[c-nsp] unusual arp behavior

[Bjørn Mork]

Alexey Toptygin <alexeyt at freeshell.org> writes:
> On Thu, 10 Feb 2005 sthaug at nethelp.no wrote:
>
>> Um, a DCHP client is supposed to ask for an IP address, and *then* it can
>> start using ARP. In the DHCP request it is common to see 0.0.0.0 as the
>> requestor address. Not in the ARP reqest, that would be very strange.
>
> Yes, a DHCP client is supposed to ask for an address first, but when it 
> gets one it can try to arp for it with requestor IP 0.0.0.0 and refuse to 
> use it if someone answers. This is called duplicate address detection, and 
> is implemented by many DHCP clients.

Yes, as suggested by RFC 2131 section 4.4.1:

                   "For example, if the client is on a network that
   supports ARP, the client may issue an ARP request for the suggested
   request.  When broadcasting an ARP request for the suggested address,
   the client must fill in its own hardware address as the sender's
   hardware address, and 0 as the sender's IP address, to avoid
   confusing ARP caches in other hosts on the same subnet."


Bjørn