Routing problem

["Carlos Fernandez Sanz" <cfs-netfilter@nisupu.com>]

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I have a small problem setting up a routing exception here.

We have a small LAN with NAT-based internet access. Nothing special here. 
The router is a Linux box, with two NICs. One of them has a private address. The other one has a WAN address (it's a requirement of our provider that we use this address even if we have public addresses). 

Anyway, one of our users needs to go out using a public IP, and NAT doesn't do, because he needs to establish a connection encrypted where the IP address is part of a signature.

We do have spare IPs. The problem is that I can't add a route to him, route returns "network is unreachable".

Suppose NIC A in the linux box (route) is 192.168.21.1. NIC B is our public IP 1 (of a pool of five) A.B.C.1. Everyone gets out using this IP and NAT.
Now I want someone in the LAN to own the public IP A.B.C.2, however he is connected to the internal switch. 
I tried to do this

route add A.B.C.2 gw A.B.C.2 dev eth0 

But I get "network unreachable".

Before you ask: I can't connect this special computer to the same place I connect the linux box (which would be the obvious solution) because the carrier expects traffic to come from one WAN IP, owned by the linux box. 

All suggestions welcome.

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