# Why are exceptions raised by X or Network Objects applications?

Graphical applications (based on Trestle/X Windows) raise the TrestleComm.Failure exception when the DISPLAY environment variable is incorrect or the X server is refusing the connection. They raise MachineIDPosix.Failure if the network configuration files are incorrectly set up, especially on LINUX; /etc/hosts must contain at least a loopback address (127.0.0.1) and the /etc/rc scripts an appropriate ifconfig command (/etc/ifconfig lo 127.0.0.1; /etc/route add 127.0.0.1). Applications with Network Objects may also raise exceptions or consume all the CPU time available when the network configuration files are incorrect.