I'm aware of PXE, but in order to do so you need either of:
- the boot image supplying server being in the same intranet as the rest of the other servers, or
- some sort of method to point the diskless server to the correct external IP address to listen to
Since the first mode is probably too unsafe, that leaves us with the second mode. Either the operator memorizes a specific IP address and types it into the BIOS each time the server is rebooted, or the IP address (and possibly the checksum of the image) are stored in a single-use pendrive that the operator carries. I wonder which of these two methods is used in this case.
So there is still one single damning piece of information stored in the servers after all - the IP address to fetch the PXE boot image from. But hey, if Mullvad finds a way to strip even that out of the servers, that'd be great