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 .. code-block:: console git clone https://gti.telent.net/dan/liminix cd liminix Now build Liminix .. code-block:: console 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: .. code-block:: console nix-shell --run "mips-vm ./result/vmlinux ./result/rootfs" 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 login prompt. You can login on the console as ``root`` (password is "secret") and poke around to see what processes are running. 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: .. code-block:: console curl https://fastly-cdn.system-rescue.org/releases/10.01/systemrescue-10.01-amd64.iso -O and run it .. code-block:: console 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 :command:`ip a` and see that it's been allocated an IP address in the range 10.3.0.0/16. * run :command:`ping 10.3.0.1` to see that the Liminix VM responds * run :command:`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. You may want to acquire a `USB TTL serial cable `_ when you start working with Liminix on real hardware. You won't *need* it 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. You have options here: the FTDI-based cables are the Rolls Royce of serial cables, whereas the ones based on PL2303 and CP2102 chipsets are cheaper but also fussier - or you could even get creative and use e.g. a `Raspberry Pi `_ or other SBC with a UART and TX/RX/GND header pins. Make sure that the voltages are compatible: this is a 3.3v device and you don't want to be sending it 5v or (even worse) 12v. 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 :file:`examples/hello-from-mt300.nix` It's instructive to compare the two configurations: .. code-block:: console 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. .. code-block:: console 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 :file:`result/` you will see a bunch of files. Most of them you can ignore for the moment, but :file:`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 :file:`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 :file:`examples/hello-from-mt300.nix`, and then create and upload a new image that has it set to something less hopeless. Final thoughts ************** * These are demonstration configs for pedagogical purposes. If you'd like to see some more realistic uses of Liminix, :file:`examples/rotuer,arhcive,extneder.nix` are based on some actual real hosts in my home network. * These example images are not writable. Later we will explain how to generate an image that can be changed after installation, and even use :command:`liminix-rebuild` (analogous to :command:`nixos-rebuild`) to keep it up to date. * 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.