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Creating a kubeconfig file for multi cluster access

To access a kubernetes/openshift cluster via a kubeconfig file, we need a cluster context to be defined. Each cluster context is a combination of:

  • cluster
  • namespace
  • user

To check the existing config:

kubectl config view

Each cluster has a set of cluster contexts already defined in the default kubeconfig and it can be viewed using:

kubectl config get-contexts

CURRENT   NAME                          CLUSTER      AUTHINFO           NAMESPACE
*         kubernetes-admin@kubernetes   kubernetes   kubernetes-admin

The cluster context defines the access parameters for a user on how they can access a cluster. Follow the next section to understand a kubeconfig file.

Alternatively, if you already understand how a kubeconfig file works, jump to this section to create a multi-cluster config.

Anatomy of a kubeconfig file

To view the existing kubeconfig for a cluster, either check the $HOME/.kube/config file or do this:

kubectl config view

Here's how a kubeconfig file looks like:

apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: <ca.cert>
    server: https://10.10.10.10:6443
  name: kubernetes
contexts:
- context:
    cluster: kubernetes
    user: kubernetes-admin
  name: kubernetes-admin@kubernetes
current-context: kubernetes-admin@kubernetes
kind: Config
preferences: {}
users:
- name: kubernetes-admin
  user:
    client-certificate-data: <client.cert>
    client-key-data: <client.key>

It has 3 sections:

  • clusters: This section contains a list of kubernetes/openshift clusters.
    • cluster.certificate-authority-data: CA cert of the server
    • cluster.server: URL of the kubernetes/openshift API server
    • name: name of this cluster that it should be identified with.
  • users: This section is where we define a list of users and their credentials. The user credentials determine what resources and verbs a user has access to in a given cluster.
    • user.client-certificate-data: client certificate for the user.
    • user.client-key-data: client key for the user.
    • name: name of this user that it should be identified with.
  • contexts: This is where we define the cluster accesses by associating a cluster, a user and a namespace. This cluster context is then used to access the resources of a kubernetes/openshift cluster.
    • context.cluster: Name of the cluster as defined in the clusters section.
    • context.user: Name of the user as defined in the users section.
    • name: name of this context that it should be identified with.

Choosing a cluster context

A kubernetes/openshift client has to run within a cluster context. kubectl runs with a pre-defined and a default set of kubeconfig contexts. To see the list of available contexts with kubectl, use this:

kubectl config get-contexts

CURRENT   NAME                          CLUSTER      AUTHINFO           NAMESPACE
*         kubernetes-admin@kubernetes   kubernetes   kubernetes-admin   
          user@kubernetes               kubernetes   kubernetes-user

To use a context out of this list, just use:

kubectl config use-context user@kubernetes

Creating a multi cluster kubeconfig file

So, with the understanding of the cluster contexts, let's now see, for two clusters cluster1 and cluster2, how we can create cluster contexts for both, to access the required resources in a single kubeconfig file. The auth information can be obtained from the respective clusters' kubeconfigs.

clusters

In the clusters section, we define two clusters: cluster1 and cluster2. For each of them, we provide the API server's URL, their ca.certs and their names. The names are user-defined.

clusters:
- cluster:
    certificate-authority-data: <cluster1-ca.cert>
    server: https://10.10.10.10:6443
  name: cluster1
- cluster:
    certificate-authority-data: <cluster2-ca.cert>
    server: https://10.10.10.11:6443
  name: cluster2

users

In the users section, we define two users, one for each cluster: c1-admin is for cluster1. Again, the name c1-admin is user-defined and can be anything (just has to be unique in the users section). We also obtain the client.cert and client.key data for this user from cluster 1's environment. Similarly, we also define another user which is for cluster2. We name this auth information as c2-admin.

users:
- name: c1-admin
  user:
    client-certificate-data: <client.cert>
    client-key-data: <client.key>
- name: c2-admin
  user:
    client-certificate-data: <client.cert>
    client-key-data: <client.key>

contexts

After defining the clusters and the users, its now time to define the contexts. We associate the cluster1 cluster with user c1-admin and name this context as cluster1-admin. We also associate cluster2 cluster with the user c2-admin and name this context as cluster2-admin.

contexts:
- context:
    cluster: cluster1
    user: c1-admin
  name: cluster1-admin
- context:
    cluster: cluster2
    user: c2-admin
  name: cluster2-admin

And, we are done! Let's combine all of the above into a single config file.

complete kubeconfig

apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: <cluster1-ca.cert>
    server: https://10.10.10.10:6443
  name: cluster1
- cluster:
    certificate-authority-data: <cluster2-ca.cert>
    server: https://10.10.10.11:6443
  name: cluster2

contexts:
- context:
    cluster: cluster1
    user: c1-admin
  name: cluster1-admin
- context:
    cluster: cluster2
    user: c2-admin
  name: cluster2-admin

kind: Config
preferences: {}

users:
- name: c1-admin
  user:
    client-certificate-data: <client.cert>
    client-key-data: <client.key>
- name: c2-admin
  user:
    client-certificate-data: <client.cert>
    client-key-data: <client.key>

Let's save this file as gslb-members.

To verify that contexts are fine, we can use this with the kubectl client from anywhere (as long as both the API servers are reachable):

kubectl --kubeconfig gslb-members config get-contexts

CURRENT   NAME             CLUSTER    AUTHINFO     NAMESPACE
          cluster1-admin   cluster1   c1-admin     default
          cluster2-admin   cluster2   c2-admin     default

This file can then be used to create a secret to be used by the AMKO pod.