The RDM standard (ANSI E1.20) as well as all the ESTA standards (including, RDMnet, sACN, E1.37 RDM messages suite) can be freely downloaded here:
https://tsp.esta.org/tsp/documents/published_docs.php
There's a number of open source projects out there but I'm not familiar with all of them. The most well known one is
https://www.openlighting.org/ and can easily be run on a Raspberry Pi I know.