Using kubernetes constraints.gatekeeper.sh with ingress

Question

Pulumi · Accepted Answer

Kubernetes, combined with the Open Policy Agent's Gatekeeper project, enables you to enforce policies on your cluster, ensuring that certain rules are followed before the Kubernetes API server will accept a resource. This can be used, for example, to enforce specific standards for `Ingress` resources to ensure they meet the requirements for security, naming, annotations, or other custom rules.

To use constraints with Gatekeeper for `Ingress` resources, you typically follow these steps:

1. **Deploy Gatekeeper**: Ensure that the Gatekeeper system is installed in your cluster. It includes a set of Kubernetes custom resource definitions (CRDs) and a controller to process and enforce the policies.
   
2. **Define ConstraintTemplates**: Write and deploy `ConstraintTemplates` which are Kubernetes CRDs that tell Gatekeeper what kind of policy you want to enforce, and provide it with the logic to do so.

3. **Create Constraints**: For each `ConstraintTemplate`, you create an instance of that template called a `Constraint`. This will be a Kubernetes resource that specifies the parameters of the policy you want to enforce with that template.

4. **Apply Ingress Resources**: Define `Ingress` resources with the specifications you require, and they should be validated by Gatekeeper automatically, based on your constraints.

Let’s put this into practice with Pulumi by writing a TypeScript program:

- We will start by defining a `ConstraintTemplate` to enforce a label on `Ingress` resources.
- We will then create a `Constraint` based on this template.
- Finally, we will apply an `Ingress` resource where the Gatekeeper will either allow or deny the creation based on our constraints.

Below is the Pulumi TypeScript program that accomplishes the above:

```typescript
import * as kubernetes from '@pulumi/kubernetes';

// You would need to have your Kubernetes cluster configured for Pulumi.
// This program assumes Pulumi is set up to use a kubeconfig file to connect to an existing cluster.

// Step 1: Register the Gatekeeper ConstraintTemplate
// The ConstraintTemplate specifies a Rego policy which will check for a specified label on Ingress resources.
const ingressLabelConstraintTemplate = new kubernetes.apiextensions.CustomResource("ingress-label-constraint-template", {
    apiVersion: "templates.gatekeeper.sh/v1beta1",
    kind: "ConstraintTemplate",
    metadata: {
        name: "k8srequiredingresslabels",
    },
    spec: {
        crd: {
            spec: {
                names: {
                    kind: "K8sRequiredIngressLabels",
                },
                validation: {
                    openAPIV3Schema: {
                        properties: {
                            labels: {
                                type: "array",
                                items: {
                                    type: "string",
                                },
                            },
                        },
                    },
                },
            },
        },
        targets: [{
            target: "admission.k8s.gatekeeper.sh",
            rego: `
package k8srequiredingresslabels

violation[{"msg": msg, "details": {"missing_labels": missing}}] {
    provided := {label | input.review.object.metadata.labels[label]}
    required := {label | label := input.parameters.labels[_]}
    missing := required - provided
    count(missing) > 0
    msg := sprintf("You must provide following labels: %v", [missing])
}`,
        }],
    },
});

// Step 2: Create an instance of Constraint based on the ConstraintTemplate.
// This Constraint requires that all Ingress resources must have the 'gatekeeper' label.
const requiredLabelsConstraint = new kubernetes.apiextensions.CustomResource("required-ingress-labels", {
    apiVersion: "constraints.gatekeeper.sh/v1beta1",
    kind: "K8sRequiredIngressLabels",
    metadata: {
        name: "must-have-gatekeeper",
    },
    spec: {
        match: {
            kinds: [
                {
                    apiGroups: [""],
                    kinds: ["Ingress"],
                },
            ],
        },
        parameters: {
            labels: ["gatekeeper"],
        },
    },
});

// Step 3: Deploy an Ingress resource, which will be validated by Gatekeeper
const ingressResource = new kubernetes.networking.v1.Ingress("example-ingress-resource", {
    // Replace with the actual specification required for your application.
    metadata: {
        name: "example-ingress",
        annotations: {
            // Add your required annotations here.
        },
        labels: {
            gatekeeper: "true", // This label is necessary to satisfy the Gatekeeper constraint.
        },
    },
    spec: {
        // Define your Ingress spec here.
    },
}, { dependsOn: [requiredLabelsConstraint] }); // This ensures the ingress is created after the constraint is applied.

// Export the Ingress hostname, if applicable
export const ingressHostname = ingressResource.status.loadBalancer.ingress[0].hostname;
```

This program defines a `ConstraintTemplate` that Gatekeeper uses to enforce that all Ingress resources must have a specific label to be admitted. Then, it applies a `Constraint` based on this template, specifying the actual label key that must be present. Finally, it tries to create an `Ingress` resource with the required label.

To understand fully how this works, you will need to be familiar with Kubernetes, Pulumi, the Open Policy Agent, and the Gatekeeper project. You can start by reading their respective documentation to help you grasp the concepts and how they all work together:

- [Pulumi with Kubernetes](https://www.pulumi.com/docs/intro/cloud-providers/kubernetes/)
- [Open Policy Agent (OPA)](https://www.openpolicyagent.org/)
- [Gatekeeper: Policy Controller for Kubernetes](https://open-policy-agent.github.io/gatekeeper/website/docs/)

Remember to have Gatekeeper installed in your Kubernetes cluster before applying these policies. These constraints will have no effect if Gatekeeper is not actively monitoring your `Ingress` resources. The installation guide can be found in the Gatekeeper documentation.