[libvirt] macvtap - no incoming ipv6 traffic processed on kvm host unless i start tcpdump on interface

Michal Privoznik mprivozn at redhat.com
Thu Apr 9 08:47:28 UTC 2015

On 09.04.2015 10:39, Stefan Bauer wrote:
> On Thu, 2015-04-09 at 10:12 +0200, Michal Privoznik wrote:
>> I mildly recalls seeing a bug like this. The problem was in intel's
>> kernel driver. A NIC by defaul checks incoming packets whether they
>> match NIC's MAC. So if a TAP device was created over a NIC, it had to be
>> put into promisc mode (automatically done by the driver) to allow
>> different MACs and the check was done in kernel then. But since this
>> generates too much interrupts, NICs HW was extended and it can be
>> programmed with multiple MACs to let through. However, there was a bug
>> which I recall of, that intel driver was not always putting the TAP MAC
>> into the NIC HW correctly. Obviously, the bug was not visible if the NIC
>> was put into promisc mode. And this may be what you are seeing. Let me
>> see if I can find the bug.
> FYI - enabling promiscious mode on eth0 does not help. I have to enable
> it on the macvtap0 which is on top of eth0.

Ah, so that's not the bug then. Have you perhaps changed the vNIC MAC
inside the guest after it was started? Even though libvirt will nowadays
adapt to such change, older version of libvirt haven't this implemented
causing guest connectivity to break.


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