Support SkyDNS' stubzones

This implements stubzones in the same way as SkyDNS. This
also works with multiple configured domains and has tests.
Also add more configuration parameters for TLS and path prefix and
enabling stubzones.  Run StubUpdates as a startup command to keep up to
date with the list in etcd.

middleware/etcd/handler.go

@@ -1,6 +1,8 @@
 package etcd
 
 import (
+	"strings"
+
 	"github.com/miekg/coredns/middleware"
 
 	"github.com/miekg/dns"
@@ -9,6 +11,20 @@ import (
 
 func (e Etcd) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 	state := middleware.State{W: w, Req: r}
+
+	// We need to check stubzones first, because we may get a request for a zone we
+	// are not auth. for *but* do have a stubzone forward for. If we do the stubzone
+	// handler will handle the request.
+	name := state.Name()
+	if len(*e.Stubmap) > 0 {
+		for zone, _ := range *e.Stubmap {
+			if strings.HasSuffix(name, zone) {
+				stub := Stub{Etcd: e, Zone: zone}
+				return stub.ServeDNS(ctx, w, r)
+			}
+		}
+	}
+
 	zone := middleware.Zones(e.Zones).Matches(state.Name())
 	if zone == "" {
 		return e.Next.ServeDNS(ctx, w, r)