In the

X Window System The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X originated as part of Project Athena at Massachusetts Institute of Technology (MIT) in 1984. The X protocol has been at ...

, an X display manager is a graphical login manager which starts a

login session In computing, a login session is the period of activity between a user logging in and logging out of a (multi-user) system. On Unix and Unix-like operating systems, a login session takes one of two main forms: * When a textual user interface is ...

on an

X server X server may refer to: * a display server for the X Window System * X.Org Server, the X.Org Foundation's display server for the X Window System * HPE Integrity Superdome X Server, a line of HPE Integrity Servers HPE Integrity Servers is a ser ...

from the same or another

computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...

. A display manager presents the user with a login screen. A session starts when a user successfully enters a valid combination of

username A user is a person who uses a computer or Computer network, network Service (systems architecture), service. A user often has a user account and is identified to the system by a username (or user name). Some software products provide serv ...

and

password A password, sometimes called a passcode, is secret data, typically a string of characters, usually used to confirm a user's identity. Traditionally, passwords were expected to be memorized, but the large number of password-protected services t ...

. When the 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 and on character-mode terminals. When the display manager runs on a remote computer, it acts like a

telnet Telnet (sometimes stylized TELNET) is a client-server application protocol that provides access to virtual terminals of remote systems on local area networks or the Internet. It is a protocol for bidirectional 8-bit communications. Its main ...

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 terminal X, or x, is the twenty-fourth letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''ex'' (pronounced ), plural ''exes''."X", ' ...

s, just coming onto the market. Various display managers continue in routine use to provide a graphical login prompt on standalone

computer workstation A workstation is a special computer designed for technical or scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating systems. The term ''workstat ...

s running X. X11R4 introduced the X Display Manager Control Protocol (XDMCP) in December 1989 to fix problems in the X11R3 implementation.

History

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. In X11R3, XDM only knew about an X terminal from its entry in the file, but XDM only consulted this file when it started. Thus every time a user switched a terminal off and on, the

system administrator An IT administrator, system administrator, sysadmin, or admin is a person who is responsible for the upkeep, configuration, and reliable operation of computer systems, especially multi-user computers, such as Server (computing), servers. The ...

had to send a SIGHUP

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

to XDM to instruct it to rescan . XDMCP arrived with the introduction of X11R4 (December 1989). With XDMCP, the X server must actively request a display manager connection from the host. An X server using XDMCP therefore no longer requires an entry in .

Local and remote display management

A display manager can run on the same computer where the user sits—starting one or more X servers, displaying the login screen at the beginning and (optionally) every time the user logs out—or on a remote one, working according to the XDMCP protocol. The XDMCP protocol mandates that the X server starts autonomously and connects to the display manager. In the X Window System paradigm, the server runs on the computer providing the display and input devices. A server can connect, using the XDMCP protocol, to a display manager running on another computer, requesting it to start the session. In this case, the X server acts as a graphical

client while the display manager acts like a telnet server: users start programs from the computer running the display manager, while their input and output take place on the computer where the server (and the user) sits. An administrator can typically configure an XDMCP Chooser program running on the local computer or X terminal to connect to a specific host's X display manager or to display a list of suitable hosts that the user can choose from. Most implementations enable such a list to contain: # a predefined set of hosts and their respective network addresses, and/or # a set of hosts (on the local

TCP/IP The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are ...

subnet A subnet, or subnetwork, is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identica ...

) that the XDMCP Chooser determines by a network broadcast to the available display managers. When the user selects a host from the list, the XDMCP Chooser running on the local machine will send a message to the selected remote computer's display manager and instruct it to connect the X server on the local computer or terminal.

X Display Manager Control Protocol

The X Display Manager Control Protocol (XDMCP) uses UDP

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...

177. An X server requests that a display manager start a session by sending a Query packet. If the display manager allows access for that X server, it responds by sending a Willing packet back to the X server. (The X server can also send BroadcastQuery or IndirectQuery packets to start a session - this mechanism for requesting a session resembles using

DHCP The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on Internet Protocol (IP) networks for automatically assigning IP addresses and other communication parameters to devices connected to the network using a clie ...

to request an IP address.) The display manager must authenticate itself to the server. To do this the X server sends a Request packet to the display manager, which returns an Accept packet. If the Accept packet contains the response the X server expects, the display manager is authenticated. Producing the correct response might require the display manager to have access to a

secret key A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm, can encode or decode cryptographic data. Based on the used method, the key c ...

, for example. If authentication succeeds, the X server sends a Manage packet to inform the display manager. Then the display manager displays its login screen by connecting to the X server as a regular X client. During the session, the server can send KeepAlive packets to the display manager at intervals. If the display manager fails to respond with an Alive packet within a certain time, the X server presumes that the display manager has ceased running, and can terminate the connection.

Security

One problem with XDMCP is that, similarly to

, the authentication takes place unencrypted. If snooping is possible, this leaves the system vulnerable to attack. It is more secure to use an ssh

tunnel A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ve ...

for X traffic.

Implementations

The

supplies XDM as its standard display manager. Programmers have developed other X display managers, both commercial and free, offering additional functionality over the basic display management:

Active

* GDM,

GNOME A gnome () is a mythological creature and diminutive spirit in Renaissance magic and alchemy, introduced by Paracelsus in the 16th century and widely adopted by authors, including those of modern fantasy literature. They are typically depict ...

implementation * SDDM, recommended display manager for KDE Plasma 6 and

LXQt LXQt is a free and open-source software, free and open source lightweight desktop environment. It was formed from the merger of the LXDE and Razor-qt projects. Like its GTK predecessor LXDE, LXQt does not ship or develop its own window manager ...

. Successor to KDM. * LightDM, a lightweight, modular, cross-desktop, fully themeable desktop display manager by Canonical Ltd. *

TWin Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two ...

, the TDE window manager * dtlogin (shipped with CDE)

Inactive

* KDM (part of

KDE KDE is an international free software community that develops free and open-source software. As a central development hub, it provides tools and resources that enable collaborative work on its projects. Its products include the KDE Plasma gra ...

) allows the user to graphically select a

window manager A window manager is system software that controls the placement and appearance of window (computing), windows within a windowing system in a graphical user interface. Most window managers are designed to help provide a desktop environment. They ...

desktop environment In computing, a desktop environment (DE) is an implementation of the desktop metaphor made of a bundle of programs running on top of a computer operating system that share a common graphical user interface (GUI), sometimes described as a graphi ...

in the login screen
Qingy
ultralight and very configurable graphical login independent on X Window (uses DirectFB)
XDM-OPTIONS
for XDM. Easy full install, Xhost Phonebook, X Login, X Desktop Chooser, menu-reconfig, repair utils. * LDM, the (remote) Display Manager of the Linux Terminal Server Project
MDM
a graphical display manager developed for Linux Mint. * (provided by SCO Open Desktop) also checks for expired passwords and performs some administrative tasks * WINGs Display Manager (using the WINGs widget-set used in Window Maker) * entranced/entrance (employs the architecture used in Enlightenment v.17, on hiatus since 2005)
LXDM
a lightweight cross-desktop and fully themeable display manager, part of

LXDE LXDE (abbreviation for Lightweight X11 Desktop Environment) is a Free and open-source software, free desktop environment with comparatively low resource requirements. This makes it especially suitable for use on older or resource-constrained pe ...

SLiM
an independent login manager.
CDM
an ultralight Console Display Manager for Unix
xlogin
, X Window login with separate XDMCP server
Enter
a lightweight graphical login manager
Orthos
another lightweight solution with very configurable animated themes that use OpenGL only
nodm
auto-login display manager for systems like kiosks, appliances and mobile phones
xlogin display manager
a lightweight, secure and login like console display manager for X, written in C. On some Unix distributions, the default display manager is selected in file $PREFIX/etc/X11/default-display-manager.

Sources

XDMCP specification
from the X.Org release documentation
XDM manual page
(XFree86.org) * Linda Mui and Eric Pearce, ''X Window System Volume 8: X Window System Administrator's Guide for X11 Release 4 and Release 5, 3rd edition'' (O'Reilly and Associates, July 1993; softcover )

References

External links

from th
FreeBSD Handbook

Linux login with a Windows box and XDMCP
A guide to logging into linux using windows. {{XWinSys X Window System