The only workaround is to restart GDM (by following the instructions to do that below.) The server reports that the maximum number of sessions from a host have been reached, even though the session has been entirely ended.

An inability of Xnest clients to reconnect.

An inability to login by default (with Xming clients at least.) Switching between regular GNOME and failesafe GNOME sessions repeatedly seems to fix this.

This author believes that this functionality is indeed a work in progress because he has observed the following bugs: Please note that these instructions apply to the rewrite of GDM since Ubuntu 9.10 (although they have not been tested under 9.10.) These instructions are not officially documented and therefore this functionality may be considered a work in progress.

You should now be able to query the server. Restart lightdm after this: sudo restart lightdm XDMCP is disabled by default but can be enabled by adding the following to '/etc/lightdm/nf' Ubuntu 12.04 LTS uses lightdm by default. However, the lack of compression can make XDMCP provide very fast graphics when the bandwidth is available. A 100mbit (wired) network may be necessary. XDMCP uses a large amount of bandwidth because it uses no compression.Also, consider using alternatives that feature security (and often compression) such as NX. Therefore, only use XDMCP on a wired network that you you trust. Over both, it is still somewhat easy for someone hijack your session and assume control of the sever. Over a wired LAN, this is more difficult but still possible.

They can likely read what is being display on your screen. Over wireless, anyone can listen to your traffic and easily record your keystrokes.

XDMCP is analogous to telnet, and therefore they share the same security issues. XDMCP is inherently insecure as it does not encrypt your traffic. XDMCP also needs to be implemented by the display manager. XDMCP is integrated into X.org, the default X11 server in Ubuntu. With XDMCP, one computer A running X11 can connecting computer B running X11, and interact with computer B as if one were physically at computer B.

Nx server ubuntu