webhook.md

Admission webhooks

This document describes how to deploy the Prometheus operator's admission webhook service.

The admission webhook can be used to:

• Ensure that all annotations of PrometheusRule objects are coerced into string values.
• Check that PrometheusRule objects are semantically valid.
• Check that AlertmanagerConfig objects are semantically valid.
• Convert AlertmanagerConfig objects between v1alpha1 and v1beta1 versions.

Prerequisites

This guide assumes that you have already deployed the Prometheus Operator and that admission controllers are enabled on your cluster.

The Kubernetes API server expects admission webhooks to communicate over HTTPS so this guide also assumes the following:

1. A valid TLS certificate and key has been provisioned for the admission webhook service.
2. A secret holding the TLS certificate and key has been created.

If you don't want to manually provision the TLS materials, cert-manager is a good option.

Admission webhook deployment

Assuming that the following secret exists and contains the base64-encoded PEM certificate (tls.crt) and key (tls.key) for the admission webhook service.

apiVersion: v1
data:
  tls.crt: LS0tLS...LS0tCg==
  tls.key: LS0tLS...LS0tCg==
kind: Secret
metadata:
  name: admission-webhook-certs
  namespace: default

The admission webhook's pod template should be modified to mount the secret as a volume and the container arguments should be modified to include the certificate and key:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: prometheus-operator-admission-webhook
  namespace: default
spec:
  template:
    spec:
      containers:
      - name: prometheus-operator-admission-webhook
        args:
        - --web.enable-tls=true
        - --web.cert-file=/etc/tls/private/tls.crt
        - --web.key-file=/etc/tls/private/tls.key
        volumeMounts:
        - mountPath: /etc/tls/private
          name: tls-certificates
          readOnly: true
     volumes:
     - name: tls-certificates
       secret:
         secretName: admission-webhook-certs

Webhook endpoints

caBundle note

The caBundle field contains the base64-encoded CA certificate used to sign the webhook's certificate. It is used by the Kubernetes API server to validate the certificate of the webhook service.

Certificate managers like cert-manager supports CA injection into webhook configurations and custom resource definitions.

/admission-prometheusrules/validate

The endpoint /admission-prometheusrules/validate rejects PrometheusRule objects that are not valid.

The following example deploys the validating admission webhook:

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
  name: prometheus-operator-rulesvalidation
webhooks:
  - clientConfig:
      caBundle: LS0tLS...LS0tCg==
      service:
        name: prometheus-operator
        namespace: default
        path: /admission-prometheusrules/validate
    failurePolicy: Fail
    name: prometheusrulemutate.monitoring.coreos.com
    namespaceSelector: {}
    rules:
      - apiGroups:
          - monitoring.coreos.com
        apiVersions:
          - '*'
        operations:
          - CREATE
          - UPDATE
        resources:
          - prometheusrules
    admissionReviewVersions: ["v1", "v1beta1"]
    sideEffects: None

/admission-prometheusrules/mutate

The endpoint /admission-prometheusrules/mutate mutates PrometheusRule objects so that integers and boolean yaml data elements are coerced into strings.

The following example deploys the mutating admission webhook:

apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:
  name: prometheus-operator-rulesmutation
webhooks:
  - clientConfig:
      caBundle: LS0tLS...LS0tCg==
      service:
        name: prometheus-operator
        namespace: default
        path: /admission-prometheusrules/mutate
    failurePolicy: Fail
    name: prometheusrulemutate.monitoring.coreos.com
    namespaceSelector: {}
    rules:
      - apiGroups:
          - monitoring.coreos.com
        apiVersions:
          - '*'
        operations:
          - CREATE
          - UPDATE
        resources:
          - prometheusrules
    admissionReviewVersions: ["v1", "v1beta1"]
    sideEffects: None

/admission-alertmanagerconfigs/validate

The endpoint /admission-alertmanagerconfigs/validate rejects AlertmanagerConfig objects that are not valid.

The following example deploys the validating admission webhook:

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
  name: prometheus-operator-alertmanager-config-validation
webhooks:
  - clientConfig:
      caBundle: LS0tLS...LS0tCg==
      service:
        name: prometheus-operator
        namespace: default
        path: /admission-alertmanagerconfigs/validate
    failurePolicy: Fail
    name: alertmanagerconfigsvalidate.monitoring.coreos.com
    namespaceSelector: {}
    rules:
      - apiGroups:
          - monitoring.coreos.com
        apiVersions:
          - v1alpha1
        operations:
          - CREATE
          - UPDATE
        resources:
          - alertmanagerconfigs
    admissionReviewVersions: ["v1", "v1beta1"]
    sideEffects: None

/convert

The endpoint /convert convert Alertmanagerconfig objects between v1alpha1 and v1beta1 versions.

The following example is a patch for the alertmanagerconfigs.monitoring.coreos.com CRD to configure the conversion webhook.

{
   "apiVersion": "apiextensions.k8s.io/v1",
   "kind": "CustomResourceDefinition",
   "metadata": {
      "name": "alertmanagerconfigs.monitoring.coreos.com"
   },
   "spec": {
      "conversion": {
         "strategy": "Webhook",
         "webhook": {
            "clientConfig": {
               "service": {
                  "name": "prometheus-operator-admission-webhook",
                  "namespace": "default",
                  "path": "/convert",
                  "port": 8443
               },
               "caBundle": "LS0tLS...LS0tCg=="
            },
            "conversionReviewVersions": [
               "v1beta1",
               "v1alpha1"
            ]
         }
      }
   }
}