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Description of original firmware
Original firmware is ARM Linux 2.4.18 from MontaVista with some custom applications from DLink. I didn't get reply from DLink about GPL source of Linux kernel for DNS-300, but I've found source for DNS-312H. It's the same device but with 120Gb HDD on board. Unfortunately these sources are older (2004 year) than current firmware in my device (2005 year).
Also I'm sure we will never get sources of proprietary DLink's applications, so it's difficult to add some functionality or fix some bugs in original firmware.