Netbooting clients into Archlinux from an OpenWrt device

This instruction is valid if you have ...

• A working OpenWrt router with some free storage space (1 MB is sufficient)
• Working internet connection through said router
• Basic knowledge on how to ssh into the router and configure it
• Some device (typically a laptop or desktop) that can be connected to the OpenWrt router through a networking cable
• You want to do some rescue on some existing Linux system on said device, or you want to install Archlinux on it
• Said device can do netboot through PXE (but cannot load images directly from the Internet through iPXE).

In my case I got into my hand a HP laptop with the Wrong operating system installed, and I wanted to install Archlinux on it. (In my case I was actually moving from an existing identical but slightly broken laptop, so rather than going through the official Archlinux installation guide, I just did a plain copy of the full hard disk after booting it up). I found the other article on this wiki - tftp.pxe-server - but half of what's there was simply not relevant for me. I was surprised on how easy it was to serve PXE from my OpenWRT device.

Ref section 3 in tftp.pxe-server, it's as simple as editing /etc/config/dhcp on the OpenWrt device. Mine looks like this:

config dnsmasq
    (...)
    option enable_tftp '1'
    option tftp_root '/tftp'

(...)

config boot 'archlinux'
        option filename 'archlinux.pxe'
        option serveraddress '192.168.1.1'
        option servername 'OpenWrt'

(Obviously, my openwrt device has local IPv4-adress 192.168.1.1. Note that tftp.pxe-server section 3 specifies that some devices may need some extra code in the boot-section)

It's needed to do /etc/init.d/dnsmasq restart to let the changes take effect. And that's all. Considering all the mess one needs to get through to set up a working DHCP and TFTP on a Linux server, this is very easy.

The Archlinux wiki has a page https://wiki.archlinux.org/title/Netboot describing how to boot Archlinux from the Internet. It reads “Netboot images are small (< 1 MiB) images that can be used to download the latest Arch Linux release on the fly upon system boot” and points to the download page https://archlinux.org/releng/netboot/. It also says “You need sufficient memory (probably 1.5GiB or even more) to store and run the live system, otherwise you may get a kernel panic on boot”. If you have less memory than that, there are certainly smaller images that can be loaded - check up https://netboot.xyz

Run df -h to check if there is enough disk space. On my device, I can see that / is an overlay file system consisting of a 4 MB ROM system that is 100% full, and an overlay file system with 8 MB free space on the overlay partition - so the netboot image will fit. If you want to use an external USB-device then read tftp.pxe-server on how to do that.

I've specified above that tftp will find images through /tftp, so we need to create this:

mkdir /tftp

... and then download the image from the Archlinux site. Right now it can be done like this:

wget https://archlinux.org/static/netboot/ipxe-arch.5ee66f360339.pxe

If you don't want Archlinux, try downloading https://boot.netboot.xyz/ipxe/netboot.xyz.kpxe instead

... though you may want to ensure you get the latest version of the pxe file, and that the file has a good checksum. Perhaps wget isn't available on your OpenWRT device - the page tftp.pxe-server shows how to install it, but you may also download it to your workstation/laptop and then use scp to copy it over.

Connect your device to the OpenWrt router with a network cable, power it on. On some devices it's needed to press F12 repeatedly to get into network boot mode - other buttons like escape, F1, F3, etc may also enter bios or boot menus dependent on the device.

On my HP-laptop I had to do some BIOS configuration before I could boot, namely I had to turn off “secure boot” and turn on “legacy boot”.

When booting archlinux, it presents a list of mirror sites and asks me to choose one of them. It took me some time to find a nearby mirror that actually worked. Perhaps the netboot.xyz site is better ... it would be nice with a boot image that just works without having to do manual steps during the boot process (and possibly with mirror sites running anycast). Downloading the image also took quite some time.