Tutorial

Liminix is very configurable, which can make it initially quite daunting - especially if you’re learning Nix or Linux or networking concepts at the same time. In this section we build some “worked example” Liminix images to introduce the concepts. If you follow the examples exactly, they should work. If you change things as you go along, they may work differently or not at all, but the experience should be educational either way.

Requirements

You will need a reasonably powerful computer running Nix. Target devices for Liminix are unlikely to have the CPU power and disk space to be able to build it in situ, so the build process is based around “cross-compilation” from another computer. The build machine can be any reasonably powerful desktop/laptop/server PC running NixOS. Standalone Nixpkgs installations on other Linux distributions - or on MacOS, or even in a Docker container - also ought to work but are untested.

Running in Qemu

You can try out Liminix without even having a router to play with. Clone the Liminix git repository and change into its directory

git clone https://gti.telent.net/dan/liminix
cd liminix

Now build Liminix

nix-build -I liminix-config=./examples/hello-from-qemu.nix \
 --arg device "import ./devices/qemu" -A outputs.default

In this command liminix-config points to the desired software configuration (e.g. services, users, filesystem, secrets) and device describes the hardware (or emulated hardware) to run it on. outputs.default tells Liminix that we want the default image output for flashing to the device: for the Qemu “hardware” it’s an alias for outputs.vmbuild, which creates a directory containing a root filesystem image and a kernel.

Tip

The first time you run this it may take several hours, because it builds all of the dependencies including a full MIPS gcc and library toolchain. Once those intermediate build products are in the nix store, subsequent builds will be much faster - practically instant, if nothing has changed.

Now you can try it:

./result/run.sh

This starts the Qemu emulator with a bunch of useful options, to run the Liminix configuration you just built. It connects the emulated device’s serial console and the QEMU monitor to stdin/stdout.

You should now see Linux boot messages and after a few seconds be presented with a root shell prompt. You can run commands to look at the filesystem, see what processes are running, view log messages (in :file:/run/uncaught-logs.current), etc. To kill the emulator, press ^P (Control P) then c to enter the “QEMU Monitor”, then type quit at the (qemu) prompt.

To see that it’s running network services we need to connect to its emulated network. Start the machine again, if you had stopped it, and open up a second terminal on your build machine. We’re going to run another virtual machine attached to the virtual network, which will request an IP address from our Liminix system and give you a shell you can run ssh from.

We use System Rescue in tty mode (no graphical output) for this example, but if you have some other favourite Linux Live CD ISO - or, for that matter, any other OS image that QEMU can boot - adjust the command to suit.

Download the System Rescue ISO:

curl https://fastly-cdn.system-rescue.org/releases/10.01/systemrescue-10.01-amd64.iso -O

and run it

nix-shell -p qemu --run " \
qemu-system-x86_64 \
    -echr 16 \
    -m 1024 \
    -cdrom systemrescue-10.01-amd64.iso \
    -netdev socket,mcast=230.0.0.1:1235,localaddr=127.0.0.1,id=lan \
    -device virtio-net,disable-legacy=on,disable-modern=off,netdev=lan,mac=ba:ad:3d:ea:21:01 \
    -display none -serial mon:stdio"

System Rescue displays a boot menu at which you should select the “serial console” option, then after a few moments it boots to a root prompt. You can now try things out:

• run ip a and see that it’s been allocated an IP address in the range 10.3.0.0/16.

• run ping 10.3.0.1 to see that the Liminix VM responds

• run ssh root@10.3.0.1 to try logging into it.

Congratulations! You have installed your first Liminix system - albeit it has no practical use and it’s not even real. The next step is to try running it on hardware.

Installing on hardware

For the next example, we’re going to install onto an actual hardware device. These steps have been tested using a GL.iNet GL-MT300A, which has been chosen for the purpose because it’s cheap and easy to unbrick if necessary.

Warning

There is always a risk of rendering your device unbootable by flashing it with an image that doesn’t work. The GL-MT300A has a builtin “debrick” procedure in the boot monitor and is also comparatively simple to attach serial cables to (soldering not required), so it is lower-risk than some devices. Using some other Liminix-supported MIPS hardware device also ought to work here, but you accept the slightly greater bricking risk if it doesn’t.

See Supported hardware for device support status.

You may want to read and inwardly digest the Develoment Manual section U-Boot and serial shenanigans when you start working with Liminix on real hardware. You won’t need serial access for this example, assuming it works, but it allows you to see the boot monitor and kernel messages, and to login directly to the device if for some reason it doesn’t bring its network up.

Now we can build Liminix. Although we could use the same example configuration as we did for Qemu, you might not want to plug a DHCP server into your working LAN because it will compete with the real DHCP service. So we’re going to use a different configuration with a DHCP client: this is examples/hello-from-mt300.nix

It’s instructive to compare the two configurations:

diff -u examples/hello-from-qemu.nix examples/hello-from-mt300.nix

You’ll see a new boot.tftp stanza which you can ignore, services.dns has been removed, and the static IP address allocation has been replaced by a dhcp.client service.

nix-build -I liminix-config=./examples/hello-from-mt300.nix \
 --arg device "import ./devices/gl-mt300a" -A outputs.default

Tip

The first time you run this it may take several hours. Again? Yes, even if you ran the previous example. Qemu is set up as a big-endian system whereas the MediaTek SoC on this device is little-endian - so it requires building all of the dependencies including an entirely different MIPS gcc and library toolchain to the other one.

This time in result/ you will see a bunch of files. Most of them you can ignore for the moment, but result/firmware.bin is the firmware image you can flash.

Flashing

Again, there are a number of different ways you could do this: using TFTP with a serial cable, through the stock firmware’s web UI, or using the vendor’s “debrick” process. The last of these options has a lot to recommend it for a first attempt:

• it works no matter what firmware is currently installed

• it doesn’t require plugging a router into the same network as your build system and potentially messing up your actual upstream

• no need to open the device and add cables

You can read detailed instructions on the vendor site, but the short version is:

1. turn the device off

2. connect it by ethernet cable to a computer

3. configure the computer to have static ip address 192.168.1.10

4. while holding down the Reset button, turn the device on

5. after about five seconds you can release the Reset button

6. visit http://192.168.1.1/ using a web browser on the connected computer

7. click on “Browse” and choose result/firmware.bin

8. click on “Update firmware”

9. wait a minute or so while it updates.

There’s no feedback from the web interface when the flashing is finished, but what should happen is that the router reboots and starts running Liminix. Now you need to figure out what address it got from DHCP - e.g. by checking the DHCP server logs, or maybe by pinging hello.lan or something. Once you’ve found it on the network you can ping it and ssh to it just like you did the Qemu example, but this time for real.

Warning

Do not leave the default root password in place on any device exposed to the internet! Although it has no writable storage and no default route, a motivated attacker with some imagination could probably still do something awful using it.

Congratulations Part II! You have installed your first Liminix system on actual hardware - albeit that it still has no practical use.

Exercise for the reader: change the default password by editing examples/hello-from-mt300.nix, and then create and upload a new image that has it set to something less hopeless.

Routing

The third example examples/demo.nix is a fully-functional home “WiFi router” - although you will have to edit it a bit before it will actually work for you. Copy examples/demo.nix to my-router.nix (or other name of your choice) and open it in your favourite text editor. Everywhere that the text EDIT appears is either a place you probably want to change or a place you almost certainly need to change.

There’s a lot going on in this configuration:

• it provides a wireless access point using the hostapd service: in this stanza you can change the ssid, the channel, the passphrase etc.

• the wireless lan and wired lan are bridged together with the bridge service, so that your wired and wireless clients appear to be on the same network.

Tip

If you were using a hardware device that provides both 2.4GHz and 5GHz wifi, you’d probably find that it has two wireless devices (often called wlan0 and wlan1). In Liminix we handle this by running two hostapd services, and adding both of them to the network bridge along with the wired lan. (You can see an example in examples/rotuer.nix)

• we use the combination DNS and DHCP daemon provided by the dnsmasq service, which you can configure

• the upstream network is “PPP over Ethernet”, provided by the pppoe service. Assuming that your ISP uses this standard, they will have provided you with a PPP username and password (sometimes this will be listed as “PAP” or “CHAP”) which you can edit into the configuration

• this example supports the new [1] Internet Protocol v6 as well as traditional IPv4. Configuring IPv6 seems to vary from one ISP to the next: this example expects them to be providing IP address allocation and “prefix delegation” using DHCP6.

Build it using the same method as the previous example

nix-build -I liminix-config=./my-router.nix \
 --arg device "import ./devices/gl-mt300a" -A outputs.default

and then you can flash it to the device.

Bonus: in-place updates

This configuration uses a writable filesystem (see the line rootfsType = "jffs2"), which means that once you’ve flashed it for the first time, you can make further updates over SSH onto the running router. To try this, make a small change (I’d suggest changing the hostname) and then run

nix-shell --run "liminix-rebuild root@address-of-the-device  -I liminix-config=./my-router.nix --arg device "import ./devices/gl-ar750""

(This requires the device to be network-accessible from your build machine, which for a test/demo system might involve a second network device in your build system - USB ethernet adapters are cheap - or a bit of messing around unplugging cables.)

For more information about liminix-rebuild, see the manual section Rebuilding the system.

Final thoughts

• These are demonstration configs for pedagogical purposes. If you’d like to see some more realistic uses of Liminix, examples/rotuer,arhcive,extneder.nix are based on some actual real hosts in my home network.

• The technique used here for flashing was chosen mostly because it doesn’t need much infrastructure/tooling, but it is a bit of a faff (requires physical access, vendor specific). There are slicker ways to do it that need a bit more setup - we’ll talk about that later as well.

Footnotes

[1]

RFC1883 Internet Protocol, Version 6 was published in 1995, so only “new” when Bill Clinton was US President