X display manager

In the X Window System, an X display manager is a graphical login manager which starts a login session on an X server from the same or another computer. An X display manager presents the user with a login screen. A session starts when a user successfully enters a valid combination of username and password. When the X display manager runs on the user's computer, it starts the X server before presenting the user the login screen, optionally repeating when the user logs out. In this condition, the DM realizes in the X Window System the functionality of getty and login on character-mode terminals. When the display manager runs on a remote computer, it acts like a telnet server, requesting username and password and starting a remote session. X11 Release 3 introduced display managers in October 1988 with the aim of supporting the standalone X terminals, just coming onto the market. Various display managers continue in routine use to provide a graphical login prompt on standalone computer workstations running X. X11R4 introduced the X Display Manager Control Protocol (XDMCP) in December 1989 to fix problems in the X11R3 implementation. XDM (the X Window Display Manager) originated in X11R3. This first version, written by Keith Packard of the MIT X Consortium, had several limitations, the most notable of which was that it could not detect when users switched X terminals off and on.