>>> and won't it need to use a bpf in order to [send a reply]?
>> Not necessarily.  It is likely enough to install an ARP entry for
>> the MAC address given and the IP address assigned to that client
>> (which can be done with a routing socket), and then send the packet
>> as a normal UDP packet.
> DHCP uses a four step process.  [...]  Do you really want the server
> installing an ARP entry at the DHCPOFFER stage when the client may or
> may not use the offered address?

I don't see why not.  In almost all cases, there is only one server and
the address will be used.  And even if it isn't, it does no harm:
nobody else should be using the IP address, if it's available to be
assigned, and if we assign it to someone else before the ARP entry
times out, we just need to be careful to destroy this ARP entry before
installing the new one - something you probably want to do anyway.

The only case where I can see it causing trouble is if a different DHCP
server assigns that address to some other client before the ARP entry
times out, and if you have two servers assigning the same address
within a single ARP domain, you have much larger troubles asking to
happen than just a stale ARP table entry.

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