fallbackdm

This crate is WIP and has not released any source, yet.

fallbackdm is a minimal, headless display manager that exists solely to own a seat and VT when no graphical session is running.

It prevents unintended keyboard input from reaching getty or the kernel VT layer by registering a proper greeter session with systemd-logind, activating a VT, and switching it to graphics mode — without starting X11 or Wayland.

This is primarily useful for kiosk setups, remote desktop systems, or input-virtualization scenarios where no local user interaction is intended, but correct VT semantics must still be preserved.

Problem Statement

On modern Linux systems:

Virtual terminals (VTs) still exist and have a kernel keyboard handler
If no graphical session is active, getty will attach to a VT
Input injected via uinput or forwarded from remote systems may:
- Trigger Ctrl+Alt+Fn
- Wake or interfere with getty
- Cause VT switches or text-mode interaction

Graphical compositors avoid this by:

Registering a session with systemd-logind
Owning a VT
Switching it to KD_GRAPHICS

But when no compositor or greeter is running, nothing owns the VT.

fallbackdm fills exactly this gap.

What fallbackdm Does

Registers a greeter session via PAM + pam_systemd
Acquires a seat using libseat
Activates a VT and switches it to graphics mode
Keeps the session alive while no real graphical session exists
Displays nothing and launches no compositor

Once a real display manager or compositor starts, it naturally replaces fallbackdm.

What fallbackdm Does Not Do

❌ No X11
❌ No Wayland
❌ No greeter UI
❌ No input filtering (by design)
❌ No Device Ownership Required: Unlike a real compositor, fallbackdm does not need to open /dev/dri/cardX or /dev/input/event* to do its job. It only needs the TTY. This minimizes the attack surface significantly.

It only ensures correct session, seat, and VT ownership.

When You Need This

You do not need fallbackdm if:

X11 or Wayland is already running
A display manager (gdm, sddm, greetd, etc.) is active

You do need fallbackdm if:

The system boots without a graphical stack
Input devices (especially uinput) must not reach getty
You rely on logind-correct VT behavior without a real compositor

Architecture Overview

fallbackdm
 ├─ PAM session (class=greeter)
 ├─ pam_systemd
 ├─ libseat
 │   └─ seatd or systemd-logind backend
 └─ VT activation + KD_GRAPHICS

This mirrors what real display managers do — just without launching anything graphical.

PAM Configuration

Create /etc/pam.d/fallbackdm:

session required pam_systemd.so class=greeter

This is mandatory. Without it, logind will not track the session.

systemd Service Example

[Unit]
Description=Fallback Display Manager
After=systemd-user-sessions.service
ConditionPathExists=!/run/graphical-session-active

[Service]
ExecStart=/usr/bin/fallbackdm
PAMName=fallbackdm
Restart=always

[Install]
WantedBy=multi-user.target

The condition is optional and can be replaced with more advanced logic later.

Relationship to Other Projects

Display managers (gdm, sddm, greetd) Full login stacks with UI and session spawning.
Greeters (gtkgreet, tuigreet) UI components launched by a display manager.
fallbackdm A headless, compatibility-focused DM whose only job is to own the seat.

Future Ideas

Optional status output on the VT
Signaling input-forwarding daemons (e.g. vuinputd)
Conditional exit when a real session becomes active

License

MIT