I have a RHEL6 server which we connect via XDMCP. If I start xdm (xdm &) it works but the login screen is the old xdm simple login and not gdm. Our RHEL5 servers connect to gdm just fine. The problem is that GDM isn't running. When I try to launch GDM (gdm &) it gives me the following message:** (gdm-binary:3229): WARNING **: Couldn't connect to system bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directoryI've also tried using (gdm-binary &) and get the same message.cat /etc/gdm/custom.conf# GDM configuration storage[daemon][security]DisallowTCP=falseAllowRemoteRoot=true[xdmcp]Enable=true[greeter]IncludeAll=true[chooser][debug]In /etc/X11/xdm/Xaccess I have the following uncommented:   *                                       #any host can get a login windowIn /etc/X11/xdm/xdm-config I have tried commenting and uncommenting the following:    DisplayManager.requestPort:    0I've referenced several tutorials such as:http://www.yolinux.com/TUTORIALS/GDM_XDMCP.htmlhttp://kosala-linux.blogspot.com/2013/02/x-manager-configurations-for-rheloel.html
Opening session via XDMCP to GDM
rhel;x11;session;gdm;xdmcp
This warning:** (gdm-binary:3229): WARNING **: Couldn't connect to system bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directorymeans that the system D-Bus message bus is not running. D-Bus provides message buses that programs can use to communicate with other programs: there is usually a system message bus, and a message bus per user session.On RHEL, the system message bus is started by the messagebus init script, provided by the dbus package.To start the system message bus you'll have to install the dbus package, and probably issue a service messagebus restart.