[Qemu-discuss] Packets dropped on tap interfaces

[Supriya Kher]

Hi everyone,

I have an issue with the tap interfaces in QEMU.

Setup Description -
Here is the long issue description to give you some background
I have a Ubuntu 11.04 host running QEMU emulator (qemu-system-x86_64.
Qemu version 0.14.0)
The VM running on this host machine has an internal VM running a
second Linux operating system.
This is a VM inside a VM architecture, it being so for a reason.

Now, consider two such virtual machines running on this Ubuntu host.
Both these VMs talk to each other via bridged interfaces.

brctl show on Ubuntu host:

brctl show
bridge name     bridge id                   STP enabled     interfaces
base_br6                8000.12b225a87738       no          btap_6_3

        btap_6_7

The VMs running on this Ubuntu Host are:
1) VM_A

    brctl show on VM_A
   bridge name     bridge id                STP enabled     interfaces
   base_br0        8000.001cae060703       no              btap1

       btap2

       eth3
   VM_A is also running QEMU emulator to host another VM ( alias
internal guest or VM_A_Internal )
   tipc-config -l -netid -addr -n issued on VM_A_Internal
   Links:
   broadcast link: up
   current network id: 7777
   node address: <6.1.7>
   Neighbors:
   No nodes found.

   eth0 on VM_A_Internal is configured as the bearer interface that sends out
   TIPC neighbor discovery packets to detect its neighbor and locate
VM_B_Internal as its neighbor.

   Similar settings applicable to VM_B.

2) VM_B - running QEMU emulator to host VM_B_Internal

    brctl show on VM_B
    bridge name     bridge id                STP enabled     interfaces
    base_br0        8000.001cae060300       no              btap1

       btap2

       eth3
    tipc-config -l -netid -addr -n issued on VM_B_Internal
    Links:
    broadcast link: up
    current network id: 7777
    node address: <6.1.3>
    Neighbors:
    No nodes found.

    eth0 on VM_B_Internal is configured as the bearer interface that
sends out TIPC neighbor discovery packets
    to detect its neighbor and locate VM_A_Internal as its neighbor.

ISSUE:
Now the issue is that all these packets ( TIPC control packets )
generated by the inner guest VMs are being
dropped by the tap interfaces on VM_A and VM_B ( confirmed by the Rx
drop count and Tx drop count shown by
netstat -i command) The host Ubuntu system also reports a drop of Rx &
Tx packets at the base_br6 bridge.

How do we avoid the packet drops at the tap interfaces created by QEMU
? Am I missing something here ?

Note: The two inner guest VMs can see each other. Pinging the IP
Addresses configured on the btap<x> interface works.
However the neighborhood detection does not, indicating that these
packets alone are being dropped. Firewall on the host
has been configured to FORWARD incoming and not REJECT/DROP any packet.

Appreciate any thoughts/inputs that will help me proceed further.

Thanks,
SM