Gnome Display Manager Reference Manual
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Controlling GDM

You can control GDM behavior during runtime in several different ways. You can either run certain commands, or you can talk to GDM using either a unix socket protocol, or a FIFO protocol.

Commands

To stop GDM, you can either send the TERM signal to the main daemon or run the gdm-stop command which is in the <sbin>/ directory. To restart GDM, you can either send the HUP signal to the main daemon or run the gdm-restart command which is also in the <sbin>/ directory. To restart GDM but only after all the users have logged out, you can either send the USR1 signal to the main daemon or run the gdm-safe-restart command which is in the <sbin>/ directory as well.

The gdmflexiserver command can be used to start new flexible (on demand) servers. Run gdmflexiserver --help to get a listing of possible options. This command will also lock the current screen with xscreensaver so that the user can safely walk away from the machine and let someone else log in. XFree86 will automatically switch back to the same virtual terminal (if your operating system supports it), after the new session has ended, so this should work quite transparently to the users.

The FIFO protocol

GDM also provides a FIFO called .gdmfifo in the ServAuthDir directory (usually <var>/gdm/.gdmfifo). You must be root to use this protocol, and it is mostly used for internal GDM chatter. It is a very simple protocol where you just echo a command on a single line to this file. It can be used to tell GDM things such as restart, suspend the machine, or restart all X servers next time it has a chance (which would be useful from an X configuration program).

Full and up to date documentation of the commands and their use is contained in the GDM source tree in the file daemon/gdm.h. Look for the defines starting with GDM_SOP_. The commands which require the pid of the slave as an argument are the ones that are really used for internal communication of the slave with the master and should not be used.

Socket Protocol

GDM provides a unix domain socket for communication at /tmp/.gdm_socket. Using this you can check if GDM is running, the version of the daemon, the current servers that are running and who is logged in on them, and if GDM supports it on your operating system, also the virtual terminals of all the console logins. You can also request new flexible servers to be run with this protocol, but this is best done with the gdmflexiserver command.

gdmflexiserver accepts the following commands with the --command option: 


VERSION
AUTH_LOCAL
FLEXI_XSERVER
FLEXI_XNEST
CONSOLE_SERVERS
ALL_SERVERS
UPDATE_CONFIG
GREETERPIDS
QUERY_LOGOUT_ACTION
SET_LOGOUT_ACTION
SET_SAFE_LOGOUT_ACTION
QUERY_VT
SET_VT
CLOSE

These are described in detail below, including required arguments, response format, and return codes.

VERSION


VERSION: Query version
Supported since: 2.2.4.0
Arguments:  None
Answers:
  GDM <gdm version>
  ERROR <err number> <english error description>
     200 = Too many messages
     999 = Unknown error

AUTH_LOCAL


AUTH_LOCAL: Setup this connection as authenticated for
            FLEXI_SERVER Because all full blown (non-Xnest)
            servers can be started only from users logged in
            locally, and here gdm assumes only users logged
            in from gdm.  They must pass the xauth
            MIT-MAGIC-COOKIE-1 that they were passed before
            the connection is authenticated.
Note:       The AUTH LOCAL command requires the
            --authenticate option, although only
            FLEXI XSERVER uses this currently.
Supported since: 2.2.4.0
Arguments:  <xauth cookie>
  <xauth cookie> is in hex form with no 0x prefix
Answers:
  OK
  ERROR <err number> <english error description>
     0 = Not implemented
     100 = Not authenticated
     200 = Too many messages
     999 = Unknown error

FLEXI_XSERVER


FLEXI_XSERVER: Start a new X flexible server
Only supported on connection that passed AUTH_LOCAL
Supported since: 2.2.4.0
Arguments:  <xserver type>
  If no arguments, starts the standard x server
Answers:
  OK <display>
  ERROR <err number> <english error description>
     0 = Not implemented
     1 = No more flexi servers
     2 = Startup errors
     3 = X failed
     4 = X too busy
     6 = No server binary
     100 = Not authenticated
     200 = Too many messages
     999 = Unknown error

FLEXI_XNEST


FLEXI_XNEXT: Start a new flexible Xnest server
Note:        Supported an older version from 2.2.4.0, later
             2.2.4.2, but since 2.3.90.4 you must supply 4
             arguments or ERROR 100 will be returned.  This
             will start Xnest using the XAUTHORITY file
             supplied and as the uid same as the owner of
             that file (and same as you supply).  You must
             also supply the cookie as the third argument
             for this display, to prove that you indeed are
             this user.  Also this file must be readable
             ONLY by this user, that is have a mode of 0600.
             If this all is not met, ERROR 100 is returned.
Note:        The cookie should be the MIT-MAGIC-COOKIE-1,
             the first one gdm can find in the XAUTHORITY
             file for this display.  If that's not what you
             use you should generate one first.  The cookie
             should be in hex form.
Supported since: 2.3.90.4
Arguments:  <display to run on> <uid of requesting user>
            <xauth cookie for the display> <xauth file>
Answers:
  OK <display>
  ERROR <err number> <english error description>
     0 = Not implemented
     1 = No more flexi servers
     2 = Startup errors
     3 = X failed
     4 = X too busy
     5 = Xnest can't connect
     6 = No server binary
     100 = Not authenticated
     200 = Too many messages
     999 = Unknown error

CONSOLE_SERVERS


CONSOLE_SERVERS: List all console servers, useful for linux
                 mostly.  Doesn't list xdmcp and xnest
                 non-console servers
Supported since: 2.2.4.0
Arguments:  None
Answers:
  OK <server>;<server>;...

  <server> is <display>,<logged in user>,<vt or xnest display>

  <logged in user> can be empty in case no one logged
  in yet, and <vt> can be -1 if it's not known or not
  supported (on non-linux for example).  If the display is an
  xnest display and is a console one (that is, it is an xnest
  inside another console display) it is listed and instead of
  vt, it lists the parent display in standard form.

  ERROR <err number> <english error description>
     1 = Not implemented
     200 = Too many messages
     999 = Unknown error

ALL_SERVERS


ALL_SERVERS: List all servers, including console, remote, xnest.
             This can, for example, be useful to figure out if
             the server you are on is managed by the gdm daemon,
             by seeing if it is in the list.  It is also somewhat
             like the 'w' command but for graphical sessions.
Supported since: 2.4.2.96
Arguments:  None
Answers:
  OK <server>;<server>;...

  <server> is <display>,<logged in user>

  <logged in user> can be empty in case no one logged in yet

  ERROR <err number> <english error description>
     0 = Not implemented
     200 = Too many messages
     999 = Unknown error

UPDATE_CONFIG


UPDATE_CONFIG: Tell the daemon to update config of some key.
               Any user can really request that values are
               re-read but the daemon caches the last date
               of the config file so a user can't actually
               change any values unless they can write the
               config file.
Supported keys:
  security/AllowRoot (2.3.90.2)
  security/AllowRemoteRoot (2.3.90.2)
  security/AllowRemoteAutoLogin (2.3.90.2)
  security/RetryDelay (2.3.90.2)
  security/DisallowTCP (2.4.2.0)
  daemon/Greeter (2.3.90.2)
  daemon/RemoteGreeter (2.3.90.2)
  xdmcp/Enable (2.3.90.2)
  xdmcp/Port (2.3.90.2)
  xdmcp/PARAMETERS (2.3.90.2) (pseudokey, all the parameters)
         xdmcp/MaxPending
         xdmcp/MaxSessions
         xdmcp/MaxWait
         xdmcp/DisplaysPerHost
         xdmcp/HonorIndirect
         xdmcp/MaxPendingIndirect
         xdmcp/MaxWaitIndirect
         xdmcp/PingIntervalSeconds (only affects new connections)
  daemon/TimedLogin (2.3.90.3)
  daemon/TimedLoginEnable (2.3.90.3)
  daemon/TimedLoginDelay (2.3.90.3)
  greeter/SystemMenu (2.3.90.3)
  greeter/ConfigAvailable (2.3.90.3)
  greeter/ChooserButton (2.4.2.0)
  greeter/SoundOnLoginFile (2.5.90.0)
  daemon/AddGtkModules (2.5.90.0)
  daemon/GtkModulesList (2.5.90.0)
Supported since: 2.3.90.2
Arguments:  <key>
  <key> is just the base part of the key such as
  "security/AllowRemoteRoot"
Answers:
  OK
  ERROR <err number> <english error description>
     0 = Not implemented
     50 = Unsupported key
     200 = Too many messages
     999 = Unknown error

GREETERPIDS


GREETERPIDS: List all greeter pids so that one can send HUP
             to them for config rereading.  Of course one
             must be root to do that.
Supported since: 2.3.90.2
Arguments:  None
Answers:
  OK <pid>;<pid>;...
  ERROR <err number> <english error description>
     0 = Not implemented
     200 = Too many messages
     999 = Unknown error

QUERY_LOGOUT_ACTION


QUERY_LOGOUT_ACTION: Query which logout actions are possible
Only supported on connections that passed AUTH_LOCAL.
Supported since: 2.5.90.0
Answers:
  OK <action>;<action>;...
     Where action is one of HALT, REBOOT or SUSPEND.  An
     empty list can also be returned if no action is possible.
     A '!' is appended to an action if it was already set with
     SET_LOGOUT_ACTION or SET_SAFE_LOGOUT_ACTION.  Note that
     SET_LOGOUT_ACTION has precedence over
     SET_SAFE_LOGOUT_ACTION.
  ERROR <err number> <english error description>
     0 = Not implemented
     100 = Not authenticanted
     200 = Too many messages
     999 = Unknown error

SET_LOGOUT_ACTION


SET_LOGOUT_ACTION:  Tell the daemon to halt/reboot/suspend
                    after slave process exits. 
Only supported on connections that passed AUTH_LOCAL.
Supported since: 2.5.90.0
Arguments:  <action>
  NONE           Set exit action to 'none'
  HALT           Set exit action to 'halt'
  REBOOT         Set exit action to 'reboot'
  SUSPEND        Set exit action to 'suspend'

Answers:
  OK
  ERROR <err number> <english error description>
     0 = Not implemented
     7 = Unknown logout action, or not available
     100 = Not authenticanted
     200 = Too many messages
     999 = Unknown error

SET_SAFE_LOGOUT_ACTION


SET_SAFE_LOGOUT_ACTION:  Tell the daemon to halt/reboot/suspend
                         after everybody logs out.  If only
                         one person logs out, then this is
                         obviously the same as the
                         SET_LOGOUT_ACTION.  Note that
                         SET_LOGOUT_ACTION has precendence over
                         SET_SAFE_LOGOUT_ACTION if it is set to
                         something other then NONE.  If no one
                         is logged in, then the action takes
                         effect immediately.
Only supported on connections that passed AUTH_LOCAL.
Supported since: 2.5.90.0
Arguments:  <action>
  NONE           Set exit action to 'none'
  HALT           Set exit action to 'halt'
  REBOOT         Set exit action to 'reboot'
  SUSPEND        Set exit action to 'suspend'

Answers:
  OK
  ERROR <err number> <english error description>
     0 = Not implemented
     7 = Unknown logout action, or not available
     100 = Not authenticanted
     200 = Too many messages
     999 = Unknown error

QUERY_VT


QUERY_VT:  Ask the daemon about which VT we are currently on.
           This is useful for logins which don't own
           /dev/console but are still console logins.  Only
           supported on Linux currently, other places will
           just get ERROR 8.  This is also the way to query
           if VT support is available in the daemon in the
           first place.
Only supported on connections that passed AUTH_LOCAL.
Supported since: 2.5.90.0
Arguments:  None
Answers:
  OK <vt number>
  ERROR <err number> <english error description>
     0 = Not implemented
     8 = Virtual terminals not supported
     100 = Not authenticanted
     200 = Too many messages
     999 = Unknown error

SET_VT


SET_VT:  Change to the specified virtual terminal.
         This is useful for logins which don't own /dev/console
         but are still console logins.  Only supported on Linux
         currently, other places will just get ERROR 8.
Only supported on connections that passed AUTH_LOCAL.
Supported since: 2.5.90.0
Arguments:  <vt>
Answers:
  OK
  ERROR <err number> <english error description>
     0 = Not implemented
     8 = Virtual terminals not supported
     9 = Invalid virtual terminal number
     100 = Not authenticanted
     200 = Too many messages
     999 = Unknown error

CLOSE


CLOSE Answers: None
Supported since: 2.2.4.0
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Configuration GDM Commands