Re: FAQ on linux-kvm.org has broken link

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jul 30, 2013 at 10:45:20PM +0200, folkert wrote:
> > If you keep losing network connectivity you may have a MAC or IP address
> > conflict.  The symptom is that network traffic is intermittent - for
> > example, ping might work but a full TCP connection does not.
> 
> I submitted a bug at bugzilla a while ago which I updated today with new
> findings: https://bugzilla.kernel.org/show_bug.cgi?id=60620
> This week the system ran a couple of times for 1-2 days but tonight was
> a bit of a disaster: I had to reboot the system 18 times. Sometimes it
> was fine for half an hour but most of the times after a couple of
> minutes (sometimes even during boot) the networking on that one guest
> failed.

I can't add anything besides suggesting slightly more verbose
troubleshooting steps:

1. Wait until the guest suffers from lost network connectivity.

2. Confirm the MAC/IP addresses and run tcpdump -ni $IFACE inside the
   guest.  Ping the guest from the host and check whether tcpdump
   reports ICMP ping packets.

3. Now try pinging the host from the guest and run tcpdump -ni $IFACE on
   the host.  To determine the host-side tap interface, run the
   following:

   $ virsh domiflist mauer
   Interface  Type       Source     Model       MAC
   -------------------------------------------------------
   vnet0      network    default    virtio      52:54:00:b9:c8:4d

Now you have verified tap connectivity with the guest.  We now know:

1. Tap connectivity is fine (both transmit and receive are working)
2. Either transmit or receive are broken (ping doesn't work but tcpdump
   does show incoming packets on one side).
3. Tap connectivity is broken (ping fails and tcpdump shows no ICMP
   packets).

If the result is #1 then you can continue troubleshooting the next step:
the bridge or NAT configuration on the host.

If the result is #2, check firewalls on host and guest.  Also try the
following inside the guest: disable the network interface, rmmod
virtio_net, modprobe virtio_net again, and bring the network up.

If the result is #3, check firewalls on host and guest as well as dmesg
output in host and guest.

Stefan