erynwells.me/content/blog/2020/10/boot-raspberry-pi-over-tftp.md

---
title: "Booting a Raspberry Pi Over TFTP"
date: 2020-10-13T08:31:52-07:00
description: A writeup of how I set up a Raspberry Pi to boot over TFTP to facilitate an operating system development project.
series: ["Raspberry Pi OS Development"]
categories: ["Tech"]
tags: ["Raspberry Pi", "Networking"]
---

In order to do this, I modified the [EEPROM bootloader][eeprom] bootloader
according to the instructions in the Raspberry Pi documentation. That page is
also on [GitHub][eeprom-gh] which might be a more stable location. On Raspbian
on the Raspberry Pi:

{{< figures/code >}}
```sh
fw=/lib/firmware/raspberrypi/bootloader/stable/pieeprom-2020-09-03.bin
rpi-eeprom-config $fw > ~/bootconf.txt
vi ~/bootconf.txt
rpi-eeprom-config --out ~/pieeprom-new.bin --config ~/bootconf.txt $fw
sudo rpi-eeprom-update -d -f ~/pieeprom-new.bin
sudo reboot
```
{{< /figures/code >}}

My updated `bootconf.txt` is:

{{< figures/code >}}
```cfg
[all]
BOOT_UART=1
WAKE_ON_GPIO=0
POWER_OFF_ON_HALT=0
DHCP_TIMEOUT=45000
DHCP_REQ_TIMEOUT=4000
TFTP_FILE_TIMEOUT=30000
ENABLE_SELF_UPDATE=1
DISABLE_HDMI=0
BOOT_ORDER=0xf412
```
{{< /figures/code >}}

I enabled UART debugging, and set the boot order to be: network `0x2`, SD card
`0x1`, USB mass storage `0x4`, and finally reboot `0xf`. These steps need to be
repeated if the bootloader is updated via apt.

I [enabled the TFTP server][mac-tftp] on my Mac:

{{< figures/code >}}
```sh
sudo launchctl load -F /System/Library/LaunchDaemons/tftp.plist
sudo launchctl enable System/com.apple.tftpd
sudo launchctl start com.apple.tftpd
```
{{< /figures/code >}}

I’m not sure if the `enable` command is actually necessary. This doesn't
actually start the `tftpd` daemon. Instead, macOS starts the daemon on demand
when it notices an incoming tftp request on the network. Don't be alarmed!

The tftp server looks for files to serve out of **`/private/tftpboot`**, and those
things need to be world `rwx`, i.e. `777`. By default (this is configurable) the
Raspberry Pi queries for a directory named by its serial number.

{{< figures/code >}}
```sh
mkdir /private/tftpboot/$raspberry_pi_serial
chmod 777 /private/tftpboot
chmod -R 777 /private/tftpboot/*
```
{{< /figures/code >}}

Raspberry Pi looks for files of various names in that directory, one in
particular by the name of **`start.elf`**.

Next, I had to update my Ubiquiti router's DHCP server configuration (on the
command line) to pass a `tftp-server` parameter in the DHCP payload. This step
may be optional because you can also set `TFTP_IP` in the **`bootconf.txt`** above
to specify the IP directly. On my router:

{{< figures/code >}}
```sh
configure
set service dhcp-server shared-network-name LAN subnet $lan_cidr_subnet tftp-server-name $ip_of_mac
commit
save
exit
```
{{< /figures/code >}}

I also gave my Mac a static IP, and renewed the DHCP lease so it took the new IP
to make the whole process a little more smooth.  Now, it appears the Raspberry
Pi will attempt a TFTP boot, and I see queries in the logs on my Mac.

## Further Reading

* [Hackaday's Raspberry Pi Boot Sequence Guide](https://hackaday.io/page/6372-raspberry-pi-4-boot-sequence)
* [Linuxhit Guide to Booting a Raspberry Pi with PXE](https://linuxhit.com/raspberry-pi-pxe-boot-netbooting-a-pi-4-without-an-sd-card/)

[eeprom]: https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2711_bootloader_config.md
[eeprom-gh]: https://github.com/raspberrypi/documentation/blob/master/hardware/raspberrypi/bcm2711_bootloader_config.md
[mac-tftp]: https://www.unixfu.ch/start-a-tftp-server-on-your-mac/