Re: [gentoo-user] bridge

[Joshua Murphy]

On Sun, Jul 10, 2011 at 10:50 AM, Daniel Hilst Selli
<danielhilst@×××××.com> wrote:
> Hi people, I'm using brctl to create bridges for some qemu guests...
>
> I create a br0 with brctl addbr br0
> the I attach my wireless card to it with brctl addif br0 eth1
>
> Then some times I get right ip with dhcpcd br0 (after doing 'ifconfig br0
> promisc up')
> but some times I got an strange ip
> The questions are
> What the promisc means?
>
> I can't understand for really the bridge concepts .. I just know that you
> attach
> cards to it, but can't understand how it route things
>
> cheers
> --
> "Do or do not... there is no try" Yoda Master

Well, in principle, a bridge acts like a switch or a hub, so there's
no real 'routing', though when it knows which 'port' a given MAC
address is on the other end of, it'll likely avoid sending traffic
headed towards that MAC to the other endpoints (as a switch does).

With regard to the trouble you're seeing, I've had more issues than I
can clearly recall with bridging and wireless cards when I've tried,
and it may be a similar roadblock that you're running into there. If
you don't need each VM to be 'local' to the network you're on, you
might be better off using NAT to interface the internal network of VMs
with the outside world.

'promisc' means that the interface is in promiscuous mode. Promiscuous
mode recieves every ethernet frame to handle them rather than only
accepting frames that are targetted for the interface's MAC address,
which then allows all of that traffic to be forwarded across the
bridge to other networks.

There's a very good writeup on bridges, how they work, common issues,
and a fairly extensive FAQ at the link below:

[1] http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge

-- 
Poison [BLX]
Joshua M. Murphy