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



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 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?

Cheers,

Stefan


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