Document fallthrough and fix rewrite (#537)

* Document fallthrough and fix *reverse*

While documenting the fallthrough behavior and testing it I noticed
the did not properly work. This PR does a tiny bit too much as it

- Documents fallthrough
- Fixes fallthrough in reverse
- Makes directives_generate complain on duplicate priorities
- Moved reverse *before* file in middleware.cfg
- Add a test that tests the reverse fallthrough behavior with a file
  backend

Fixes #515

* ....and fix the tests

middleware/reverse/reverse.go

@@ -13,16 +13,14 @@ import (
 
 // Reverse provides dynamic reverse DNS and the related forward RR.
 type Reverse struct {
-	Next     middleware.Handler
-	Networks networks
+	Next        middleware.Handler
+	Networks    networks
+	Fallthrough bool
 }
 
 // ServeDNS implements the middleware.Handler interface.
 func (re Reverse) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
-	var (
-		rr          dns.RR
-		fallThrough bool
-	)
+	var rr dns.RR
 
 	state := request.Request{W: w, Req: r}
 	m := new(dns.Msg)
@@ -42,7 +40,6 @@ func (re Reverse) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 		// loop through the configured networks
 		for _, n := range re.Networks {
 			if n.IPnet.Contains(ip) {
-				fallThrough = n.Fallthrough
 				rr = &dns.PTR{
 					Hdr: dns.RR_Header{Name: state.QName(), Rrtype: dns.TypePTR, Class: dns.ClassINET, Ttl: n.TTL},
 					Ptr: n.ipToHostname(ip),
@@ -54,7 +51,6 @@ func (re Reverse) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 	case dns.TypeA:
 		for _, n := range re.Networks {
 			if dns.IsSubDomain(n.Zone, state.Name()) {
-				fallThrough = n.Fallthrough
 
 				// skip if requesting an v4 address and network is not v4
 				if n.IPnet.IP.To4() == nil {
@@ -75,7 +71,6 @@ func (re Reverse) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 	case dns.TypeAAAA:
 		for _, n := range re.Networks {
 			if dns.IsSubDomain(n.Zone, state.Name()) {
-				fallThrough = n.Fallthrough
 
 				// Do not use To16 which tries to make v4 in v6
 				if n.IPnet.IP.To4() != nil {
@@ -95,14 +90,17 @@ func (re Reverse) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 	}
 
-	if rr == nil && !fallThrough {
-		return middleware.NextOrFailure(re.Name(), re.Next, ctx, w, r)
+	if rr != nil {
+		m.Answer = append(m.Answer, rr)
+		state.SizeAndDo(m)
+		w.WriteMsg(m)
+		return dns.RcodeSuccess, nil
 	}
 
-	m.Answer = append(m.Answer, rr)
-	state.SizeAndDo(m)
-	w.WriteMsg(m)
-	return dns.RcodeSuccess, nil
+	if re.Fallthrough {
+		return middleware.NextOrFailure(re.Name(), re.Next, ctx, w, r)
+	}
+	return dns.RcodeServerFailure, nil
 }
 
 // Name implements the Handler interface.