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

[Martin Kletzander]

On Wed, Apr 08, 2015 at 02:13:49PM +0200, Stefan Bauer wrote:
>Dear folks,
>
>I'm using for the first time macvtap interface for my virtual machines in bridged mode.
>
>VM -> HOST -> Router -> INTERNET
>
>This works fine for ipv4 connectivity.
>
>For ipv6 my virtual machines receive appropriate v6 address from radvd but are not able to receive answer packages from outside (ping -t -6 google.de was started inside VM).
>
>I see the ping request/response on my router:
>
>14:10:52.147834 IP6 2a01:198:200:8350:dc8b:cd82:144e:14eb > 2a00:1450:4001:806::1018: ICMP6, echo request, seq 108, length 40
>14:10:52.182073 IP6 2a00:1450:4001:806::1018 > 2a01:198:200:8350:dc8b:cd82:144e:14eb: ICMP6, echo reply, seq 108, length 40
>14:10:55.179874 IP6 2a01:198:200:350::2 > 2a00:1450:4001:806::1018: ICMP6, destination unreachable, unreachable address 2a01:198:200:8350:dc8b:cd82:144e:14eb, length 88
>
>
>But i do not receive the reply on the VM.
>
>However on the KVM host - when i start a tcpdump on the macvtap interface with
>
>root at s1:~# tcpdump -ni macvtap0 ip6
>tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
>listening on macvtap0, link-type EN10MB (Ethernet), capture size 262144 bytes
>14:12:37.134516 IP6 2a01:198:200:8350:dc8b:cd82:144e:14eb > 2a00:1450:4001:806::1018: ICMP6, echo request, seq 129, length 40
>14:12:37.188529 IP6 fe80::12fe:edff:fee6:cfa > ff02::1:ff4e:14eb: ICMP6, neighbor solicitation, who has 2a01:198:200:8350:dc8b:cd82:144e:14eb, length 32
>14:12:37.189040 IP6 2a01:198:200:8350:dc8b:cd82:144e:14eb > fe80::12fe:edff:fee6:cfa: ICMP6, neighbor advertisement, tgt is 2a01:198:200:8350:dc8b:cd82:144e:14eb, length 32
>14:12:37.189202 IP6 2a00:1450:4001:806::1018 > 2a01:198:200:8350:dc8b:cd82:144e:14eb: ICMP6, echo reply, seq 129, length 40
>
>
>packages starting to get processed and VM receives replies.
>Any idea what is happening here?
>

I'm guessing the promiscuous modes plays its part in this field.  You
can try setting the interface to promisc mode manually using 'ip l set
$dev promisc on' and see whether that helps without starting tcpdump.
Also check sysctl -a | grep 'ipv6.*forward'.

Disclaimer: all of that ^^ is just a guess :)

>Cheers,
>
>Stefan
>
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