add option to get IF_NAME from default route

src/tc-gen

@@ -508,6 +508,14 @@ while getopts ":i:u:d:b:f:q:c:C:p:xV" OPT; do
     esac
 done
 
+if [[ "${IF_NAME}" = "DEFAULT" ]]; then
+    IF_NAME="$(ip -j route | jq -r 'map(select(.dst=="default"))[0].dev' 2>&1)"
+    if [[ "$?" -ne 0 ]]; then
+	>&2 echo "ERROR: failed to determine the interface name for the default route"
+	exit 1
+    fi
+fi
+
 if [[ -z ${IF_NAME} ]]; then
     print_usage
     exit 1