sorry, don't know enough of Nim yet to give you a good answer.
just in case, here is one example I found in C : https://wxwidgets.info/cross-platform-way-of-obtaining-mac-address-of-your-machine/
Take a look at the ifconfig script in psutil-nim:
https://github.com/juancarlospaco/psutil-nim/blob/master/scripts/ifconfig.nim