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Open Policy Agent (OPA)

In Kubernetes, Gloo Gateway stores its configuration as Custom Resource Definitions (CRDs). Because these CRDs are Kubernetes objects, you can use normal Kubernetes Role Based Access Control (RBAC) to create a policy that grants users the ability to create Gloo Gateway resources, such as a VirtualService object.

However, RBAC only allows administrators to grant permissions to entire objects. With the Open Policy Agent (OPA), you can specify fine-grained control over Gloo Gateway objects. Compare the differences in the following examples.

RBAC: User john@example.com is allowed to create virtual service.
OPA: User john@example.com is allowed to create virtual services that only point to the domain example.com.

You can use both Kubernetes RBAC and OPA together.

In this guide, you create a simple OPA policy that dictates that all virtual services must not have a prefix re-write.

Before you begin

Install Gloo Gateway in a Kubernetes cluster.
Follow the OPA documentation to set up OPA as an admission controller in your cluster. Then, OPA validates Kubernetes objects before they become visible to other controllers that act on them, including Gloo Gateway.

Depending on your security preferences, you might want to create a separate cluster role and role binding to restrict Gloo Gateway resources to view-only permissions, such as in the following example.

Click for an example YAML file for a cluster role and cluster role binding you might add to your OPA admission controller setup.

Policy

OPA Policies are written in Rego, a language specifically designed for policy decisions.

Write a Rego policy, which denies virtual services that include a prefix re-write.

package kubernetes.admission

operations = {"CREATE", "UPDATE"}

deny[msg] {
	input.request.kind.kind == "VirtualService"
	operations[input.request.operation]
	input.request.object.spec.virtualHost.routes[_].options.prefixRewrite
	msg := "prefix re-write not allowed"
}

Table: Understanding this Rego policy

Part of the policy	Description
`operations = {"CREATE", "UPDATE"}`	Applies the policy only to objects that are created or updated.
`deny[msg] {}`	Starts a policy to deny to creating or updating objects, if all of the conditions in the braces are met.
`input.request.kind.kind == "VirtualService"`	Specifies that the object must be a VirtualService.
`operations[input.request.operation]`	Specifies that the object has a create or update operation.
`input.request.object.spec.virtualHost.routes[_].options.prefixRewrite`	Specifies that the object has a prefixRewrite stanza.
`msg := "prefix re-write not allowed"`	Returns the `"prefix re-write not allowed"` message when all the conditions are true and the object is denied.

Apply Policy

Apply the policy by writing it to a configmap in the opa namespace.

kubectl --namespace=opa create configmap vs-no-prefix-rewrite --from-file=vs-no-prefix-rewrite.rego

After a moment, check that the policy status changes to ok.

kubectl get configmaps -n opa vs-no-prefix-rewrite -o yaml

apiVersion: v1
data:
  vs-no-prefix-rewrite.rego: "package kubernetes.admission\n\noperations = {\"CREATE\",
    \"UPDATE\"}\n\ndeny[msg] {\n\tinput.request.kind.kind == \"VirtualService\"\n\toperations[input.request.operation]\n\tinput.request.object.spec.virtualHost.routes[_].options.prefixRewrite\n\tmsg
    := \"prefix re-write not allowed\"\n}\n"
kind: ConfigMap
metadata:
  annotations:
    openpolicyagent.org/policy-status: '{"status":"ok"}'
  creationTimestamp: "2019-08-20T11:10:55Z"
  name: vs-no-prefix-rewrite
  namespace: opa
  resourceVersion: "39558874"
  selfLink: /api/v1/namespaces/opa/configmaps/vs-no-prefix-rewrite
  uid: 2de8732f-c33b-11e9-8be1-42010a8000dc

Verify

Time to test!

Click and save the following two virtual service configuration files to test valid and denied scenarios.

Valid VirtualService (vs-ok.yaml).

apiVersion: gateway.solo.io/v1
kind: VirtualService
metadata:
  name: default
  namespace: gloo-system
spec:
  virtualHost:
    domains:
    - '*'
    routes:
    - matchers:
       - exact: /sample-route-1
      routeAction:
        single:
          upstream:
            name: default-petstore-8080
            namespace: gloo-system

Denied VirtualService (vs-err.yaml), includes a prefix rewrite.

apiVersion: gateway.solo.io/v1
kind: VirtualService
metadata:
  name: default
  namespace: gloo-system
spec:
  virtualHost:
    domains:
    - '*'
    routes:
    - matchers:
       - exact: /sample-route-1
      routeAction:
        single:
          upstream:
            name: default-petstore-8080
            namespace: gloo-system
      options:
        prefixRewrite: /api/pets

Apply the valid virtual service in your cluster, and verify that the virtual service is created.

kubectl apply -f vs-ok.yaml
virtualservice.gateway.solo.io/default created

Apply the denied virtual service in your cluster. Verify that the virtual service is denied because it includes a prefix rewrite that your OPA policy does not allow.

kubectl apply -f vs-err.yaml
Error from server (prefix re-write not allowed): error when applying patch:
{"metadata":{"annotations":{"kubectl.kubernetes.io/last-applied-configuration":"{\"apiVersion\":\"gateway.solo.io/v1\",\"kind\":\"VirtualService\",\"metadata\":{\"annotations\":{},\"name\":\"default\",\"namespace\":\"gloo-system\"},\"spec\":{\"virtualHost\":{\"domains\":[\"*\"],\"routes\":[{\"matchers\":[{\"exact\":\"/sample-route-1\"}],\"options\":{\"prefixRewrite\":\"/api/pets\"},\"routeAction\":{\"single\":{\"upstream\":{\"name\":\"default-petstore-8080\",\"namespace\":\"gloo-system\"}}}}]}}}\n"}},"spec":{"virtualHost":{"routes":[{"matchers":[{"exact":"/sample-route-1"}],"options":{"prefixRewrite":"/api/pets"},"routeAction":{"single":{"upstream":{"name":"default-petstore-8080","namespace":"gloo-system"}}}}]}}}
to:
Resource: "gateway.solo.io/v1, Resource=virtualservices", GroupVersionKind: "gateway.solo.io/v1, Kind=VirtualService"
Name: "default", Namespace: "gloo-system"
Object: &{map["apiVersion":"gateway.solo.io/v1" "kind":"VirtualService" "metadata":map["annotations":map["kubectl.kubernetes.io/last-applied-configuration":"{\"apiVersion\":\"gateway.solo.io/v1\",\"kind\":\"VirtualService\",\"metadata\":{\"annotations\":{},\"name\":\"default\",\"namespace\":\"gloo-system\"},\"spec\":{\"virtualHost\":{\"domains\":[\"*\"],\"routes\":[{\"matchers\":[{\"exact\":\"/sample-route-1\"}],\"routeAction\":{\"single\":{\"upstream\":{\"name\":\"default-petstore-8080\",\"namespace\":\"gloo-system\"}}}}]}}}\n"] "creationTimestamp":"2020-01-29T14:41:28Z" "generation":'\x06' "name":"default" "namespace":"gloo-system" "resourceVersion":"7076134" "selfLink":"/apis/gateway.solo.io/v1/namespaces/gloo-system/virtualservices/default" "uid":"6ed4d802-42a5-11ea-84a5-56542bf21e7d"] "spec":map["virtualHost":map["domains":["*"] "routes":[map["matchers":[map["exact":"/sample-route-1"]] "routeAction":map["single":map["upstream":map["name":"default-petstore-8080" "namespace":"gloo-system"]]]]]]] "status":map["reported_by":"gateway" "state":'\x01' "subresource_statuses":map["*v1.Proxy.gloo-system.gateway-proxy":map["reported_by":"gloo" "state":'\x01']]]]}
for: "vs-err.yaml": admission webhook "validating-webhook.openpolicyagent.org" denied the request: prefix re-write not allowed

Cleanup

If you no longer want the OPA admission controller, you can uninstall it from your cluster. For example, you might use a teardown script, such as the following example.

kubectl delete validatingwebhookconfiguration  opa-validating-webhook
kubectl delete namespace opa
kubectl delete clusterrolebinding opa-viewer opa-gloo-viewer

rm ca.crt ca.key ca.srl server.conf server.crt server.csr server.key

kubectl label namespace kube-system openpolicyagent.org/webhook-

Next Steps

Now that you know how to configure a basic policy with OPA, you can continue learning about policies, or check out some of the other security features in Gloo Gateway.

You might also want to learn about the various features available to Routes on a Virtual Service in the Traffic Management guides.