auto ip configuration

[Unknown <netfilter-devel@demultiplexer.de>]

dear list,

you know about people, which have some static ip configuration on their
notebooks. they would like to connect to a smart gateway
without the need to change anything. 

I would ask you whether this is possible to implement such a gateway
using netfilter.

I alredy build a working example with some tools and scripting
but it isn't really performant and it have a lot of "design errors".
Simple speaking it is a farpd running box, where all incoming requests
are logged through the bridging code. The Syslog output is piped
into a script and the requested ip is assigned to the lan interface.
Evil thing ;-)

I would preffer something like inverse SNAT/MASQUERADE.
The difference would be, that:
	it applys to PREROUTING
	it does mapping from a big subnet (maybe 0/0) to a singe address or a
range of addresses.

The farpd would point all clients to the gateway.
Having rp_filter disabled all misconfigured packets incoming on a lan
interface would be translated into a valid ip address.
Then the usual routing would take place.
The response packets would be back-natted into the clients idea of the
gateway address.

Would please someone give me a pointer.
Am I completly wrong with this?

regards
jaroslaw