[Qemu-discuss] Using multiple network interfaces on a guest

[Pavel Troller]

Hi!
  I have a strange problem. I need to run a guest with two separate network
cards. On the host, their tap ends are connected to two separate bridges.
It's on qemu-2.0.0. Related part of the command line looks like this:
qemu-system-x86_64...
...-net nic,model=virtio,macaddr=xx-xx-xx-xx-xx-xx -net tap,ifname=alpha-tap..
...-net nic,model=virtio,macaddr=yy-yy-yy-yy-yy-yy -net tap,ifname=bravo-tap..
They are incorporated to the bridges on the host:
address@hidden:# brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.300bff486546       no              eth1
                                                        delta-tap
                                                        bravo-tap
                                                        echo-tap
br1             8000.bc03d650794a       no              eth0
                                                        alpha-tap

The br1 bridge is anonymous one, it doesn't have an IP address and serves just
to bridge the traffic from the guest to a dedicated physical card (eth0).
delta-tap and echo-tap come from another guests. The br1 bridge never contains
more interfaces.

On the guest, there is no bridging between its virtual nics. Every one has its
own IP address and some set of routes, there is just one default route,
leading to the dedicated network card on the host. But NO BRIDGING. There is
no bridge interface activated at all. 

And now, to the problem: Immediately after bringing the guest up and adding
its interfaces to the appropriate bridges, it starts to bridge them together!
All the traffic (including arp queries, DHCP transactions etc.) visible on
br0 starts to be visible on br1 and vice versa. It makes a lot of serious
network problems - confused ARP tables, unstable routing, even forwarding
the local DHCP server (coming from delta-tap on br0) to the dedicated card
(eth0) on br1, etc. Bringing either alpha-tap or bravo-tap down stops the
bridging, bringing them up starts it again. Bringing down the virtual nics
on the guest DOESN'T stop the bridging at all.

  I can't understand, why this happens. How to prevent the bridging ? Is it
a feature of tap interfaces, that all ones are automatically bridged together?
Or is it a qemu bug ? I'm really confused and a lot of hours spent by
searching the problem on the net didn't bring anything useful, it seems that
nobody ever tried more virtual nics on one guest.
  Sorry for such a long post. Any useful reply will be highly appreciated.

With regards,
  Pavel