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This document provides information on how to use kubeadm’s advanced options.

Running kubeadm init bootstraps a Kubernetes cluster. This consists of the following steps:

  1. kubeadm runs a series of pre-flight checks to validate the system state before making changes. Some checks only trigger warnings, others are considered errors and will exit kubeadm until the problem is corrected or the user specifies --skip-preflight-checks.

  2. kubeadm generates a token that additional nodes can use to register themselves with the master in future. Optionally, the user can provide a token.

  3. kubeadm generates a self-signed CA using openssl to provision identities for each node in the cluster, and for the API server to secure communication with clients.

  4. Outputting a kubeconfig file for the kubelet to use to connect to the API server, as well as an additional kubeconfig file for administration.

  5. kubeadm generates Kubernetes resource manifests for the API server, controller manager and scheduler, and placing them in /etc/kubernetes/manifests. The kubelet watches this directory for static resources to create on startup. These are the core components of Kubernetes, and once they are up and running we can use kubectl to set up/manage any additional components.

  6. kubeadm installs any add-on components, such as DNS or discovery, via the API server.

Running kubeadm join on each node in the cluster consists of the following steps:

  1. Use the token to talk to the API server and securely get the root CA certificate.

  2. Creates a local key pair. Prepares a certificate signing request (CSR) and sends that off to the API server for signing.

  3. Configures the local kubelet to connect to the API server

Usage

Fields that support multiple values do so either with comma separation, or by specifying the flag multiple times.

kubeadm init

It is usually sufficient to run kubeadm init without any flags, but in some cases you might like to override the default behaviour. Here we specify all the flags that can be used to customise the Kubernetes installation.

By default, kubeadm init automatically detects IP addresses and uses these to generate certificates for the API server. This uses the IP address of the default network interface. If you would like to access the API server through a different IP address, or through a hostname, you can override these defaults with --api-advertise-addresses and --api-external-dns-names. For example, to generate certificates that verify the API server at addresses 10.100.245.1 and 100.123.121.1, you could use --api-advertise-addresses=10.100.245.1,100.123.121.1. To allow it to be accessed with a hostname, --api-external-dns-names=kubernetes.example.com,kube.example.com Specifying --api-advertise-addresses disables auto detection of IP addresses.

Currently, kubeadm init does not provide autodetection of cloud provider. This means that load balancing and persistent volumes are not supported out of the box. You can specify a cloud provider using --cloud-provider. Valid values are the ones supported by controller-manager, namely "aws", "azure", "cloudstack", "gce", "mesos", "openstack", "ovirt", "rackspace", "vsphere". In order to provide additional configuration for the cloud provider, you should create a /etc/kubernetes/cloud-config.json file manually, before running kubeadm init. kubeadm automatically picks those settings up and ensures other nodes are configured correctly. You must also set the --cloud-provider and --cloud-config parameters yourself by editing the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file appropriately.

By default, kubeadm deploys a single node etcd cluster on the master to store Kubernetes state. This means that any failure on the master node requires you to rebuild your cluster from scratch. Currently kubeadm init does not support automatic deployment of a highly available etcd cluster. If you would like to use your own etcd cluster, you can override this behaviour with --external-etcd-endpoints. kubeadm supports etcd client authentication using the --external-etcd-cafile, --external-etcd-certfile and --external-etcd-keyfile flags.

For certain networking solutions the Kubernetes master can also play a role in allocating network ranges (CIDRs) to each node. This includes many cloud providers and flannel. You can specify a subnet range that will be broken down and handed out to each node with the --pod-network-cidr flag. This should be a minimum of a /16 so controller-manager is able to assign /24 subnets to each node in the cluster. If you are using flannel with this manifest you should use --pod-network-cidr=10.244.0.0/16. Most CNI based networking solutions do not require this flag.

You can use the --service-cidr flag to override the subnet Kubernetes uses to assign pods IP addresses. If you do, you will also need to update the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file to reflect this change else DNS will not function correctly.

By default, kubeadm init deploys a cluster that assigns services with DNS names <service_name>.<namespace>.svc.cluster.local. You can use the --service-dns-domain to change the DNS name suffix. Again, you will need to update the /etc/systemd/system/kubelet.service.d/10-kubeadm.conf file accordingly else DNS will not function correctly.

By default, kubeadm runs a series of preflight checks to validate the system before making any changes. Advanced users can use this flag to bypass these if necessary.

By default, kubeadm init automatically generates the token used to initialise each new node. If you would like to manually specify this token, you can use the --token flag. The token must be of the format <6 character string>.<16 character string>.

kubeadm was originally built for Kubernetes version v1.4.0, older versions are not supported. With this flag you can try any future version, e.g. v1.5.0-beta.1 whenever it comes out (check releases page for a full list of available versions).

kubeadm join

When you use kubeadm join, you must supply the token used to secure cluster boostrap as a mandatory flag, and the master IP address as a mandatory argument.

Here’s an example on how to use it:

kubeadm join --token=the_secret_token 192.168.1.1

By default, kubeadm runs a series of preflight checks to validate the system before making any changes. Advanced users can use this flag to bypass these if necessary.

By default, when kubeadm init runs, a token is generated and revealed in the output. That’s the token you should use here.

Automating kubeadm

Rather than copying the token you obtained from kubeadm init to each node, as in the basic kubeadm tutorials, you can parallelize the token distribution for easier automation. To implement this automation, you must know the IP address that the master will have after it is started.

  1. Generate a token. This token must have the form <6 character string>.<16 character string>

    Here is a simple python one-liner for this:

    python -c 'import random; print "%0x.%0x" % (random.SystemRandom().getrandbits(3*8), random.SystemRandom().getrandbits(8*8))'

  2. Start both the master node and the worker nodes concurrently with this token. As they come up they should find each other and form the cluster.

Once the cluster is up, you can grab the admin credentials from the master node at /etc/kubernetes/admin.conf and use that to talk to the cluster.

Environment variables

There are some environment variables that modify the way that kubeadm works. Most users will have no need to set these.

Variable Default Description
KUBE_KUBERNETES_DIR /etc/kubernetes Where most configuration files are written to and read from
KUBE_HOST_PKI_PATH /etc/kubernetes/pki Directory for master PKI assets
KUBE_HOST_ETCD_PATH /var/lib/etcd Local etcd state for Kubernetes cluster
KUBE_HYPERKUBE_IMAGE `` If set, use a single hyperkube image with this name. If not set, individual images per server component will be used.
KUBE_DISCOVERY_IMAGE gcr.io/google_containers/kube-discovery-<arch>:1.0 The bootstrap discovery helper image to use.
KUBE_ETCD_IMAGE gcr.io/google_containers/etcd-<arch>:2.2.5 The etcd container image to use.
KUBE_COMPONENT_LOGLEVEL --v=4 Logging configuration for all Kubernetes components

Releases and release notes

If you already have kubeadm installed and want to upgrade, run apt-get update && apt-get upgrade or yum update to get the latest version of kubeadm.

Troubleshooting

# cat /etc/sysctl.d/k8s.conf
net.bridge.bridge-nf-call-ip6tables = 1
net.bridge.bridge-nf-call-iptables = 1

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