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THEORY OF OPERATION:
NetworkManager attempts to keep an active network connection available at all
times. It is intended only for the desktop use-case, and is not intended for
usage on servers. 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. 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.
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.
For wireless networking support, NetworkManager keeps two lists of wireless
networks: a Trusted list, and a Preferred list. The Trusted list contains
networks the user specifically adds to it, while the preferred list contains
networks the user forces NetworkManager to connect to. For example, while the
company's wireless network and WEP/WPA key would be preloaded into the Trusted
Networks list, if the user wished to use the wireless network in a Starbucks,
the user would explicitly tell NetworkManager to associate with that network.
NetworkManager does not try to use _any_ available network in the area (a
possible security risk), but will associate with any Trusted Network first, and
Preferred Networks later. Preferred Networks are ones the user has explicitly
made NetworkManager associate with at some previous time. So if the user walks
into a Starbucks and explicitly asks NetworkManager to associate with that
Starbucks network, NetworkManager will remember the 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.
STRUCTURE:
NetworkManager runs as a root-user system level daemon, since it
must manipulate hardware directly. It communicates over DBUS with a
desktop-level per-user process, nm-applet. Since Trusted and 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
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
provide to NetworkManager the Trusted and 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 Trusted and Preferred Networks in
GConf and WEP/WPA keys in gnome-keyring, and proxies that information to
NetworkManager upon request.
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