This tutorial shows how to deploy OPA as an admission controller from scratch. It covers the OPA-kubernetes version that uses kube-mgmt. The OPA Gatekeeper version has its own docs. For the purpose of the tutorial we will deploy two policies that ensure:
- Ingress hostnames must be on allowlist on the Namespace containing the Ingress.
- Two ingresses in different namespaces must not have the same hostname.
💡 Kubernetes does not guarantee consistency across resources. If two ingresses are created in parallel, there is no guarantee that OPA (or any other admission controller) will observe the creation of one ingress before the other. This means that it’s not possible to enforce these policies during admission control 100% of the time. There will be a small window of time (usually on the order of milliseconds) when the eventually consistent cache inside of OPA (or any other admission controller) is out-of-date. To catch these violations we recommend you periodically audit the state of the cluster against your policies. Offline auditing is one of the features provided by the OPA Gatekeeper project.
Prerequisites
This tutorial requires Kubernetes 1.20 or later. To run the tutorial locally ensure you start a cluster with Kubernetes version 1.20+, we recommend using minikube or KIND.
Steps
1. Enable recommended Kubernetes Admission Controllers
To implement admission control rules that validate Kubernetes resources during create, update, and delete operations, you must enable the ValidatingAdmissionWebhook when the Kubernetes API server is started. The ValidatingAdmissionWebhook admission controller is included in the recommended set of admission controllers to enable
Start minikube:
minikube start
Make sure that the minikube ingress addon is enabled:
minikube addons enable ingress
2. Create a new Namespace to deploy OPA into
kubectl create namespace opa
Configure kubectl
to use this namespace:
kubectl config set-context opa-tutorial --user minikube --cluster minikube --namespace opa
kubectl config use-context opa-tutorial
3. Create TLS credentials for OPA
Communication between Kubernetes and OPA must be secured using TLS. To configure TLS, use openssl
to create a
certificate authority (CA) and certificate/key pair for OPA:
openssl genrsa -out ca.key 2048
openssl req -x509 -new -nodes -sha256 -key ca.key -days 100000 -out ca.crt -subj "/CN=admission_ca"
Generate the TLS key and certificate for OPA:
cat >server.conf <<EOF
[ req ]
prompt = no
req_extensions = v3_ext
distinguished_name = dn
[ dn ]
CN = opa.opa.svc
[ v3_ext ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage = clientAuth, serverAuth
subjectAltName = DNS:opa.opa.svc,DNS:opa.opa.svc.cluster,DNS:opa.opa.svc.cluster.local
EOF
openssl genrsa -out server.key 2048
openssl req -new -key server.key -sha256 -out server.csr -extensions v3_ext -config server.conf
openssl x509 -req -in server.csr -sha256 -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 100000 -extensions v3_ext -extfile server.conf
Note: the Common Name value and Subject Alternative Name you give to openssl MUST match the name of the OPA service created below.
Create a Secret to store the TLS credentials for OPA:
kubectl create secret tls opa-server --cert=server.crt --key=server.key --namespace opa
4. Define OPA policy
Let’s define a couple of policies to test admission control. First create a new folder to store our policies:
mkdir policies && cd policies
Policy 1: Restrict Hostnames
Create a policy that restricts the hostnames that an ingress can use. Only hostnames matching the specified regular expressions will be allowed.
ingress-allowlist.rego:
xxxxxxxxxx
package kubernetes.admission
import rego.v1
import data.kubernetes.namespaces
operations := {"CREATE", "UPDATE"}
deny contains msg if {
input.request.kind.kind == "Ingress"
operations[input.request.operation]
host := input.request.object.spec.rules[_].host
not fqdn_matches_any(host, valid_ingress_hosts)
msg := sprintf("invalid ingress host %q", [host])
}
valid_ingress_hosts := {host |
allowlist := namespaces[input.request.namespace].metadata.annotations["ingress-allowlist"]
hosts := split(allowlist, ",")
host := hosts[_]
}
fqdn_matches_any(str, patterns) if {
fqdn_matches(str, patterns[_])
}
fqdn_matches(str, pattern) if {
pattern_parts := split(pattern, ".")
pattern_parts[0] == "*"
suffix := trim(pattern, "*.")
endswith(str, suffix)
}
fqdn_matches(str, pattern) if {
not contains(pattern, "*")
str == pattern
}
Policy 2: Prohibit Hostname Conflicts
Now let’s define another policy to test admission control. The following policy prevents Ingress objects in different namespaces from sharing the same hostname.
ingress-conflicts.rego:
xxxxxxxxxx
package kubernetes.admission
import rego.v1
import data.kubernetes.ingresses
deny contains msg if {
some other_ns, other_ingress
input.request.kind.kind == "Ingress"
input.request.operation == "CREATE"
host := input.request.object.spec.rules[_].host
ingress := ingresses[other_ns][other_ingress]
other_ns != input.request.namespace
ingress.spec.rules[_].host == host
msg := sprintf("invalid ingress host %q (conflicts with %v/%v)", [host, other_ns, other_ingress])
}
Combine Policies
Let’s define a main policy that imports the Restrict Hostnames and Prohibit Hostname Conflicts policies and provides an overall policy decision.
main.rego:
xxxxxxxxxx
package system
import rego.v1
import data.kubernetes.admission
main := {
"apiVersion": "admission.k8s.io/v1",
"kind": "AdmissionReview",
"response": response,
}
default uid := ""
uid := input.request.uid
response := {
"allowed": false,
"uid": uid,
"status": {"message": reason},
} if {
reason = concat(", ", admission.deny)
reason != ""
}
else := {"allowed": true, "uid": uid}
⚠️ When OPA receives a request, it executes a query against the document defined
data.system.main
by default.
5. Build and Publish OPA Bundle
Build an OPA bundle containing policies defined in the previous step. In our setup, OPA will download policies from the
bundle service and the kube-mgmt
container will load Kubernetes resources into OPA. Since we load policy and data into
OPA from multiple sources, we need to scope the bundle to a subset of OPA’s policy and data cache by defining a manifest.
More information about this can be found here. Run the
following commands in the policies
folder created in the previous step.
cat > .manifest <<EOF
{
"roots": ["kubernetes/admission", "system"]
}
EOF
opa build -b .
We will now serve the OPA bundle using Nginx.
docker run --rm --name bundle-server -d -p 8888:80 -v ${PWD}:/usr/share/nginx/html:ro nginx:latest
6. Deploy OPA as an Admission Controller
Next, use the file below to deploy OPA as an admission controller.
admission-controller.yaml
:
# Grant OPA/kube-mgmt read-only access to resources. This lets kube-mgmt
# replicate resources into OPA so they can be used in policies.
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
name: opa-viewer
roleRef:
kind: ClusterRole
name: view
apiGroup: rbac.authorization.k8s.io
subjects:
- kind: Group
name: system:serviceaccounts:opa
apiGroup: rbac.authorization.k8s.io
---
# Define role for OPA/kube-mgmt to update configmaps with policy status.
kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
namespace: opa
name: configmap-modifier
rules:
- apiGroups: [""]
resources: ["configmaps"]
verbs: ["update", "patch"]
---
# Grant OPA/kube-mgmt role defined above.
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
namespace: opa
name: opa-configmap-modifier
roleRef:
kind: Role
name: configmap-modifier
apiGroup: rbac.authorization.k8s.io
subjects:
- kind: Group
name: system:serviceaccounts:opa
apiGroup: rbac.authorization.k8s.io
---
kind: Service
apiVersion: v1
metadata:
name: opa
namespace: opa
spec:
selector:
app: opa
ports:
- name: https
protocol: TCP
port: 443
targetPort: 8443
---
apiVersion: apps/v1
kind: Deployment
metadata:
labels:
app: opa
namespace: opa
name: opa
spec:
replicas: 1
selector:
matchLabels:
app: opa
template:
metadata:
labels:
app: opa
name: opa
spec:
containers:
# WARNING: OPA is NOT running with an authorization policy configured. This
# means that clients can read and write policies in OPA. If you are
# deploying OPA in an insecure environment, be sure to configure
# authentication and authorization on the daemon. See the Security page for
# details: https://www.openpolicyagent.org/docs/security.html.
- name: opa
image: openpolicyagent/opa:0.67.1
args:
- "run"
- "--server"
- "--tls-cert-file=/certs/tls.crt"
- "--tls-private-key-file=/certs/tls.key"
- "--addr=0.0.0.0:8443"
- "--addr=http://127.0.0.1:8181"
- "--set=services.default.url=http://host.minikube.internal:8888"
- "--set=bundles.default.resource=bundle.tar.gz"
- "--log-format=json-pretty"
- "--set=status.console=true"
- "--set=decision_logs.console=true"
volumeMounts:
- readOnly: true
mountPath: /certs
name: opa-server
readinessProbe:
httpGet:
path: /health?plugins&bundle
scheme: HTTPS
port: 8443
initialDelaySeconds: 3
periodSeconds: 5
livenessProbe:
httpGet:
path: /health
scheme: HTTPS
port: 8443
initialDelaySeconds: 3
periodSeconds: 5
- name: kube-mgmt
image: openpolicyagent/kube-mgmt:2.0.1
args:
- "--replicate-cluster=v1/namespaces"
- "--replicate=networking.k8s.io/v1/ingresses"
volumes:
- name: opa-server
secret:
secretName: opa-server
⚠️ If using
kind
to run a local Kubernetes cluster, the bundle service URL should behttp://host.docker.internal:8888
.
kubectl apply -f admission-controller.yaml
When OPA starts, the kube-mgmt
container will load Kubernetes Namespace and Ingress objects into OPA. You can
configure the sidecar to load any kind of Kubernetes object into OPA. The sidecar establishes watches on the
Kubernetes API server so that OPA has access to an eventually consistent cache of Kubernetes objects.
Next, generate the manifest that will be used to register OPA as an admission controller. This webhook will ignore
any namespace with the label openpolicyagent.org/webhook=ignore
.
cat > webhook-configuration.yaml <<EOF
kind: ValidatingWebhookConfiguration
apiVersion: admissionregistration.k8s.io/v1
metadata:
name: opa-validating-webhook
webhooks:
- name: validating-webhook.openpolicyagent.org
namespaceSelector:
matchExpressions:
- key: openpolicyagent.org/webhook
operator: NotIn
values:
- ignore
rules:
- operations: ["CREATE", "UPDATE"]
apiGroups: ["*"]
apiVersions: ["*"]
resources: ["*"]
clientConfig:
caBundle: $(cat ca.crt | base64 | tr -d '\n')
service:
namespace: opa
name: opa
admissionReviewVersions: ["v1"]
sideEffects: None
EOF
The generated configuration file includes a base64 encoded representation of the CA certificate created in Step 3 so that TLS connections can be established between the Kubernetes API server and OPA.
Next label kube-system
and the opa
namespace so that OPA does not control the resources in those namespaces.
kubectl label ns kube-system openpolicyagent.org/webhook=ignore
kubectl label ns opa openpolicyagent.org/webhook=ignore
Finally, register OPA as an admission controller:
kubectl apply -f webhook-configuration.yaml
You can follow the OPA logs to see the webhook requests being issued by the Kubernetes API server:
# ctrl-c to exit
kubectl logs -l app=opa -c opa -f
7. Exercise Restrict Hostnames policy
Now let’s exercise the Restrict Hostnames policy by creating two new namespaces.
qa-namespace.yaml:
apiVersion: v1
kind: Namespace
metadata:
annotations:
ingress-allowlist: "*.qa.acmecorp.com,*.internal.acmecorp.com"
name: qa
production-namespace.yaml:
apiVersion: v1
kind: Namespace
metadata:
annotations:
ingress-allowlist: "*.acmecorp.com"
name: production
kubectl create -f qa-namespace.yaml
kubectl create -f production-namespace.yaml
Next, define two Ingress objects. One of the Ingress objects will be permitted and the other will be rejected.
ingress-ok.yaml:
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
name: ingress-ok
spec:
rules:
- host: signin.acmecorp.com
http:
paths:
- pathType: ImplementationSpecific
path: /
backend:
service:
name: nginx
port:
number: 80
ingress-bad.yaml:
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
name: ingress-bad
spec:
rules:
- host: acmecorp.com
http:
paths:
- pathType: ImplementationSpecific
path: /
backend:
service:
name: nginx
port:
number: 80
Finally, try to create both Ingress objects:
kubectl create -f ingress-ok.yaml -n production
kubectl create -f ingress-bad.yaml -n qa
The second Ingress is rejected because its hostname does not match the allowlist in the qa
namespace.
It will report an error as follows:
Error from server: error when creating "ingress-bad.yaml": admission webhook "validating-webhook.openpolicyagent.org"
denied the request: invalid ingress host "acmecorp.com"
8. Exercise Prohibit Hostname Conflicts policy
Test the Prohibit Hostname Conflicts policy by verifying that you cannot create an Ingress in another namespace with the same hostname as the one created earlier.
staging-namespace.yaml:
apiVersion: v1
kind: Namespace
metadata:
annotations:
ingress-allowlist: "*.acmecorp.com"
name: staging
kubectl create -f staging-namespace.yaml
kubectl create -f ingress-ok.yaml -n staging
The above command will report an error as follows:
Error from server (BadRequest): error when creating "ingress-ok.yaml": admission webhook
"validate.nginx.ingress.kubernetes.io" denied the request: host "signin.acmecorp.com" and
path "/" is already defined in ingress production/ingress-ok
Wrap Up
Congratulations for finishing the tutorial!
This tutorial showed how you can leverage OPA to enforce admission control decisions in Kubernetes clusters without modifying or recompiling any Kubernetes components. Furthermore, with OPA’s Bundle feature policies can be periodically downloaded from remote servers to satisfy changing operational requirements.
For more information about deploying OPA on top of Kubernetes, see Deployments - Kubernetes.
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