Universal Client Gateway

[Jacob S. Barrett]

I am trying to make what some call a universal client gateway.  Finding 
anything via google on the subject is turning up nothing.  Basically I want 
setup a gateway that will masquerade IP from any host reguardless of its IP 
configuration.  For example if a host is configured with IP 192.168.2.2 and a 
gateway of 192.168.2.1 my gateway would reply to ARP requests for 
192.168.2.1.  When the host forwards its IP traffic to me I would masquerade 
the packet with my IP and forward it.  When the reply comes back my gateway 
would de-masquerade the packet and forward it back the host.  I have it all 
working except for the return forwarding.

For proxying the configured gateway address for incoming packets I running 
arpd on the LAN interface.  It replies to all unclaimed IP addresses.  So 
when the host ARPs for 192.168.2.2 it replies with my gateways MAC.  This 
works great.

For the return path I have tried a few things with no luck.
ARP hacks:
I first looked into adding an ARP entry using arp(8), but since no interfaces 
have subnets that matches the hosts IP it won't add it.  Also arp(8) doesn't 
support specifying an interface to force assignment.  If I wrote my own 
program to force an entry into the ARP table with the correct interface would 
that work?
Route hacks:
I then tried adding a route entry for the LAN interface.  I used the command:
route add -host 192.168.2.2 -interface vlan1000
This produces a route entry that looks promising:
192.168.2.2        vlan1000:0.xx.xx.xx.xx.aa UHLS        0        0 vlan10
But when you dump the arp tables we see:
? (192.168.2.2) at 00:xx:xx:xx:xx:aa on vlan1000 permanent [vlan]
Which is the MAC address of the gateway and not the host.
What I really want is an routing entry that looks like a normal local host:
192.168.0.27       0.xx.xx.xx.xx.bb    UHLW        1     4953 vlan10    904
I have tried several combinations of commands with route(8) with no luck.  Is 
there a way to add the route as a direct route to 0.xx.xx.xx.xx.cc?  Can I do 
it if I write my own program to add the route to the table?

I don't want to go the route of adding the faked gateway address to the 
interface with matching subnet to fit the client's IP in.  The problem with 
that is if a client is configure with IP 10.0.0.1/8 with a gateway of 
10.255.255.254 the smallest subnet I could create would be /8.  And that 
would mostlikely cause problems for connectly configured hosts trying to 
communicate to 10/8.

So does anyone understand what I am trying to do?  Do you know how to do it?  
Am I going about this all wrong?

-- 
Jacob S. Barrett
jbarrett at amduat.net
www.amduat.net

"I don't suffer from insanity, I enjoy every minute of it."