# 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|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]