On Fri, Jan 05, 2007 at 10:14:24AM -0000, Andrew Ball wrote:
> 
> Hello,
> 
> I'm preparing to move up to a NetBSD 3.1 machine.  As a
> temporary measure, I have connected the new machine to my
> existing one via a serial cable and (with help! :-) built
> a SLIP connection between them...
> 
> ,--------------,         ,--------------,
> | tinman       |         | almond       |
> | ~~~~~~       |         | ~~~~~~       |
> | 192.168.0.129|--rs232--|192.168.0.128 |
> |              |         | 192.168.0.254|--10baseT--> to NAT
> `--------------'         `--------------'            gateway
> 
>       ...my existing machine, almond, has a 10baseT card and
> is connected to the Internet via a (non-NetBSD) NAT gateway.
> 

You have choosen static ip numbers.
So you don't need proxy-arp.

Configure almond as a router. 

Tell tinman, that it's default route has next hop almond.
Tell almond the static route to tinman.

Either activate routed on almond and let almond and your nat-box exchange routing information via RIP or RIPv2.

Or Tell your nat-box statically, that the next hop to tinman is almond.

I prefer the rip-solution, since network tend to grow once you have started. 
You already have two routers in your setup. Adding a third will start a growing
static route conplexity. Maybe you like to add a backup-route to internet via (nowadays cheep) isdn or modem router? Them dynamic routing will switch from your standard route to backup automatically for you. Nice isn't it?

Andreas
-- 
NetBSD: If you happen to have any problem with your uptime.