plugin/forward: Forward NODATA responses to Next handler (#8065)

plugin/forward/forward.go

@@ -44,6 +44,7 @@ type Forward struct {
 	ignored []string
 
 	nextAlternateRcodes []int
+	nextOnNodata        bool
 
 	tlsConfig                  *tls.Config
 	tlsServerName              string
@@ -245,6 +246,14 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 			}
 		}
 
+		if f.nextOnNodata && f.Next != nil {
+			if ret.Rcode == dns.RcodeSuccess && isEmpty(ret) {
+				if _, ok := f.Next.(*Forward); ok {
+					return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
+				}
+			}
+		}
+
 		w.WriteMsg(ret)
 		return 0, nil
 	}
@@ -277,6 +286,19 @@ func (f *Forward) isAllowedDomain(name string) bool {
 	return true
 }
 
+func isEmpty(r *dns.Msg) bool {
+	if len(r.Answer) == 0 {
+		return true
+	}
+
+	for _, r := range r.Answer {
+		if r != nil {
+			return false
+		}
+	}
+	return true
+}
+
 // ForceTCP returns if TCP is forced to be used even when the request comes in over UDP.
 func (f *Forward) ForceTCP() bool { return f.opts.ForceTCP }