[CentOS-virt] CentOS 6 and KVM woes

Sun Jul 17 04:22:29 UTC 2011
Trey Dockendorf <treydock at gmail.com>

Nataraj , I'm going to work on your suggestions, but I have a bit more
troubleshooting info that may help (or to me just makes this stranger), and
also to answer Eric's suggestion.

With the VM running this is the brctl table...

# brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.001b21a1cf76       no              eth3
virbr0          8000.fe5400345b8c       yes             vnet1

So that looks good.

The arp table from the VM only shows its own interface, nothing else.

>From the KVM host, I get this (with IPs removed)...

# arp
Address                  HWtype  HWaddress           Flags Mask
 Iface           ether   52:54:00:34:5b:8c   C
---.---.---.--- (VM on br0)                   (incomplete)
---.---.---.--- (gateway)           ether   00:13:xx:xx:xx:xx   C

To clarify a bit about the networking on this server, there are 4 physical
interfaces (eth0-3).  eth0 is what I use for management.  eth3 is linked to
br0 and that is the bridge the VM is associated to.  The VM currently has
two interfaces (for testing in this case).  the VM's eth0 is attached to
host's br0 and the VMs eth1 is attached to the host's NAT.

What makes this situation very confusing to me is that from the VM, with NAT
now working...I can ping my servers on other subnets, and resolve DNS (ie
ping google.com).  However I am unable to ping the gateway of the local
subnet or the KVM host, from the VM, even though networking is functional
using NAT.  I am also unable to ping the VM from the KVM host via the
bridged IP.  This is what leads me to believe the university switches could
be blocking something.  I've been told they increased the MAC address limit
on my port, but due to restrictions on this subnet I am not able to
directly correspond or troubleshoot this issue with the University IT , but
rather have to go through someone else who only knows Windows (*sigh*).

I'll work on troubleshooting with tcpdump and traceroute, but have never
used those tools previously so will have to do a bit of quick
learning-on-the-go to provide useful information.

Thanks for the help thus far!

- Trey

On Sat, Jul 16, 2011 at 8:24 PM, Nataraj <incoming-centos at rjl.com> wrote:

> **
> On 07/16/2011 04:58 PM, Trey Dockendorf wrote:
> On Fri, Jul 15, 2011 at 6:24 PM, Emmanuel Noobadmin <
> centos.admin at gmail.com> wrote:
>> On 7/16/11, Trey Dockendorf <treydock at gmail.com> wrote:
>> > I have successfully bridged one of the server's NICs to br0, and I can
>> ping
>> > the IP remotely that is assigned to br0, but none of the VMs that worked
>> in
>> > 5.6's KVM are able to access the network.  Please let me know what
>> > information would be useful to troubleshoot this.
>>  Could you try creating a new VM using the GUI tool, then check if the
>> networking works from it?
>> I was having problems with KVM and part of the troubleshooting process
>> got me to try it on SL6, which finally led me to discover that the
>> command line tool generated XML doesn't work as well as the GUI tool
>> for some reason. So there's the possibility that it could be that the
>> definitions created through virsh in 5.6 has the same issues in CentOS
>> 6.
>>  _______________________________________________
>> CentOS-virt mailing list
>> CentOS-virt at centos.org
>> http://lists.centos.org/mailman/listinfo/centos-virt
>  I did try another VM (CentOS 6) via virt-manager with the same results.
>  However I setup a test server at home, and am able to get both bridging and
> NAT to work so this may be an issue with the network on my server.  It's a
> University network and their switches tend to play havoc with virtual
> servers even though I've been assured enough MAC addresses have been allowed
> on my port.
>  How does one troubleshoot or provide debug information on a correctly or
> incorrectly functioning network bridge?  As I contact my University's
> helpdesk I'd like to be able to point out the fault is not with my KVM
> server.
>  Thanks
> - Trey
> _______________________________________________
> CentOS-virt mailing listCentOS-virt at centos.orghttp://lists.centos.org/mailman/listinfo/centos-virt
>  If both the VM's and the server are on the same bridge and they can't talk
> to each other, I would from both the server and VM end, ping the opposite
> end and check the arp table to see if arp entries are getting resolved, then
> I would run tcpdump on each respective end and send packets from the other
> end and see if they are getting through.  If not, then their is either a
> problem with either the VM's config file or the networking/bridge config on
> the server.  (Of course if you have any kind of ipfilter access lists, then
> I would check those).
> Once you've got the above working, I would attempt to perform similar tests
> to the outside.  If you happen to have a login on another host on the same
> subnet, you can ping your VM and check the arp table to see if there is arp
> resolution.  (Also check that you don't have duplicate ip address
> assignments).  If there is arp resolution, then run tcp dump on the vm (or
> the physical interface of the server) and see if you can see the packets
> from outside.  Checking the reverse direction is harder if you don't have
> root access on the remote end.
> If your going through gateways, then run traceroute to see how far your
> getting.
> As I thought about it more, it's unclear weather your VM's are on a
> seperate bridge from your server's external interface or they are bridged
> directly onto it.  If the VM's are on a bridge that also has an external
> interface on it, then you don't use NAT.  NAT would be if you wanted your
> server to act as a router/firewall for the VM's in which case the VM's would
> be on a separate bridge and the server would have another external interface
> and would act as a router between the bridge network and the external
> network.
> This should be a start anyway.
> Nataraj
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