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< Kubernetes

API Server and Role Base Access Control

To prevent unauthorized users from modifying the cluster state, RBAC is used, defining roles and role bindings for a user. A service account resource is created for a pod to determine how it has control over the cluster state. For example, the default service account will not allow you to list the services in a namespace.

The Kubernetes API server provides CRUD actions (Create, Read, Update, Delete) interface for interacting with cluster state over a RESTful API. API calls can come only from 2 sources:

  • kubectl
  • POD

There is 4 stage process

  1. Authentication
  2. Authorization
  3. Admission
  4. Writing the configuration state CRUD actions to persistent store etcd database

Example plugins:

  • serviceaccount plugin applies default serviceaccount to pods that don't explicitly specify

RBAC is managed by 4 resources, divided over 2 groups

RBAC resources
Group-1 namespace resources Group-2 cluster level resources resources type
roles cluster roles defines what can be done
role bindings cluster role bindings defines who can do it

When deploying a pod a default serviceaccount is assigned if not specified in the pod manifest. The serviceaccount represents an identity of an app running on a pod. Token file holds authentication token. Let's create a namespace and create a test pod to try to list available services.

kubectl create ns rbac
kubectl run apitest --image=nginx -n rbac #create test container, to run API call test from

Each pod has serviceaccount, the API authentication token is on a pod. When a pod makes API call uses the token, this allows to assumes the serviceaccount, so it gets identity. You can preview the token on the pod.

kubectl -n rbac1 exec -it apitest-<UID> -- /bin/sh  #connect to the container shell

#display token and namespace that allows to connect to API server from this pod
root$ cat /var/run/secrets/{token,namespace} 

#call API server to list K8s services in 'rbac' namespace
root$ curl localhost:8001/api/v1/namespaces/rbac/services

List all serviceaccounts. Serviceaccounts can only be used within the same namespace.

kubectl get serviceaccounts -n rbac
kubectl get secrets
NAME                  TYPE                                  DATA   AGE
default-token-qqzc7   3      39h
kubectl get secrets default-token-qqzc7 -o yaml #display secrets