A lot of times I've seen questions about how this works when you have multiple nodes or a CDN, it can be quite tricky in theory if you have random IPs or several IPs. The way certbot works at least for non-DNS challenges is that it will hit a random server that it resolves to, you have no control over which one it hits. If certbot hits node 1 at first to tell it to create the well-known file, then checks node 2 or any other node, you will find auhorization fails.........

*Update so this doesn't work it must be something to do with the path of nfs or something else but the installer fails with "Installer crashed" at the end whereas with the CD/USB it works. This assumes you've already installed and configured a separate PXE/DHCP server somewhere else and your /tftpboot directory is setup. This is for Linux Mint 18.1 but generally applies to most versions although you may have tro change things like "casper"........