[ale] turn linux into a router??

[djinn at djinnspace.com]

Here's what I need to do:

assume: machine with eth0   a.b.c.d     and eth1  1.2.3.1   (both
external ranges)
assume: three machines with external range IP 1.2.3.2 - 1.2.3.5
network:
                            INTERNET
                                        |
                                a.b.c.d (eth0)
                                1.2.3.1 (eth1)
                                        |
                                        |
                    ----------------
                    |                    |                        |
            1.2.3.2            1.2.3.3            1.2.3.4



-take requests on port 53 and route them to 1.2.3.2:53
-take requests on ports 80/443/21 and route them to 1.2.3.4
-stateful inspection of packets (I've already got this bit in
place...using iptables and some custom rules based on bastille)

Please note, both network a.b.c.d and 1.2.3.4-5 are externally visible
IP addresses, in other words, a.b.c.d site in front of 1.2.3.4 to act
as  a firewall but 1.2.3.4 is visible from the outside.

I've looked at the Linux Routing Project but it seems to be overly
concerned with NAT...which isn't *exactly* what I'm doing since people
from the outside will be querying 1.2.3.x directly with no knowledge of
a.b.c.d, and receiving responses from 1.2.3.x directly...so a.b.c.d
needs to be transparent here to this process.

I'm so confused.  I'm not 100% sure exactly what I need to accomplish
this.  And I want to do it with a linux box.  And I need to have it done
days ago. ;)
If it helps any, we've got one IP assigned from our co-lo on the a.b.c.d
range, and then 4 IP's on the 1.2.3.x range that expect to use 1.2.3.1
as the gateway back to the internet.

Help???
TIA
jenn

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