:

1. Using kubectl get csr, you can see that the original CSR is in the Pending state.

   kubectl get csr
   

   The output is similar to this:

   NAME                                                   AGE       REQUESTOR                 CONDITION
   node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ   18s       system:bootstrap:878f07   Pending
   

2. kubectl certificate approve allows the admin to approve CSR.This action tells a certificate signing controller to issue a certificate to the requestor with the attributes requested in the CSR.

   kubectl certificate approve node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ
   

   The output is similar to this:

   certificatesigningrequest "node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ" approved
   

3. This would change the CRS resource to Active state.

   kubectl get csr
   

   The output is similar to this:

   NAME                                                   AGE       REQUESTOR                 CONDITION
   node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ   1m        system:bootstrap:878f07   Approved,Issued
   

This forces the workflow that kubeadm join will only succeed if kubectl certificate approve has been run.

Turning off public access to the cluster-info ConfigMap

In order to achieve the joining flow using the token as the only piece of validation information, a ConfigMap with some data needed for validation of the control-plane node's identity is exposed publicly by default. While there is no private data in this ConfigMap, some users might wish to turn it off regardless. Doing so will disable the ability to use the --discovery-token flag of the kubeadm join flow. Here are the steps to do so:

• Fetch the cluster-info file from the API Server:

kubectl -n kube-public get cm cluster-info -o jsonpath='{.data.kubeconfig}' | tee cluster-info.yaml

The output is similar to this:

apiVersion: v1
kind: Config
clusters:
- cluster:
    certificate-authority-data: <ca-cert>
    server: https://<ip>:<port>
  name: ""
contexts: []
current-context: ""
preferences: {}
users: []

• Use the cluster-info.yaml file as an argument to kubeadm join --discovery-file.

• Turn off public access to the cluster-info ConfigMap:

kubectl -n kube-public delete rolebinding kubeadm:bootstrap-signer-clusterinfo

These commands should be run after kubeadm init but before kubeadm join.

Using kubeadm join 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 join 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 JoinConfiguration structure. 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.

{{% heading "whatsnext" %}}

• kubeadm init to bootstrap a Kubernetes control-plane node.
• kubeadm token to manage tokens for kubeadm join.
• kubeadm reset to revert any changes made to this host by kubeadm init or kubeadm join.