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131 lines
3.6 KiB
Markdown
131 lines
3.6 KiB
Markdown
Netboot HowTo
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=============
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This document describes how to set up network booting U-Boot devices on
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a LAN, e.g., when working with an evaluation board or other embedded
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system. The most secure way to do this is with a local LAN between a PC
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and the device.
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Instead of setting up everything in U-Boot to download the Linux Image,
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device tree, and initramfs, we will let U-Boot download a script with
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instructions to run. When you have multiple systems (boards) this
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quickly becomes a lot easier to manage.
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> [!NOTE]
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> Instructions in this HowTo assume a Debian based development system,
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> e.g., Ubuntu or Linux Mint.
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## Network Interface Setup
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For two dedicated network interfaces, here `eth2` and `eth3` (ymmv), we
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create an old-style interfaces config[^1] with the following content:
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```
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# /etc/network/interfaces.d/gimli
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auto eth2
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iface eth2 inet static
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address 192.168.0.1
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netmask 255.255.255.0
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auto eth3
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iface eth3 inet manual
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```
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> [!TIP]
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> Use configuration file names in `.d/` directories that make sense and
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> can easily be remembered. Here we use the hostname of the PC.
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## DHCP/TFTP Server Setup
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The examples given here use `dnsmasq`, which provides both DHCP and TFTP
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server support. The same can be achieved with other implementations.
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Similar to `interfaces.d`, dnsmasq has an `/etc/dnsmasq.d` directory so
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we can use "snippets" instead of modifying `/etc/dnsmasq.conf` directly.
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Add a file called `/etc/dnsmasq.d/gimli`.
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Initial content:
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```
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# Remember IP address handed out to BOOTP clients
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bootp-dynamic
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# Disable check-if-ip-address-is-already-used
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no-ping
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# Enable TFTP server, use /srv/ftp, same as any FTP server, useful
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# when using the same images for system upgrade as for netbooting.
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enable-tftp
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tftp-root=/srv/ftp
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```
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> [!CAUTION]
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> First of all, make sure you DO NOT accidentally set up dnsmasq so that
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> it starts acting as a DHCP server also on your office LAN! Follow the
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> instructions below for more details.
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If you have many interfaces used for lab equipment and only one office
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LAN interface, then use something like this:
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```
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# Disable DHCP server on loopback and office LAN (eth0)
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except-interface=lo
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except-interface=eth0
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```
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To further lock this down, we only run the DHCP server on each of the
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interfaces used for lab equipment, with a dedicated IP range as well:
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```
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# Currently I have an imx8mp-evk on eth2, so on my system I have a
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# symlink bootfile-eth2 -> netboot.scr
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interface=eth2
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dhcp-range=192.168.0.100,192.168.0.199,1h
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dhcp-boot=tag:eth2,bootfile-eth2
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```
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## Bootfile netboot.scr
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The bootfile U-Boot retrieves from the TFTP server is a script that
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looks like this, `netboot.sh`:
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```sh
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setenv ramdisk_addr_r 0x58000000
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setenv fdt_addr_r 0x50400000
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setenv autoboot off
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tftp ${fdt_addr_r} imx8mp-evk/imx8mp-evk.dtb
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tftp ${kernel_addr_r} imx8mp-evk/Image
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tftp ${ramdisk_addr_r} imx8mp-evk/rootfs.squashfs
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setenv bootargs console=ttymxc1,115200 root=/dev/ram0 brd.rd_size=500000 rauc.slot=net
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booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r}
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```
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U-Boot cannot read script files directly, so we need to wrap it with a
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FIT format header, this is done by first converting it on the PC:
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```
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$ mkimage -T script -d netboot.sh netboot.scr
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```
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The output is `netboot.scr` which we symlink to above in the dnsmasq
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setup step.
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## U-Boot Commands
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U-Boot is a maze of environment variables, some with values, some wrap
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commands, and most are undocumented. We will use a prefix for our
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variables to ensure we do not overwrite anything you may want to use
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later.
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```sh
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==> setenv ixboot 'dhcp && source \${fileaddr}'
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==> saveenv
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```
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[^1]: To prevent NetworkManager from automatically managing the interfaces.
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