backend: fix root zone usage (#4039)

properly concatenate labels by using dnsutil.Join instead of '+'

Fixes: #3316
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/backend_lookup.go

@@ -422,7 +422,7 @@ func NS(ctx context.Context, b ServiceBackend, zone string, state request.Reques
 	old := state.QName()
 
 	state.Clear()
-	state.Req.Question[0].Name = "ns.dns." + zone
+	state.Req.Question[0].Name = dnsutil.Join("ns.dns.", zone)
 	services, err := b.Services(ctx, state, false, opt)
 	if err != nil {
 		return nil, nil, err
@@ -462,12 +462,8 @@ func SOA(ctx context.Context, b ServiceBackend, zone string, state request.Reque
 
 	header := dns.RR_Header{Name: zone, Rrtype: dns.TypeSOA, Ttl: ttl, Class: dns.ClassINET}
 
-	Mbox := hostmaster + "."
-	Ns := "ns.dns."
-	if zone[0] != '.' {
-		Mbox += zone
-		Ns += zone
-	}
+	Mbox := dnsutil.Join(hostmaster, zone)
+	Ns := dnsutil.Join("ns.dns", zone)
 
 	soa := &dns.SOA{Hdr: header,
 		Mbox:    Mbox,