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kubeadm join

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

• What's next

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

Synopsis

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 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 Master. 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 calcuated 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 Master.

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 used for both parts. In this case, the –token flag can be used instead of specifying each token individually.

kubeadm join [flags]

Options

The joining workflow

kubeadm join bootstraps a Kubernetes worker node and joins it to the cluster. This action consists of the following steps:

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. If kubeadm is invoked with --feature-gates=DynamicKubeletConfig enabled, it first retrieves the kubelet init configuration from the master and writes it to the disk. When kubelet starts up, kubeadm updates the node Node.spec.configSource property of the node. See Set Kubelet parameters via a config file and Reconfigure a Node’s Kubelet in a Live Cluster for more information about Dynamic Kubelet Configuration.

3. 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.

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 Kubernetes 1.8 and above. 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 command:

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

Advantages:

• Allows bootstrapping nodes to securely discover a root of trust for the master 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 master 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 though.

Token-based discovery without CA pinning

This was the default in Kubernetes 1.7 and earlier, but comes with some important caveats. This mode relies only on the symmetric token to sign (HMAC-SHA256) the discovery information that establishes the root of trust for the master. It’s still possible in Kubernetes 1.8 and above using the --discovery-token-unsafe-skip-ca-verification flag, but 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 master 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 master 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 master and bootstrapping nodes. Consider using this mode if you are building automated provisioning using kubeadm.

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 master even if the network or other worker nodes are compromised.

Disadvantages:

• Requires that you have some way to carry the discovery information from the master to the bootstrapping nodes. This might be possible, for example, via your cloud provider or provisioning tool. The information in this file is not secret, but HTTPS or equivalent is required to ensure its integrity.

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 clusterrole kubeadm:node-autoapprove-bootstrap

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

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

$ kubectl certificate approve node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ
certificatesigningrequest "node-csr-c69HXe7aYcqkS1bKmH4faEnHAWxn6i2bHZ2mD04jZyQ" approved

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

Only after kubectl certificate approve has been run, kubeadm join can proceed.

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 master’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
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: <ca-cert>
    server: https://<ip>:<port>
  name: ""
contexts: []
current-context: ""
kind: Config
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 alpha 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 in the --config option.

apiVersion: kubeadm.k8s.io/v1alpha1
kind: NodeConfiguration
caCertPath: <path|string>
discoveryFile: <path|string>
discoveryToken: <string>
discoveryTokenAPIServers:
- <address|string>
- <address|string>
nodeName: <string>
tlsBootstrapToken: <string>
token: <string>
discoveryTokenCACertHashes:
- <SHA-256 hash|string>
- <SHA-256 hash|string>
discoveryTokenUnsafeSkipCAVerification: <bool>

What's next

• kubeadm init to bootstrap a Kubernetes master 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