How to configure routing with two exit points (Internet and closed network)?

Question

Debian

Given:

• Local network (192.168.1.X) with two servers: Windows 2k3 SB SP2 and Debian 6.
  
• DHCP is not configured, addresses on computers are registered statically.
  
• Two exits to the outside world:
  

1. Internet via Linux server.
   
2. Access to a closed network through a special router (a closed network has its own set of addresses like 192.168.15.X, for example).
   

Needed:
For the Linux server to take DNS records from the router and give them to computers on the local network if they want to access the sites of the closed part.
____
Now those who need a private part have routes manually registered (and in the network connection settings there are 2 ip-addresses, 2 dns and 2 gateways on one interface). As I understand it, to work with the private part, it's better to set DHCP and transfer the entire network to the addresses of the private part before blocking the redirect or is it not necessary?

Answer 1

What prevents to solve this question by routing? Make a route on the Linux gateway to the 192.168.15.X network through your "special router".

Answer 2

the task described above can be solved through iptables
nat prerouting
1. First, configure routing from the gateway, through route add
2. Then configure NAT to this network.