diff --git a/include/net/tcp.h b/include/net/tcp.h
index f75a04d..2757a11 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -252,6 +252,7 @@
 extern int sysctl_tcp_cookie_size;
 extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
+extern int sysctl_tcp_challenge_ack_limit;
 
 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;