SDB:FreeNX to existing display

By now it is commonly known FreeNX does not yet support the connection to X display :0 or other native running displays. This article describes a workaround for this problem.

Tested on openSUSE

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Situation

NX is designed to use an internal VNC server and client to make it possible to connect to a already running X session, normally display :0 (X sessions start to count at 0). In the open-source version FreeNX this feature is still not working like it should. This article describes how to use an external VNC server and client for the same functionality.

Good side

This allows you to have the ease of VNC desktop sharing, the security from SSH (NX uses SSH encryption) and the reaction speed and compression from NX. Ideal for slow and high latency connections.

Bad side

Once you have a local X session running, you won't be able to create a new FreeNX session. But if you have a system running where you don't need the local display to be ran, you don't want to read this article. You just want to have a regular NX connection which you can suspend and re-connect to.

When do you want to connect (with FreeNX) to a local display?:

Somebody is actualy working at the computer and you use this solution for cases you want to help him from outside a secured network.
You use GUI software which is automatic loaded during boot and needs to keep running. You want to check up from time to time, form a remote location

Prerequisite

You are already able to log in to your computer using FreeNX. Read FreeNX server if not. This article describes the use of NoMachine's NX client, but should be the same for other client software.

All actions performed from here on are done on the server side, unless otherwise stated.

Software

Since we are going to use our own VNC software and tunnel it over the NX connection, we need the proper VNC software installed. On top of the already installed software you need to open FreeNX connections you will need the following packages:

tightvnc
x11vnc

Install them with:

zypper in tightvnc x11vnc

Configure x11vnc

We are going to use x11vnc to serve our VNC display. X11vnc can take a lot of options. Both on the command line or in a configuration file. Options are the same for both, only the command line way need the "-" suffix. Eg. -localhost on the command line is the same like localhost in the configuration file.

Configuration file

X11vnc automaticly looks for $HOME/.x11vncrc when invoked. For our configuration it is more easy to create this file and type the required options there. Open your favorite text editor and type the following lines:

display :0
shared
forever
localhost
rfbauth /home/user/.vnc/passwd

display :0 Sets the display number. :0 being the first one.
shared Enables the possibility for more than VNC session, not really necessary, but can come in handy.
forever Means x11vnc keeps running when you disconnect. Default is to allow only 1 connection and after that stop x11vnc.
localhost x11vnc doesn't listen to any external interfaces. Bind adress is set to 127.0.0.1 and the only allowed host is 127.0.0.1.
rfbauth /home/user/.vnc/passwd Replace user with your username. This is the path where the VNC password is stored. rfbauth sets the type of password.

Password file

Now we need to create the above mentioned password file. The password just gives you some extra security. The client will use the same file (since it will be run on the same system and as the same user), so you will not have to type the password every time you connect. It just makes sure nobody else can hijack your session. (Eg. other users that have a SSH connection counts as a localhost connection for x11vnc). Run in a shell:

x11vnc -storepasswd

Use the default mentioned location, this is like we set above. Enter the same password twice, only the first 8 characters will be used. So entering longer passwords is useless.

Check the permissions of the file (should be OK by default).

ls -l ~/.vnc

Should give an output like this:

-rw------- 1 user users 8 2010-09-11 16:30 passwd
 ^^
 This should be the only permissions.

If you see more over there, run:

chmod 600 ~/.vnc/passwd

Configure vncviewer

Vncviewer is installed with the "thightvnc" package. We need vncviewer, because NX client somehow fails to connect to a x11vnc. Also executing vncviewer directly, instead of a window manager, does not work. For this we can use a workaround. We create a shell script, which executes vncviewer. We let NX client execute this script (and we tell NX client this script is a window manager) to get the VNC connection working.

Open you favorite text editor and type:

#!/bin/sh
exec vncviewer -passwd /home/user/.vnc/passwd localhost::5900

And save it as /home/user/bin/nxvnc. Make the file executable:

chmod +x /home/user/bin/nxvnc

Vncviewer accepts the same file format for the password, so we point it to the same password file as x11vnc: /home/user/.vnc/passwd, with the -passwd option. localhost::5900 points to the x11vnc port number. 5900 resembles display :0.

Warning: You need the double colon behing localhost. A single collon will represent a display number, but can cause problems when a ssh connection is active with X forwarding enabled. (Somehow it will try to connect to display :10, which is reserved for this ssh connection.)

Running

x11vnc

Issue command:

x11vnc

And x11vnc is loaded with the pre-setting given in the earlier config file. X11vnc is dropped once you close the shell. You are probably using ssh and it's certainly not what you want. You can use screen. Screen is a virtual tty/console inside bash, which can be detached but remains active in the background. Serving x11vnc until you re-attach and kill screen or x11vnc.

screen x11vnc

This loads x11vnc in screen. You can check the progress. If everything looks OK hit CTRL+A+D to detach. If you want to re-attach again issue: (as the same user!)

screen -D -R

You can off course configure your favorite window or display manager to auto-load x11vnc.

NX client

In this example NoMachine's NX client is used. It is obtainable from NoMachine download page. Although it's free, it is not free and opensource.

	Open NX Connettion wizard. You will be shown the welcome window and press next.
	Give your connection a name. Fill in the address and port number of the server. The address can be an IP address or Internet address
	Set the session type as Unix and Other. Press the Settings... button.
	Select the Run the following command option. Type `/home/user/bin/nxvnc` in the text box. Select the New virtual desktop option. Press OK to close the window again.
	In the next screen you need to select Show Advanced Configuration dialog and click finish.
	Under dispay select fullscreen. It makes live lots easier, since the remote desktop is not going to adjust it's size to your window. This gives you some more room over big desktops (equal or more resolution then the local one). Press OK to close this window. Now press Login and see the magic happening!