RE: [Xen-users] Strange network issue; Guest/DomU outgoing traffic

["Fischer, Anna" <anna.fischer@xxxxxx>]

> -----Original Message-----
> From: Luke S Crawford [mailto:lsc@xxxxxxxxx]
> Sent: 25 March 2009 16:08
> To: Fischer, Anna
> Cc: eric van blokland; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Strange network issue; Guest/DomU outgoing
> traffic
> 
> "Fischer, Anna" <anna.fischer@xxxxxx> writes:
> 
> > You are tracing this within the guest OS? Or are you tracing on the
> bridge within Dom0?
> 
> 
> Assuming the original poster has the same problem I do, within the
> DomU.
> 
> only one guest has a problem at a time, and the problem remains even if
> i
> 'xm save' and then 'xm restore' the guest.
> 
> > Again, you are tracing this within the guest OS? What do the stats on
> the guest network interface say? Do you see packet counters increasing
> (both within the guest OS eth0, and on vifX.0 in Dom0?) when you are
> sending packet?
> 
> after you reset the interfaces within the guest, the transmitted packet
> count stays at 1.
> 
> > If you only have a single guest failing on Xen, then I would guess
> this is an issue with the guest OS and not with anything relating to
> Xen networking. What kernel does the guest OS run?
> 
> 
> In my case, CentOS 5.1 kernel-xen   (I don't remember exact uname.
> sorry, I know that is important.)
> 
> 
> > > Does anyone have an idea about the cause or a solution to this
> problem?
> >
> > I have seen strange things with libpcap under certain guest OS
> versions when used with Xen's netfront driver... libpcap is used for
> tcpdump and some network testing tools (potentially the ones you are
> using). It might be worth trying to also trace packets on the backend
> devices in Dom0, and also have a look at packet counters on all
> interfaces (Dom0/DomU).
> 
> 
> the packet counters within the DomU, in my case, would increment for
> rx,
> for tx, they would not increment.   this looked the same if you looked
> at
> the iface from within the domu or within the dom0  (which makes sense,
> if the
> DomU doesn't think it's transmitting, the dom0 is unlikely to see a
> packet)

If you are tracing within DomU, assuming you are doing a simple Linux "ping 
x.x.x.x" command where x.x.x.x is on the same network as your DomU, then the 
only reason that you would not see an ICMP packet would be that you do not have 
an ARP table entry for x.x.x.x on DomU. What does your ARP table show? If there 
is no entry, then you should see an ARP packet in the trace. If you do not see 
an ARP packet, then it could be that your routing is not set up properly. What 
does ip route show?

Do your interface counters / netstat values show any TX errors at all?

>From your description I don't think this issue is Xen related.

When I had problems with libpcap, I actually saw interface packet counters 
increasing, but no packets in libpcap. So this is a different issue.

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