Support for RDMnet protocol dissection has been accepted into the Wireshark mainline for their upcoming 3.3.0 release. This release is not available as a stable download yet, but if you are feeling adventurous, you can download one of the nightly development builds from here:
https://www.wireshark.org/download/automated/
As a reminder, ETC ships a suite of RDMnet test applications as well as an RDMnet C/C++ library. The test applications can be downloaded from here:
https://etclabs.github.io/RDMnet/
I highly recommend downloading the "latest" version for full compliance with the RDMnet standard.