Re: [lwip-devel] IP conflict detecting

[Bernhard Wiegel]

Hi Piero,
>     yes. When there is a packet, directed to your IP but to a
>     different MAC address then it is pretty clear there is someone
>     using your IP.
>     usually this is done on Broadcast packets so you can detect it
>     also in switched networks. Basically ARP is used.
>
>
> i didn't understand... 
> i configured emac on my microcontroller to filter ethernet packets, 
> so, only packets for my macaddress OR broadcast ethernet packets are 
> forwarded to microcontroller.
> So, packets with different mac and different IP are not received, and 
> broadcast ethernet packets have FFFFFFFFFF as mac address (i.e. arp 
> packets), so i cannot detect ip conflict....
ARP uses the broadcast address as it's target address.

You can detect a conflict easily if your device simply does an ARP 
request with it's own IP address.
If someone else answers, then there is a conflict.

So let's make an example of a device which has the static IP (a.b.c.d)

ARP request: Who has a.b.c.d? Please tell ...

If the device receives a reply ( ARP reply: a.b.c.d is at HW address) 
the devices IP will conflict with another one on the same subnet.
If there is no reply then the IP is not used by someone else at the moment.

So if you implement this process at startup and your device gets no ARP 
answer it can be sure that no one else uses this static IP at the moment.

Hope this make it more clear.

Regards,
Bernhard

--
Dipl-Ing Bernhard Wiegel

Universität Ulm - Ulm University
Institut für Organisation und Management von Informationssystemen (OMI) - 
Institute for Information Resource Management

Albert-Einstein-Allee 43
89081 Ulm, Germany 
Tel: +49 731 - 50 28788
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