Note: doesn't build anymore since remote is sockaddr_storage now.

--- ./dataxfer.c.orig	2014-10-06 10:58:24.919982485 +0200
+++ ./dataxfer.c	2014-10-06 11:01:35.091981560 +0200
@@ -1480,6 +1480,7 @@
     port_info_t *port = (port_info_t *) data;
     socklen_t len;
     char *err = NULL;
+    char buffer[256];
     int optval;
 
     if (port->tcp_to_dev_state != PORT_UNCONNECTED) {
@@ -1489,7 +1490,8 @@
 	  /* Wait the port to be unconnected and clean, go back to main loop*/
 	  return;
       }
-	err = "Port already in use\n\r";
+	snprintf(buffer, 256, "Port already in use by %s.\r\n", inet_ntoa(port->remote.sin_addr));
+	err = buffer;
     } else if (is_device_already_inuse(port)) {
 	err = "Port's device already in use\n\r";
     }