If you *know* the IP address of the PC, you could use etharp_request() which directly sends an ARP request. You can also use that to re-request the addresses in the ARP table. If you put that code into etharp_tmr (with a counter per entry), you have a periodic refresh of the whole table.
Good idea, or, i can call a fuction wich refresh arp table when a lot of time without packets received is elapsed. My only question is: is it thread safe? in my opinion the only way to avoid problem is call thsi function in lwip_thread, right? or, how i can ask to lwip_thread to execute this function from another thread, (API works in this way and they are thread safe) ?
Only one question: why would you need that? If you set ARP_MAXAGE low enough, arp entries automatically get invalidated after a timeout and will be re-requested if reused.
Did you read my previous email, which start this discussion? My problem is to force packet traffic TO lwip board if it's connected directly with a pc.
Anyway... ARP_MAXAGE is only for clear arp table, it doesn't force refresh, right?