Rename reverse zone constants (#1568)

Rename the constants to IP4arpa and IP6arpa (shorter and exported) and
make IsReverse return the type of the reverse zone which could be handy
for some callers.

Also add tests for IsReverse()

plugin/kubernetes/setup.go

@@ -9,9 +9,10 @@ import (
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	"github.com/coredns/coredns/plugin/pkg/parse"
-
 	"github.com/coredns/coredns/plugin/pkg/upstream"
+
 	"github.com/mholt/caddy"
 	"github.com/miekg/dns"
 	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -113,7 +114,7 @@ func ParseStanza(c *caddy.Controller) (*Kubernetes, error) {
 
 	k8s.primaryZoneIndex = -1
 	for i, z := range k8s.Zones {
-		if strings.HasSuffix(z, "in-addr.arpa.") || strings.HasSuffix(z, "ip6.arpa.") {
+		if dnsutil.IsReverse(z) > 0 {
 			continue
 		}
 		k8s.primaryZoneIndex = i