The Gnome Display Manager 1. Theory of operation gdm is a replacement for xdm, the X Display Manager. Unlike its competitors (x3dm, kdm, wdm) gdm was written from scratch and does not contain any original xdm code. gdm was written with simplicity and security in mind. The overall design concept is this: When gdm starts it parses the config file gdm.conf. For each of the local displays gdm forks a slave process. The main gdm process will listen to XDMCP requests from remote displays and monitor the local display sessions. The gdm slave process starts an Xserver according to information read from the config file. gdm sets up proper X authentication and starts up the greeter window requesting the user for login and password. The gdm master and slave processes are deliberately kept small and they are believed to be secure. The program providing the user interface is significantly more complex and is linked to several unaudited libraries. Therefore it runs as a dedicated gdm user and communicates with gdm through a pipe. 2. Overview of the config directory. The configuration files for gdm are located in the /etc/gdm/ directory. This is a listing of the config directory contents: Init/ PostSession/ PreSession/ Sessions/ gdm.conf gdm.conf is the main gdm configuration file. The options will be described later in this document. The remaining configuration is done by dropping scripts in the subdirectories of the gdm folder. This approach makes it easy for package management systems to install window managers and different session types without requiring the sysadmin/user to edit files. In this section we will explain the Init, PreRoot and PostRoot directories as they are very similar. When the X server has been successfully started, gdm will try to run the script called Init/. I.e. Init/:0 for the first local display. If this file is not found, gdm will attempt to to run Init/Default. The script will be run as root and gdm blocks until it terminates. Use the Init/* script for programs that are supposed to run alongside with the gdm login window. xconsole for instance. Commands to set the background etc. goes in this file too. It is up to the sysadmin to decide whether clients started by the Init script should be killed before starting the user session. This is controlled with the KillInitClient option in gdm.conf. When the user has been successfully authenticated, gdm tries to run the PreSession script. Similar to the Init-scripts, PreSession/ will be executed first, if that is not found gdm will attempt to run PreSession/Default. The script will be run as root and gdm blocks until it terminates. Use this script for local session management or accounting stuff. The USER environment variable contains the login of the authenticated user. The script should return 0 on success. Any other value will cause gdm to terminate the current login process. Then the session script is run. Session scripts are located in the etc/gdm/Session directory. Which one gdm runs depends on the session the user chose in the Sessions-menu in the gdm greeter. If no session is selected and the user has no last session stored in his ~/.gnome/gdm file, the system will choose or first script found or -- if Sessions/Default exists -- this will be run. For instance you can create a symlink from Gnome to Default to make Gnome the default desktop environment. When the user terminates his session the PostSession script will be run. Operation is similar to Init and PreSession. That is, gdm will attempt to execute the script PostSession/ and if that doesn't exist: PostSession/Default. Again the script will be run with root priviledges, gdm will block and the USER environment variable will contain the name of the user who just logged out. Neither of the Init, PreSession or PostSession scripts are necessary and can be left out. At least one session script is required for proper operation. 3. The Login Window gdm supports two different login modes (which happen to be selected by changing greeter programs). The gdmlogin program is a bare bones gui application providing only a few menus and a login prompt. A graphical version of the textmode login program, if you wish. gdmlogin was designed for use in environments, where usernames can't be exposed. gdmgreeter is slightly more powerful. It consists of a browser window containing faces of all users on the system. ~user/.gnome/photo is expected to contain an Imlib supported image. gdmgreeter will scale down large images to the sysadmin specified maximum. 4. The config file 4.1 Section: [appearance] LogoImage=@pixmapdir@/gnome-logo-large.png Filename to display in the logo box. The image must be in an Imlib supported format and it must be readable for the gdm user. Quiver=1 Controls whether gdmgreeter should shake the display when an incorrect username/password is entered. Default: 1. Iconify=1 The greeter window can be iconified (For TV and fish). You can turn the iconify option off by setting this to 0. Default: 1. IconFile=@pixmapdir@/gdm.xpm File to use for gdmgreeter when it's in iconified state. The image must be in an Imlib supported format and it must be readable for the gdm user. Gtkrc= Path to a gtkrc containing the theme for use in gdmgreeter/gdmlogin/gdmchooser. NoFaceImage=@pixmapdir@/nophoto.png (Only used by gdmgreeter) Default icon file for users without a personal picture in ~/gnome/photo. The image must be in an Imlib supported format and it must be readable for the gdm user. GlobalImageDir=@datadir@/faces/ (Only used by gdmgreeter) Systemwide directory for icon files. The sysadmin can place icons for users here without touching their homedirs. The iconname represents a user name. I.e. GlobalImageDir/johndoe would contain the icon for the user johndoe (No extension!). The images must be in an Imlib supported format and they must be readable for the gdm user. A user's own icon file will take precedence over the sysadmin provided one. 4.2 Section: [system] ShutdownMenu=0 Turns the Shutdown/Halt menu on/off. Default: 0 SuspendCommand= If specified a Suspend item will appear in the System menu in gdmgreeter. The command specified here will be executed asynchronously. Please use full path! For instance SuspendCommand=/usr/bin/apm --suspend UserFileCutoffSize=65535 User files larger than this value (in bytes) will be ignored by gdm/gdmgreeter. Handy for users putting gigantic pictures in their ~/.gnome/photo. In addition to the size check both gdm and gdmgreeter are extremely picky about accessing files in user directories. Neither will follow symlinks and they refuse to read files and directories writable by other than the owner. UserIconMaxWidth=128 UserIconMaxHeight=128 Specifies the maximum icon sizes in the face browser. DefaultPath=@bindir@:/usr/local/bin:/usr/bin/X11:/usr/bin:/bin Specifies the path which will be set in the user's session. VerboseAuth=0 Specifies whether gdm should print authentication errors. Depending on authentication type usernames might be exposed when this option is on. AllowRoot=0 Set to 1 to enable root logins. RelaxPermissions=0 By default gdm ignores files/dirs writable to other users than the owner. Changing the value of RelaxPermissions makes it possible to alter this behaviour: 0 - Paranoia option. Only accepts user owned files and dirs. 1 - Allow group writable files/dirs 2 - Allow world writable files/dirs RetryDelay=3 The number of seconds gdm should wait before reactivating the entry field after a failed login. 4.3 Section: [messages] Welcome=Welcome to %h Controls which text to display next to the logo image in the greeter. `%h' will be expanded to the hostname `%d' will be replaced by the display's hostname `%%' will print the %-character 4.4 Section: [daemon] SessionDir=@sysconfdir@/gdm/Sessions Directory containing the Session scripts. PidFile=/var/run/gdm.pid Name of the gdm daemon pidfile. Greeter=@bindir@/gdmgreeter Path and name of the login program executable. @bindir@/gdmlogin for the secure login window. @bindir@/gdmgreeter for the face browser. Chooser=@bindir@/gdmchooser Path and name of the gdmchooser executable. User=gdm The username under which gdm is run. Group=gdm The group id under which gdmgreeter is run. DisplayInitDir=@sysconfdir@/gdm/Init Directory containing the Init scripts. KillInitClients=1 Determines whether gdm should kill X clients started by the Init scripts when the user logs in. Default: 1. PreSessionScriptDir=@sysconfdir@/gdm/PreSession Directory containing the PreSession scripts. PostSessionScriptDir=@sysconfdir@/gdm/PostSession Directory containing the PostSession scripts. AuthDir=@authdir@ Directory containing the X authentication files for the displays. Should be owned by gdm.gdm with permissions 750. LogDir=@authdir@ Directory containing the log files for the displays. By default this is the same as the AuthDir. 4.5 Section: [servers] 0=/usr/bin/X11/X 1=/usr/bin/X11/X -bpp 8 Control section for local X servers. Each line indicates the local display number and which command needs to be run to start the X server(s). 4.6 Section: [xdmcp] Enable=1 Enables XDMCP support allowing remote displays/X terminals to be managed by gdm. gdm listens for requests on UDP port 177. Access from remote displays is controlled by the TCP Wrappers library. The service name is `gdm'. You should add gdm: .my.domain or something similar to /etc/hosts.allow. See the hosts_access(5) man page for details. Please note that XDMCP is not a particularly secure protocol and that it is a good idea to block UDP port 177 on your firewall unless you really need it. MaxPending=4 To avoid denial of service attacks, gdm has fixed size queue of pending connections. Only MaxPending displays can start at the same time. Please note that this parameter does *not* limit the number of remote displays which can be managed. It only limits the number of simultaneous displays initiating a connection. MaxManageWait=20 When gdm is ready to manage a display an ACCEPT packet is sent to it containing a unique session id which will be used in future conversations. gdm will then place the session id in the pending queue waiting for the display to respond with a MANAGE request. If no response is received within MaxManageWait seconds, gdm will declare the display dead and erase it from the pending queue freeing up the slot for other displays. MaxSessions=4 Determines the maximum number of remote display connections which will be accepted. Port=177 The UDP port number gdm should listen to for XDMCP requests. 4.7 Section: [chooser] ImageDir=@datadir@/hosts Repository for host icon files. The sysadmin can place icons for remote hosts here and they will appear in gdmchooser. The file name must match the FQDN for the host (No extension!). Icons must be in an Imlib supported format and must be readable to the gdm user. DefaultImage=@pixmapdir@/nohost.png File name for the default host icon. $Id$