FirewallBuilder

The Firewall Builder binary does not start. Error "fwbuilder: cannot connect to X server localhost:0.0" or similar

The Firewall Builder GUI is an X application, that is, it needs X server to display it on the screen. The program determines how to connect to the X server using environment variable DISPLAY; you probably do not have this environment variable if you get an error like that. The simplest way to avoid this problem is to start fwbuilder from the shell window in Gnome or KDE environment.

It may also be that the environment variable DISPLAY is set, but the program fwbuilder cannot connect to the X server. In this situation you won't be able to run any application using X, check if that's the case by trying to start "xclock". This may be happening because of many different reasons, such as X server is not running, X authentication failure, or DISPLAY variable reassigned its value by the shell login script or many others. This problem falls outside the scope of this document, please search on the Internet for the answer. Here are few URLs to make troubleshooting easier: