The lwIP core isn't thread safe. If we have to use lwIP in a multithread environment, we should (or HAVE TO) use "upper" API layers (netconn or sockets). When using raw API you need to protect yourself the core. Some others informations to remember: * Some special functions can also cause problems regarding multithreading: netif_xxx and dhcp_xxx. These functions use the same variables as tcpip_thread. Other examples To avoid the previous 2 problems, you could use "netifapi" which is very close to "netif" and "dhcp" API.