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NetworkManager attempts to keep an active network connection available at all
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times.  The point of NetworkManager is to make networking configuration and
setup as painless and automatic as possible.  If using DHCP, NetworkManager is
_intended_ to replace default routes, obtain IP addresses from a DHCP server,
and change nameservers whenever it sees fit.  In effect, the goal of
NetworkManager is to make networking Just Work.  If you have special needs,
we'd like to hear about them, but understand that NetworkManager is not
intended to serve the needs of all users.
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From a list of all adapters currently installed on the system, NetworkManager
will first try a wired and then a wireless adapter.  Wireless adapters that
support wireless scanning are preferred over ones that cannot.  NetworkManager
does not try to keep a connection up as long as possible, meaning that plugging
into a wired network will switch the connection to the wired network away from
the wireless one.

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For wireless networking support, NetworkManager keeps a list of wireless
networks, the preferred list.  Preferred Networks are wireless networks that
the user has explicitly made NetworkManager associate with at some previous
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time.  So if the user walks into a Starbucks and explicitly asks NetworkManager
to associate with that Starbucks network, NetworkManager will remember the
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Starbucks network information from that point on.  Upon returning to that
Starbucks, NetworkManager will attempt to associate _automatically_ with the
Starbucks network since it is now in the Preferred Networks list.  The point of
this is to ensure that only the user can determine which wireless networks to
associate with, and that the user is aware which networks are security risks
and which are not.

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NetworkManager runs as a root-user system level daemon, since it
must manipulate hardware directly.  It communicates over DBUS with a
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desktop-level per-user process, nm-applet.  Since Preferred Networks are
user-specific, there must be some mechanism of getting this information
per-user.  NetworkManager cannot store that information as it is user-specific,
and therefore communicates over DBUS to the user daemon which provides those
lists.  NetworkManager also provides an API over DBUS for any DBUS-aware
application to determine the current state of the network, including available
wireless networks the computer is aware of and specific details about those
networks.  This API also provides the means for forcing NetworkManager to
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associate with a specific wireless network.  Use of DBUS allows separation of
NetworkManager, which requires no user-interface, and the parts of the user
interface which might be desktop environment specific.

The nm-applet provides a DBUS service called NetworkManagerInfo, which should
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provide to NetworkManager the Preferred Networks lists upon request.  It also
should be able to display a dialog to retrieve a WEP/WPA key or passphrase from
the user when NetworkManager requests it.  The GNOME version of
NetworkManagerInfo, for example, stores Preferred Networks in GConf and
WEP/WPA keys in gnome-keyring, and proxies that information to NetworkManager
upon request.