[ovs-discuss] OVS not forwarding packets to port

Gurucharan Shetty shettyg at nicira.com
Wed Mar 18 15:27:03 UTC 2015

Fwiw, I had seen a similar issue a couple of years ago. It turned out
to be a system bug. I debugged (with help from Ben) like this:
ps -ef | grep kvm : Choose the process id of the kvm
ls -ltr /proc/$pid/fd

This will show you the fd of /dev/net/tun
So the KVM process will be listening on that fd for all the packets
that comes from host and is headed to VM.

"strace -o logfile -p $Pid" for a few seconds.

If you open the logfile, you will see a select system call on that fd
and then read system calls on that fd.

In my case, there was nothing being read. I don't remember very well
what exactly was the problem. The fix was to use a different network
device .

On Wed, Mar 18, 2015 at 8:14 AM, Chris Picton <chris at picton.nom.za> wrote:
> On 03/18/15 16:52, Ben Pfaff wrote:
>> On Wed, Mar 18, 2015 at 12:44:08PM +0200, Chris Picton wrote:
>>> On 03/18/15 08:47, Chris Picton wrote:
>>>> The relevant flow is:
>>>> recirc_id(0),skb_priority(0),in_port(2),eth(src=00:15:17:91:05:4c,dst=52:54:00:14:54:c6),eth_type(0x8100),vlan(vid=3,pcp=0),encap(eth_type(0x0806)),
>>>> packets:2, bytes:120, used:0.014s, actions:pop_vlan,7
>>>> I can't find any definitive documentation for an action element
>>>> consisting
>>>> purely of a number - in this case "7".
>>> I have just re-read the man page, and found the following:
>>> "The  target  may  be  a  decimal  port  number  designating the physical
>>> port on which to  output the packet."
>>> so the packet should, according to the above, be forwarded to port 7,
>>> which
>>> is vnet0.
>>> However, the packet is not arriving on vnet0.  Any ideas where I can look
>>> to
>>> find out why not?
>> I'd start by checking the kernel log (with "dmesg").
> Hi Ben
> I had already done system level debugging to try find out where issues may
> lie.  There are no kernel logs indicating dropped packets or transmit
> failures.
> A diff between the contents of /sys/class/net/vnet0 (which is the port that
> gets no traffic) and /sys/class/net/veth-carbon1-3/ (which is the port that
> gets traffic) shows very few differences at the kernel level (rx/tx
> counts/address/etc differ, but type/features/carrier/flags/etc are the same)
> What I know so far is that while openvswitch should be forwarding packets
> onto vnet0 (which is what the flow indicates), vnet0 does not receive those
> packets (as indicated with tcpdump).  Also the tx_packets counter for vnet0
> does not increment (as it should when packets get sent to the interface and
> transmitted on)
> Is this a kernel/openvswitch bug?  I can probably get the interface working
> by migrating the VM to a different host and migrating it back, but I would
> rather leave it in a non working state to try and pinpoint the issue.
> Regards
> Chris
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