Re: [lwip-users] DHCP sends ARP requests with source address of 0.0.0.0 on start-up

[Simon Goldschmidt]

"R. Diez" wrote:
> I noticed that, when the DHCP client gets the IP address, it sends an
> ARP request, but the source address is 0.0.0.0. Is that the right thing
> to do?

Yes. See RFC 5227 for example.

> I would have thought that, if we don't have an IP address yet, there is
> no point sending an ARP request with a source IP of 0.0.0.0, or is that
> valid ARP behaviour?

Yes. See RFC 5227 for example.

> The reason I noticed is because I am checking that the gratuitous ARP
> request does get sent correctly in version 2.0.2 too.

I don't really understand that sentence. Is there a problem or not?

> The ARP request above seems to remain in the ARP queue, and gets resent
> a few times afterwards. On those retries, the request looks exactly the
> same as the gratuitous ARP request. So it seems that lwIP is sending a
> few gratuitous ARP requests after getting the IP address over DHCP.

Oh, no no no. There are 2 kinds of ARP requests sent:
a) Before using the address to check if it is really free for use. The device 
does not have an address here so can only use 0.0.0.0. This is called "probe".
b) After applying the address to tell everyone "I'm here". This is called 
"gratuitous".

> If the DHCP server offers an address that is already in
> use, can we actually reject it and request a different one? Or will the
> DHCP server probably send us the same one again?

This is kind of out of scope here. We follow RFCs and implement them. Can only 
assume it will work eventually.

I still don't understand if you actually have a problem or not.

Simon