diff --git a/usr.sbin/ypserv/yp_main.c b/usr.sbin/ypserv/yp_main.c
index 057174ba908..7473e6fc439 100644
--- a/usr.sbin/ypserv/yp_main.c
+++ b/usr.sbin/ypserv/yp_main.c
@@ -41,6 +41,7 @@ __FBSDID("$FreeBSD$");
  */
 
 #include <sys/types.h>
+#include <sys/mman.h>
 #include <sys/queue.h>
 #include <sys/socket.h>
 #include <sys/wait.h>
@@ -525,6 +526,9 @@ main(int argc, char *argv[])
 		unregister();
 	}
 
+	if (madvise(NULL, 0, MADV_PROTECT) != 0)
+		_msgout("madvise(): %s", strerror(errno));
+
 	/*
 	 * Create RPC service for each transport.
 	 */