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THEORY OF OPERATION: Networking that Just Works
--------------------------
NetworkManager attempts to keep an active network connection available at all NetworkManager attempts to keep an active network connection available at all
times. The point of NetworkManager is to make networking configuration and times. The point of NetworkManager is to make networking configuration and
setup as painless and automatic as possible. If using DHCP, NetworkManager is setup as painless and automatic as possible. NetworkManager is intended to
_intended_ to replace default routes, obtain IP addresses from a DHCP server, replace default route, replace other routes, set IP addresses, and in general
and change nameservers whenever it sees fit. In effect, the goal of configure networking as NM sees fit (with the possibility of manual override as
NetworkManager is to make networking Just Work. If you have special needs, necessary). In effect, the goal of NetworkManager is to make networking Just
we'd like to hear about them, but understand that NetworkManager is not Work with a minimum of user hassle, but still allow customization and a high
intended to serve the needs of all users. level of manual network control. If you have special needs, we'd like to hear
about them, but understand that NetworkManager is not intended for every
use-case.
NetworkManager will attempt to keep every network device in the system up and
active, as long as the device is available for use (has a cable plugged in,
the killswitch isn't turned on, etc). Network connections can be set to
'autoconnect', meaning that NetworkManager will make that connection active
whenever it and the hardware is available.
"Settings services" store lists of user- or administrator-defined "connections",
which contain all the settings and parameters required to connect to a specific
network. NetworkManager will _never_ activate a connection that is not in this
list, or that the user has not directed NetworkManager to connect to.
From a list of all adapters currently installed on the system, NetworkManager How it works:
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 a list of wireless The NetworkManager daemon runs as a privileged service (since it must access
networks, the preferred list. Preferred Networks are wireless networks that and control hardware), but provides a D-Bus interface on the system bus to
the user has explicitly made NetworkManager associate with at some previous allow for fine-grained control of networking. NetworkManager does not store
time. So if the user walks into a Starbucks and explicitly asks NetworkManager connections or settings, it is only the mechanism by which those connections
to associate with that Starbucks network, NetworkManager will remember the are selected and activated.
Starbucks network information from that point on. Upon returning to that
Starbucks, NetworkManager will attempt to associate _automatically_ with the To store pre-defined network connections, two separate services, the "system
Starbucks network since it is now in the Preferred Networks list. The point of settings service" and the "user settings service" store connection information
this is to ensure that only the user can determine which wireless networks to and provide these to NetworkManager, also via D-Bus. Each settings service
associate with, and that the user is aware which networks are security risks can determine how and where it persistently stores the connection information;
and which are not. for example, the GNOME applet stores its configuration in GConf, and the system
settings service stores it's config in distro-specific formats, or in a distro-
agnostic format, depending on user/administrator preference.
A variety of other system services are used by NetworkManager to provide
network functionality: wpa_supplicant for wireless connections and 802.1x
wired connections, pppd for PPP and mobile broadband connections, DHCP clients
for dynamic IP addressing, dnsmasq for proxy nameserver and DHCP server
functionality for internet connection sharing, and avahi-autoipd for IPv4
link-local addresses. Most communication with these daemons occurs, again,
via D-Bus.
STRUCTURE: Why doesn't my network Just Work?
Driver problems are the #1 cause of why NetworkManager sometimes fails to
connect to wireless networks. Often, the driver simply doesn't behave in a
consistent manner, or is just plain buggy. NetworkManager supports _only_
those drivers that are shipped with the upstream Linux kernel, because only
those drivers can be easily fixed and debugged. ndiswrapper, vendor binary
drivers, or other out-of-tree drivers may or may not work well with
NetworkManager, precisely because they have not been vetted and improved by the
open-source community, and becuase problems in these drivers usually cannot
be fixed.
Sometimes, command-line tools like 'iwconfig' will work, but NetworkManager will
fail. This is again often due to buggy drivers, becuase these drivers simply
aren't expecting the dynamic requests that NetworkManager and wpa_supplicant
make. Driver bugs should be filed in the bug tracker of the distribution being
run, since often distributions customize their kernel and drivers.
Sometimes, it really is NetworkManager's fault. If you think that's the case,
please file a bug at http://bugzilla.gnome.org and choose the NetworkManager
component. Attaching the output of /var/log/messages or /var/log/daemon.log
(wherever your distribution directs syslog's 'daemon' facility output) is often
very helpful, and (if you can get) a working wpa_supplicant config file helps
enormously.
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 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 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.