plugin/tsig: reject non-final TSIG records (#8585)

This PR returns FORMERR before dispatching requests that contain a non-final TSIG.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/tsig/tsig.go

@@ -32,6 +32,14 @@ func (t TSIGServer) Name() string { return pluginName }
 
 // ServeDNS implements plugin.Handler
 func (t *TSIGServer) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	for i, rr := range r.Extra {
+		if rr.Header().Rrtype == dns.TypeTSIG && i != len(r.Extra)-1 {
+			resp := new(dns.Msg).SetRcode(r, dns.RcodeFormatError)
+			w.WriteMsg(resp)
+			return dns.RcodeSuccess, nil
+		}
+	}
+
 	var (
 		state  = request.Request{Req: r, W: w}
 		tsigRR = r.IsTsig()

plugin/tsig/tsig_test.go

@@ -216,6 +216,37 @@ func TestServeDNS(t *testing.T) {
 	}
 }
 
+func TestServeDNSRejectsNonFinalTSIG(t *testing.T) {
+	nextCalled := false
+	tsig := TSIGServer{
+		Zones: []string{"."},
+		Next: test.HandlerFunc(func(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+			nextCalled = true
+			return testHandler()(ctx, w, r)
+		}),
+	}
+
+	r := new(dns.Msg)
+	r.SetQuestion("test.example.", dns.TypeA)
+	r.SetTsig("test.key.", dns.HmacSHA256, 300, time.Now().Unix())
+	r.Extra = append(r.Extra, test.OPT(42, true))
+
+	w := dnstest.NewRecorder(&test.ResponseWriter{})
+	if _, err := tsig.ServeDNS(context.Background(), w, r); err != nil {
+		t.Fatal(err)
+	}
+
+	if nextCalled {
+		t.Error("expected non-final TSIG to be rejected before calling the next plugin")
+	}
+	if w.Msg == nil {
+		t.Fatal("expected a FORMERR response")
+	}
+	if w.Msg.Rcode != dns.RcodeFormatError {
+		t.Errorf("expected FORMERR, got %s", dns.RcodeToString[w.Msg.Rcode])
+	}
+}
+
 func TestServeDNSTsigErrors(t *testing.T) {
 	clientNow := time.Now().Unix()