This blog post describes the nuances of how users and groups are configured in Kubernetes and how the role-based access control (RBAC) mechanism applies for them.

We will also dive into the usage of the kubectl command line tool to check API access in Kubernetes. It especially focuses on the difference between the --user and --as options in kubectl.

Prerequisites

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one using minikube.

OpenSSL command line utility will be used to view the x509 certificates.

Role-based access control

Role-based access control (RBAC) is a method of regulating access to computer or network resources based on the roles of individual users within your organisation.

You may choose to go through the Kubernetes RBAC documentation before reading ahead.

Checking API Access

The kubectl auth can-i command can be used to determine whether a user has permissions to execute a certain action.

Scenario for the Default Admin User

In the first example, we will work with the default admin user.

Check the contexts available in the minikube cluster:

kubectl config get-contexts minikube

You should see an output similar to:

CURRENT   NAME       CLUSTER    AUTHINFO   NAMESPACE
*         minikube   minikube   minikube   default

Check the name of the admin user that is created with our basic minikube installation:

kubectl config view -o jsonpath='{.contexts[?(@.name=="minikube")].context.user}'

You should see an output similar to:

minikube

Check the certificate of the minikube user:

kubectl config view -o jsonpath='{.users[?(@.name=="minikube")].user.client-certificate}'

You should see an output similar to:

/Users/adityasamant/.minikube/profiles/minikube/client.crt

View the Subject of this certificate (use the path generated in the previous command):

openssl x509 -in /Users/adityasamant/.minikube/profiles/minikube/client.crt -text -noout | grep Subject | grep -v "Public Key Info"

You should see an output similar to:

Subject: O=system:masters, CN=minikube-user

Use the kubectl auth can-i command to verify a few scenarios.

Check permissions to create pods:

kubectl auth can-i create pods

yes

Check permissions to create deployments:

kubectl auth can-i create deployments

yes

Check permissions to delete secrets:

kubectl auth can-i delete secrets

yes

Scenario for a Normal User

Configure a normal user and verify how the kubectl auth can-i commands can be used to check the access. To do this we need to issue a certificate for the user.

Create a private key and a csr file:

openssl genrsa -out jane.key 2048
openssl req -new -key jane.key -out jane.csr -subj "/CN=jane"

This will generate a private key named jane.key and a certificate signing request named jane.csr.

Get the base64 encoded value of the CSR file content:

cat jane.csr | base64 | tr -d "\n"

Create a CertificateSigningRequest:

cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: jane
spec:
  request: <base64 encoded csr>
  signerName: kubernetes.io/kube-apiserver-client
  expirationSeconds: 86400  # one day
  usages:
  - client auth
EOF

Get the CSR:

kubectl get csr jane

NAME   AGE   SIGNERNAME                            REQUESTOR       REQUESTEDDURATION   CONDITION
jane   55s   kubernetes.io/kube-apiserver-client   minikube-user   24h                 Pending

Approve the CSR:

kubectl certificate approve jane

kubectl get csr jane

NAME   AGE     SIGNERNAME                            REQUESTOR       REQUESTEDDURATION   CONDITION
jane   2m17s   kubernetes.io/kube-apiserver-client   minikube-user   24h                 Approved,Issued

Granting permissions via RBAC

Create a clusterrole granting permissions to only create pods:

kubectl create clusterrole createpods --verb=create --resource=pods

clusterrole.rbac.authorization.k8s.io/createpods created

Create a clusterrolebinding to bind the clusterrole with user jane:

kubectl create clusterrolebinding createpods --clusterrole=createpods --user=jane

clusterrolebinding.rbac.authorization.k8s.io/createpods created

Difference between ‘--as’ and ‘--user’ options of kubectl

kubectl has a number of global options that can be passed as an argument to any kubectl command. The list can be found with the following command:

kubectl options

Two of the options are --user and --as. It is important to understand the difference between them.

--user='':
The name of the kubeconfig user to use

--as='':
Username to impersonate for the operation. 
User could be a regular user or a service account in a namespace.

Let’s put the theory into action with the help of the user we created.

Using ‘--as’ to Check Access

Check permissions to create pods:

kubectl auth can-i create pods --as=jane

yes

Check permissions to create deployments:

kubectl auth can-i create deployments --as=jane

no

Check permissions to delete secrets:

kubectl auth can-i delete secrets --as=jane

no

Due to the fact that we explicitly assigned the clusterrole createpods to user jane, we see that jane has access to create pods, but no access to create deployments or delete secrets. Great, this is as expected.

Using ‘--user’ to Check Access

Try the same commands, but this time using the --user option:

kubectl auth can-i create pods --user=jane

error: auth info "jane" does not exist

The command leads to an error. This is because we have not configured the user jane in the kubeconfig file.

We will fix this in the next step.

Adding the user to kubeconfig

Get the user’s certificate:

kubectl get csr jane -o jsonpath='{.status.certificate}'| base64 -d > jane.crt

Add the new credentials to kubeconfig:

kubectl config set-credentials jane --client-key=jane.key --client-certificate=jane.crt --embed-certs=true

User "jane" set.

Add the context to kubeconfig:

kubectl config set-context jane --cluster=minikube --user=jane

Context "jane" created.

Let’s try the kubectl auth can-i command once again to verify the permissions on user jane:

Check permissions to create pods:

kubectl auth can-i create pods --user=jane

yes

Check permissions to create deployments:

kubectl auth can-i create deployments --user=jane

no

Check permissions to delete secrets:

kubectl auth can-i delete secrets --user=jane

no

Now everything works as expected.

Example for a non-existent user

If the --as option is used for a non-existent user, there is no error thrown as shown below.

kubectl auth can-i create pods --as=nobody

no

Scenario for a Custom Admin User

Configure a new admin user and verify the behaviour of the kubectl auth can-i commands.

This time we will check the permissions that a user inherits via the group it is attached to.

Issue a certificate for the new admin user.

Create a private key and a csr file:

openssl genrsa -out poweruser.key 2048
openssl req -new -key poweruser.key -out poweruser.csr -subj "/CN=poweruser/O=system:masters"

Create a CertificateSigningRequest:

cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: poweruser
spec:
  request: <base64 encoded csr>
  signerName: kubernetes.io/kube-apiserver-client
  expirationSeconds: 86400  # one day
  usages:
  - client auth
EOF

The above command throws an error as the kube-apiserver blocks any CertificateSigningRequest that attempts to add a user as part of the system:masters group.

Error from server (Forbidden): error when creating “STDIN”: certificatesigningrequests.certificates.k8s.. “poweruser” is forbidden: use of kubernetes.io/kube-apiserver-client signer with system:masters group is not allowed

Granting permissions via RBAC through groups

In order to create a new admin user we will create a custom admin group that replicates the behaviour of the system:masters group. Let’s call it example:masters

To do this, create a new clusterrolebinding as below:

cat <<EOF | kubectl apply -f -
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: example-cluster-admin
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
- apiGroup: rbac.authorization.k8s.io
  kind: Group
  name: example:masters
EOF

The next step is to add the new admin user to the example:masters group.

Delete the previous files created for poweruser:

rm poweruser.key poweruser.csr

Create a new private key and a csr file:

openssl genrsa -out poweruser.key 2048
openssl req -new -key poweruser.key -out poweruser.csr -subj "/CN=poweruser/O=example:masters"

Create a CertificateSigningRequest:

cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: poweruser
spec:
  request: <base64 encoded csr>
  signerName: kubernetes.io/kube-apiserver-client
  expirationSeconds: 86400  # one day
  usages:
  - client auth
EOF

Approve the CSR:

kubectl certificate approve poweruser

certificatesigningrequest.certificates.k8s.io/poweruser approved

Get the certificate:

kubectl get csr poweruser -o jsonpath='{.status.certificate}'| base64 -d > poweruser.crt

View the Subject of this certificate:

openssl x509 -in poweruser.crt -text -noout | grep Subject | grep -v "Public Key Info"

Subject: O=example:masters, CN=poweruser

Add the new admin user to kubeconfig.

Add the new credentials:

kubectl config set-credentials poweruser --client-key=poweruser.key --client-certificate=poweruser.crt --embed-certs=true

User "poweruser" set.

Add the context:

kubectl config set-context poweruser --cluster=minikube --user=poweruser

Context "poweruser" created.

Try the kubectl auth can-i command to verify the permissions on the new admin user:

Check permissions to create pods:

kubectl auth can-i create pods --user=poweruser

yes

Check permissions to create deployments:

kubectl auth can-i create deployments --user=poweruser

yes

Check permissions to delete secrets:

kubectl auth can-i delete secrets --user=poweruser

yes

Try the same commands but with using the --as option.

Check permissions to create pods:

kubectl auth can-i create pods --as=poweruser

no

Check permissions to create deployments:

kubectl auth can-i create deployments --as=poweruser

no

Check permissions to delete secrets:

kubectl auth can-i delete secrets --as=poweruser

no

Strange!!! The expected output was yes for all 3 commands, as poweruser is an admin user with full access to the cluster.

We can prove this as follows:

Switch the context to work with the poweruser:

kubectl config use-context poweruser

Create a pod:

kubectl run nginx --image=nginx

pod/nginx created

Create the deployment:

kubectl create deployment nginx-deploy --image=nginx

deployment.apps/nginx-deploy created

Create and delete a secret:

kubectl create secret generic test-secret --from-literal=secret=1234

secret/test-secret created

kubectl delete secrets test-secret

secret "test-secret" deleted

My first thought was that this is a defect in Kubernetes.

I raised issue #122579 to Kubernetes for confirmation.

The reality is that the behaviour is as-expected. The API server has no knowledge of group membership apart from what is encoded directly in the credential or provided by a token webhook.

To overcome this you need to pass the group you want to impersonate with the --as-group flag.

The ‘--as-group’ option of kubectl

Try the same commands but this time append the --as-group option as well.

Check permissions to create pods:

kubectl auth can-i create pods --as=poweruser --as-group=example:masters

yes

Check permissions to create deployments:

kubectl auth can-i create deployments --as=poweruser --as-group=example:masters

yes

Check permissions to delete secrets:

kubectl auth can-i delete secrets --as=poweruser --as-group=example:masters

yes

Now the results are as expected.

Summary

The kubectl auth can-i command behaves differently for the --user and --as options.

The --user option checks for the actual presence of the user in the kubeconfig file and has the ability to check permissions derived from the group of the user.

The --as option can be used to check permissions for any user irrespective of its presence in the kubeconfig file. It checks permissions which are directly bound to the user through RBAC, and does not check permissions that are derived from the user’s group. The API server has no knowledge of group membership apart from whatever is encoded directly in the credential or provided by a token webhook.

The --as-group option should be used to check for permissions that are derived from the user’s group.

Cleaning up

Delete the resources created during this lab:

rm jane*
rm poweruser*
kubectl delete pod nginx
kubectl delete deployment nginx-deploy

Optionally, you can delete the entire minikube cluster:

minikube delete --all