AH HA! I think this is the solution from the NAT howto.  I thought there
was a redirect thing going to happen. It turns out, according to this
that the solution is to SNAT these packets to the NAT box as well as the
DNAT. I don't have time to think it through right now and write a rule,
but this should get you close.

It is the last section the applies to your situation I belive.


10. Destination NAT Onto the Same Network

If you are doing port forwarding back onto the same network, you need to
make sure that both future packets and reply packets pass through the
NAT box (so they can be altered). The NAT code will now (since
2.4.0-test6), block the outgoing ICMP redirect which is produced when
the NAT'ed packet heads out the same interface it came in on, but the
receiving server will still try to reply directly to the client (which
won't recognize the reply).

The classic case is that internal staff try to access your `public' web
server, which is actually DNAT'ed from the public address (1.2.3.4) to
an internal machine (192.168.1.1), like so:

# iptables -t nat -A PREROUTING -d 1.2.3.4 \
        -p tcp --dport 80 -j DNAT --to 192.168.1.1

One way is to run an internal DNS server which knows the real (internal)
IP address of your public web site, and forward all other requests to an
external DNS server. This means that the logging on your web server will
show the internal IP addresses correctly.

The other way is to have the NAT box also map the source IP address to
its own for these connections, fooling the server into replying through
it. In this example, we would do the following (assuming the internal IP
address of the NAT box is 192.168.1.250):

# iptables -t nat -A POSTROUTING -d 192.168.1.1 -s 192.168.1.0/24 \
        -p tcp --dport 80 -j SNAT --to 192.168.1.250

Because the PREROUTING rule gets run first, the packets will already be
destined for the internal web server: we can tell which ones are
internally sourced by the source IP addresses.





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