Creating constraints

This topic shows how to define Policy Controller constraints.

Overview

Policy Controller allows you to enforce policy for a Kubernetes cluster by defining one or more constraint objects. Once a constraint is installed, requests to the API server are checked against the constraint and will be rejected if they do not comply. Pre-existing non-compliant resources will be reported at audit time.

Every constraint is backed by a constraint template that defines the schema and logic of the constraint. Constraint templates can be sourced from Google, third parties, or you can write your own. See Writing a constraint template for more information creating new templates.

Before you begin

Using the constraint template library

When you define a constraint, you specify the constraint template it extends. A library of common constraint templates developed by Google is installed by default, and many organizations do not need to create custom constraint templates directly in Rego. Constraint templates provided by Google have the label configmanagement.gke.io/configmanagement. To list them, use the following command:

kubectl get constrainttemplates \
  -l="configmanagement.gke.io/configmanagement=config-management"

To describe a constraint template and check its required parameters:

kubectl describe constrainttemplate [CONSTRAINT-TEMPLATE-NAME]

You can view all constraint templates in the library.

Defining a constraint

You define a constraint using YAML, and you do not need to understand or write Rego. Instead, a constraint invokes a constraint template and provides it with parameters specific to the constraint.

  • The lowercased kind matches the name of a constraint template.
  • The metadata.name is the name of the constraint.
  • The match field defines which objects the constraint applies to. All conditions specified must be matched before an object is in-scope for a constraint. match conditions are defined by the following sub-fields:
    • kinds are the kinds of resources the constraint applies to, determined by two fields: apiGroups is a list of Kubernetes API groups that will match and kinds is a list of kinds that will match. "*" matches everything. If at least one apiGroup and one kind entry match, the kinds condition is satisfied.
    • namespaces is a list of namespace names the object can belong to. The object must belong to at least one of these namespaces. Namespace resources are treated as if they belong to themselves.
    • excludedNamespaces is a list of namespaces that the object cannot belong to.
    • labelSelector is a Kubernetes label selector that the object must satisfy.
    • namespaceSelector is a label selector on the namespace the object belongs to. If the namespace does not satisfy the object, it will not match. Namespace resources are treated as if they belong to themselves.
  • The parameters field defines the arguments for the constraint, based on what the constraint template expects.

The following constraint, called ns-must-have-geo invokes a constraint template called K8sRequiredLabels, which is included in the constraint template library provided by Google. The constraint defines parameters that the constraint template uses to evaluate whether namespaces have the geo label set to some value.

# ns-must-have-geo.yaml

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: ns-must-have-geo
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
  parameters:
    labels:
      - key: "geo"

To create the constraint, apply it using kubectl apply -f:

kubectl apply -f ns-must-have-geo.yaml

Auditing a constraint

If the constraint is configured and installed correctly, its status.byPod[].enforced field is set to true, whether the constraint is configured to enforce or only test the constraint.

Constraints are enforced by default, and a violation of a constraint prevents a given cluster operation. You can set a constraint's spec.enforcementAction to dryrun to report violations in the status.violations field without preventing the operation.

To learn more about auditing, see Auditing using constraints

Caveats when syncing constraints

Keep the following caveats in mind when syncing constraints.

Eventual consistency

You can commit constraints to the repo, and can limit their effects using ClusterSelectors or NamespaceSelectors. Because syncing is eventually consistent, keep the following caveats in mind:

  • If a cluster operation triggers a constraint whose NamespaceSelector refers to a namespace that hasn't been synced, the constraint is enforced and the operation is prevented. In other words, a missing namespace "fails closed."
  • If you change the labels of a namespace, the cache may contain outdated data for a brief time.

Minimize the need to rename a namespace or change its labels, and test constraints that impact a renamed or relabeled namespace to ensure they work as expected.

Configure Policy Controller for referential constraints

Before you can enable referential constraints, you must create a Config that tells Policy Controller what kinds of objects to watch, such as namespaces.

Save the following YAML manifest to a file, and apply it with kubectl. The manifest configures Policy Controller to watch namespaces and Ingresses. Create an entry with group, version, and kind under spec.sync.syncOnly, with the values for each type of object you want to watch.

apiVersion: config.gatekeeper.sh/v1alpha1
kind: Config
metadata:
  name: config
  namespace: "gatekeeper-system"
spec:
  sync:
    syncOnly:
      - group: ""
        version: "v1"
        kind: "Namespace"
      - group: "extensions"
        version: "v1beta1"
        kind: "Ingress"

Enabling referential constraints

A referential constraint references another object in its definition. For example, you could create a constraint that requires Ingress objects in a cluster to have unique hostnames. The constraint is referential if its constraint template contains the string data.inventory in its Rego.

Referential constraints are disabled by default in Policy Controller. Referential constraints are only guaranteed to be eventually consistent, and this creates risks:

  • On an overloaded API server, the contents of Policy Controller's cache may become stale, causing a referential constraint to "fail open", meaning that the enforcement action appears to be working when it isn't. For example, you can create Ingresses with duplicate hostnames too quickly to allow the admission controller to detect the duplicates.

  • The order in which constraints are installed and the order in which the cache is updated are both random.

If you understand these risks and still want to enable support for referential constraints, set policyController.referentialRulesEnabled to true in the Operator object:

apiVersion: configmanagement.gke.io/v1
kind: ConfigManagement
metadata:
  name: config-management
  namespace: config-management-system
spec:
  clusterName: my-cluster
  channel: dev
  policyController:
    enabled: true
    referentialRulesEnabled: true

Listing all constraints

To list all constraints installed on a cluster, use kubectl.

kubectl get constraint

Removing a constraint

To find all constraints using a constraint template, list all objects with the same kind as the constraint template's metadata.name:

kubectl get [CONSTRAINT-TEMPLATE-NAME]

To remove a constraint, specify its kind and name:

kubectl delete [CONSTRAINT-TEMPLATE-NAME] [CONSTRAINT-NAME]

If you want to delete the constraint template the constraint was using, make a note of the constraint's kind.

When you remove a constraint, it stops being enforced as soon as the API server marks the constraint as deleted.

Removing all constraint templates

Set spec.policyController.templateLibraryInstalled to false. This prevents the Operator from automatically reinstalling the library.

To remove all constraint templates and all constraints:

kubectl delete constrainttemplate --all

Restoring the constraint template library

If you disabled the constraint template library or uninstalled all constraint templates, you can restore it by setting spec.policyController.templateLibraryInstalled to true in the Operator config.

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