Kubernetes Admission Controller Webhook Demo
This repository contains a small HTTP server that can be used as a Kubernetes MutatingAdmissionWebhook.
The logic of this demo webhook is fairly simple: it enforces more secure defaults for running
containers as non-root user. While it is still possible to run containers as root, the webhook
ensures that this is only possible if the setting runAsNonRoot
is explicitly set to false
in the securityContext
of the Pod. If no value is set for runAsNonRoot
, a default of true
is applied, and the user ID defaults to 1234
.
Prerequisites
A cluster on which this example can be tested must be running Kubernetes 1.9.0 or above,
with the admissionregistration.k8s.io/v1beta1
API enabled. You can verify that by observing that the
following command produces a non-empty output:
kubectl api-versions | grep admissionregistration.k8s.io/v1beta1
In addition, the MutatingAdmissionWebhook
admission controller should be added and listed in the admission-control
flag of kube-apiserver
.
For building the image, GNU make and Go are required.
Deploying the Webhook Server
- Bring up a Kubernetes cluster satisfying the above prerequisites, and make
sure it is active (i.e., either via the configuration in the default location, or by setting
the
KUBECONFIG
environment variable). - Run
./deploy.sh
. This will create a CA, a certificate and private key for the webhook server, and deploy the resources in the newly createdwebhook-demo
namespace in your Kubernetes cluster.
Verify
- The
webhook-server
pod in thewebhook-demo
namespace should be running:
$ kubectl -n webhook-demo get pods
NAME READY STATUS RESTARTS AGE
webhook-server-6f976f7bf-hssc9 1/1 Running 0 35m
- A
MutatingWebhookConfiguration
nameddemo-webhook
should exist:
$ kubectl get mutatingwebhookconfigurations
NAME AGE
demo-webhook 36m
- Deploy a pod that neither sets
runAsNonRoot
norrunAsUser
:
$ kubectl create -f examples/pod-with-defaults.yaml
Verify that the pod has default values in its security context filled in:
$ kubectl get pod/pod-with-defaults -o yaml
...
securityContext:
runAsNonRoot: true
runAsUser: 1234
...
Also, check the logs that the pod had in fact been running as a non-root user:
$ kubectl logs pod-with-defaults
I am running as user 1234
- Deploy a pod that explicitly sets
runAsNonRoot
tofalse
, allowing it to run as theroot
user:
$ kubectl create -f examples/pod-with-override.yaml
$ kubectl get pod/pod-with-override -o yaml
...
securityContext:
runAsNonRoot: false
...
$ kubectl logs pod-with-override
I am running as user 0
- Attempt to deploy a pod that has a conflicting setting:
runAsNonRoot
set totrue
, butrunAsUser
set to 0 (root). The admission controller should block the creation of that pod.
$ kubectl create -f examples/pod-with-conflict.yaml
Error from server (InternalError): error when creating "examples/pod-with-conflict.yaml": Internal error
occurred: admission webhook "webhook-server.webhook-demo.svc" denied the request: runAsNonRoot specified,
but runAsUser set to 0 (the root user)
Build the Image from Sources (optional)
An image can be built by running make
.
If you want to modify the webhook server for testing purposes, be sure to set and export
the shell environment variable IMAGE
to an image tag for which you have push access. You can then
build and push the image by running make push-image
. Also make sure to change the image tag
in deployment/deployment.yaml.template
, and if necessary, add image pull secrets.