# Creating Custom rootfs and kernel Images

## Creating a kernel Image

Currently, Firecracker supports only uncompressed, ELF kernel images. You can
build an uncompressed Linux kernel image with:

```bash
make vmlinux
```

Here's a quick step-by-step guide to building your own kernel that Firecracker
can boot:
1. Get the Linux source code:

   ```bash
   git clone https://github.com/torvalds/linux.git linux.git
   cd linux.git
   ```

2. Check out the Linux version you want to build (e.g. we'll be using v4.20
   here):

   ```bash
   git checkout v4.20
   ```

3. You will need to configure your Linux build. You can start from
   [our recommended config](../resources/microvm-kernel-config) - just copy
   it to `.config` (under the Linux sources dir). You can make interactive
   config adjustments using:

   ```bash
   make menuconfig
   ```

   *Note*: there are many ways of building a kernel config file, other than
   `menuconfig`. You are free to use whichever one you choose.

4. Build the uncompressed kernel image:

   ```bash
   make vmlinux
   ```

5. Upon a successful build, you can find the uncompressed kernel image under
   `./vmlinux`.


## Creating a rootfs Image

A rootfs image is just a file system image, that hosts at least an init
system. For instance, our getting started guide uses an EXT4 FS image with
OpenRC as an init system. Note that, whichever file system you choose to use,
support for it will have to be compiled into the kernel, so it can be mounted
at boot time.

To build an EXT4 image:
1. Prepare a properly-sized file. We'll use 50MiB here, but this depends
   on how much data you'll want to fit inside:

   ```bash
   dd if=/dev/zero of=/rootfs.ext4 bs=1M count=50
   ```

2. Create an empty file system on the file you created:

   ```bash
   mkfs.ext4 rootfs.ext4
   ```

You now have an empty EXT4 image in `rootfs.ext4`, so let's prepare to
populate it. First, you'll need to mount this new file system, so you
can easily access its contents:

```bash
mkdir /tmp/my-rootfs
sudo mount rootfs.ext4 /tmp/my-rootfs
```

The minimal init system would be just an ELF binary, placed at `/sbin/init`.
The final step in the Linux boot process executes `/sbin/init` and expects it
to never exit. More complex init systems build on top of this, providing
service configuration files, startup / shutdown scripts for various services,
and many other features.

For the sake of simplicity, let's set up an Alpine-based rootfs, with OpenRC
as an init system. To that end, we'll use the official Docker image for
Alpine Linux:
1. First, let's start the Alpine container, bind-mounting the EXT4 image
   created earlier, to `/my-rootfs`:

   ```bash
   docker run -it --rm -v /tmp/my-rootfs:/my-rootfs alpine
   ```

2. Then, inside the container, install the OpenRC init system, and some basic
   tools:

   ```bash
   apk add openrc
   apk add util-linux
   ```

3. And set up userspace init (still inside the container shell):

   ```bash
   # Set up a login terminal on the serial console (ttyS0):
   ln -s agetty /etc/init.d/agetty.ttyS0
   echo ttyS0 > /etc/securetty
   rc-update add agetty.ttyS0 default

   # Make sure special file systems are mounted on boot:
   rc-update add devfs boot
   rc-update add procfs boot
   rc-update add sysfs boot

   # Then, copy the newly configured system to the rootfs image:
   for d in bin etc lib root sbin usr; do tar c "/$d" | tar x -C /my-rootfs; done
   for dir in dev proc run sys var; do mkdir /my-rootfs/${dir}; done

   # All done, exit docker shell
   exit
   ```

4. Finally, unmount your rootfs image:

   ```bash
   sudo umount /tmp/my-rootfs
   ```

You should now have a kernel image (`vmlinux`) and a rootfs image
(`rootfs.ext4`), that you can boot with Firecracker.
