diff --git a/libc/kernel/uapi/linux/tcp.h b/libc/kernel/uapi/linux/tcp.h
index cb26f97..c71715c 100644
--- a/libc/kernel/uapi/linux/tcp.h
+++ b/libc/kernel/uapi/linux/tcp.h
@@ -71,6 +71,7 @@
 #define TCP_AO_INFO 40
 #define TCP_AO_GET_KEYS 41
 #define TCP_AO_REPAIR 42
+#define TCP_IS_MPTCP 43
 #define TCP_REPAIR_ON 1
 #define TCP_REPAIR_OFF 0
 #define TCP_REPAIR_OFF_NO_WP - 1