Developer Diary Excerpts: machinectl auto-login

Bringing up a root filesystem with systemd-nspawn -b -D and consequently logging into the container necessitates that either a password is set for the desired user, or that an autologin override is present for the console-getty service.

If a container is brought up with machinectl start instead, the console-getty override has no effect for consequent machinectl login invocations, as container-getty@.service is in charge of the pseudo terminals that login attaches to instead.

    State: running
     Jobs: 0 queued
   Failed: 0 units
    Since: Sat 2019-03-16 17:11:26 CET; 16min ago
   CGroup: /
           ├─init.scope
           │ └─1 /lib/systemd/systemd
           └─system.slice
             ├─console-getty.service
             │ └─48 /sbin/agetty --noclear --keep-baud console 115200,38400,9600 vt220
             └─system-container\x2dgetty.slice
               └─container-getty@0.service
                 ├─53 /bin/login -f
                 └─58 -bash

Shortened systemctl status output of container.

Logging into the container without a password can still be achieved by using the shell subcommand of machinectl, though the login subcommand remains the only option for Linux distributions that ship with older versions of systemd, which do not include shell (before v224).

An ExecStart override for container-getty@.service can be used to replicate the behaviour of shell with login.

systemctl edit container-getty@.service

Execute inside the container, not the host system (a text editor has to be installed).

[Service]
ExecStart= 
ExecStart=-/sbin/agetty --noclear --autologin root --keep-baud pts/%I 115200,38400,9600 $TERM

The empty space after the first ExecStart is there on purpose, as it tells systemd to clear the pre-existing ExecStart content.
Substituting root with another user name will instead enable automatic login for the specified user.

Notes

systemd as well as dbus should be installed in the container root file-system, otherwise both login and shell will fail (Failed to get shell PTY: Protocol error).
The override doesn't necessarily have to be created with the edit subcommand of systemd. Creating and subsequenly populating the contents of /etc/systemd/system/container-getty@.service.d/override.conf also works just as well.