multicast VPN breaks IPv6 Duplicate Address Detection

Bug #761469 reported by Neil Wilson on 2011-04-15
This bug affects 2 people
Affects Status Importance Assigned to Milestone
Won't Fix

Bug Description

The multicast VPN facility in Qemu uses Multicast Loopback to make sure that other Qemu processes on the Host server receive the transmission. The side effect of this is that the process sending the packet also gets the packet back on its receive channel and currently this is not filtered but passed directly to the Virtual Machine.

You can see this effect by running tcpdump in the virtual machine. Every packet sent appears to be duplicated.

For IPv4 it doesn't appear to cause much of a problem, however with IPv6 the duplicate neighbor solicitation packet is precisely what the system uses to detect duplicate addresses. So IPv6 addresses fail to establish.

If you run 'ip addr' on a virtual Linux machine with IPv6 enabled you will see 'dadfailed' next to the link local address. 'ping6' will then not work.

Checked against 0.12.1.

Description of problem:

Virtual NIC of type "mcast" receives copies of what it sent, resulting in many disastrous behaviors if you add one to a Linux software ethernet bridge.

Version-Release number of selected component (if applicable):

How reproducible:


Steps to Reproduce:
1. create a kvm/qemu guest with an mcast NIC as eth0
2. create a bridge br0 in the guest, enabling STP
3. add eth0 to br0 as its only slave

Actual results:

Once STP starts sending frames, the host starts reporting:

eth0: received packet with own address as source address

Expected results:

eth0 should not receive copies of what it sends.

Additional info:

This happens as above when the NIC goes into promiscuous mode. I have not bothered to verify whether it happens for multicast or broadcast traffic in non-promiscuous mode.

Things become more obvious and destructive if you add another slave to the bridge. Any incoming broadcast or multicast traffic from the other slave gets bridged into the mcast NIC, reflected back, and creating a kind of psuedo-loop. This confuses the user, upsets everyone on the other LAN, and also confuses the Linux software bridge as it starts falsely discovering all the reflected MACs as if they are reachable behind the mcast NIC.

With a manually created list of the MAC addresses actually behind the mcast LAN, one can create draconian ebtables filters to drop all reflected packets via a command like:

ebtables -t nat -A PREROUTING -i ethX --among-src-file ! /etc/mcast-mac-addrs -j dnat --dnat-target DROP

This will drop the bad frames before they confuse the bridging code, and you end up with a working bridged mcast network. This proves that the problem is reflected packets coming back to the sender via the mcast NIC.

This would presumably break all IPv6 duplicate address detection, so guests don't work with IPv6?

I am not using IPv6 in my test environment, so I cannot confirm nor deny. The network goes haywire very rapidly if one does this experiment without the ebtables rule above, leading one to rip cables out of walls as fast as one can.

It is a great DoS attack on the victim LAN that you bridge into such an mcast tunnel, as it reflects all broadcast traffic such as mDNS and probably confuses hardware switches as well as the Linux software bridge. As such, I won't be performing more detailed tests of this failure mode in my environment!

Your initial description doesn't entirely make sense. The QEMU 'mcast' nic type is totally isolated from the host network, but in the steps to reproduce you're talking about setting up bridges in the host which implies the 'tap' nic type in QEMU.

Please provide the exact QEMU command line you are invoking, and the output of 'ifconfig -a', 'brctl show' and 'route -n' on the host OS.

No, I described setting up a bridge in the guest with the mcast NIC!

So the bug-triggering guest needs only one NIC, eth0, which is defined like this:

    <interface type='mcast'>
      <mac address='...'/>
      <source address='...' port='5558'/>

(with actual local MAC and multicast address assignments).

After booting that guest, use brctl to create a bridge br0, and simply slave that virtual eth0 to the bridge (as its only "physical" port). This will exhibit the problem, with no real networks involved.

When I mentioned the host LAN, I had created a second NIC in a privileged guest, so it had one mcast NIC and one tap NIC already bridged into the host LAN by libvirt. Then that guest bridged its two NICs, allowing other unprivileged guests to use an mcast NIC to participate in the host LAN. Because of this mcast bug, you can only do this once this privileged guest has the severe ebtables workaround I reported.

But this extra step is unnecessary to demonstrate the bug. It is only interesting in that it does something useful once the ebtables workaround is in place (allowing unprivileged guests to participate in a real LAN).

This package has changed ownership in the Fedora Package Database. Reassigning to the new owner of this component.

This bug appears to have been reported against 'rawhide' during the Fedora 13 development cycle.
Changing version to '13'.

More information and reason for this action is here:

I found this bug too.

Simple way how to reproduce:

Run kvm guest 1 and 2, with net something like this:
g1:-net nic,vlan=0,model=virtio,macaddr=52:54:00:00:01:01 -net socket,vlan=0,mcast=

g2: -net nic,vlan=0,model=virtio,macaddr=52:54:00:00:02:01 -net socket,vlan=0,mcast=

This should give you two running vm on same net.

Now in each guest configure ip addresses:
g1: ifconfig eth0
g2: ifconfig eth0

now on g2 ping

and on g1 run tcpdump -i eth0 icmp
and every second you will get:
time ip ... ICMP echo request
time ip ... ICMP echo reply
time ip ... ICMP echo reply

There shouldn't be two echo replies, but only one.

Another effect is, that Duplicate Address Detection (for ipv6) simply doesn't work (this is what original bug is talking about).

Of course, solution is not to disable loopback for mcast socket, because there must be chance to run more then one VM on same host on same mcast address:port.

Solution may be to detect sending socket ip:port pair and simply drop packets received from this ip:port.

Just one extra note, I was able to reproduce this bug on qemu-system-x86-0.13.0-1.fc14.i686

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Bug is reproducible on fc14 too

F14 is end-of-life now. If anyone is still affected by this bug with newer qemu versions, I'd recommend following up in the (still open) upstream bug report mentioned in Comment #10

Changed in fedora:
importance: Unknown → High
status: Unknown → Won't Fix
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