diff --git a/tools/testing/selftests/net/forwarding/lib.sh b/tools/testing/selftests/net/forwarding/lib.sh
index 59272824ef37d93231bd066b8148b03f18709d44..5f4b7ed2c65a6aec8dbdd7b9c3398f068ab608f6 100644
--- a/tools/testing/selftests/net/forwarding/lib.sh
+++ b/tools/testing/selftests/net/forwarding/lib.sh
@@ -156,6 +156,19 @@ check_fail()
 	fi
 }
 
+check_err_fail()
+{
+	local should_fail=$1; shift
+	local err=$1; shift
+	local what=$1; shift
+
+	if ((should_fail)); then
+		check_fail $err "$what succeeded, but should have failed"
+	else
+		check_err $err "$what failed"
+	fi
+}
+
 log_test()
 {
 	local test_name=$1