Ask for step-by-step instructions to connect between local X server and remote display manager via XDMCP

[Heng Zhou]

I want set up an environment in which a local X server is connecting a
remote display manager via XDMCP protocol, just like the second picture in
the wikipedia item "X display manager" (
http://en.wikipedia.org/wiki/X_display_manager_%28program_type%29) shows.
The specific configuration is described in the following.

A computer is running ubuntu 14.04 LTS in front of me. Call it "local
machine".

A headless server is running ubuntu 14.04 LTS at another geographical
place. Call it "remote machine". The remote machine has no graphics card,
no attached monitor, but I have full control over it. Concretely, it is an
Amazon EC2 ubuntu instance.

The local machine has a physical graphics card and an attached monitor. It
is running ordinary ubuntu desktop so the X Window System -- X.Org Server
-- is running after boot, presumably.

The remote machines has installed LightDM, the display manager of ubuntu.
And I am sure it is running over there by checking the results returned by
command ps aux|grep lightdm.

That's the configuration of the local and remote machines. Now I need to
follow the steps in the wikipedia item "X display manager" (please see the
link above) to try to connect the local X server with the remote display
manager LightDM. What I wish to happen should be, according to my
understanding of the wikipedia webpage, as follows:

(1) The local machines run a XDMCP Chooser program to select the display
manager running now on the remote machine to connect.

(2) The X server on the local machine uses XDMCP protocol to shake hands
with the remote display manager LightDM.

(3) The remote display manager LightDM should respond as expected:

a) it connects to the local X server as a client

b) display a login GUI on the local machine, with the assistance of the
local X server

c) start an X session after I log in

d) run X applications like xterm in the session as directed; display its
graphical output on the local machine, using the local X display server.

e) If possible, I hope in addition to show the unity desktop on the local
machine, just like a normal ubuntu desktop, but the executable code is
actually running on the remote machine.

That's all I want. But I have no idea how all these can be done. I searched
the internet and found something like http://wiki.ubuntu.com/xdmcp and
http://danilodellaquila.com/blog/how-to-remote-login-via-xdmcp-in-ubuntu,
but they are either too old or too too old, and therefore useless for my
configuration. So I wonder if anyone familiar with XDMCP and having a
workable experience could please share his/her ideas on how to setting up
this environment. Please be specific. Thanks a lot. If you need any other
info, please ask.

PS: The display manager on the remote machine doesn't have to be LightDM. I
specifies it just because it's light weight and I'm familiar with ubuntu
desktop. You can use other option, say, GDM, if you are working or once
worked with it, to set up the environment.

Looking forward to any workable instructions. Thanks.
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