/usr/share/doc/iptables/html/NAT-HOWTO-10.html (2525B)
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.