https://sourceforge.net/p/vtun/bugs/61/
--- cfg_file.y.orig
+++ cfg_file.y
@@ -624,7 +624,7 @@ int clear_nat_hack_client(void *d, void
 }

 /* Clear the VTUN_NAT_HACK flag which are not relevant to the current operation mode */
-inline void clear_nat_hack_flags(int svr)
+extern inline void clear_nat_hack_flags(int svr)
 {
 	if (svr)
 		llist_trav(&host_list,clear_nat_hack_server,NULL);