Troubleshoot GKE on Bare Metal webhook issues

This page shows you how to resolve issues with problematic or unsafe webhooks in Google Distributed Cloud Virtual for Bare Metal.

If you need additional assistance, reach out to Cloud Customer Care.

Types of problematic webhooks

Admission webhooks, or webhooks in Kubernetes, are a type of admission controller that can be used in Kubernetes clusters to validate or mutate requests to the control plane prior to a request being persisted. It is common for third-party applications to use webhooks that operate on system-critical resources and namespaces. Incorrectly configured webhooks can impact control plane performance and reliability. For example, an incorrectly configured webhook created by a third-party application could prevent Google Distributed Cloud Virtual for Bare Metal from creating and modifying resources in the managed kube-system namespace, which could degrade the functionality of the cluster.

Problematic webhooks include the following types:

Webhooks that operate, but have no endpoints available. Follow the instructions to check webhooks with no available endpoints.
Webhooks that are considered unsafe as they operate on system critical resources and namespaces.

The following webhooks are considered unsafe:
- Webhooks that intercept Pods and leases in the kube-system namespace.
- Webhooks that intercept leases in the kube-node-lease namespace.
- Webhooks that intercept Nodes, TokenReviews, SubjectAccessReviews, and CertificateSigningRequests resources.
Follow the instructions to check webhooks that are considered unsafe.

Webhooks that have no available endpoints

If a webhook has no available endpoints, the Service that backs the webhook endpoint has one or more Pods which aren't running. To make the webhook endpoints available, follow the instructions to find and troubleshoot the Pods of the Service that is backing this webhook endpoint:

Find the serving Pods for the Service associated with the webhook. Run the following command to describe the Service:
```
kubectl describe svc SERVICE_NAME -n SERVICE_NAMESPACE
```
Replace the following:
- SERVICE_NAME with the name of the Service.
- SERVICE_NAMESPACE with the name of the namespace.
If you can't find the Service name listed in the webhook, the unavailable endpoint might be caused by a mismatch between the name listed in the configuration and the actual name of the Service. To fix the endpoint availability, update the Service name in the webhook configuration to match the correct Service object.
Inspect the serving Pods for this Service. Identify which Pods aren't running by listing the Deployment:
```
kubectl get deployment -n SERVICE_NAMESPACE
```
Or, run the following command to list the Pods:
```
kubectl get pods -n SERVICE_NAMESPACE -o wide
```
For any Pods that aren't running, inspect the Pod logs to see why the Pod isn't running.

Webhooks that are considered unsafe

If a webhook intercepts any resources in system-managed namespaces, we recommend that you update the webhooks to avoid intercepting these resources.

Inspect the webhook configuration. Run the following kubectl command to get the webhook configuration:
```
kubectl get validatingwebhookconfigurations CONFIGURATION_NAME -o yaml
```
Replace CONFIGURATION_NAME with the name of the webhook configuration.

If this command doesn't return anything, run the command again, replacing validatingwebhookconfigurations with mutatingwebhookconfigurations.

In the webhooks section of the output, one or more webhooks are listed.
Edit the configuration, depending on the reason the webhook is considered unsafe:

Exclude kube-system and kube-node-lease namespaces

A webhook is considered unsafe if scope is *, or if scope is Namespaced and either of the following conditions are true:
- The operator condition is NotIn and values omits kube-system and kube-node-lease, as in the following example:
```
webhooks:
- admissionReviewVersions:
  ...
  namespaceSelector:
    matchExpressions:
    - key: kubernetes.io/metadata.name
      operator: NotIn
      values:
      - blue-system # add 'kube-system' and 'kube-node-lease' if `NotIn`
  objectSelector: {}
  rules:
  - apiGroups:
    ...
    scope: '*' # 'Namespaced'
  sideEffects: None
  timeoutSeconds: 3
```
  Ensure that scope is set to Namespaced, not *, so that the webhook only operates in specific namespaces. Ensure that if operator is NotIn, kube-system and kube-node-lease are included in values.
- The operator condition is In and values includes kube-system and kube-node-lease, as in the following example:
```
namespaceSelector:
    matchExpressions:
    - key: kubernetes.io/metadata.name
      operator: In
      values:
      - blue-system
      - kube-system # remove as operator is `In`
      - kube-node-lease # remove as operator is `In`
```
  Ensure that scope is set to Namespaced, not *, so that the webhook only operates in specific namespaces. Ensure that if operator is In, kube-system and kube-node-lease are not included in values.
Exclude matched resources

A webhook is also considered unsafe if nodes, tokenreviews, subjectaccessreviews, or certificatesigningrequests are listed under resources, as in the following example:
```
- admissionReviewVersions:
...
    resources:
    - 'pods' # keep, remove everything else
    - 'nodes'
    - 'tokenreviews'
    - 'subjectacessreviews'
    - 'certificatesigningrequests'
    scope: '*'
  sideEffects: None
  timeoutSeconds: 3
```
Remove nodes, tokenreviews, subjectaccessreviews, and certificatesigningrequests from the resource section.

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