Netboot HowTo¶

This document describes how to set up network booting U-Boot devices on a LAN, e.g., when working with an evaluation board or other embedded system. The most secure way to do this is with a local LAN between a PC and the device.

Instead of setting up everything in U-Boot to download the Linux Image, device tree, and initramfs, we will let U-Boot download a script with instructions to run. When you have multiple systems (boards) this quickly becomes a lot easier to manage.

Note

Instructions in this HowTo assume a Debian based development system, e.g., Ubuntu or Linux Mint.

Network Interface Setup¶

For two dedicated network interfaces, here eth2 and eth3 (ymmv), we create an old-style interfaces config¹ with the following content:

# /etc/network/interfaces.d/gimli
auto eth2
iface eth2 inet static
        address 192.168.0.1
        netmask 255.255.255.0

auto eth3
iface eth3 inet manual

Tip

Use configuration file names in .d/ directories that make sense and can easily be remembered. Here we use the hostname of the PC.

DHCP/TFTP Server Setup¶

The examples given here use dnsmasq, which provides both DHCP and TFTP server support. The same can be achieved with other implementations.

Similar to interfaces.d, dnsmasq has an /etc/dnsmasq.d directory so we can use "snippets" instead of modifying /etc/dnsmasq.conf directly. Add a file called /etc/dnsmasq.d/gimli.

Initial content:

# Remember IP address handed out to BOOTP clients
bootp-dynamic
# Disable check-if-ip-address-is-already-used
no-ping

# Enable TFTP server, use /srv/ftp, same as any FTP server, useful
# when using the same images for system upgrade as for netbooting.
enable-tftp
tftp-root=/srv/ftp

Caution

First of all, make sure you DO NOT accidentally set up dnsmasq so that it starts acting as a DHCP server also on your office LAN! Follow the instructions below for more details.

If you have many interfaces used for lab equipment and only one office LAN interface, then use something like this:

# Disable DHCP server on loopback and office LAN (eth0)
except-interface=lo
except-interface=eth0

To further lock this down, we only run the DHCP server on each of the interfaces used for lab equipment, with a dedicated IP range as well:

# Currently I have an imx8mp-evk on eth2, so on my system I have a
# symlink bootfile-eth2 -> netboot.scr
interface=eth2
dhcp-range=192.168.0.100,192.168.0.199,1h
dhcp-boot=tag:eth2,bootfile-eth2

Bootfile netboot.scr¶

The bootfile U-Boot retrieves from the TFTP server is a script that looks like this, netboot.sh:

setenv ramdisk_addr_r 0x58000000
setenv fdt_addr_r     0x50400000

setenv autoboot off
tftp ${fdt_addr_r}     imx8mp-evk/imx8mp-evk.dtb
tftp ${kernel_addr_r}  imx8mp-evk/Image
tftp ${ramdisk_addr_r} imx8mp-evk/rootfs.squashfs

setenv bootargs console=ttymxc1,115200 root=/dev/ram0 brd.rd_size=500000 rauc.slot=net
booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r}

U-Boot cannot read script files directly, so we need to wrap it with a FIT format header, this is done by first converting it on the PC:

$ mkimage -T script -d netboot.sh netboot.scr

The output is netboot.scr which we symlink to above in the dnsmasq setup step.

U-Boot Commands¶

U-Boot is a maze of environment variables, some with values, some wrap commands, and most are undocumented. We will use a prefix for our variables to ensure we do not overwrite anything you may want to use later.

==> setenv ixboot 'dhcp && source \${fileaddr}'
==> saveenv

To prevent NetworkManager from automatically managing the interfaces. ↩