diff -up flow-tools-0.68.5.1/src/flow-filter.c.extern flow-tools-0.68.5.1/src/flow-filter.c
--- flow-tools-0.68.5.1/src/flow-filter.c.extern	2008-01-27 13:48:55.000000000 -0700
+++ flow-tools-0.68.5.1/src/flow-filter.c	2020-01-27 20:44:13.176636051 -0700
@@ -56,7 +56,7 @@
 int debug;
 int ip_net_only;
 
-struct acl_list acl_list;
+extern struct acl_list acl_list;
 
 int yyparse (void);
 void usage(void);