linux-container-runtime-from-scratch

Building a linux container runtime like @docker or @rkt from scratch

What are container runtime ?

Container Runtime are the system responsible for running and managing containers.

What are Containers ?

Containers are the tarballed images containing

• App metadata (Instructions and details of the containerised application).
• Filesystem (Containerised App + an operating system having no kernel and no init)

Steps to create a container image and container runtime :

• Step 1.: Create a simple linux system in a directory as a chroot.

  • A Script named "1_rootfs_setup.sh" is provided to automate this step. Note : The script is for Fedora based systems.

• Step 2.: Configuring Namespaces

  • The chroots of other systems :

    • Process Trees :

      sudo unshare -p -f
      --mount-proc=$PWD/rootfs/proc
      chroot rootfs /bin/bash

    • Network interfaces :

    • Mounted volumes

Further ,

• Find the process by

  • ps aux | grep /bin/bash

• Export PID as PID=1234 in environment

• Check the process Namespaces by

  • ls /proc/$PID/ns

• Step 3.: Entering "Namespaces" :

  • Namespaces are composable .

  Like in Kubernetes pod :

  • Multiple processes with different chroots are there .
  • Same network and mount namespace.

Further , for entering and seperating the "Process Namespace" :

  nsenter \
    --pid=/proc/$PID/ns/pid \
    --mount=/proc/$PID/ns/mnt \
    chroot $PWD/rootfs /bin/bash

• Step 4.: Mounting volumes :

  • nsenter --mount=/proc/$PID/ns/mnt
    mount --bind -o ro
    $PWD/readonlyfiles
    $PWD/rootfs/var/readonlyfiles