Bug 674539

I could not reproduce this bug with (qemu-kvm-0.12.1.2-2.129.el6.x86_64) by the steps in comment #0.

> Steps to Reproduce:
> 1. create multiple guests connected to a bridge: A,B,C,D
> 2. force arp in guest A to match guest B
> arp -i eth0 -s <ip for guest B> <mac for guest B>

execute in guest A ?
>   repeat for guests C, D

execute in guest C ?
> 3. ping guest B, we should get back packets
>    e.g. with -c 1

ping from guest A ?
>    repeat for C, D

ping from guest A ?
> 3. ifdown guest B

guest B) # ifconfig eth0 down
>     repeat for C,D

guest D) # ifconfig eth0 down
> 4. log into guest A
>  ping -f -c 2000 -s 1000 B_ip -i 0.01

ping from guest A?
>    repeat for C, D

ping from guest A?
> Actual results:
> outgoing networking from A is blocked.
> E.g. A does not reply to ping.

ping from host?
> Expected results:
> networking from A to C,D is blocked,
> networking from A to host works

qemu commandline
# qemu-kvm -name guesta -chardev socket,id=human_monitor_Knqg,path=/tmp/monitor-humanmonitor1-20110219-174500-2Nk6,server,nowait -mon chardev=human_monitor_Knqg,mode=readline -chardev socket,id=serial_2QM1,path=/tmp/s1,server,nowait -device isa-serial,chardev=serial_2QM1 -drive file=/home/autotest-devel/client/tests/kvm/images/RHEL-Server-6.0-64-virtio.qcow2,index=0,if=none,id=drive-virtio-disk1,media=disk,cache=none,snapshot=on,format=qcow2,aio=native -device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk1,id=virtio-disk1 -device virtio-net-pci,netdev=idkZj2Gr,mac=9a:76:6e:ab:5c:3f,netdev=idkZj2Gr,id=ndev00idkZj2Gr,bus=pci.0,addr=0x3 -netdev tap,id=idkZj2Gr,vhost=on,ifname=t0-174500-2Nk6,script=/home/autotest-devel/client/tests/kvm/scripts/qemu-ifup-switch,downscript=no -m 512 -smp 2,cores=1,threads=1,sockets=2 -cpu cpu64-rhel6,+sse2,+x2apic -vnc :0 -rtc base=utc,clock=host,driftfix=none -boot order=cdn,once=c,menu=off -usbdevice tablet -no-kvm-pit-reinjection -enable-kvm

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    New Contents:
Cause: qemu-kvm tap code default for 'sndbuf' can prevent another guest from transmitting any packets.

Consequence: sending packets from guest to another guest that do not consume the packets can cause all networking from said guest to get blocked.

Fix: default of 'sndbuf' option changed to 0, disabling sndbuf.

Result: guest networking is not blocked even when it is sending packets to another guest that is not consuming the packets.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2011-0534.html

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2011-0534.html