diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 3f08e038..4830a468 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -759,10 +759,8 @@ netconn_alloc(enum netconn_type t, netconn_callback callback)
   sys_mbox_set_invalid(&conn->acceptmbox);
 #endif
   conn->state        = NETCONN_NONE;
-#if LWIP_SOCKET
   /* initialize socket to -1 since 0 is a valid socket */
-  conn->socket       = -1;
-#endif /* LWIP_SOCKET */
+  conn->callback_arg.socket = -1;
   conn->callback     = callback;
 #if LWIP_TCP
   conn->current_msg  = NULL;
diff --git a/src/api/sockets.c b/src/api/sockets.c
index 78e0f883..bf9e484d 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -661,8 +661,8 @@ lwip_accept(int s, struct sockaddr *addr, socklen_t *addrlen)
    * so nsock->rcvevent is >= 1 here!
    */
   SYS_ARCH_PROTECT(lev);
-  recvevent = (s16_t)(-1 - newconn->socket);
-  newconn->socket = newsock;
+  recvevent = (s16_t)(-1 - newconn->callback_arg.socket);
+  newconn->callback_arg.socket = newsock;
   SYS_ARCH_UNPROTECT(lev);
 
   if (newconn->callback) {
@@ -1729,7 +1729,7 @@ lwip_socket(int domain, int type, int protocol)
     set_errno(ENFILE);
     return -1;
   }
-  conn->socket = i;
+  conn->callback_arg.socket = i;
   done_socket(&sockets[i - LWIP_SOCKET_OFFSET]);
   LWIP_DEBUGF(SOCKETS_DEBUG, ("%d\n", i));
   set_errno(0);
@@ -2478,7 +2478,7 @@ event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len)
 
   /* Get socket */
   if (conn) {
-    s = conn->socket;
+    s = conn->callback_arg.socket;
     if (s < 0) {
       /* Data comes in right away after an accept, even though
        * the server task might not have created a new socket yet.
@@ -2486,16 +2486,16 @@ event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len)
        * will use the data later. Note that only receive events
        * can happen before the new socket is set up. */
       SYS_ARCH_PROTECT(lev);
-      if (conn->socket < 0) {
+      if (conn->callback_arg.socket < 0) {
         if (evt == NETCONN_EVT_RCVPLUS) {
           /* conn->socket is -1 on initialization
              lwip_accept adjusts sock->recvevent if conn->socket < -1 */
-          conn->socket--;
+          conn->callback_arg.socket--;
         }
         SYS_ARCH_UNPROTECT(lev);
         return;
       }
-      s = conn->socket;
+      s = conn->callback_arg.socket;
       SYS_ARCH_UNPROTECT(lev);
     }
 
diff --git a/src/include/lwip/api.h b/src/include/lwip/api.h
index 739f8be5..be8c22aa 100644
--- a/src/include/lwip/api.h
+++ b/src/include/lwip/api.h
@@ -246,10 +246,10 @@ struct netconn {
       all threads when closing while threads are waiting. */
   int mbox_threads_waiting;
 #endif
-  /** only used for socket layer */
-#if LWIP_SOCKET
-  int socket;
-#endif /* LWIP_SOCKET */
+  union {
+    int socket;
+    void *ptr;
+  } callback_arg;
 #if LWIP_SO_SNDTIMEO
   /** timeout to wait for sending data (which means enqueueing data for sending
       in internal buffers) in milliseconds */
@@ -373,6 +373,9 @@ err_t   netconn_err(struct netconn *conn);
 #define netconn_clear_flags(conn, clr_flags)   do { (conn)->flags = (u8_t)((conn)->flags & (u8_t)(~(clr_flags) & 0xff)); } while(0)
 #define netconn_is_flag_set(conn, flag)        (((conn)->flags & (flag)) != 0)
 
+#define netconn_set_callback_arg(conn, arg)   do { (conn)->callback_arg.ptr = (arg); } while(0)
+#define netconn_get_callback_arg(conn)        ((conn)->callback_arg.ptr)
+
 /** Set the blocking status of netconn calls (@todo: write/send is missing) */
 #define netconn_set_nonblocking(conn, val)  do { if(val) { \
   netconn_set_flags(conn, NETCONN_FLAG_NON_BLOCKING); \