How to obtain the enable admission controller list in kubernetes? - kubernetes

Question

AFAIK, the admission controller is the last pass before the submission to the database.
However I cannot know which one is enabled, Is there a way to know which one is taking effect?
Thanks.

Answer 1

The kube-apiserver is running in your kube-apiserver-< example.com > container.
The application does not have a get method at the moment to obtain the enabled admission plugins, but you can get the startup parameters from its command line.
kubectl -n kube-system describe po kube-apiserver-example.com
Another way, to see what is in the container: unfortunately there is no "ps" command in the container, but you can get the initial process command parameters from /proc , something like that:
kubectl -n kube-system exec kube-apiserver-example.com -- sed 's/--/\n/g' /proc/1/cmdline
It will be probably like :
enable-admission-plugins=NodeRestriction

Answer 2

There isn't an admissionscontroller k8s object exposed directly in kubectl.
To get a list of admissions controllers, you have to hit the k8s master API directly with the right versions supported by your k8s installation:
kubectl get --raw /apis/admissionregistration.k8s.io/v1/validatingwebhookconfigurations | jq
For our environment, we run open policy agent as an admissions controller and we can see the webhook object here:
kubectl get --raw /apis/admissionregistration.k8s.io/v1/validatingwebhookconfigurations | jq '.items[] | select(.metadata.name=="open-policy-agent-latest-helm-opa")'
Which outputs the JSON object:
{
"metadata": {
"name": "open-policy-agent-latest-helm-opa",
"selfLink": "/apis/admissionregistration.k8s.io/v1/validatingwebhookconfigurations/open-policy-agent-latest-helm-opa",
"uid": "02139b9e-b282-4ef9-8017-d698bb13882c",
"resourceVersion": "150373119",
"generation": 93,
"creationTimestamp": "2021-03-18T06:22:54Z",
"labels": {
"app": "open-policy-agent-latest-helm-opa",
"app.kubernetes.io/managed-by": "Helm",
"chart": "opa-1.14.6",
"heritage": "Helm",
"release": "open-policy-agent-latest-helm-opa"
},
"annotations": {
"meta.helm.sh/release-name": "open-policy-agent-latest-helm-opa",
"meta.helm.sh/release-namespace": "open-policy-agent-latest"
},
"managedFields": [
{
"manager": "Go-http-client",
"operation": "Update",
"apiVersion": "admissionregistration.k8s.io/v1beta1",
"time": "2021-03-18T06:22:54Z",
"fieldsType": "FieldsV1",
"fieldsV1": {
"f:metadata": {
"f:annotations": {
".": {},
"f:meta.helm.sh/release-name": {},
"f:meta.helm.sh/release-namespace": {}
},
"f:labels": {
".": {},
"f:app": {},
"f:app.kubernetes.io/managed-by": {},
"f:chart": {},
"f:heritage": {},
"f:release": {}
}
},
"f:webhooks": {
".": {},
"k:{\"name\":\"webhook.openpolicyagent.org\"}": {
".": {},
"f:admissionReviewVersions": {},
"f:clientConfig": {
".": {},
"f:caBundle": {},
"f:service": {
".": {},
"f:name": {},
"f:namespace": {},
"f:port": {}
}
},
"f:failurePolicy": {},
"f:matchPolicy": {},
"f:name": {},
"f:namespaceSelector": {
".": {},
"f:matchExpressions": {}
},
"f:objectSelector": {},
"f:rules": {},
"f:sideEffects": {},
"f:timeoutSeconds": {}
}
}
}
}
]
},
"webhooks": [
{
"name": "webhook.openpolicyagent.org",
"clientConfig": {
"service": {
"namespace": "open-policy-agent-latest",
"name": "open-policy-agent-latest-helm-opa",
"port": 443
},
"caBundle": "LS0BLAH="
},
"rules": [
{
"operations": [
"*"
],
"apiGroups": [
"*"
],
"apiVersions": [
"*"
],
"resources": [
"namespaces"
],
"scope": "*"
}
],
"failurePolicy": "Ignore",
"matchPolicy": "Exact",
"namespaceSelector": {
"matchExpressions": [
{
"key": "openpolicyagent.org/webhook",
"operator": "NotIn",
"values": [
"ignore"
]
}
]
},
"objectSelector": {},
"sideEffects": "Unknown",
"timeoutSeconds": 20,
"admissionReviewVersions": [
"v1beta1"
]
}
]
}
You can see from above the clientConfig endpoint in k8s which is what the admissions payload is sent to. Tail the logs of the pods that serve that endpoint and you'll see your admissions requests being processed.
To get mutating webhooks, hit the version of the API of interest again:
# get v1 mutating webhook configurations
kubectl get --raw /apis/admissionregistration.k8s.io/v1/mutatingwebhookconfigurations | jq

Answer 3

This is the official explanation:
https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/#which-plugins-are-enabled-by-default
Notes: You should get the stdout by exec in container
kubectl exec -it kube-apiserver-your-machine-name -n kube-system -- kube-apiserver -h | grep enable-admission-plugins

Answer 4

You may find the list of default enabled admission controllers in doc:
https://kubernetes.io/docs/reference/command-line-tools-reference/kube-apiserver/#options, search for "--enable-admission-plugins";
or equivalently in code:
https://github.com/kubernetes/kubernetes/blob/master/pkg/kubeapiserver/options/plugins.go#L131-L145
For customized ones, you may run cmd in any master node:
cat /etc/kubernetes/manifests/kube-apiserver.yaml | grep -E "(enable|disable)-admission-plugins".

Answer 5

ImagePolicyWebhook uses a configuration file to set options for the behavior of the backend
Create one of these pods by running kubectl create -f examples/<name>.yaml. In this you can verify the user id under which the pod ran by inspecting the logs, for example:
$ kubectl create -f examples/pod-with-defaults.yaml
$ kubectl logs pod-with-defaults

Answer 6

Not sure why it was not stated before, but it's even in the kubernetes docs:
kubectl exec -it kube-apiserver-<your-machine-name> -n kube-system -- kube-apiserver -h | grep enable-admission-plugins
It does exactly what you want.