GNOME Display Manager Reference Manual

The GNOME Display Manager (GDM) is a display manager that implements all significant features required for managing attached and remote displays. GDM was written from scratch and does not contain any XDM or X Consortium code.

Note that GDM is configurable, and many configuration settings have an impact on security. Issues to be aware of are highlighted in this document.

Please note that some Operating Systems configure GDM to behave differently than the default values as described in this document. If GDM does not seem to behave as documented, then check to see if any related configuration may be different than described here.

For further information about GDM, refer to the project website at http://wiki.gnome.org/Projects/GDM.

For discussion or queries about GDM, refer to the

Please submit any bug reports or enhancement requests to the "gdm" category in http://bugzilla.gnome.org.

GDM 2.20 and earlier supported stable configuration interfaces. However, the codebase was completely rewritten for GDM 2.22, and is not completely backward compatible with older releases. This is in part because things work differently, so some options just don't make sense, in part because some options never made sense, and in part because some functionality has not been reimplemented yet.

Interfaces which continue to be supported in a stable fashion include the Init, PreSession, PostSession, PostLogin, and Xsession scripts. Some daemon configuration options in the <etc>/gdm/custom.conf file continue to be supported. Also, the ~/.dmrc, and face browser image locations are still supported.

GDM 2.20 and earlier supported the ability to manage multiple displays with separate graphics cards, such as used in terminal server environments, login in a window via a program like Xnest or Xephyr, the gdmsetup program, XML-based greeter themes, and the ability to run the XDMCP chooser from the login screen. These features were not added back during the 2.22 rewrite.

GDM is responsible for managing displays on the system. This includes authenticating users, starting the user session, and terminating the user session. GDM is configurable and the ways it can be configured are described in the "Configuring GDM" section of this document. GDM is also accessible for users with disabilities.

GDM provides the ability to manage the main console display, and displays launched via VT. It is integrated with other programs, such as the Fast User Switch Applet (FUSA) and gnome-screensaver to manage multiple displays on the console via the Xserver Virtual Terminal (VT) interface. It also can manage XDMCP displays.

Regardless of the display type, GDM will do the following when it manages the display. It will start an Xserver process, then run the Init script as the root user, and start the greeter program on the display.

The greeter program is run as the unprivileged "gdm" user/group. This user and group are described in the "Security" section of this document. The main functions of the greeter program are to provide a mechanism for selecting an account for log in and to drive the dialogue between the user and system when authenticating that account. The authentication process is driven by Pluggable Authentication Modules (PAM). The PAM modules determine what prompts (if any) are shown to the user to authenticate. On the average system, the greeter program will request a username and password for authentication. However some systems may be configured to use supplemental mechanisms such as a fingerprint or SmartCard readers. GDM can be configured to support these alternatives in parallel with greeter login extensions and the --enable-split-authentication ./configure option, or one at a time via system PAM configuration.

The smartcard extension can be enabled or disabled via the org.gnome.display-manager.extensions.smartcard.active gsettings key.

Likewise, the fingerprint extension can be enabled or disabled via the org.gnome.display-manager.extensions.fingerprint.active gsettings key.

GDM and PAM can be configured to not require any input, which will cause GDM to automatically log in and simply start a session, which can be useful for some environments, such as single user systems or kiosks.

In addition to authentication, the greeter program allows the user to select which session to start and which language to use. Sessions are defined by files that end in the .desktop suffix and more information about these files can be found in the "GDM User Session and Language Configuration" section of this document. By default, GDM is configured to display a face browser so the user can select their user account by clicking on an image instead of having to type their username. GDM keeps track of the user's default session and language in the user's ~/.dmrc and will use these defaults if the user did not pick a session or language in the login GUI.

After authenticating a user, the daemon runs the PostLogin script as root, then runs the PreSession script as root. After running these scripts, the user session is started. When the user exits their session, the PostSession script is run as root. These scripts are provided as hooks for distributions and end-users to customise how sessions are managed. For example, using these hooks you could set up a machine which creates the user's $HOME directory on the fly, and erases it on logout. The difference between the PostLogin and PreSession scripts is that PostLogin is run before the pam_open_session call so is the right place to do anything which should be run before the user session is initialised. The PreSession script is called after session initialisation.

The GDM greeter program displays a panel docked at the bottom of the screen which provides additional functionality. When a user is selected, the panel allows the user to select which session, language, and keyboard layout to use after logging in. The keyboard layout selector also changes the keyboard layout used when typing your password. The panel also contains an area for login services to leave status icons. Some example status icons include a battery icon for current battery usage, and an icon for enabling accessibility features. The greeter program also provides buttons which allow the user to shutdown or restart the system. It is possible to configure GDM to not provide the shutdown and restart buttons, if desired. GDM can also be configured via PolicyKit (or via RBAC on Oracle Solaris) to require the user have appropriate authorization before accepting the shutdown or restart request.

Note that keyboard layout features are only available on systems that support libxklavier.

GDM supports "Accessible Login", allowing users to log into their desktop session even if they cannot easily use the screen, mouse, or keyboard in the usual way. Accessible Technology (AT) features such as an on-screen keyboard, screen reader, screen magnifier, and Xserver AccessX keyboard accessibility are available. It is also possible to enable large text or high contrast icons and controls, if needed. Refer to the "Accessibility Configuration" section of the document for more information how various accessibility features can be configured.

On some Operating Systems, it is necessary to make sure that the GDM user is a member of the "audio" group for AT programs that require audio output (such as text-to-speech) to be functional.

The Face Browser is the interface which allows users to select their username by clicking on an image. This feature can be enabled or disabled via the org.gnome.login-screen disable-user-list GSettings key and is on by default. When disabled, users must type their complete username by hand. When enabled, it displays all local users which are available for login on the system (all user accounts defined in the /etc/passwd file that have a valid shell and sufficiently high UID) and remote users that have recently logged in. The face browser in GDM 2.20 and earlier would attempt to display all remote users, which caused performance problems in large, enterprise deployments.

The Face Browser is configured to display the users who log in most frequently at the top of the list. This helps to ensure that users who log in frequently can quickly find their login image.

The Face Browser supports "type-ahead search" which dynamically moves the face selection as the user types to the corresponding username in the list. This means that a user with a long username will only have to type the first few characters of the username before the correct item in the list gets selected.

The icons used by GDM can be installed globally by the sysadmin or can be located in the user's home directories. If installed globally they should be in the <share>/pixmaps/faces/ directory and the filename should be the name of the user. Face image files should be a standard image that GTK+ can read, such as PNG or JPEG. Face icons placed in the global face directory must be readable to the GDM user.

If there is no global icon for the user, GDM will look in the user's $HOME directory for the image file. GDM will first look for the user's face image in ~/.face. If not found, it will try ~/.face.icon. If still not found, it will use the value defined for "face/picture=" in the ~/.gnome2/gdm file.

If a user has no defined face image, GDM will use the "stock_person" icon defined in the current GTK+ theme. If no such image is defined, it will fallback to a generic face image.

Please note that loading and scaling face icons located in remote user home directories can be a very time-consuming task. Since it not practical to load images over NIS or NFS, GDM does not attempt to load face images from remote home directories.

When the browser is turned on, valid usernames on the computer are exposed for everyone to see. If XDMCP is enabled, then the usernames are exposed to remote users. This, of course, limits security somewhat since a malicious user does not need to guess valid usernames. In some very restrictive environments the face browser may not be appropriate.

The GDM daemon can be configured to listen for and manage X Display Manage Protocol (XDMCP) requests from remote displays. By default XDMCP support is turned off, but can be enabled if desired. If GDM is built with TCP Wrapper support, then the daemon will only grant access to hosts specified in the GDM service section in the TCP Wrappers configuration file.

GDM includes several measures making it more resistant to denial of service attacks on the XDMCP service. A lot of the protocol parameters, handshaking timeouts, etc. can be fine tuned. The default configuration should work reasonably on most systems.

GDM by default listens for XDMCP requests on the normal UDP port used for XDMCP, port 177, and will respond to QUERY and BROADCAST_QUERY requests by sending a WILLING packet to the originator.

GDM can also be configured to honour INDIRECT queries and present a host chooser to the remote display. GDM will remember the user's choice and forward subsequent requests to the chosen manager. GDM also supports an extension to the protocol which will make it forget the redirection once the user's connection succeeds. This extension is only supported if both daemons are GDM. It is transparent and will be ignored by XDM or other daemons that implement XDMCP.

If XDMCP seems to not be working, make sure that all machines are specified in /etc/hosts.

Refer to the "Security" section for information about security concerns when using XDMCP.

GDM uses syslog to log errors and status. It can also log debugging information, which can be useful for tracking down problems if GDM is not working properly. Debug output can be enabled by setting the debug/Enable key to "true" in the <etc>/gdm/custom.conf file.

Output from the various Xservers is stored in the GDM log directory, which is normally <var>/log/gdm/. Any Xserver messages are saved to a file associated with the display value, <display>.log.

The session output is piped through the GDM daemon to the ~/$XDG_CACHE_HOME/gdm/session.log file which usually expands to ~/.cache/gdm/session.log. The file is overwritten on each login, so logging out and logging back into the same user via GDM will cause any messages from the previous session to be lost.

Note that if GDM can not create this file for some reason, then a fallback file will be created named ~/$XDG_CACHE_HOME/gdm/session.log.XXXXXXXX where the XXXXXXXX are some random characters.

GDM allows multiple users to be logged in at the same time. After one user is logged in, additional users can log in via the User Switcher on the GNOME Panel, or from the "Switch User" button in Lock Screen dialog of GNOME Screensaver. The active session can be changed back and forth using the same mechanism. Note that some distributions may not add the User Switcher to the default panel configuration. It can be added using the panel context menu.

Note this feature is available on systems that support Virtual Terminals. This feature will not function if Virtual Terminals is not available.

For security reasons a dedicated user and group ID are recommended for proper operation. This user and group are normally "gdm" on most systems, but can be configured to any user or group. All GDM GUI programs are run as this user, so that the programs which interact with the user are run in a sandbox. This user and group should have limited privileges.

The only special privilege the "gdm" user requires is the ability to read and write Xauth files to the <var>/run/gdm directory. The <var>/run/gdm directory should have root:gdm ownership and 1777 permissions.

You should not, under any circumstances, configure the GDM user/group to a user which a user could easily gain access to, such as the user nobody. Any user who gains access to an Xauth key can snoop on and control running GUI programs running in the associated session or perform a denial-of-service attack on it. It is important to ensure that the system is configured properly so that only the "gdm" user has access to these files and that it is not easy to login to this account. For example, the account should be setup to not have a password or allow non-root users to login to the account.

The GDM greeter configuration is stored in GConf. To allow the GDM user to be able to write configuration, it is necessary for the "gdm" user to have a writable $HOME directory. Users may configure the default GConf configuration as desired to avoid the need to provide the "gdm" user with a writable $HOME directory. However, some features of GDM may be disabled if it is unable to write state information to GConf configuration.

GDM uses PAM for login authentication. PAM stands for Pluggable Authentication Module, and is used by most programs that request authentication on your computer. It allows the administrator to configure specific authentication behavior for different login programs (such as SSH, login GUI, screensaver, etc.)

PAM is complicated and highly configurable, and this documentation does not intend to explain this in detail. Instead, it is intended to give an overview of how PAM configuration relates with GDM, how PAM is commonly configured with GDM, and known issues. It is expected that a person needing to do PAM configuration would need to do further reading of PAM documentation to understand how to configure PAM and to understand terms used in this section.

PAM configuration has different, but similar, interfaces on different Operating Systems, so check the pam.d or pam.conf man page for details. Be sure you read the PAM documentation and are comfortable with the security implications of any changes you intend to make to your configuration.

Note that, by default, GDM uses the "gdm" PAM service name for normal login and the "gdm-autologin" PAM service name for automatic login. These services may not be defined in your pam.d or pam.conf configured file. If there is no entry, then GDM will use the default PAM behavior. On most systems this should work fine. However, the automatic login feature may not work if the gdm-autologin service is not defined.

The PostLogin script is run before pam_open_session is called, and the PreSession script is called after. This allows the system administrator to add any scripting to the login process either before or after PAM initialises the session.

If you wish to make GDM work with other types of authentication mechanisms (such as a fingerprint or SmartCard reader), then you should implement this by using a PAM service module for the desired authentication type rather than by trying to modify the GDM code directly. Refer to the PAM documentation on your system. How to do this is frequently discussed on the

PAM does have some limitations regarding being able to work with multiple types of authentication at the same time, like supporting the ability to accept either SmartCard and the ability to type the username and password into the login program. There are techniques that are used to make this work, and it is best to research how this problem is commonly solved when setting up such a configuration.

If automatic login does not work on a system, check to see if the "gdm-autologin" PAM stack is defined in the PAM configuration. For this to work, it is necessary to use a PAM module that simply does no authentication, or which simply returns PAM_SUCCESS from all of its public interfaces. Assuming your system has a pam_allow.so PAM module which does this, a PAM configuration to enable "gdm-autologin" would look like this:

       gdm-autologin auth  required    pam_unix_cred.so.1
       gdm-autologin auth  sufficient  pam_allow.so.1
       gdm-autologin account  sufficient  pam_allow.so.1
       gdm-autologin session  sufficient  pam_allow.so.1
       gdm-autologin password  sufficient  pam_allow.so.1

The above setup will cause no lastlog entry to be generated. If a lastlog entry is desired, then use the following for the session:

       gdm-autologin session required pam_unix_session.so.1

If the computer is used by several people, which makes automatic login unsuitable, you may want to allow some users to log in without entering their password. This feature can be enabled as a per-user option in the users-admin tool from the gnome-system-tools; it is achieved by checking that the user is member a Unix group called "nopasswdlogin" before asking for a password. For this to work, the PAM configuration file for the "gdm" service must include a line such as:

      gdm auth  sufficient  pam_succeed_if.so  user ingroup nopasswdlogin

GDM generates utmp and wtmp User Accounting Database entries upon session login and logout. The utmp database contains user access and accounting information that is accessed by commands such as finger, last, login, and who. The wtmp database contains the history of user access and accounting information for the utmp database. Refer to the utmp and wtmp man pages on your system for more information.

Xserver authorization files are stored in a newly created subdirectory of <var>/run/gdm at start up. These files are used to store and share a "password" between X clients and the Xserver. This "password" is unique for each session logged in, so users from one session can't snoop on users from another.

GDM only supports the MIT-MAGIC-COOKIE-1 Xserver authentication scheme. Normally little is gained from the other schemes, and no effort has been made to implement them so far. Be especially careful about using XDMCP because the Xserver authentication cookie goes over the wire as clear text. If snooping is possible, then an attacker could simply snoop your authentication password as you log in, regardless of the authentication scheme being used. If snooping is possible and undesirable, then you should use ssh for tunneling an X connection rather then using XDMCP. You could think of XDMCP as a sort of graphical telnet, having the same security issues. In most cases, ssh -Y should be preferred over GDM's XDMCP features.

Even though your display is protected by cookies, XEvents and keystrokes typed when entering passwords will still go over the network in clear text. It is trivial to capture these.

XDMCP is primarily useful for running thin clients such as in terminal labs. Those thin clients will only ever need the network to access the server, and so it seems like the best security policy to have those thin clients on a separate network that cannot be accessed by the outside world, and can only connect to the server. The only point from which you need to access outside is the server. This type of set up should never use an unmanaged hub or other sniffable network.

XDMCP access control is done using TCP wrappers. It is possible to compile GDM without TCP wrapper support, so this feature may not be supported on some Operating Systems.

You should use the daemon name gdm in the <etc>/hosts.allow and <etc>/hosts.deny files. For example, to deny computers from .evil.domain from logging in, then add

gdm: .evil.domain

to <etc>/hosts.deny. You may also need to add

gdm: .your.domain

to your <etc>/hosts.allow if you normally disallow all services from all hosts. See the hosts.allow(5) man page for details.

Even though GDM tries to outsmart potential attackers trying to take advantage of XDMCP, it is still advised that you block the XDMCP port (normally UDP port 177) on your firewall unless really needed. GDM guards against denial of service attacks, but the X protocol is still inherently insecure and should only be used in controlled environments. Also each remote connection takes up lots of resources, so it is much easier to do a denial of service attack via XDMCP than attacking a webserver.

It is also wise to block all of the Xserver ports. These are TCP ports 6000+ (one for each display number) on your firewall. Note that GDM will use display numbers 20 and higher for flexible on-demand servers.

X is not a very safe protocol when using it over the Internet, and XDMCP is even less safe.

GDM may be configured to use PolicyKit to allow the system administrator to control whether the login screen should provide the shutdown and restart buttons on the greeter screen.

These buttons are controlled by the org.freedesktop.consolekit.system.stop-multiple-users and org.freedesktop.consolekit.system.restart-multiple-users actions respectively. Policy for these actions can be set up using the polkit-gnome-authorization tool, or the polkit-auth command line program.

GDM may be configured to use RBAC instead of PolicyKit. In this case the RBAC configuration is used to control whether the login screen should provide the shutdown and restart buttons on the greeter screen.

For example, on Oracle Solaris, the "solaris.system.shutdown" authorization is used to control this. Simply modify the /etc/user_attr file so that the "gdm" user has this authorization.

The GDM script integration points can be found in the <etc>/gdm/ directory:

Xsession
Init/
PostLogin/
PreSession/
PostSession/

The Init, PostLogin, PreSession, and PostSession scripts all work as described below.

For each type of script, the default one which will be executed is called "Default" and is stored in a directory associated with the script type. So the default Init script is <etc>/gdm/Init/Default. A per-display script can be provided, and if it exists it will be run instead of the default script. Such scripts are stored in the same directory as the default script and have the same name as the Xserver DISPLAY value for that display. For example, if the <Init>/:0 script exists, it will be run for DISPLAY ":0".

All of these scripts are run with root privilege and return 0 if run successfully, and a non-zero return code if there was any failure that should cause the login session to be aborted. Also note that GDM will block until the scripts finish, so if any of these scripts hang, this will cause the login process to also hang.

When the Xserver for the login GUI has been successfully started, but before the login GUI is actually displayed, GDM will run the Init script. This script is useful for starting programs that should be run while the login screen is showing, or for doing any special initialization if required.

After the user has been successfully authenticated GDM will run the PostLogin script. This is done before any session setup has been done, including before the pam_open_session call. This script is useful for doing any session initialisation that needs to happen before the session starts. For example, you might set up the user's $HOME directory if needed.

After the user session has been initialised, GDM will run the PreSession script. This script is useful for doing any session initialisation that needs to happen after the session has been initialised. It can be used for session management or accounting, for example.

When a user terminates their session, GDM will run the PostSession script. Note that the Xserver will have been stopped by the time this script is run, so it should not be accessed.

Note that the PostSession script will be run even when the display fails to respond due to an I/O error or similar. Thus, there is no guarantee that X applications will work during script execution.

All of the above scripts will set the $RUNNING_UNDER_GDM environment variable to yes. If the scripts are also shared with other display managers, this allows you to identify when GDM is calling these scripts, so you can run specific code when GDM is used.

The <share>/gdm/autostart/LoginWindow directory contains files in the format specified by the "FreeDesktop.org Desktop Application Autostart Specification". Standard features in the specification may be used to specify programs that should auto-restart or only be launched if a GConf configuration value is set, etc.

Any .desktop files in this directory will cause the associated program to automatically start with the login GUI greeter. By default, GDM is shipped with files which will autostart the gdm-simple-greeter login GUI greeter itself, the gnome-power-manager application, the gnome-settings-daemon, and the Metacity window manager. These programs are needed for the greeter program to work. In addition, desktop files are provided for starting various AT programs if the configuration values specified in the Accessibility Configuration section below are set.

There is also an Xsession script located at <etc>/gdm/Xsession which is called between the PreSession and the PostSession scripts. This script does not support per-display like the other scripts. This script is used for actually starting the user session. This script is run as the user, and it will run whatever session was specified by the Desktop session file the user selected to start.

The GDM daemon is configured using the <etc>/gdm/custom.conf file. Default values are stored in GConf in the gdm.schemas file. It is recommended that end-users modify the <etc>/gdm/custom.conf file because the schemas file may be overwritten when the user updates their system to have a newer version of GDM.

Note that older versions of GDM supported additional configuration options which are no longer supported in the latest versions of GDM.

The <etc>/gdm/custom.conf file is in the keyfile format. Keywords in brackets define group sections, strings before an equal sign (=) are keys and the data after equal sign represents their value. Empty lines or lines starting with the hash mark (#) are ignored.

The file <etc>/gdm/custom.conf supports the "[daemon]", "[security]", and "[xdmcp]" group sections. Within each group, there are particular key/value pairs that can be specified to modify how GDM behaves. For example, to enable timed login and specify the timed login user to be a user named "you", you would modify the file so it contains the following lines:

[daemon]
TimedLoginEnable=true
TimedLogin=you

A full list of supported configuration keys follow:

The GDM default greeter is called the simple Greeter and is configured via GConf. Default values are stored in GConf in the gdm-simple-greeter.schemas file. These defaults can be overridden if the "gdm" user has a writable $HOME directory to store GConf settings. These values can be edited using the gconftool-2 or gconf-editor programs. The following configuration options are supported:

This section describes the accessibility configuration options available in GDM.

AutostartCondition=GNOME /desktop/gnome/applications/at/screen_keyboard_enabled
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_magnifier_enabled
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_reader_enabled

[Desktop Entry]
Encoding=UTF-8
Name=Onboard Onscreen Keyboard
Comment=Use an on-screen keyboard
TryExec=onboard
Exec=onboard --size 500x180 -x 20 -y 10
Terminal=false
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Accessibility;
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_keyboard_enabled

[Desktop Entry]
Encoding=UTF-8
Name=Software Mouse-Clicks
Comment=Perform clicks by dwelling with the pointer
TryExec=mousetweaks
Exec=mousetweaks --enable-dwell -m window -c -x 20 -y 240 
Terminal=false
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Accessibility;
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_keyboard_enabled

The GDM Greeter uses some of the same framework that your desktop session will use. And so, it is influenced by a number of the same GConf settings. For each of these settings the Greeter will use the default value unless it is specifically overridden by a) GDM's installed mandatory policy b) system mandatory policy. GDM installs its own mandatory policy to lock down some settings for security.

GDM enables the following gnome-settings-daemon plugins: a11y-keyboard, background, sound, xsettings.

These are responsible for things like the background image, font and theme settings, sound events, etc.

Plugins can also be disabled using GConf. For example, if you want to disable the sound plugin then unset the following key: /apps/gdm/simple-greeter/settings-manager-plugins/sound/active.

GDM sessions are specified using the FreeDesktop.org Desktop Entry Specification, which can be referenced at the following URL: http://www.freedesktop.org/wiki/Specifications/desktop-entry-spec.

By default, GDM will install desktop files in the <share>/xsessions directory. GDM will search the following directories in this order to find desktop files: <etc>/X11/sessions/, <dmconfdir>/Sessions, <share>/xsessions, and <share>/gdm/BuiltInSessions. By default the <dmconfdir> is set to <etc>/dm/ unless GDM is configured to use a different directory via the "--with-dmconfdir" option.

A session can be disabled by editing the desktop file and adding a line as follows: Hidden=true.

GDM desktop files support a GDM-specific extension, a key named "X-GDM-BypassXsession". If the key is not specified in a desktop file, the value defaults to "false". If this key is specified to be "true" in a desktop file, then GDM will launch the program specified by the desktop file "Exec" key directly when starting the user session. It will not run the program via the <etc>/gdm/Xsession script, which is the normal behavior. Since bypassing the <etc>/gdm/Xsession script avoids setting up the user session with the normal system and user settings, sessions started this way can be useful for debugging problems in the system or user scripts that might be preventing a user from being able to start a session.

The user's default session and language choices are stored in the ~/.dmrc file. When a user logs in for the first time, this file is created with the user's initial choices. The user can change these default values by simply changing to a different value when logging in. GDM will remember this change for subsequent logins.

The ~/.dmrc file is in the standard INI format. It has one section called [Desktop] which has two keys: Session and Language.

The Session key specifies the basename of the session .desktop file that the user wishes to normally use without the .desktop extension. The Language key specifies the language that the user wishes to use by default. If either of these keys is missing, the system default is used. The file would normally look as follows:

[Desktop]
Session=gnome
Language=cs_CZ.UTF-8

The GDM package provides the following commands in sbindir intended to be run by the root user:

There are a many problems that can cause GDM to fail to start, but this section will discuss a few common problems and how to approach tracking down a problem with GDM starting. Some problems will cause GDM to respond with an error message or dialogue when it tries to start, but it can be difficult to track down problems when GDM fails silently.

First make sure that the Xserver is configured properly. The GDM configuration file contains a command in the [server-Standard] section that is used for starting the Xserver. Verify that this command works on your system. Running this command from the console should start the Xserver. If it fails, then the problem is likely with your Xserver configuration. Refer to your Xserver error log for an idea of what the problem may be. The problem may also be that your Xserver requires different command-line options. If so, then modify the Xserver command in the GDM configuration file so that it is correct for your system.

Also make sure that the /tmp directory has reasonable ownership and permissions, and that the machine's file system is not full. These problems will cause GDM to fail to start.