If you're trying to get XDMCP working on CentOS 7 or RHEL 7, you must switch lightdm and xfce b/c GDM and Gnome (and I assume KDE) require direct hardware access. I kept getting a black screen when trying to connect. Switching to lightdm resolved this.
This page explains it best.
https://www.netsarang.com/forum/xmanager/4076/how_to_configure_xmanager_to_centos7
