Netbooting Raspberry Pi with NFS root

How set up a Raspberry Pi for network boot without local storage and run it with NFS root filesystem. This should mostly be somewhat universal across Raspberry Pi 3 & 4, but the local "make it netboot" prep is different between Pi 3 & Pi 4 and this is mostly about Raspberry Pi 4.

preconditions

• A Raspberry Pi 4 (duh!) with a running Raspbian installation.
• A machine to serve as NFS, TFTP & DHCP server
• Said machine already set up as NFS, TFTP & DHCP server

preparing the Pi

• You may need to update the bootloader, so run rpi-eeprom-update -a and reboot the Pi
• enable network boot by running rpi-eeprom-config -e and setting BOOT_ORDER=0xf21. Note that this has the divergent procedure for Raspberry Pi 3.
• Grab the ethernet address of the ethernet interface from the output of ip link show.
• Grab the last 8 digits of the serial number from the output of cat /proc/cpuinfo |grep Serial.

DHCP

• Add a standard host entry for the Pi here, using the ethernet address we extracted before.
• To that host entry, add the following lines:
  • TFTP server name: option tftp-server-name "tftp-server";
  • NFS root path: option root-path "/nfsroots/raspi-box";

NFS server

• Create a directory /nfsroots/raspi-box and copy the filesystem from Raspberry Pi there, including /boot.
• Add an entry for it to /etc/exports and reload that config.

TFTP server

• Create a directory in /srv/tftp named with the last 8 digits of the serial number we extracted above, e.g. 12345678
• Add a bind mount entry for it to /etc/fstab:

        /nfsroots/raspi-box/boot /srv/tftp/12345678  none defaults,bind 0 0
        
    

• Mount that fstab entry - this is where the Raspberry Pi fetches the files it needs to boot via TFTP

Let's go!