diff --git a/src/libcharon/plugins/tnc_tnccs/tnc_tnccs_manager.c b/src/libcharon/plugins/tnc_tnccs/tnc_tnccs_manager.c
index 36bab7068..8db3731b2 100644
--- a/src/libcharon/plugins/tnc_tnccs/tnc_tnccs_manager.c
+++ b/src/libcharon/plugins/tnc_tnccs/tnc_tnccs_manager.c
@@ -13,6 +13,8 @@
  * for more details.
  */
 
+#define _GNU_SOURCE /* for asprintf() */
+
 #include "tnc_tnccs_manager.h"
 
 #include <tnc/tnc.h>