Kapsel (German for capsule) is a simplified Linux container implemented from scratch in C++. The code and implementation are primarily based on the following resources:

• Go: Building a container from scratch in Go [video]
• Python: A workshop on Linux containers: Rebuild Docker from Scratch
• Ccont: A Linux Container Implemented in C

I have found the first two links in this repository, which contains numerous tutorials. It is definitely worth looking into for anyone who wants to implement their own tools from ground up.

I started this project mainly to practice C++ and familiarize myself with the basic principles behind Docker containers. Since I am new to both C++ and Linux, the code is far from being perfect. As a disclaimer, please be aware that this is only a toy project and the container that I implemented is incomplete. Therefore, I would highly recommended not to use it in a production environment of any kind.

If you want to try Kapsel yourself, follow the steps below:

• Clone this repository and make sure that cmake (> 3.19) is installed.
• Enter the following commands in the terminal:

$ cd kapsel && mkdir build && cd build
$ cmake .. && make -j

• To start the most basic ubuntu container in an interactive bash shell, enter the following command. Note that you have to execute the command with sudo, otherwise container initialization will fail.

$ sudo ./kapsel run /bin/bash
Starting container vllrscbn4aca with pid 9467
Container vllrscbn4aca initialized
Executing command: /bin/bash 
root@vllrscbn4aca:/# id
uid=0(root) gid=0(root) groups=0(root)
root@vllrscbn4aca:/# ps
    PID TTY          TIME CMD
      1 ?        00:00:00 kapsel
      2 ?        00:00:00 sh
      3 ?        00:00:00 bash
      6 ?        00:00:00 ps
root@vllrscbn4aca:/# 

./kapsel [OPTION...] [cmd-type] [args]

$ sudo ./kapsel -t alpine run /bin/sh

Start a alpine container in an interactive shell. The container will be removed after use.

$ sudo ./kapsel -b -i container run /bin/bash

Start a ubuntu container named container whose file system will be compressed and stored after exiting from the shell.

$ sudo ./kapsel ls
#              Image ID        Size                   Last Modified
0             container      26.7MB        Sun Aug 29 20:37:58 2021

List the container images that have been built.

$ sudo ./kapsel rm container
Removed image with ID container

Removes the image with the ID container.

Kapsel containers support the following features:

• Isolation in Linux namespaces:
  • UTS
  • PID
  • MOUNT
  • NETWORK
• Control over CGroup resources:
  • pids.max
  • memory.limit_in_bytes
  • memory.memsw.limit_in_bytes
  • cpu.shares
• Filesystem isolation with chroot and pivot_root.
• Access to the Internet.
• Being able to run, save and delete a stored container image as a tar archive.

• Sometimes, container network initialization will be stuck possibly due to a bug in the use of the POSIX semaphores. Restarting your computer will likely solve the issue.