hanneseichblatt.de About Pages Tags Contact RSS

Underappreciated Systemd Features: systemd-nspawnd

17 August 2017

systemd(1) does contain something that looks very similar to a minimal container solution. It is not meant to be a full blown container solution but merely meant to

cover testing, debugging, building, installing, recovering. That’s what you should use it for and what it is really good at, and where it is a much much nicer alternative to chroot(1).

You can think of systemd-nspawn(1) more like LXC, less than Docker. Changes to your filesystem are persistent across container boots and there is no clearly defined repository for operating system images for example. Nevertheless it provides a built-in (since most Linux distributions use systemd(1) nowadays) way to start a Linux system without sprinkling your filesystem with the remnants of your experiments.

Your containers will live in a directory for each one under /var/lib/machines. Suppose we want to create an Arch Linux container, we can use pacman(8) to bootstrap our root filesystem:

# cd /var/lib/machines
# mkdir ./archcontainer
# pacstrap -i -c -d /var/lib/machines/archcontainer --noconfirm base base-devel

This will install the base and base-devel groups, essential for every Arch Linux system into our directory under /var/lib/machines. We then need to make a couple of configuration changes. First, we activate systemd-networkd(8), systemd(1)’s built-in network manager. At this point, you could also use NetworkManager(8). Additionally, we activate systemd-resolved(8), a local DNS resolver. We use chroot(1) to execute the command within the container.

# chroot /var/lib/machines/archcontainer sh -c "systemctl enable systemd-networkd systemd-resolved"

To be able to log in to your container afterwards, we have to set a passwort for the root account to “r00tpw”. We use chroot(1) again.

# chroot /var/lib/machines/archcontainer sh -c "echo root:r00tpw | chpasswd"

Since our login will not happen via ssh(1) or local login, we need to add /dev/pts(4) to the list of secure tty(4)s in /etc/securetty(5).

# echo "pts/0" >> /var/lib/archcontainer/etc/securetty

Since we used the default location for the directory containing our root filesystem, the container is automatically recognized as a machine by machinectl(1).

# machinectl start archcontainer
# machinectl list

The last command should show your container. There are two ways to get access to your running container from the host system. You can directly start a shell session in your container.

# machinectl shell archcontainer

You could also get a console with a login prompt for the machine by issuing the following command.

# machinectl login archcontainer

If you chose the latter way, you can disconnect from the console by pressing Ctrl+5 three times within one second. You can stop the container by issuing the following command.

# machinectl stop archcontainer

There is another way to start the container besides using machinectl(1), unsurprisingly also using systemd(1), in this case instantiated units.

# systemctl start systemd-nspawn@archcontainer
# systemctl status systemd-nspawn@archcontainer
# systemctl stop systemd-nspawn@archcontainer

Do not mix the machinectl(1) and systemd-nspawn(1) ways of controlling the container.

One interesting effect of the integration of all these concepts in systemd(1), we can use our host’s systemctl(1) to control the container’s systemd(1). For example you can list all units running in the container by issuing the following command on the host.

# systemctl -M archcontainer status

Nevertheless, always keep in mind that this is not a full-blown container technology but a nice way to start a lightweight container to test a task or two.