Kubernetes service external-name link to another namespaces service not work.

[BSWANG]

kubernetes plugin can not resolve service which using externalName to link another service. I guess this issue is similar to #1866 .

Cluster using kubedns cannot reproduce this issue.

How to reproduce

➜  kubectl get svc  # there is a service in default namespace named `kubernetes`
NAME          TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)    AGE
kubernetes    ClusterIP   172.21.0.1     <none>        443/TCP    9d

➜  kubectl create -f -<<EOF # create a service link to service in default namespace
heredoc> kind: Service
apiVersion: v1
metadata:
  name: servicetest
  namespace: testdns
spec:
  type: ExternalName
  externalName: kubernetes.default.svc.cluster.local
heredoc> EOF
service "servicetest" created

➜  kubectl run -it --image registry.aliyuncs.com/wangbs/netdia --namespace testdns --rm busybox 
If you don't see a command prompt, try pressing enter.
/ # ping kubernetes.default.svc.cluster.local # `kubernetes` service can resolved directly
PING kubernetes.default.svc.cluster.local. (172.21.0.1): 56 data bytes
^C
--- kubernetes.default.svc.cluster.local. ping statistics ---
2 packets transmitted, 0 packets received, 100% packet loss

/ # ping servicetest.testdns.svc.cluster.local # service linked to `kubernetes` cannot resolved
ping: bad address 'servicetest.testdns.svc.cluster.local'

kubernetes version && coredns version

k8s=1.10.4
coredns=1.2.0

[PFacheris]

Just an additional note here, I upgraded to 1.3.1 in my EKS cluster and was still getting SERVFAIL for this use-case. I resolved it by updating the upstream configuration in the kubernetes block of my Corefile from:

kubernetes cluster.local in-addr.arpa ip6.arpa {
      pods insecure
      upstream
      fallthrough in-addr.arpa ip6.arpa
    }

to

kubernetes cluster.local in-addr.arpa ip6.arpa {
      pods insecure
      upstream /etc/resolv.conf
      fallthrough in-addr.arpa ip6.arpa
    }

I'm not sure why the behavior described in the docs for upstream did not work for me:

If no ADDRESS is given, CoreDNS will resolve External Services against itself.