What is a LightDM Greeter?
LightDM is a lightweight display manager that splits the responsibilities of a display manager between the server and the UI, which is known as a greeter. When you login to Ubuntu you are shown the Unity Greeter, but all the back-end session management and authentication is provided by the LightDM daemon, and functionality that a greeter needs is provided by liblightdm. LightDM exposes a set of APIs that allow you easily and quickly write your own greeter using your choice of languages. As long as you can interact with LightDMs GObject API, you can use any language you want.
Why Write a Greeter?
Given all that, why would you want to write a greeter? The Unity Greeter looks pretty nice after all. In my case, I had special requirements that necessitated a change to the UI and some extra processing for the session. During the course of this work, I wrote a greeter based on the sample GTK greeter that comes with LightDM and then applied my modifications. I originally wrote it in C, but then moved to Python for easier integration into my other code. My hope is that you can re-use some of my learnings below and write your own greeter that suits the needs of your distro or project.
Let’s Get Started
This post is designed to give you an overview of the greeter and how it works. The code for my very simple example greeter, which is available here, is commented to the point where I hope it will answer your code questions. After reading this post, walk through the code and I think the process will make sense. Once you understand this code and want to do more advanced things, you should check out the GTK example greeter, which is part of the LightDM GTK+ Greeter project.
Note: You can also get my code from launchpad here, or via bzr with bzr branch lp:~mfisch/+junk/example-greeter
How LightDM Authentication Works
The first thing to understand is the way LightDM authenticates is a multi-step process and that this process will guide how we write our greeter.
- User enters a username and clicks the Login button.
- LightDM passes the username to PAM and responds back to the greeter with a prompt, such as “Password:”.
- User enters the password and the greeter passes it back to LightDM.
- LightDM authenticates with PAM and returns success or failure back to the greeter.
- LightDM may also return a message, such as “Login failed”, which needs to be displayed to the user.
- If the authentication succeeds, the greeter starts a session.
The diagram below shows the flow between the greeter, the top row, and the LightDM backend, the bottom section. This flow assumes a successful login as we don’t show the failure case for simplicity’s sake.
Looking at the Code
At this point you probably want to look at the code and compare it to the flow in the diagram. The code is heavily commented and hopefully the diagram makes it easy to follow.
Screenshots of the Example Greeter
As you can see our greeter isn’t pretty, but it works. Hopefully this gives you an idea of the prompt re-use, which is pretty standard in greeters I’ve seen. Making this look nice is left as an exercise to the artistic reader.
Installing Your Greeter
Your greeter will minimally include an executable and a desktop file. The executable is usually installed somewhere in the path, like /usr/bin. The .desktop file goes into /usr/share/xgreeters and tells LightDM how to launch your greeter. The name of the .desktop file defines what you need to put in LightDM’s conf file. Additionally your greeter may install a configuration file, typically in /etc/lightdm and a Gtk UI file, if you’re using GTK. Once your greeter is in place you need to tell LightDM to use it. If you’re just testing things, you can edit /etc/lightdm/lightdm.conf and set greeter-session=example-greeter. What you set here has to match the name of your desktop file as mentioned above. If you plan to do this in a package install, you need to use /usr/lib/lightdm-set-defaults, which allows you to set a few defaults via a script. In this case we’d call /usr/lib/lightdm/lightdm-set-defaults –greeter=example-greeter. You can run that script with no arguments to see it’s full capabilities and options.
Once you install your greeter, you need to restart LightDM with sudo initctl restart lightdm. If it works, great! If not, check out the Debugging section below.
A few hints on debugging your greeter. Anything you print to stderr in your greeter ends up in /var/log/lightdm/x-0-greeter.log. I find that keeping that open in a separate console dedicated to looking at this log always helps. Since the logs get deleted every time the greeter restarts, this is your best option: sudo tail -f /var/log/lightdm/x-0-greeter.log
LightDM itself may have useful logs if your configuration is broken, they’re in /var/log/lightdm/lightdm.log, and again you need sudo to view the logs.
Debugging a greeter with a debugger can be tricky, so I’ve found that printing debug into to stderr is your best option. However, the greeter seems simple enough that this method usually works fine.
Also you almost always want to do your development of a greeter in a VM. Alternatively, you can run lightdm in test mode with lightdm –test-mode, which allows you to run lightdm from the command-line as an unprivileged user. This will start your greeter in a window inside your existing session.
Finally you may get your greeter into a state where it keeps trying to launch and keeps failing. In a VM this will result in the window popping between X and a text terminal as it keeps failing. Just wait a bit and LightDM will eventually give up on your greeter and you can drop into a console and see what’s wrong.
LightDM’s API is defined here. Everything I used in the example greeter focuses on the Greeter section. If you want to do more advanced things like allow the user to shutdown the system from the greeter, or query the list of available sessions, please check the other sections.
I also highly recommend walking through the GTK Greeter for seeing how some of these more advanced options work.
A word of caution if you’re going to use Python, in version 1.0.6 of LightDM, the introspection bindings for Python were broken. They’re fixed on trunk, but I don’t think a fix has made it into a release yet (as of Feb 6, 2012).
This example code barely covers the surface of what you need to do in order to make a good greeter, but it should help you get started. I recommend for follow-up that you read through example the GTK example greeter, which comes in the source for the LightDM package. You can use that knowledge to make your greeter look much nicer and add features like accessibility (a11y) and options like shutdown/restart/etc from your greeter.