Debugging tools on Flatcar Container Linux

    You can use common debugging tools like tcpdump or strace with Toolbox. Using the filesystem of a specified Docker container Toolbox will launch a container with full system privileges including access to system PIDs, network interfaces and other global information. Inside of the toolbox, the machine’s filesystem is mounted to /media/root.

    Quick debugging

    By default, Toolbox uses the stock Fedora Docker container. To start using it, simply run:


    NOTE: For Fedora, it’s recommended to use at least 2048 MB RAM to avoid the following dnf operation being killed by the OOM manager.

    You’re now in the namespace of Fedora and can install any software you’d like via dnf. For example, if you’d like to use tcpdump:

    [root@srv-3qy0p ~]# dnf -y install tcpdump
    [root@srv-3qy0p ~]# tcpdump -i ens3
    tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
    listening on ens3, link-type EN10MB (Ethernet), capture size 65535 bytes

    Specify a custom Docker image

    Create a .toolboxrc in the user’s home folder to use a specific Docker image:

    $ cat .toolboxrc
    $ /usr/bin/toolbox
    Pulling repository

    You can also specify this in a Butane Config:

    variant: flatcar
    version: 1.0.0
        - path: /home/core/.toolboxrc
          mode: 0644
            inline: |

    Under the hood

    Behind the scenes, toolbox downloads, prepares and exports the container image you specify (or the default fedora image), then creates a container from that extracted image by calling systemd-nspawn. The exported image is retained in /var/lib/toolbox/[username]-[image name]-[image tag], e.g. the default image run by the core user is at /var/lib/toolbox/core-fedora-latest.

    This means two important things:

    • Changes made inside the container will persist between sessions
    • The container filesystem will take up space on disk (a few hundred MiB for the default fedora container)

    Spawn a toolbox with tmux in the background

    Since toolbox can only be started once it is not straightforward to use tmux for long-running jobs or sharing a debugging session with someone else.

    To keep user processes running in the background after logging out with SSH, you need to start them via systemd-run because process lingering is disabled by default in logind and all non-service user processes are killed on logout. Spawn a user service to persist the toolbox container with the tmux process even when you log out with SSH. The following command line will ensure tmux, strace and pidof are installed in the container, then create a new tmux session to which you can later attach, and keep the service active by waiting with strace until the tmux process exits.

    systemd-run --user toolbox sh -c 'dnf install -y tmux strace procps-ng; TERM=tmux tmux new-session -d -s sharedsession; strace -p "$(pidof tmux)"'

    With -d we tell tmux to not allocate a TTY now (needed for systemd-run) but run a new session in the background. Because tmux forks away, we cannot use wait in the shell to wait for children but need to use strace to have a foreground process running that prevents toolbox from quitting.

    Once this is running you can can attach to the tmux session as often as you want from any SSH connection.

    sudo nsenter -t "$(pidof tmux | cut -d ' ' -f 1)" -a tmux a

    As usual with tmux you can attach and detach to the session as many times as you want because detaching still keeps tmux running in the background. But keep in mind that if you exit the session, the process started with systemd-run will terminate and you’ll have to start the service again with systemd-run.

    SSH directly into a toolbox

    Advanced users can SSH directly into a toolbox by setting up an /etc/passwd entry:

    useradd bob -m -p '*' -s /usr/bin/toolbox -U -G sudo,docker,rkt

    To test, SSH as bob:

    ssh [email protected]
    Flatcar Container Linux by Kinvolk alpha (2671.0.0)
    Downloading sha256:ee7e8933710 [=============================] 63.4 MB / 63.4 MB
    Spawning container bob-fedora-latest on /var/lib/toolbox/bob-fedora-latest.
    Press ^] three times within 1s to kill container.
    [root@srv-3qy0p ~]# dnf -y install emacs-nox
    [root@srv-3qy0p ~]# emacs /media/root/etc/systemd/system/newapp.service