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[Bug 761469] Re: multicast VPN breaks IPv6 Duplicate Address Detection

From: Thomas Huth
Subject: [Bug 761469] Re: multicast VPN breaks IPv6 Duplicate Address Detection
Date: Thu, 12 Nov 2020 08:59:36 -0000

The QEMU project is currently considering to move its bug tracking to another 
system. For this we need to know which bugs are still valid and which could be 
closed already. Thus we are setting older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch the state 
back to "New" within the next 60 days, otherwise this report will be marked as 
"Expired". Or mark it as "Fix Released" if the problem has been solved with a 
newer version of QEMU already. Thank you and sorry for the inconvenience.

** Changed in: qemu
       Status: New => Incomplete

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  multicast VPN breaks IPv6 Duplicate Address Detection

Status in QEMU:
Status in Fedora:
  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

  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.

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