How to troubleshoot a macvtap network in kvm/virt-manager

I have a virtual machine running under qemu-kvm, on a 13.10 host. I am using virt-manager to define the machine, and I have added a virtual NIC, using the new macvtap driver. I have the following definition:

<interface type='direct'> <mac address='52:54:00:1f:dd:c4'/> <source dev='eth0' mode='bridge'/> <model type='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

On the host machine, I can see that the tap interface is created:

$ip link
5: macvtap0@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT qlen 500 link/ether 52:54:00:1f:dd:c4 brd ff:ff:ff:ff:ff:ff
$ls /dev/tap5
/dev/tap5

But in the virtual machine, interface eth0 is unable to access the network. dhcp fails. Static configuration obviously works, but then I cannot ping anything.

I have no iptables rules, neither on the host, nor in the virtual machine.

So, how can I troubleshoot this problem ?

1 Answer

I have same problem with Ubuntu 13.10, I also update to 14.04 development branch to try but still does not work.

I do not know a good way debug, for my problem, I found eth0 does not enter promiscuous mode, I can not find message like device eth0 entered promiscuous mode in dmesg output. So I force eth0 into promiscuous mode, the problem then disappears.

This is what I did:

1. Force eth0 into promiscuous mode: sudo ifconfig eth0 promisc
2. Use virt-manager to start the VM
3. ifconfig -a and ping outside to see whether it works for you.

I do not know why libvirt does not put eth0 into promiscuous mode, maybe a bug of it.