Re: [lwip-users] Suppress ARP

[Kieran Mansley]

On Thu, 2009-09-24 at 12:53 +0200, Mathias Zenger wrote:
> I have a special problem with ARP.
> 
> My application uses UDP protocol and decides at start-up how to configure
> the stack/network interface. There is one mode where it uses DHCP/ARP and
> then requests packets from a server. Running an other mode it only receives
> multicast addressed packets and doesn't necessarily need the DHCP and ARP
> options. In this mode a simple dummy IP and gateway address are configured
> at start-up.
> 
> For some multicast packets I should send a reply to the server. When calling
> sento() from the socket API the stack sends out an ARP request which cannot
> be answered by anyone due to the used dummy addresses. Accordingly my reply
> packet won't be sent.
> 
> Question: is there a suitable way to suppress the ARP request for this
> special case and send out the datagram instead? Is ARP required at all or
> should I simply not define it in lwipopts.h for both modes?

Sounds to me like your problem is more fundamental: you're trying to use
IP without a valid IP address.

However, if you're sure you want to do that, you could probably add an
ARP cache entry for the server address you're trying to send to.  The
stack should already know this because it's recently received a
multicast address from the server.  Then ARP would not need to query the
server's address.

The proper solution would be to give your stack a valid IP address
however.

Kieran