diff --git a/lib/sockopt.h b/lib/sockopt.h
index 9538279..cb05c6f 100644
--- a/lib/sockopt.h
+++ b/lib/sockopt.h
@@ -89,6 +89,7 @@
 			             unsigned int ifindex
 					 /* optional: if non-zero, may be used
 					 	instead of if_addr */);
+extern int setsockopt_ipv4_tos(int sock, int tos);
 
 /* Ask for, and get, ifindex, by whatever method is supported. */
 extern int setsockopt_ifindex (int, int, int);