RPI4B PXE Boot

Raspberry Pi 4 can be booted by PXE boot. You can either make the entire file system on the remote server or just make the boot partition remote.

In this article, we choose the latter since we will mostly only update the kernel of the Raspberry Pi.

Prerequisites

We assume there is a DHCP server in the network and assign fixed IP address to the remote PXE server.
(You can also run a DHCP server on the PXE server so the Raspberry Pi can automatically detect the address. But for a normal home network setup, this configuration is not always available. You may refer to this post for more detailed and advanced setup.)
You should have you RPI4 booted into Ubuntu/Raspbian first to configure the PXE boot.
(If you have your RPI4 booted into other OS, you should know how to install the rpi-eeprom package or modify the eeprom directly.)

Raspberry Pi configuration

EEPROM Configuration

Raspberry Pi 4, 400 and Compute Module 4 computers use an EEPROM to boot the system.

To enable PXE boot, you should first modify the EEPROM configuration.

sudo apt install rpi-eeprom

rpi-eeprom-config --edit

Then modify/add the following entries:

[all]

BOOT_UART=1

WAKE_ON_GPIO=1

POWER_OFF_ON_HALT=0

BOOT_ORDER=0xf412

TFTP_IP=PXE_SERVER_IP

The BOOT_ORDER option decides the boot order of the RPi4. It will be decided by from the least significant byte to higher ordered bytes.

For example, 0x2 means boot from NETWORK, 0x1 means boot from SD CARD and 0x4 means boot from USB. So 0xf412 above specifies the boot order of NETWORK->SD CARD->USB. 0xf means reboot if all previous boot mode fail.

For detailed boot order, refer to the RPI official documents.

Replace PXE_SERVER_IP with the IP address of your PXE server.

Copy the boot partition of your RPi to your PXE server

The boot partition of the RPi locates at /boot/firmware. You should copy the entire directory to your PXE server.

Note Down the Serial Number of Your RPI

RPI uses its serial number to locate the boot partition on the PXE server. To get the serial number, run

cat /proc/cpuinfo | grep Serial | awk -F ': ' '{print $2}' | tail -c 8

Note down the serial number. It will be used later.

GRACEFULLY SHUTDOWN YOUR RPI with shutdown now or halt -p so the changes are written to the EEPROM.

PXE Server Configuration

TFTP Server Configuration

On your PXE server, install tftpd-hpa TFTP server:

sudo apt install tftpd-hpa

Then configure the server

vi /etc/default/tftpd-hpa

You should configure the TFTP server with the following option

# /etc/default/tftpd-hpa

TFTP_USERNAME="tftp"

TFTP_DIRECTORY="/srv/tftp"

TFTP_ADDRESS=":69"

TFTP_OPTIONS="--secure"

Create the Boot Partition for the RPI

As we mentioned above, the RPI looks up the boot partition on the PXE server using its serial number. You should create a directory using the serial number of your RPI as the name in the tftp root directory and copy all the files in the boot partition into it.

mkdir -p /srv/tftp/RPI_SERIAL

cp -a firmware/* /srv/tftp/RPI_SERIAL

Now you can test your TFTP server on another client in the network by:

tftp PXE_SERVER_IP

> get RPI_SERIAL/cmdline.txt

If you have the cmdline.txt downloaded, you are one step away from getting your RPI network booted.

Boot Your Raspberry Pi

Simply boot your RPi with a ethernet cable plugged in. You should see the following output from your serial console:

ARP PXE_SERVER_IP PXE_SERVER_MAC

NET RPI_IP_ADDRESS 255.255.255.0 gw 0.0.0.0 tftp PXE_SERVER_IP

RX: 26 IP: 0 IPV4: 30 MAC: 2 UDP: 2 UDP RECV: 2 IP_CSUM_ERR: 0 UDP_CSUM_ERR: 0

TFTP_GET: PXE_SERVER_MAC PEX_SERVER_IP RPI_SERIAL/start4.elf

You are now boot your RPI from the Network.