diff --git a/zebra/ipforward.h b/zebra/ipforward.h
index a772337..8a935c1 100644
--- a/zebra/ipforward.h
+++ b/zebra/ipforward.h
@@ -22,14 +22,14 @@
 #ifndef _ZEBRA_IPFORWARD_H
 #define _ZEBRA_IPFORWARD_H
 
-int ipforward ();
-int ipforward_on ();
-int ipforward_off ();
+extern int ipforward (void);
+extern int ipforward_on (void);
+extern int ipforward_off (void);
 
 #ifdef HAVE_IPV6
-int ipforward_ipv6 ();
-int ipforward_ipv6_on ();
-int ipforward_ipv6_off ();
+extern int ipforward_ipv6 (void);
+extern int ipforward_ipv6_on (void);
+extern int ipforward_ipv6_off (void);
 #endif /* HAVE_IPV6 */
 
 #endif /* _ZEBRA_IPFORWARD_H */