Re: Network link detection

[Dan Williams <dcbw@redhat.com>]

On Thu, 2011-03-03 at 23:29 +0100, Nico Schümann wrote:
> On Thu, Mar 03, 2011 at 02:01:06PM -0800, David Miller wrote:
> > From: Chris Friesen <chris.friesen@genband.com>
> > Date: Thu, 03 Mar 2011 15:38:35 -0600
> > 
> > > You might look at whether you could write a kernel module to register
> > > for NETDEV_CHANGE notifications and pass that back to userspace.
> > 
> > This is the kind of responses you get when you ask networking specific
> > questions and don't CC: netdev :-/
> > 
> 
> Thank you for CC.
> 
> > There is this thing called netlink, you can listen for arbitrary
> > network state change events on a socket, and get the link state
> > notifications you are looking for.  It's in use by many real
> > applications like NetworkManager and co.
> 
> That really looks like what I'm looking for. I was already wondering 
> where NetworkManager gets the link state changes from, but I just 
> expected it to poll. So now I'll read a bit of documentation and 
> hopefully get it work.

http://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/nm-netlink-monitor.c

NM uses libnl as the basic library for parsing netlink messages and
handling communication with the kernel.  Which is why you'll see a lot
of nl_* calls in there.  NM sets up the netlink connection using libnl,
then creates a GIOChannel to handle communication over the netlink
socket.  When something comes in (to event_handler()) the code handles
error conditions on the socket, then dispatches to libnl for processing.
libnl then calls back into NM to handle the actual message in
event_msg_ready().

Dan