diff --git a/extensions/xt_pknock.h b/extensions/xt_pknock.h
index 57f036a..e5051d3 100644
--- a/extensions/xt_pknock.h
+++ b/extensions/xt_pknock.h
@@ -22,8 +22,8 @@
 #define IPT_PKNOCK_CLOSESECRET	0x40
 
 #define IPT_PKNOCK_MAX_PORTS		15
-#define IPT_PKNOCK_MAX_BUF_LEN		32
-#define IPT_PKNOCK_MAX_PASSWD_LEN	32
+#define IPT_PKNOCK_MAX_BUF_LEN		31
+#define IPT_PKNOCK_MAX_PASSWD_LEN	31
 
 #define DEBUG 1