Introduction to Kubernetes: RBAC
Create Kubernetes cluster
kind create cluster --name rbac --image kindest/node:v1.20.2
Kubernetes CA Certificate
Kubernetes does not have a concept of users, instead it relies on certificates and would only
trust certificates signed by its own CA.
To get the CA certificates for our cluster, easiest way is to access the master node.
Because we run on kind, our master node is a docker container.
The CA certificates exists in the /etc/kubernetes/pki folder by default.
If you are using minikube you may find it under ~/.minikube/.
Access the master node:
docker exec -it rbac-control-plane bash
ls -l /etc/kubernetes/pki
total 60
-rw-r--r-- 1 root root 1135 Sep 10 01:38 apiserver-etcd-client.crt
-rw------- 1 root root 1675 Sep 10 01:38 apiserver-etcd-client.key
-rw-r--r-- 1 root root 1143 Sep 10 01:38 apiserver-kubelet-client.crt
-rw------- 1 root root 1679 Sep 10 01:38 apiserver-kubelet-client.key
-rw-r--r-- 1 root root 1306 Sep 10 01:38 apiserver.crt
-rw------- 1 root root 1675 Sep 10 01:38 apiserver.key
-rw-r--r-- 1 root root 1066 Sep 10 01:38 ca.crt
-rw------- 1 root root 1675 Sep 10 01:38 ca.key
drwxr-xr-x 2 root root 4096 Sep 10 01:38 etcd
-rw-r--r-- 1 root root 1078 Sep 10 01:38 front-proxy-ca.crt
-rw------- 1 root root 1679 Sep 10 01:38 front-proxy-ca.key
-rw-r--r-- 1 root root 1103 Sep 10 01:38 front-proxy-client.crt
-rw------- 1 root root 1675 Sep 10 01:38 front-proxy-client.key
-rw------- 1 root root 1679 Sep 10 01:38 sa.key
-rw------- 1 root root 451 Sep 10 01:38 sa.pub
exit the container
Copy the certs out of our master node:
cd kubernetes/rbac
docker cp rbac-control-plane:/etc/kubernetes/pki/ca.crt ca.crt
docker cp rbac-control-plane:/etc/kubernetes/pki/ca.key ca.key
Kubernetes Users
As mentioned before, Kubernetes has no concept of users, it trusts certificates that is signed by its CA.
This allows a lot of flexibility as Kubernetes lets you bring your own auth mechanisms, such as OpenID Connect or OAuth.
This allows managed Kubernetes offerings to use their cloud logins to authenticate.
So on Azure, I can use my Microsoft account, GKE my Google account and AWS EKS my Amazon account.
You will need to consult your cloud provider to setup authentication.
Example Azure AKS
User Certificates
First thing we need to do is create a certificate signed by our Kubernetes CA.
We have the CA, let's make a certificate.
Easy way to create a cert is use openssl and the easiest way to get openssl is to simply run a container:
docker run -it -v ${PWD}:/work -w /work -v ${HOME}:/root/ --net host alpine sh
apk add openssl
Let's create a certificate for Bob Smith:
#start with a private key
openssl genrsa -out bob.key 2048
Now we have a key, we need a certificate signing request (CSR).
We also need to specify the groups that Bob belongs to.
Let's pretend Bob is part of the Shopping team and will be developing
applications for the Shopping
openssl req -new -key bob.key -out bob.csr -subj "/CN=Bob Smith/O=Shopping"
Use the CA to generate our certificate by signing our CSR.
We may set an expiry on our certificate as well
openssl x509 -req -in bob.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out bob.crt -days 1
Building a kube config
Let's install kubectl in our container to make things easier:
apk add curl nano
curl -LO https://storage.googleapis.com/kubernetes-release/release/`curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt`/bin/linux/amd64/kubectl
chmod +x ./kubectl
mv ./kubectl /usr/local/bin/kubectl
We'll be trying to avoid messing with our current kubernetes config.
So lets tell kubectl to look at a new config that does not yet exists
export KUBECONFIG=~/.kube/new-config
Create a cluster entry which points to the cluster and contains the details of the CA certificate:
kubectl config set-cluster dev-cluster --server=https://127.0.0.1:52807 \
--certificate-authority=ca.crt \
--embed-certs=true
#see changes
nano ~/.kube/new-config
kubectl config set-credentials bob --client-certificate=bob.crt --client-key=bob.key --embed-certs=true
kubectl config set-context dev --cluster=dev-cluster --namespace=shopping --user=bob
kubectl config use-context dev
kubectl get pods Error from server (Forbidden): pods is forbidden: User "Bob Smith" cannot list resource "pods" in API group "" in the namespace "shopping"
Give Bob Smith Access
cd kubernetes/rbac
kubectl create ns shopping
kubectl -n shopping apply -f .\role.yaml
kubectl -n shopping apply -f .\rolebinding.yaml
Test Access as Bob
kubectl get pods No resources found in shopping namespace.
Kubernetes Service Accounts
So we've covered users, but what about applications or services running in our cluster ?
Most business apps will not need to connect to the kubernetes API unless you are building something that integrates with your cluster, like a CI/CD tool, an autoscaler or a custom webhook.
Generally applications will use a service account to connect.
You can read more about Kubernetes Service Accounts.
Let's deploy a service account
kubectl -n shopping apply -f serviceaccount.yaml
Now we can deploy a pod that uses the service account
kubectl -n shopping apply -f pod.yaml
Now we can test the access from within that pod by trying to list pods:
kubectl -n shopping exec -it shopping-api -- bash
# Point to the internal API server hostname
APISERVER=https://kubernetes.default.svc
# Path to ServiceAccount token
SERVICEACCOUNT=/var/run/secrets/kubernetes.io/serviceaccount
# Read this Pod's namespace
NAMESPACE=$(cat ${SERVICEACCOUNT}/namespace)
# Read the ServiceAccount bearer token
TOKEN=$(cat ${SERVICEACCOUNT}/token)
# Reference the internal certificate authority (CA)
CACERT=${SERVICEACCOUNT}/ca.crt
# List pods through the API
curl --cacert ${CACERT} --header "Authorization: Bearer $TOKEN" -s ${APISERVER}/api/v1/namespaces/shopping/pods/
# we should see an error not having access
Now we can allow this pod to list pods in the shopping namespace
kubectl -n shopping apply -f serviceaccount-role.yaml
kubectl -n shopping apply -f serviceaccount-rolebinding.yaml
If we try run curl command again we can see now we are able to get a json
response with pod information