The GTK+ Greeter is the default graphical user interface that is
presented to the user. The greeter contains a menu at the top, an
optional face browser, an optional logo and a text entry
widget.
The text entry field is used for entering logins, passwords,
passphrases etc. gdmlogin is controlled by the
underlying daemon and is basically stateless. The daemon controls the
greeter through a simple protocol where it can ask the greeter for a
text string with echo turned on or off. Similarly, the daemon can
change the label above the text entry widget to correspond to the
value the authentication system wants the user to enter.
The menu bar in the top of the greeter enables the user to select the
requested session type/desktop environment, select an appropriate
locale/language and optionally shutdown/reboot/suspend the computer,
configure GDM (given the user knows the root password), change
the GTK+ theme, or start an XDMCP chooser.
Optionally the greeter can provide a face browser containing icons for
all the users on a system. The icons can be installed globally by the
sysadmin or in the users' home directories.  If installed globally
they should be in the <share>/pixmaps/faces/
directory (though this can be configured with the
GlobalFaceDir
configuration option) and the filename should be the name of the user,
optionally with a .png appended.
The users can place their icons in a file called
~/.face, and they can use the command
gdmphotosetup to graphically configure this.
Face icons placed in the global face directory must be readable to
the GDM user.  However, the daemon, proxies user pictures to the
greeter and thus those don't have be be readable by the GDM user, but
root.
Please note that loading and scaling face icons located in user home
directories can be a very time consuming task.  Especially on large
systems or systems running NIS. The browser feature is only intended
for systems with relatively few users.  Also if home directories are
on an on demand mounted filesystem like AFS, then GDM may mount all
the home directories just to check for pictures if the face browser is
on.  GDM will try to give up after 5 seconds of activity however and
only display the users whose pictures it has gotten so far.
To control the faces that get displayed, there are a number of
configuration options that can be used.  If the
Include option contains a list of users
separated by commas, then only the specified users will be displayed.
If the IncludeAll option is set to true, then
the password file will be scanned and all users will be displayed.
Any user listed in the Exclude option and users
whose UID's is lower than MinimalUID will be
filtered out.
When the browser is turned on, valid usernames on the computer are
inherently exposed to a potential intruder.  This may be a bad idea if
you don't know who can get to a login screen.  This is especially true
if you run XDMCP.  However you should never run XDMCP on an open
network anyway.
The greeter can optionally display a logo in the login window.  The
image must be in a format readable to the gdk-pixbuf library (GIF,
JPG, PNG, TIFF, XPM and possibly others), and it must be readable to
the GDM user. See the Logo option in the
reference section below for details.
