Is there a reversal of `ExternalName` service in Kubernetes? - kubernetes

Question

I'm trying to move my development environment to Kubernetes to be more in line with existing deployment stages. In that context I need to call a service by its Ingress DNS name internally, while this DNS name resolves to an IP unreachable from the cluster itself. I would like to create a DNS alias inside the cluster which would point to the service, basically a reversal of a ExternalName service.
Example:
The external DNS name is my-service.my-domain.local, resolving to 127.0.0.1
Internal service is my-service.my-namespace.svc.cluster.local
A process running in a pod can't reach my-service.my-domain.local because of the resolved IP, but could reach my-service.my-namespace.svc.cluster.local, but needs to be accessing the former by name
I would like to have a cluster-internal DNS name my-service.my-domain.local, resolving to the service my-service.my-namespace.svc.cluster.local (ExternalName service would do the exact opposite).
Is there a way to implement this in Kubernetes?

Answer 1

You can use the core dns and add the entry over there using configmap
apiVersion: v1
kind: ConfigMap
metadata:
annotations:
labels:
eks.amazonaws.com/component: coredns
k8s-app: kube-dns
name: coredns
namespace: kube-system
data:
Corefile: |
domain-name:port {
errors
cache 30
forward . <IP or custom DNS>
reload
}
To test you can start one busy box pod
kubectl run busybox --restart=Never --image=busybox:1.28 -- sleep 3600
hit the domain name from inside of busy box
kubectl exec busybox -- nslookup domain-name
Official doc ref : https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/
Nice article for ref : https://coredns.io/2017/05/08/custom-dns-entries-for-kubernetes/
Or
you can map the domain to the service name using rewrite, rewrite name example.io service.default.svc.cluster.local
Use the Rewrite plug-in of CoreDNS to resolve a specified domain name
to the domain name of a Service.
apiVersion: v1
data:
Corefile: |-
.:5353 {
bind {$POD_IP}
cache 30
errors
health {$POD_IP}:8080
kubernetes cluster.local in-addr.arpa ip6.arpa {
pods insecure
fallthrough in-addr.arpa ip6.arpa
}
rewrite name example.io service.default.svc.cluster.local
loadbalance round_robin
prometheus {$POD_IP}:9153
forward . /etc/resolv.conf
reload
}
kind: ConfigMap
metadata:
labels:
app: coredns
k8s-app: coredns
kubernetes.io/cluster-service: "true"
kubernetes.io/name: CoreDNS
release: cceaddon-coredns
name: coredns
namespace: kube-system