Tiny TCP proxy server

Recently I needed to set up a small transparent TCP proxy to forward requests from a specified port on a public host to another host working behind nat firewall in a local network. The task seems to be easy once have root privileges to set up required iptables rules. But even than, you need to be familiar with things like prerouting, postrouting, source nat and destination nat - otherwise packet forwarding will not work as expected. I needed something to let me set up port forwarding easily, without root access and without remembering all necessary firewall rules. I also wanted it to work seemlessly on my Linksys router with Tomato firmware. So I created a small utility in ANSI C, which can be compiled for Linux with standard gcc, for Windows with Cygwin, and for Openwrt/Tomato firmware with appropriate toolchain. In case you need such utility you can find it on Github.