Force kubernetes ingress cname format - kubernetes

Question

With Kubernetes, in a multi-tenant env., controlled by RBAC, when creating a new Ingress cname, I would like to force cname format like:
${service}.${namespace}.${cluster}.kube.infra
apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
name: ${servce}
spec:
tls:
- hosts:
- ${service}.${namespace}.${cluster}.kube.infra
secretName: conso-elasticsearch-ssl
rules:
- host: ${service}.${namespace}.${cluster}.kube.infra
http:
paths:
- path: /
backend:
serviceName: ${service}
servicePort: 9200
Is it possible?

Answer 1

You can do by it by writing a validating admission webhook which validates the ingress yaml and rejects it if the cname format is not as per the way you want. A better way to is to use Open Policy agent(OPA) and write rego policy. Here is a guide on how to perform policy driven validation of ingress using OPA.
package kubernetes.admission
import data.kubernetes.namespaces
operations = {"CREATE", "UPDATE"}
deny[msg] {
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 = {
// valid hosts
}
fqdn_matches_any(str, patterns) {
fqdn_matches(str, patterns[_])
}
fqdn_matches(str, pattern) {
// validation logic
}
fqdn_matches(str, pattern) {
not contains(pattern, "*")
str == pattern
}