I know this thread is old, but I am adding onto it with my research and findings.
My experience with this issue is only with HP computers, so no idea how other manufacturers do this.
HP’s firmware is written in a way that MAC address passthrough only works using HP thunderbolt docks, on Windows, using the correct dock drivers. MAC address passthrough also does not work with USB-C to Ethernet adapters because the adapters are not thunderbolt. Given this information, FOS out of the box would not be able to access the MAC address passthrough.
Judging by the Linux kernel patches I have seen online, adding this support to FOS would be pretty difficult since it seems like it depends on the network controller of each dock. This means that each dock needs to be programmed into the kernel and changes to that network controller driver need to be made as well.
The other part of the issue is that iPXE gets the MAC address of the device in a different way than FOS. In my experience, iPXE gets the system MAC address (the same address that should get passthrough), while FOS gets the address that Linux gives through the ip addr
command.
I have read other threads where iPXE gives a different MAC address, but it wasn’t HP so I can’t check on that.
I have a workaround that I did for HP computers which I have been testing for a couple weeks. The workaround is working for me, but I would rather do some more testing before putting out a tutorial.