Error from server (Forbidden):User "system: anonymous" cannot list nodes at the cluster scope even after granting permission

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06-10-2024

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"Forbidden" Error in Kubernetes: Why Anonymous Users Can't List Nodes

Kubernetes clusters often need to interact with external tools or services, leading to the use of "anonymous" users. This enables these external entities to interact with the cluster without requiring explicit user authentication. However, you might encounter a frustrating "Forbidden" error when attempting to perform basic operations like listing nodes.

This article aims to explain why this error occurs and how to grant the necessary permissions for anonymous users to access node information.

Scenario and Code:

Imagine you're trying to access the list of nodes in your cluster from an external script or tool. The script, using the kubectl command, attempts to retrieve the node list with the following command:

kubectl get nodes

The response you get, instead of the expected list of nodes, is a cryptic "Error from server (Forbidden): User "system: anonymous" cannot list nodes at the cluster scope."

Why Does This Happen?

Kubernetes follows the principle of least privilege, meaning users and entities should only have access to the resources they absolutely need. By default, anonymous users (those who don't authenticate with the cluster) are assigned minimal permissions. This helps maintain the security and integrity of the cluster.

Providing Access:

To allow anonymous users to list nodes, you need to explicitly grant them the necessary permissions. This is done through Role-Based Access Control (RBAC).

Here's how to achieve this:

1. Create a Role: Define a role named node-viewer that allows only listing nodes:

   apiVersion: rbac.authorization.k8s.io/v1
   kind: Role
   metadata:
     name: node-viewer
   rules:
   - apiGroups: ["""] # "" indicates the core API group
     resources: ["nodes"]
     verbs: ["get", "list", "watch"]
   

2. Create a RoleBinding: Bind the node-viewer role to the system:anonymous user:

   apiVersion: rbac.authorization.k8s.io/v1
   kind: RoleBinding
   metadata:
     name: node-viewer-binding
   subjects:
   - kind: User
     name: system:anonymous
   roleRef:
     kind: Role
     name: node-viewer
     apiGroup: rbac.authorization.k8s.io
   

3. Apply the YAML files: Apply these YAML files to your cluster using kubectl:

   kubectl apply -f node-viewer.yaml
   kubectl apply -f node-viewer-binding.yaml 
   

Important Considerations:

• Security: While granting anonymous users access to list nodes might be necessary for certain use cases, carefully consider the potential security implications.
• Scope: The node-viewer role only grants access to list nodes. If you need anonymous users to interact with nodes in other ways (e.g., create, update, delete), you'll need to adjust the rules in the role definition.
• Alternatives: If possible, avoid using anonymous access altogether. Explore alternatives like creating a dedicated service account or using authenticated user credentials.

Additional Resources:

• Kubernetes RBAC documentation
• Kubernetes Roles and RoleBindings

Conclusion:

Understanding the principles of RBAC and applying them correctly can help you overcome the "Forbidden" error when dealing with anonymous users in Kubernetes. Carefully evaluate your security needs and grant the minimum level of access necessary for your specific use cases. By following these steps and considering security implications, you can ensure a secure and efficient Kubernetes environment.