xephyr and lightdm

[planarian]

I'm trying to figure how to nest X sessions within lightdm using Xephyr, employing two separate accounts. When I execute

ssh -XfC  account-name@my-computer lightdm-session

on the client side I get a cascade of errors, so clearly I either need to make changes to my lightdm configuration, or to use something other than lightdm-session in the ssh command. Pointers, anyone?

[Toz]

If you are looking at running a local nested X session, you can simply run:

dm-tool add-nested-seat

...to open up a second nested session.



If you want to run a remote nested session, it gets a little more complicated. On the remote host, create the file /usr/local/bin/nestedX with the following content:

#!/bin/bash
export XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
/usr/bin/dm-tool add-nested-seat

...and make the file executable. Then from the guest system:

ssh -XfC  account-name@my-computer nestedX

(Note: ensure that openssh-server is installed and properly configured and that port 22 is allowed through the firewall).

dm-tool (part of lightdm) seems tailored to this but I can't seem to find much in the way of documentation. Passing it --help shows some info:

$ dm-tool --help
Usage:
  dm-tool [OPTION...] COMMAND [ARGS...] - Display Manager tool

Options:
  -h, --help        Show help options
  -v, --version     Show release version
  --session-bus     Use session D-Bus

Commands:
  switch-to-greeter                   Switch to the greeter
  switch-to-user USERNAME [SESSION]   Switch to a user session
  switch-to-guest [SESSION]           Switch to a guest session
  lock                                Lock the current seat
  list-seats                          List the active seats
  add-nested-seat                     Start a nested display
  add-local-x-seat DISPLAY_NUMBER     Add a local X seat
  add-seat TYPE [NAME=VALUE...]       Add a dynamic seat

The one thing I haven't been able to figure out is how to change the viewable screen dimensions of the nested session.

[planarian]

Thanks for bringing dm-tool to my attention!

I used your remote-host instructions because although I'm working on a single machine, I want to import a session from another login. Unfortunately, although

ssh -XfC  account-name@my-computer nestedX

produces the right login prompt, when I enter the password it opens a nested session onto my current desktop, not the one I logged in for....

[Toz]

I used your remote-host instructions because although I'm working on a single machine, I want to import a session from another login. Unfortunately, although

ssh -XfC  account-name@my-computer nestedX

produces the right login prompt, when I enter the password it opens a nested session onto my current desktop, not the one I logged in for....

Just to confirm, you are trying to connect to an existing login session on the same computer but have it display in a nested session? If so, I'm not sure thats possible with lightdm.

[planarian]

Yes that is what I'm trying to do. But here's the odd thing: If I reverse the client and server, the process works. In fact if I merely execute

dm-tool add-nested-seat

from the second account (without ssh), the first account appears in a Xephyr window without even asking me for a password. Not only is that bizarre, it would seem to be a significant security hole!

[EDIT: I may have misunderstood your question. I'm trying to use an existing login, but NOT to connect to an existing session. Rather than login to a virtual terminal, I'd like to access my second account from a window within the first]

[Toz]

Yes that is what I'm trying to do. But here's the odd thing: If I reverse the client and server, the process works. In fact if I merely execute

dm-tool add-nested-seat

from the second account (without ssh), the first account appears in a Xephyr window without even asking me for a password. Not only is that bizarre, it would seem to be a significant security hole!

I am unable to replicate that. Whenever I create a nested seat (from either login), I am greeted with the login screen.

[planarian]

I just realized that my primary account had autologin enabled. Once I switched it off, the greeter appears no matter which account I start from (the desired result). Thanks very much! Now if only I can figure a way to increase the display area....

[planarian]

Well, I have it straight from the horse's mouth that there's currently no way to change Xephyr's display area when it's invoked by dm-tool. At the project leader's suggestion, I've submitted a bug report. If anyone here would like to cast a vote, it would probably help speed things along....

[Toz]

Well, I have it straight from the horse's mouth that there's currently no way to change Xephyr's display area when it's invoked by dm-tool. At the project leader's suggestion, I've submitted a bug report. If anyone here would like to cast a vote, it would probably help speed things along....

Thanks. I've added my name to the affected users list.

[planarian]

While we're waiting for a proper update, I decided to go ahead and roll my own. Please understand that I'm not an experienced C hacker -- caveat emptor -- but the attached executable/src ought to do the trick:

dm-tool.modifed add-nested-seat -screen 800x600

dm-tool.modifed add-nested-seat -fullscreen

My changes only add accommodation for -screen and -fullscreen. The code was taken from version 1.6.0.