Logins in gdm default to DISPLAY :1 Customer applications are expecting DISPLAY :0

This document (000020370) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 15 SP3
SUSE Linux Enterprise Server 15 SP2
SUSE Linux Enterprise Server 15 SP1

Situation

When gdm runs in SLES 15 the greeter session (login screen) is started as display :0 and user logins then get display :1 or higher.

Resolution

This is working as designed, see Cause section.

If you want to go back to the old method that is possible, but this method may be removed in future versions of SLES so update applications to make use of the new default method.

To go back to the previous behavior do the following:

Run "systemctl edit display-manager.server" and enter the following (which gets added as a display-manager.service override file in /etc/systemd/system/)

[Service]
Environment="GDM_DISABLE_USER_DISPLAY_SERVER=1"

Check that your changes are correct:
SLES15SP3:~ # systemctl cat display-manager.service
# /usr/lib/systemd/system/display-manager.service
[Unit]
Description=X Display Manager
Requires=var-run.mount
Conflicts=getty@tty7.service plymouth-quit.service
After=ypbind.service gpm.service time-sync.target winbind.service acpid.service
Wants=remote-fs.target dbus.socket systemd-user-sessions.service systemd-logind.service
After=remote-fs.target dbus.socket systemd-user-sessions.service systemd-logind.service getty@tty7.service plymouth->
OnFailure=plymouth-quit.service

[Service]
Type=forking
PIDFile=/run/displaymanager.pid
ExecStart=/usr/lib/X11/display-manager start
ExecStop=/usr/lib/X11/display-manager stop
ExecReload=/usr/lib/X11/display-manager reload
KillMode=process

[Install]
WantedBy=graphical.target
Alias=xdm.service

# /etc/systemd/system/display-manager.service.d/override.conf
[Service]
Environment="GDM_DISABLE_USER_DISPLAY_SERVER=1"

Reboot the system and login.

Cause

"GDM's behavior was changed in 3.16 so that instead of running one X server on the login screen which is then transformed into the user's session, we now start a new X session from within the user's session, which means that we need to start the user's session on another TTY/VT."

Disclaimer

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