--- reviewers: - mikedanese - luxas - jbeda title: kubeadm join content_template: templates/concept weight: 30 --- {{% capture overview %}} This command initializes a Kubernetes worker node and joins it to the cluster. {{% /capture %}} {{% capture body %}} {{< include "generated/kubeadm_join.md" >}} ### 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. 1. 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](/docs/tasks/administer-cluster/kubelet-config-file/) and [Reconfigure a Node's Kubelet in a Live Cluster](/docs/tasks/administer-cluster/reconfigure-kubelet/) for more information about Dynamic Kubelet Configuration. 1. 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:`. 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](https://tools.ietf.org/html/rfc7469#section-2.4)) and can also be calculated by 3rd party tools or provisioning systems. For example, using the OpenSSL CLI: ```bash 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:** ```bash 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 {#securing-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: ```console $ kubectl delete clusterrole kubeadm:node-autoapprove-bootstrap ``` After that, `kubeadm join` will block until the admin has manually approved the CSR in flight: ```console $ 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: ```console $ 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: server: https://: 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: ```console $ 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 {#config-file} {{< caution >}} **Caution:** The config file is still considered alpha 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 in the `--config` option. ```yaml apiVersion: kubeadm.k8s.io/v1alpha1 kind: NodeConfiguration caCertPath: discoveryFile: discoveryToken: discoveryTokenAPIServers: - - nodeName: tlsBootstrapToken: token: discoveryTokenCACertHashes: - - discoveryTokenUnsafeSkipCAVerification: ``` {{% /capture %}} {{% capture whatsnext %}} * [kubeadm init](/docs/reference/setup-tools/kubeadm/kubeadm-init/) to bootstrap a Kubernetes master node * [kubeadm token](/docs/reference/setup-tools/kubeadm/kubeadm-token/) to manage tokens for `kubeadm join` * [kubeadm reset](/docs/reference/setup-tools/kubeadm/kubeadm-reset/) to revert any changes made to this host by `kubeadm init` or `kubeadm join` {{% /capture %}}