Require Unique External DNS Services

 1apiVersion: kyverno.io/v1
 2kind: ClusterPolicy
 3metadata:
 4  name: unique-external-dns
 5  annotations:
 6    policies.kyverno.io/title: Require Unique External DNS Services
 7    policies.kyverno.io/category: Other
 8    policies.kyverno.io/severity: medium
 9    policies.kyverno.io/subject: Service
10    kyverno.io/kyverno-version: 1.5.1
11    kyverno.io/kubernetes-version: "1.21"
12    policies.kyverno.io/minversion: 1.6.0
13    policies.kyverno.io/description: >-
14      ExternalDNS, part of Kubernetes SIGs, triggers the creation of external DNS records in supported
15      providers when the annotation`external-dns.alpha.kubernetes.io/hostname` is present. Like with
16      internal DNS, duplicates must be avoided. This policy requires every such Service have a cluster-unique
17      hostname present in the value of the annotation.
18spec:
19  validationFailureAction: Audit
20  background: false
21  rules:
22    - name: ensure-valid-externaldns-annotation
23      match:
24        any:
25        - resources:
26            kinds:
27              - Service
28            annotations:
29              external-dns.alpha.kubernetes.io/hostname: "*"
30      context:
31        # Looks up external DNS entries.
32        - name: alldns
33          apiCall:
34            urlPath: "/api/v1/services"
35            jmesPath: "items[?[metadata.namespace, metadata.name] != ['{{request.object.metadata.namespace}}', '{{request.object.metadata.name}}']].metadata.annotations.\"external-dns.alpha.kubernetes.io/hostname\""
36      preconditions:
37        all:
38        - key: "{{ request.operation || 'BACKGROUND' }}"
39          operator: AnyIn
40          value:
41            - CREATE
42            - UPDATE
43      validate:
44        message: >-
45          External DNS entry "{{request.object.metadata.annotations."external-dns.alpha.kubernetes.io/hostname"}}" is already
46          taken by another service in the cluster.
47        deny:
48          conditions:
49            all:
50              # Deny if "external-dns.alpha.kubernetes.io/hostname" annotation value is already taken
51              - key: "{{request.object.metadata.annotations.\"external-dns.alpha.kubernetes.io/hostname\"}}"
52                operator: AnyIn
53                value: "{{alldns}}"