XDM

XDM stands for X Display Manager, a graphical login manager for the X Window System. It is responsible for presenting a login prompt, starting the X server, and launching a user session. XDM was part of early X11 distributions and remains available as a simple, lightweight display manager in many Unix-like systems, though it has largely been superseded by more feature-rich alternatives such as GDM and LightDM.

XDM supports XDMCP (X Display Manager Control Protocol) for initiating sessions from remote hosts, enabling remote logins to the local X session. However, XDMCP is generally considered insecure on untrusted networks, and many deployments disable it by default.

Configuration for XDM is handled via a small set of files, typically located under /etc/X11/xdm, including Xservers (which lists the servers to start), Xaccess (which hosts are allowed to connect), and Xsession (which defines the login session command). The xdm program itself can be started by init or via the system’s service manager on modern distributions.

Other uses: The acronym XDM is also used by other software projects, notably Xtreme Download Manager, a cross-platform download manager; context usually clarifies the reference. See also related display managers such as GDM, LightDM, and SDDM.