As I hoped, it looks like XDMCP was fixed as a side effect of the dock- icon fixing. I can do: /usr/X11/bin/Xquartz -query 10.0.10.1 and get a login window that I can interact with. You don't get the full "X11.app" menus, etc... and the icon is a default icon, but it works. However it looks like I broke it when executing from the .app: /Applications/Utilities/X11.app/Contents/MacOS/X11 -query 10.0.10.1 TransformProcessType: Success TransformProcessType: Success X11.app starting: X.Org Xquartz X Server 1.3.0-apple4 Build Date: 20071207 Check-in failed: Permission denied Fatal server error: XDMCP fatal error: Session declined No valid address AbortDDX Quitting XQuartz... ... yes, I intend to fix it... but I thought those of you interested in XDMCP should know...