Bug 97961

Summary:	macvtap does not transport IPv4 multicast packets
Product:	Networking	Reporter:	Marc Haber (mh+kernel-bugzilla)
Component:	IPV4	Assignee:	Stephen Hemminger (stephen)
Status:	NEW ---
Severity:	normal
Priority:	P1
Hardware:	All
OS:	Linux
Kernel Version:	4.0.2	Subsystem:
Regression:	No	Bisected commit-id:

Description Marc Haber 2015-05-08 19:50:20 UTC

Hi,

I have a system running Debian Jessie and kernel 4.0.2 (self-built). When I start a VM (also Debian Jessie) in kvm which has its network connected via macvtap, IPv4 multicast packets do not show up on the macvtap interface and also not inside the VM. This can easily be observed when avahi is in use.

tcpdump on enp3s0:
21:44:24.855171 IP6 2a01:238:4071:321e:224:d7ff:fed0:5adc.5353 > ff02::fb.5353: 0 A (QM)? parada.local. (30)
21:44:24.855242 IP 192.168.251.21.5353 > 224.0.0.251.5353: 0 A (QM)? parada.local. (30)
21:44:25.856546 IP6 2a01:238:4071:321e:224:d7ff:fed0:5adc.5353 > ff02::fb.5353: 0 A (QM)? parada.local. (30)
21:44:25.856617 IP 192.168.251.21.5353 > 224.0.0.251.5353: 0 A (QM)? parada.local. (30)
21:44:27.858923 IP6 2a01:238:4071:321e:224:d7ff:fed0:5adc.5353 > ff02::fb.5353: 0 A (QM)? parada.local. (30)
21:44:27.858995 IP 192.168.251.21.5353 > 224.0.0.251.5353: 0 A (QM)? parada.local. (30)

tcpdump on macvtap0:
21:44:40.113076 IP6 2a01:238:4071:321e:224:d7ff:fed0:5adc.5353 > ff02::fb.5353: 0 A (QM)? parada.local. (30)
21:44:41.114809 IP6 2a01:238:4071:321e:224:d7ff:fed0:5adc.5353 > ff02::fb.5353: 0 A (QM)? parada.local. (30)
21:44:43.117490 IP6 2a01:238:4071:321e:224:d7ff:fed0:5adc.5353 > ff02::fb.5353: 0 A (QM)? parada.local. (30)

The avahid on the VM does not answer the IPv6 packets since it is configured not to answer A queries over IPv6.

Using a regular bridge to connect the VM via the shared device model works.

I do not know whether this is a regression since I never tried with older kernels, I have never tried macvtap before.

Greetings
Marc