diff --git a/src/cfgparse-tcp.c b/src/cfgparse-tcp.c
index 0265e7d19..40419cc51 100644
--- a/src/cfgparse-tcp.c
+++ b/src/cfgparse-tcp.c
@@ -10,6 +10,7 @@
  *
  */
 
+#define _GNU_SOURCE // for TCP_MD5SIG_MAXKEYLEN
 #include <ctype.h>
 #include <errno.h>
 #include <stdio.h>