Tc bits (#617)

* middleware/erratic: allow TC bit to be set

Add `truncate` as an option.

Fixes #593

middleware/erratic/erratic_test.go

@@ -39,7 +39,41 @@ func TestErraticDrop(t *testing.T) {
 		}
 
 		if tc.drop && rec.Msg != nil {
-			t.Errorf("Test %d: Expected dropped packet, but got %q", i, rec.Msg.Question[0].Name)
+			t.Errorf("Test %d: Expected dropped message, but got %q", i, rec.Msg.Question[0].Name)
+		}
+	}
+}
+
+func TestErraticTruncate(t *testing.T) {
+	e := &Erratic{truncate: 2} // 50% drops
+
+	tests := []struct {
+		expectedCode int
+		expectedErr  error
+		truncate     bool
+	}{
+		{expectedCode: dns.RcodeSuccess, expectedErr: nil, truncate: true},
+		{expectedCode: dns.RcodeSuccess, expectedErr: nil, truncate: false},
+	}
+
+	ctx := context.TODO()
+
+	for i, tc := range tests {
+		req := new(dns.Msg)
+		req.SetQuestion("example.org.", dns.TypeA)
+
+		rec := dnsrecorder.New(&test.ResponseWriter{})
+		code, err := e.ServeDNS(ctx, rec, req)
+
+		if err != tc.expectedErr {
+			t.Errorf("Test %d: Expected error %q, but got %q", i, tc.expectedErr, err)
+		}
+		if code != int(tc.expectedCode) {
+			t.Errorf("Test %d: Expected status code %d, but got %d", i, tc.expectedCode, code)
+		}
+
+		if tc.truncate && !rec.Msg.Truncated {
+			t.Errorf("Test %d: Expected truncated message, but got %q", i, rec.Msg.Question[0].Name)
 		}
 	}
 }