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LightDM Design

What is a Display Manager?

The responsibilities of a display manager are:

  • Starting and managing local instances of the X server.
  • Authenticating users.

  • Starting and managing user sessions. Common use cases:

  • Starting a single X server on boot and starting a session (kiosk mode).

  • Starting a single X server instance on boot, displaying a greeter GUI (username and password), and starting the user session when connected (traditional).
  • Supporting multiple simultaneous logins by exposing what users are logged in, and starting new X servers for each user (user switching).
  • Running a thin-client server by allowing X servers to connect using XDMCP, and connecting greeters and sessions to those X servers.

Design Goals

  • Fast - The display manager should add no noticeable delay to startup time.
  • Fail-safe - Failures should be handled gracefully.
  • Secure - Resistant to malicious users.
  • Flexible - Able to support a range of use-cases.
  • Extensible - Able to support rarer use-cases though plugins.
  • Simple - Configuration should be easy and the code should be simple to understand and modify.

Definitions

  • Display Manager - A daemon that manages the displays on a system.
  • Display - A combination of an X server, greeter and a user session.
  • User session - An application that runs on a display and allows the user to run applications.
  • Greeter - An application to run on a display and prompt for authentication and session options.

Requirements

Daemon:

  • Able to run as as system service without user interaction
  • Provide logging information for debugging
  • Launch and monitor X servers
  • Launch and monitor greeter applications for displays without a user session
  • Launch and monitor user sessions after user is authenticated
  • Authenticate users
  • Provide an interface to greeter applications
  • Provide an interface to user switchers
  • Store a database of active displays
  • Load sessions from /usr/share/xsessions
  • Store session configuration in ~/.dmrc
  • Support .dmrc not being readable before login
  • Advertise display database to Software/ConsoleKit if available

  • Use PAM for authentication Session environment:

  • Set USER to the username

  • Set HOME to users home directory
  • Set SHELL to the users shell
  • Set PATH to /usr/bin:/bin
  • Set LANG to the users language
  • Set DISPLAY to the X servers address

  • Set environment variables from /etc/environment Static Display Module:

  • Allow 0-N displays to be enabled at all time

  • Support automatic/timed login User Switcher Module:

  • Interface to switch to existing local display for a logged in user

  • Start new static display if user not logged in XDMCP Server Module:

  • Implement XDMCP protocol Module interface:

  • Expose display database Greeter application/interface:

  • User authentication

  • Session choice
  • Session language
  • Session keyboard layout
  • Logged in users

Implementation

Daemon:

  • Implemented in C+GObject

  • Single process Greeter Interface:

  • D-Bus GTK+ Greeter:

Inter-process Relationships

[[!img LightDM interprocess relationships.png]