diff --git a/lib/sockunion.h b/lib/sockunion.h
index 96b9a0d..db145cf 100644
--- a/lib/sockunion.h
+++ b/lib/sockunion.h
@@ -99,6 +99,7 @@
 extern int sockunion_stream_socket (union sockunion *);
 extern int sockopt_reuseaddr (int);
 extern int sockopt_reuseport (int);
+extern int sockopt_v6only (int family, int sock);
 extern int sockunion_bind (int sock, union sockunion *, 
                            unsigned short, union sockunion *);
 extern int sockopt_ttl (int family, int sock, int ttl);