Description of problem:

No Kubernetes ExternalName Services can be resolved via DNS when they point to in-cluster resources.

An example of the kind of ExternalName Service that this breaks:

apiVersion: v1
kind: Service
metadata:
  name: external-internal
spec:
  externalName: image-registry.openshift-image-registry.svc.cluster.local
  type: ExternalName


This is an upstream bug in CoreDNS, which OpenShift moved to in 4.0.

Upstream bug report - https://github.com/coredns/coredns/issues/2038

Upstream fix - https://github.com/coredns/coredns/pull/2040

Pull request to move us to a newer version of CoreDNS with this fix in our openshift/coredns fork - https://github.com/openshift/coredns/pull/6


Version-Release number of selected component (if applicable):

Anything in 4.0



How reproducible:

This can be reproduced every time.


Steps to Reproduce:

# Create two ExternalName services - one pointing to a domain
# inside the cluster and one pointing outside the cluster
cat <<EOF | oc apply -f -
---
apiVersion: v1
kind: Service
metadata:
  name: external-internal
spec:
  externalName: image-registry.openshift-image-registry.svc.cluster.local
  type: ExternalName
---
apiVersion: v1
kind: Service
metadata:
  name: external-external
spec:
  externalName: redhat.com
  type: ExternalName
EOF

# Start a busybox pod so we can test DNS resolution
oc run busybox -it --rm=true --restart=Never --image=busybox

# From inside the running pod, try pinging a few addresses

# PASS: resolve the ExternalName service that points to an external domain
ping -c 1 external-external | grep PING
PING external-external (209.132.183.105): 56 data bytes

# FAIL: resolve the ExternalName service that points to an internal domain
ping -c 1 external-internal | grep PING
ping: bad address 'external-internal'

# PASS: resolve the internal domain that the external-internal service points to
ping -c 1 image-registry.openshift-image-registry.svc.cluster.local | grep PING
PING image-registry.openshift-image-registry.svc.cluster.local (172.30.3.178): 56 data bytes


Actual results:

The external-internal domain cannot resolve. That is the ExternalName service that points to an in-cluster resource.


Expected results:

The external-internal domain should resolve. I've verified that when running a newer CoreDNS inside OpenShift 4 (built from the openshift/coredns PR linked above), it does resolve.