XDMCP starts session displaying gdm login screen on host not Xserver

Dear Ubuntu developers,

I am preparing local host and about 20 clients to use Ubuntu based "X terminals" to teach parallel programing class.

I was used XDMCPfor openSUSE and Ubuntu as well with MAC OS X and Ubuntu remote X servers(X-terminals) using xdm
 in the past

With Ubuntu 15.04 there is an issue. XDCMP request form X -terminal contacts host, but host does not connect to remote X-server and instead starts local session for host and therefore stops running unity and reach login screen with System runs in low resolution mode error message

Is there any way to change such behavior using gdm , because it is only display manager I was able to use pam-ldap login with "Not Listed/Someone else" login name option with history of recently logged users

I look forward hearing form you

Yours faithfully

Peter Fodrek