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kubeadm join

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

• What's next

Synopsis

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 1.2.3.4:6443, 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 “<hash-type>:<hex-encoded-value>”, 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]

Options

Options inherited from parent commands

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.

1. 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.

4. Adding this node to the ClusterStatus of the kubeadm cluster.

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, kubadm 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

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 returned in the kubeadm join command printed at the end of kubeadm init or in the output of kubeadm token create --print-join-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 that is easy to copy and paste.

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.

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:

kubectl get csr

The output is similar to this:

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

kubectl certificate approve node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ

The output is similar to this:

certificatesigningrequest "node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ" approved

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 yaml | grep "kubeconfig:" -A11 | grep "apiVersion" -A10 | sed "s/    //" | 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.

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.

To print the default values of JoinConfiguration run the following command:

kubeadm config print join-defaults

For details on individual fields in JoinConfiguration see the godoc.

What's next

• 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

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