Connecting with X windows

If you get a message about not being able to open or connect to a display, then LCMgui is probably installed on a remote computer. Installation-dependent security measures may cause this. Otherwise, you may only need to inform your local computer and remote computer about each other and X windows.

All of this can be avoided by using a display directly connected to the computer with LCMgui. If this is impractical, then try informing the two partners with the following two commands:

Telling your local display

Log onto your local computer and then enter:

   xhost +yyy.yyy.yyy.yyy

Here, yyy.yyy.yyy.yyy is the IP address of your remote computer. An example is 142.87.102.2.

(You cannot use symbolic host names of the form mycomp.mlab.myu.edu; only the numeric IP addresses of the form yyy.yyy.yyy.yyy will work. In the long term, you may want to replace xhost with a more secure method.)

Telling your remote computer

Now, still on your local computer, connect to your remote computer with

    ssh  -X  -l your_remote_username  yyy.yyy.yyy.yyy

where -X is upper case, -l is lower case L and yyy.yyy.yyy.yyy is again the IP address of your remote computer.

Then start lcmgui with

    ~/.lcmodel/lcmgui

If there is an error message of the form "

...couldn't connect to display
"localhost:??.0"

", enter

    export DISPLAY=127.0.0.1:??.0

that is, you simply replace localhost with 127.0.0.1; so, be sure to replace ??.0 above with the actual number at the end of the error message (often 10.0). Put no spaces on either side of the = sign.

If you get a message that the command export is not found, then enter:

    setenv DISPLAY 127.0.0.1:??.0

If, instead, there is an error message of the form "

... couldn't 
connect to display...

", but with "localhost" missing from the message, then enter

    export DISPLAY=xxx.xxx.xxx.xxx:0

where xxx.xxx.xxx.xxx is the IP address of your local computer.

In either case, now try again with

    ~/.lcmodel/lcmgui

Possible additional Linux settings

Recent Linux distributions can require that a configuration file be edited. With the GNOME window manager, the file is typically /etc/X11/gdm.conf or /etc/opt/gnome/gdm/gdm.conf (or /etc/gdm/custom.conf with Fedora Core 5). Change the line DisallowTCP=true to

   DisallowTCP=false

Reportedly with KDE, the file is typically /etc/kde3/kdm/kdmrc (but the directory may very well differ). Comment out ServerArgsLocal=-nolisten tcp with a #:

   #ServerArgsLocal=-nolisten tcp

Then restart GNOME or X windows.

Older Linux distributions will not have these strings and should not need changes. High-security installations may need more changes. Newer distributions may need other changes. Sometimes remote X windows will work with some applications (such as xclock) and not work with others (such as lcmgui). Ask your system manager. Internet search engines can also help you find a solution for your Linux distribution.

Further help

If problems or questions remain, send the complete error message to sp@lcmodel.CA

Last modified: 18 November 2016