Routing intp private subnet

[Andreas Herrmann]

Hi there,

I want to setup a gateway / firewall solution with current FreeBSD. The 
network has following structure:

Several host (host[1,...,x].domain.net) are defined within the DNS and 
all of them have the same A-Record with the IP 1.2.3.4
The gateway is listening on its external network interface with the IP 
1.2.3.4 and has an internal interface with a private subnet 
(192.168.0.0/24). The hosts (host[1,...,x].) are addressed in this subnet.

How can it be solved, that the gateway opens a tunnel to the special 
host in the private subnet (let.s say 192.168.0.3) if there is a query 
for host3.domain.net?

In my opinion this cannot be done because the client queries the DNS and 
simply opens the connection to the IP 1.2.3.4 and the gateway has now 
hints how to decide to which internal host the tunnel should be opened.

But this setup is possible because Microsoft ISA Server exactly does 
this job?

I have have no idea how to solve this. First idea was a kernel bridge 
between the interfaces.

Do you have any hints for me?

Thanks a lot!

Andreas