Disable the user list on the CentOS7 login screen.
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Create this file , Add this content to that file:
# vi /etc/dconf/db/gdm.d/00-login-screen
[org/gnome/login-screen]
# Do not show the user list
disable-user-list=true
#
Run this command and restart X, I just rebooted
# dconf update
# systemctl restart gdm
Disable the user list on the CentOS6 login screen.
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# gconftool-2 --direct --config-source=xml:readwrite:/etc/gconf/gconf.xml.defaults --type bool --set /apps/gdm/simple-greeter/disable_user_list true