Accessing Port Forwarded Servers on a DLink DSL-2740B rev C2 (Local Loopback)
I just replaced my Asus DSL-N13 with a new DLink DSL-2740B ADSL modem as the Asus unit would not allow me to access servers (such as mail servers) that are hosted on my local network and port forwarded from the router, when the computer attempting the connection is also within the local network.
Imagine my dismay when the new router had a similar web interface, and exhibited the same problem!
Fortunately, I do have a solution. It turns out the firmware (2.34) for the DLink unit on the Australian website is not the most recent. Instead, go to DLink's Taiwanese website and download the latest firmware for the C3 revision, which works just fine on my C2 hardware (4.15 at the time of writing).
The new firmware adds a number of SNAT rules in a new chain "NAT_LOOPBACK" which correspond to each port forwarded. These rules make it look like the router is originating the connection to the local servers.