[c-nsp] Windows Vista, Gratuitous ARP and DHCP conflicts

[Phil Mayers]

> 202 ADSL modem.  Previously we used the 200ES and 201 models.  The 202 
> was significantly different.  Unfortunately it also came with a builtin 
> proxy-arp function that can't be disabled.
> 
>> router: OFFER src=gwip dst=255.255.255.255 dmac=? [1]
>> client: ARP request ip=leaseip
> 
> It's all good up until this point.  The g-arp that Vista sends uses a BS 
> MAC (a multi-cast MAC no less) for the THA (target hardware address). It 

That's odd.

> uses 0.0.0.0 for the SPA (source protocol address).  Herein lies the 

That's normal.

> problem.  Any device that performs proxy-arp functions will respond to 
> this ARP because 1) it has a connected network to the target device in 
> question and 2) it knows based on the SPA that the sending host is not 
> already in that subnet (otherwise the local host would respond to the 
> ARP itself).  When the proxy-arp device responds, in our case the 

You're saying that gateways with proxy arp enabled will respond to ARP 
requests when the target IP is INSIDE the subnet?

It should be apparent that's not the case, or nothing would work on that 
subnet ever.