Re: [lwip-users] ARP requests but no reply

[David Empson]

"Robert Schilling" <address@hidden> wrote:
> I'm trying to port port lwIP to a ARM Cortex M3 NXP LPC1768.
> I'm using following Example: 
> http://www.coocox.com/EXAMPLE/NXP_LPC1766_ARMCC.htm
>
> If I want to ping the device, ARP requests of the host computer are coming 
> in, but the device doesn't reply them.
>
> I could isolate the problem to the function update_arp_entry() where the 
> reply should be started.
> There is a loop, where the queued packets should be sent: while 
> (arp_table[i].q != NULL).
>
> In my case arp_table[0] gets initialized but arp_table[0].q is always 
> NULL.
> For this reason I don't get an ARP reply.

Hi Robert

You misunderstand the purpose of that queue.

arp_table[i].q is used to hold outgoing IP packets which have not yet been 
sent because LWIP didn't know the MAC address of the destination device. In 
this situation, LWIP issues an ARP request while queueing the outgoing IP 
packet, and when the ARP reply arrives from the other device, pending 
packets can be sent. In your case there is nothing on that queue because 
LWIP hasn't tried to send any IP packets yet.

Your problem is the opposite way around: the other device is sending an ARP 
request, and LWIP is not sending the ARP reply.

When an ARP request is received, LWIP calls update_arp_entry() as a side 
effect: the incoming ARP request provides enough information for LWIP to 
update an entry in its own (outgoing) address table, saving LWIP from having 
to issue its own ARP request if it needs to send something to the IP address 
of the other device.

This call to update_arp_entry() is from etharp_arp_input(). The code to send 
the ARP reply is a little further down that function. Find "case 
ARP_REQUEST". The code there construct an ARP reply and calls 
netif->linkoutput() to send it. You should trace through that code and work 
out why the transmission is not occurring.

The most likely explanation is that for some reason your Ethernet driver is 
unable to transmit anything.