Demo Validating Admission Webhook created with Go

Kubernetes documentation about admission webhooks: https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers

Generate CA and sign a cert:

openssl genrsa -out certs/tls.key 2048
openssl req -new -key certs/tls.key -out certs/tls.csr -subj "/CN=demo-webhook-svc.default.svc"
openssl x509 -req -extfile <(printf "subjectAltName=DNS:demo-webhook-svc.default.svc") -in certs/tls.csr -signkey certs/tls.key -out certs/tls.crt

kubectl create secret tls demo-webhook-tls \
    --cert "certs/tls.crt" \
    --key "certs/tls.key" -n default 

kubectl get secret demo-webhook-tls -n default -oyaml > k8s/tls-secret.yaml

ENCODED_CA=$(cat certs/tls.crt | base64 | tr -d '\n')

ValidatingWebhookConfiguration:

The service default path is /, our server listens to /validate on port 443 https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#service-reference

How to run this demo

1. Docker and Go installed (optionally tmux and k9s for task demo)
2. Install taskfile
3. Generate the CA and certs as explained in the first section or use CertManager
4. Run task demo
5. Run task k8s-up
6. Try to deploy the example pods allowed-pod.yaml and unallowed-pod.yaml
7. See the messages you recieve when trying to apply the unallowed-pod yaml
8. In the admission-go pod you can see the logs that show what as sent as a request and the reason for the aproval/rejection

Possible Improvements

• verify the resource is a pod
• check all containers in the pod not only the first (main.go#L72)
• Check for a minum resource value
• CertManager CA injector as an automated alternative of generating the certs