In Knative 0.3.x or later, all Routes with a domain suffix of
svc.cluster.local will only be visible inside the cluster.
This can be done by changing the config-domain config map as instructed
here.
You can also set the label serving.knative.dev/visibility=cluster-local on
your Route or KService to achieve the same effect.
For example, if you didn't set a label when you created the Route
helloworld-go and you want to make it local to the cluster, run:
kubectl label route helloworld-go serving.knative.dev/visibility=cluster-local