Creating a cluster with kubeadm
kubeadm, you can create a minimum viable Kubernetes cluster that conforms to best practices. In fact, you can use
kubeadm to set up a cluster that will pass the Kubernetes Conformance tests.
kubeadm also supports other cluster
lifecycle functions, such as bootstrap tokens and cluster upgrades.
kubeadm tool is good if you need:
- A simple way for you to try out Kubernetes, possibly for the first time.
- A way for existing users to automate setting up a cluster and test their application.
- A building block in other ecosystem and/or installer tools with a larger scope.
You can install and use
kubeadm on various machines: your laptop, a set
of cloud servers, a Raspberry Pi, and more. Whether you're deploying into the
cloud or on-premises, you can integrate
kubeadm into provisioning systems such
as Ansible or Terraform.
Before you begin
To follow this guide, you need:
- One or more machines running a deb/rpm-compatible Linux OS; for example: Ubuntu or CentOS.
- 2 GiB or more of RAM per machine--any less leaves little room for your apps.
- At least 2 CPUs on the machine that you use as a control-plane node.
- Full network connectivity among all machines in the cluster. You can use either a public or a private network.
You also need to use a version of
kubeadm that can deploy the version
of Kubernetes that you want to use in your new cluster.
Kubernetes' version and version skew support policy applies to
kubeadm as well as to Kubernetes overall.
Check that policy to learn about what versions of Kubernetes and
are supported. This page is written for Kubernetes v1.21.
kubeadm tool's overall feature state is General Availability (GA). Some sub-features are
still under active development. The implementation of creating the cluster may change
slightly as the tool evolves, but the overall implementation should be pretty stable.
Note: Any commands under
kubeadm alphaare, by definition, supported on an alpha level.
- Install a single control-plane Kubernetes cluster
- Install a Pod network on the cluster so that your Pods can talk to each other
Installing kubeadm on your hosts
See "Installing kubeadm".
If you have already installed kubeadm, run
apt-get update && apt-get upgradeor
yum updateto get the latest version of kubeadm.
When you upgrade, the kubelet restarts every few seconds as it waits in a crashloop for kubeadm to tell it what to do. This crashloop is expected and normal. After you initialize your control-plane, the kubelet runs normally.
Initializing your control-plane node
- (Recommended) If you have plans to upgrade this single control-plane
kubeadmcluster to high availability you should specify the
--control-plane-endpointto set the shared endpoint for all control-plane nodes. Such an endpoint can be either a DNS name or an IP address of a load-balancer.
- Choose a Pod network add-on, and verify whether it requires any arguments to
be passed to
kubeadm init. Depending on which third-party provider you choose, you might need to set the
--pod-network-cidrto a provider-specific value. See Installing a Pod network add-on.
- (Optional) Since version 1.14,
kubeadmtries to detect the container runtime on Linux by using a list of well known domain socket paths. To use different container runtime or if there are more than one installed on the provisioned node, specify the
kubeadm init. See Installing runtime.
- (Optional) Unless otherwise specified,
kubeadmuses the network interface associated with the default gateway to set the advertise address for this particular control-plane node's API server. To use a different network interface, specify the
kubeadm init. To deploy an IPv6 Kubernetes cluster using IPv6 addressing, you must specify an IPv6 address, for example
- (Optional) Run
kubeadm config images pullprior to
kubeadm initto verify connectivity to the gcr.io container image registry.
To initialize the control-plane node run:
kubeadm init <args>
Considerations about apiserver-advertise-address and ControlPlaneEndpoint
--apiserver-advertise-address can be used to set the advertise address for this particular
control-plane node's API server,
--control-plane-endpoint can be used to set the shared endpoint
for all control-plane nodes.
--control-plane-endpoint allows both IP addresses and DNS names that can map to IP addresses.
Please contact your network administrator to evaluate possible solutions with respect to such mapping.
Here is an example mapping:
192.168.0.102 is the IP address of this node and
cluster-endpoint is a custom DNS name that maps to this IP.
This will allow you to pass
kubeadm init and pass the same DNS name to
kubeadm join. Later you can modify
cluster-endpoint to point to the address of your load-balancer in an
high availability scenario.
Turning a single control plane cluster created without
--control-plane-endpoint into a highly available cluster
is not supported by kubeadm.
For more information about
kubeadm init arguments, see the kubeadm reference guide.
kubeadm init with a configuration file see Using kubeadm init with a configuration file.
To customize control plane components, including optional IPv6 assignment to liveness probe for control plane components and etcd server, provide extra arguments to each component as documented in custom arguments.
kubeadm init again, you must first tear down the cluster.
If you join a node with a different architecture to your cluster, make sure that your deployed DaemonSets have container image support for this architecture.
kubeadm init first runs a series of prechecks to ensure that the machine
is ready to run Kubernetes. These prechecks expose warnings and exit on errors.
then downloads and installs the cluster control plane components. This may take several minutes.
After it finishes you should see:
Your Kubernetes control-plane has initialized successfully! To start using your cluster, you need to run the following as a regular user: mkdir -p $HOME/.kube sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config sudo chown $(id -u):$(id -g) $HOME/.kube/config You should now deploy a Pod network to the cluster. Run "kubectl apply -f [podnetwork].yaml" with one of the options listed at: /docs/concepts/cluster-administration/addons/ You can now join any number of machines by running the following on each node as root: kubeadm join <control-plane-host>:<control-plane-port> --token <token> --discovery-token-ca-cert-hash sha256:<hash>
To make kubectl work for your non-root user, run these commands, which are
also part of the
kubeadm init output:
mkdir -p $HOME/.kube sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config sudo chown $(id -u):$(id -g) $HOME/.kube/config
Alternatively, if you are the
root user, you can run:
Warning: Kubeadm signs the certificate in the
Subject: O = system:masters, CN = kubernetes-admin.
system:mastersis a break-glass, super user group that bypasses the authorization layer (e.g. RBAC). Do not share the
admin.conffile with anyone and instead grant users custom permissions by generating them a kubeconfig file using the
kubeadm kubeconfig usercommand.
Make a record of the
kubeadm join command that
kubeadm init outputs. You
need this command to join nodes to your cluster.
The token is used for mutual authentication between the control-plane node and the joining
nodes. The token included here is secret. Keep it safe, because anyone with this
token can add authenticated nodes to your cluster. These tokens can be listed,
created, and deleted with the
kubeadm token command. See the
kubeadm reference guide.
Installing a Pod network add-on
This section contains important information about networking setup and deployment order. Read all of this advice carefully before proceeding.
You must deploy a Container Network Interface (CNI) based Pod network add-on so that your Pods can communicate with each other. Cluster DNS (CoreDNS) will not start up before a network is installed.
Take care that your Pod network must not overlap with any of the host networks: you are likely to see problems if there is any overlap. (If you find a collision between your network plugin's preferred Pod network and some of your host networks, you should think of a suitable CIDR block to use instead, then use that during
--pod-network-cidrand as a replacement in your network plugin's YAML).
kubeadmsets up your cluster to use and enforce use of RBAC (role based access control). Make sure that your Pod network plugin supports RBAC, and so do any manifests that you use to deploy it.
If you want to use IPv6--either dual-stack, or single-stack IPv6 only networking--for your cluster, make sure that your Pod network plugin supports IPv6. IPv6 support was added to CNI in v0.6.0.
Note: Kubeadm should be CNI agnostic and the validation of CNI providers is out of the scope of our current e2e testing. If you find an issue related to a CNI plugin you should log a ticket in its respective issue tracker instead of the kubeadm or kubernetes issue trackers.
Several external projects provide Kubernetes Pod networks using CNI, some of which also support Network Policy.
See a list of add-ons that implement the Kubernetes networking model.
You can install a Pod network add-on with the following command on the control-plane node or a node that has the kubeconfig credentials:
kubectl apply -f <add-on.yaml>
You can install only one Pod network per cluster.
Once a Pod network has been installed, you can confirm that it is working by
checking that the CoreDNS Pod is
Running in the output of
kubectl get pods --all-namespaces.
And once the CoreDNS Pod is up and running, you can continue by joining your nodes.
If your network is not working or CoreDNS is not in the
Running state, check out the
Control plane node isolation
By default, your cluster will not schedule Pods on the control-plane node for security reasons. If you want to be able to schedule Pods on the control-plane node, for example for a single-machine Kubernetes cluster for development, run:
kubectl taint nodes --all node-role.kubernetes.io/master-
With output looking something like:
node "test-01" untainted taint "node-role.kubernetes.io/master:" not found taint "node-role.kubernetes.io/master:" not found
This will remove the
node-role.kubernetes.io/master taint from any nodes that
have it, including the control-plane node, meaning that the scheduler will then be able
to schedule Pods everywhere.
Joining your nodes
The nodes are where your workloads (containers and Pods, etc) run. To add new nodes to your cluster do the following for each machine:
- SSH to the machine
- Become root (e.g.
sudo su -)
- Run the command that was output by
kubeadm init. For example:
kubeadm join --token <token> <control-plane-host>:<control-plane-port> --discovery-token-ca-cert-hash sha256:<hash>
If you do not have the token, you can get it by running the following command on the control-plane node:
kubeadm token list
The output is similar to this:
TOKEN TTL EXPIRES USAGES DESCRIPTION EXTRA GROUPS 8ewj1p.9r9hcjoqgajrj4gi 23h 2018-06-12T02:51:28Z authentication, The default bootstrap system: signing token generated by bootstrappers: 'kubeadm init'. kubeadm: default-node-token
By default, tokens expire after 24 hours. If you are joining a node to the cluster after the current token has expired, you can create a new token by running the following command on the control-plane node:
kubeadm token create
The output is similar to this:
If you don't have the value of
--discovery-token-ca-cert-hash, you can get it by running the following command chain on the control-plane node:
openssl x509 -pubkey -in /etc/kubernetes/pki/ca.crt | openssl rsa -pubin -outform der 2>/dev/null | \ openssl dgst -sha256 -hex | sed 's/^.* //'
The output is similar to:
Note: To specify an IPv6 tuple for
<control-plane-host>:<control-plane-port>, IPv6 address must be enclosed in square brackets, for example:
The output should look something like:
[preflight] Running pre-flight checks ... (log output of join workflow) ... Node join complete: * Certificate signing request sent to control-plane and response received. * Kubelet informed of new secure connection details. Run 'kubectl get nodes' on control-plane to see this machine join.
A few seconds later, you should notice this node in the output from
kubectl get nodes when run on the control-plane node.
(Optional) Controlling your cluster from machines other than the control-plane node
In order to get a kubectl on some other computer (e.g. laptop) to talk to your cluster, you need to copy the administrator kubeconfig file from your control-plane node to your workstation like this:
scp root@<control-plane-host>:/etc/kubernetes/admin.conf . kubectl --kubeconfig ./admin.conf get nodes
The example above assumes SSH access is enabled for root. If that is not the case, you can copy the
admin.conffile to be accessible by some other user and
scpusing that other user instead.
admin.conffile gives the user superuser privileges over the cluster. This file should be used sparingly. For normal users, it's recommended to generate an unique credential to which you grant privileges. You can do this with the
kubeadm alpha kubeconfig user --client-name <CN>command. That command will print out a KubeConfig file to STDOUT which you should save to a file and distribute to your user. After that, grant privileges by using
kubectl create (cluster)rolebinding.
(Optional) Proxying API Server to localhost
If you want to connect to the API Server from outside the cluster you can use
scp root@<control-plane-host>:/etc/kubernetes/admin.conf . kubectl --kubeconfig ./admin.conf proxy
You can now access the API Server locally at
If you used disposable servers for your cluster, for testing, you can
switch those off and do no further clean up. You can use
kubectl config delete-cluster to delete your local references to the
However, if you want to deprovision your cluster more cleanly, you should first drain the node and make sure that the node is empty, then deconfigure the node.
Remove the node
Talking to the control-plane node with the appropriate credentials, run:
kubectl drain <node name> --delete-local-data --force --ignore-daemonsets
Before removing the node, reset the state installed by
The reset process does not reset or clean up iptables rules or IPVS tables. If you wish to reset iptables, you must do so manually:
iptables -F && iptables -t nat -F && iptables -t mangle -F && iptables -X
If you want to reset the IPVS tables, you must run the following command:
Now remove the node:
kubectl delete node <node name>
If you wish to start over, run
kubeadm init or
kubeadm join with the
Clean up the control plane
You can use
kubeadm reset on the control plane host to trigger a best-effort
reference documentation for more information about this subcommand and its
- Verify that your cluster is running properly with Sonobuoy
- See Upgrading kubeadm clusters
for details about upgrading your cluster using
- Learn about advanced
kubeadmusage in the kubeadm reference documentation
- Learn more about Kubernetes concepts and
- See the Cluster Networking page for a bigger list of Pod network add-ons.
- See the list of add-ons to explore other add-ons, including tools for logging, monitoring, network policy, visualization & control of your Kubernetes cluster.
- Configure how your cluster handles logs for cluster events and from applications running in Pods. See Logging Architecture for an overview of what is involved.
- For bugs, visit the kubeadm GitHub issue tracker
- For support, visit the #kubeadm Slack channel
- General SIG Cluster Lifecycle development Slack channel: #sig-cluster-lifecycle
- SIG Cluster Lifecycle SIG information
- SIG Cluster Lifecycle mailing list: kubernetes-sig-cluster-lifecycle
Version skew policy
kubeadm tool of version v1.21 may deploy clusters with a control plane of version v1.21 or v1.20.
kubeadm v1.21 can also upgrade an existing kubeadm-created cluster of version v1.20.
Due to that we can't see into the future, kubeadm CLI v1.21 may or may not be able to deploy v1.22 clusters.
These resources provide more information on supported version skew between kubelets and the control plane, and other Kubernetes components:
The cluster created here has a single control-plane node, with a single etcd database running on it. This means that if the control-plane node fails, your cluster may lose data and may need to be recreated from scratch.
Regularly back up etcd. The etcd data directory configured by kubeadm is at
/var/lib/etcdon the control-plane node.
kubeadm deb/rpm packages and binaries are built for amd64, arm (32-bit), arm64, ppc64le, and s390x following the multi-platform proposal.
Multiplatform container images for the control plane and addons are also supported since v1.12.
Only some of the network providers offer solutions for all platforms. Please consult the list of network providers above or the documentation from each provider to figure out whether the provider supports your chosen platform.
If you are running into difficulties with kubeadm, please consult our troubleshooting docs.