Re: [lwip-users] UDP not accepting repsonses to a broadcast

[Leon Woestenberg]

Hello Paul,

> The packet goes out ok but the replies I get back never get back to the 
> udp receive function.
> ... 
> The issue is with pcb->remote_ip set to IP_ADDR_BROADCAST  then
> ip_addr_isany(&pcb->remote_ip)   fails so the input is rejected.
Correct. Read on.

> The alternative is to set pcb->remote_ip to IP_ADDR_ANY but then
> the udp packet is sent to  0.0.0.0 and no hosts reply (only windows 
> tested).
>
> Can anyone make a suggestion on how to fix or work around this?

Yes, both your assumptions are correct, but there is a simple
workaround (also see dhcp.c/dhcp_discover() which does *exactly* the
same): Just re-associate your PCB directly after sending.

    udp_bind(dhcp->pcb, IP_ADDR_ANY, DHCP_CLIENT_PORT);
    /* send broadcast to any DHCP server */
    udp_connect(dhcp->pcb, IP_ADDR_BROADCAST, DHCP_SERVER_PORT);
    udp_send(dhcp->pcb, dhcp->p_out);

    /* remove remote address association, i.e. accept from ANY*/
    udp_connect(dhcp->pcb, IP_ADDR_ANY, DHCP_SERVER_PORT);

The above is for DHCP in 0.7.1, by the way.


By pure coincidence, I have implemented udp_sendto() today, which
allows you to send to a specified destination address, without
affecting the remote address association of the PCB.

This code is in CVS HEAD, together with a modified DHCP client using
this in dhcp_discover() and dhcp_bind().

We need testers, so please (also) use CVS HEAD to test this.

Regards,

Leon.