plugin/{forward,proxy}: check for truncated (#1644)

Check for trunacted in the lookup function as well and use the Match
function here as well.

plugin/forward/forward.go

@@ -92,20 +92,7 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 			child.Finish()
 		}
 
-		// If you query for instance ANY isc.org; you get a truncated query back which miekg/dns fails to unpack
-		// because the RRs are not finished. The returned message can be useful or useless. Return the original
-		// query with some header bits set that they should retry with TCP.
-		if err == dns.ErrTruncated {
-			// We may or may not have something sensible... if not reassemble something to send to the client.
-			if ret == nil {
-				ret = new(dns.Msg)
-				ret.SetReply(r)
-				ret.Truncated = true
-				ret.Authoritative = true
-				ret.Rcode = dns.RcodeSuccess
-			}
-			err = nil // and reset err to pass this back to the client.
-		}
+		ret, err = truncated(ret, err)
 
 		if err != nil {
 			// Kick off health check to see if *our* upstream is broken.

plugin/forward/lookup.go

@@ -31,15 +31,22 @@ func (f *Forward) Forward(state request.Request) (*dns.Msg, error) {
 		}
 
 		ret, err := proxy.connect(context.Background(), state, f.forceTCP, true)
+
+		ret, err = truncated(ret, err)
+
 		if err != nil {
 			if fails < len(f.proxies) {
 				continue
 			}
 			break
-
 		}
 
-		return ret, nil
+		// Check if the reply is correct; if not return FormErr.
+		if !state.Match(ret) {
+			return state.ErrorMessage(dns.RcodeFormatError), nil
+		}
+
+		return ret, err
 	}
 	return nil, errNoHealthy
 }

plugin/forward/truncated.go

@@ -0,0 +1,25 @@
+package forward
+
+import "github.com/miekg/dns"
+
+// truncated looks at the error and if truncated return a nil errror
+// and a possible reconstructed dns message if that was nil.
+func truncated(ret *dns.Msg, err error) (*dns.Msg, error) {
+	// If you query for instance ANY isc.org; you get a truncated query back which miekg/dns fails to unpack
+	// because the RRs are not finished. The returned message can be useful or useless. Return the original
+	// query with some header bits set that they should retry with TCP.
+	if err != dns.ErrTruncated {
+		return ret, err
+	}
+
+	// We may or may not have something sensible... if not reassemble something to send to the client.
+	m := ret
+	if ret == nil {
+		m = new(dns.Msg)
+		m.SetReply(ret)
+		m.Truncated = true
+		m.Authoritative = true
+		m.Rcode = dns.RcodeSuccess
+	}
+	return m, nil
+}

plugin/proxy/lookup.go

@@ -92,6 +92,11 @@ func (p Proxy) lookup(state request.Request) (*dns.Msg, error) {
 			atomic.AddInt64(&host.Conns, -1)
 
 			if backendErr == nil {
+
+				if !state.Match(reply) {
+					return state.ErrorMessage(dns.RcodeFormatError), nil
+				}
+
 				return reply, nil
 			}