FAQ
I have decided to migrate my latest KVM server to CentOS 6.0 and am
beginning to get a little frustrated with some issues that worked perfectly
in 5.6.

Right now I've given up on getting virbr0 and NAT to work, but now I need
networking bridging to work, but nothing seems to fix the issue. I have not
had much experience with troubleshooting KVM so could really use some
pointers on resolving this issue.

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.

Here's what I have so far...

ifcfg-eth3...
---------------------------------
DEVICE="eth3"
HWADDR="00:1B:21:A1:CF:76"
ONBOOT="yes"
BRIDGE=br0

ifcfg-br0
--------------------------------
DEVICE=br0
TYPE=Bridge
BOOTPROTO=static
ONBOOT=yes
IPADDR=....
BROADCAST=....
NETMASK=....
NETWORK=....


# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.001b21a1cf76 no eth3


sysctl.conf has these 3 additional lines...
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0


Again I can ping br0's IP remotely , but no VMs assigned to it can access
even that network's gateway. I have disabled iptables and ip6tables for now
while trying to get this to work. The VMs , worked just fine in 5.6 and I
have done nothing different but import them with virsh.

Thanks
- Trey
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  • Eric Searcy at Jul 15, 2011 at 5:20 pm
    No experience with 6 here, but do your virsh-imported libvirt VM configs show <devices>...<interface type=bridge>...<source bridge='br0'/> ? i.e. the bridge there matches the bridge name you're created with ifcfg scripts?

    Eric
  • Trey Dockendorf at Jul 15, 2011 at 7:15 pm

    On Fri, Jul 15, 2011 at 4:20 PM, Eric Searcy wrote:

    No experience with 6 here, but do your virsh-imported libvirt VM configs
    show <devices>...<interface type=bridge>...<source bridge='br0'/> ? i.e.
    the bridge there matches the bridge name you're created with ifcfg scripts?

    Eric
    _______________________________________________
    CentOS-virt mailing list
    CentOS-virt at centos.org
    http://lists.centos.org/mailman/listinfo/centos-virt
    Yes, the name matches, the KVM hosts both used br0 on 5.6 and now on 6.0.
    Here's what's in the XML file

    <interface type='bridge'>
    <mac address='52:54:00:31:6f:16'/>
    <source bridge='br0'/>
    <target dev='vnet0'/>
    <model type='virtio'/>
    <alias name='net0'/>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
    function='0x0'/>
    </interface>


    Thanks
    - Trey
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  • Jussi Hirvi at Jul 16, 2011 at 3:00 am

    On 16.7.2011 2.15, Trey Dockendorf wrote:
    Yes, the name matches, the KVM hosts both used br0 on 5.6 and now on 6.0.
    Here's what's in the XML file

    <interface type='bridge'>
    <mac address='52:54:00:31:6f:16'/>
    <source bridge='br0'/>
    <target dev='vnet0'/>
    <model type='virtio'/>
    <alias name='net0'/>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
    function='0x0'/>
    </interface>
    I am not sure if yo have the same kind of virtual network structure as I
    have. Anyway, my xml file (CentOS 5.6 on both host and guest) says simply:
    <interface type='bridge'>
    <mac address='54:52:00:3d:aa:46'/>
    <source bridge='br0'/>
    <model type='virtio'/>
    </interface>

    I have one physical nic, one bridge (br0), one external ip address per
    guest. No NAT.

    BTW, I now notice that I have wrong MAC address (54:52), and you have it
    right (52:54), I probably got mine from "man virt-install" which
    contains an error:

    https://bugzilla.redhat.com/show_bug.cgi?idb0838

    Anyway, my vguests work fine.

    - Jussi
  • Akemi Yagi at Jul 15, 2011 at 5:20 pm

    On Fri, Jul 15, 2011 at 2:07 PM, Trey Dockendorf wrote:
    I have decided to migrate my latest KVM server to CentOS 6.0 and am
    beginning to get a little frustrated with some issues that worked perfectly
    in 5.6. (snip)
    Here's what I have so far...
    ifcfg-eth3...
    ---------------------------------
    DEVICE="eth3"
    HWADDR="00:1B:21:A1:CF:76"
    ONBOOT="yes"
    BRIDGE=br0
    ifcfg-br0
    --------------------------------
    DEVICE=br0
    TYPE=Bridge
    BOOTPROTO=static
    ONBOOT=yes
    IPADDR=....
    BROADCAST=....
    NETMASK=....
    NETWORK=....
    I do not see GATEWAY=.... in your ifcfg-br0. Do you actually have it?
    Again I can ping br0's IP remotely , but no VMs assigned to it can access
    even that network's gateway.
    Akemi
  • Trey Dockendorf at Jul 15, 2011 at 7:16 pm

    On Fri, Jul 15, 2011 at 4:20 PM, Akemi Yagi wrote:
    On Fri, Jul 15, 2011 at 2:07 PM, Trey Dockendorf wrote:
    I have decided to migrate my latest KVM server to CentOS 6.0 and am
    beginning to get a little frustrated with some issues that worked perfectly
    in 5.6. (snip)
    Here's what I have so far...
    ifcfg-eth3...
    ---------------------------------
    DEVICE="eth3"
    HWADDR="00:1B:21:A1:CF:76"
    ONBOOT="yes"
    BRIDGE=br0
    ifcfg-br0
    --------------------------------
    DEVICE=br0
    TYPE=Bridge
    BOOTPROTO=static
    ONBOOT=yes
    IPADDR=....
    BROADCAST=....
    NETMASK=....
    NETWORK=....
    I do not see GATEWAY=.... in your ifcfg-br0. Do you actually have it?
    Again I can ping br0's IP remotely , but no VMs assigned to it can access
    even that network's gateway.
    Akemi
    _______________________________________________
    CentOS-virt mailing list
    CentOS-virt at centos.org
    http://lists.centos.org/mailman/listinfo/centos-virt
    The GATEWAY is specified in /etc/sysconfig/network file. Should I include
    it in the ifcfg-br0 as well? In CentOS 5.x I always left it in the
    /etc/sysconfig/network file which seems to be the default from fresh install
    of CentOS 6.

    Thanks
    - Trey
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  • Emmanuel Noobadmin at Jul 15, 2011 at 7:24 pm

    On 7/16/11, Trey Dockendorf 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.
  • Trey Dockendorf at Jul 16, 2011 at 7:58 pm

    On Fri, Jul 15, 2011 at 6:24 PM, Emmanuel Noobadmin wrote:
    On 7/16/11, Trey Dockendorf 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
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  • Eric Searcy at Jul 16, 2011 at 9:04 pm
    When you showed the output of brctl show earlier only eth0 showed up. Does the VM NIC showed up attached to br0 when the VM is running?

    If so, then you can ping the VM from your host? That wouldn't involve the university switches so it would be a good first step before digging into packet dumping arp traffic...

    Sent from my mobile phone
    On Jul 16, 2011, at 4:58 PM, Trey Dockendorf wrote:



    On Fri, Jul 15, 2011 at 6:24 PM, Emmanuel Noobadmin wrote:
    On 7/16/11, Trey Dockendorf 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 list
    CentOS-virt at centos.org
    http://lists.centos.org/mailman/listinfo/centos-virt
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  • Nataraj at Jul 16, 2011 at 9:24 pm

    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 <mailto: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 list
    CentOS-virt at centos.org
    http://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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  • Trey Dockendorf at Jul 17, 2011 at 12:22 am
    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
    vnet0
    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
    192.168.122.20 ether 52:54:00:34:5b:8c C
    virbr0
    ---.---.---.--- (VM on br0) (incomplete)
    eth0
    ---.---.---.--- (gateway) ether 00:13:xx:xx:xx:xx C
    eth0


    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 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 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


    _______________________________________________
    CentOS-virt mailing list
    CentOS-virt at centos.org
    http://lists.centos.org/mailman/listinfo/centos-virt
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  • Trey Dockendorf at Jul 17, 2011 at 12:40 am

    On Sat, Jul 16, 2011 at 11:22 PM, Trey Dockendorf wrote:

    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
    vnet0
    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
    192.168.122.20 ether 52:54:00:34:5b:8c C
    virbr0
    ---.---.---.--- (VM on br0) (incomplete)
    eth0
    ---.---.---.--- (gateway) ether 00:13:xx:xx:xx:xx C
    eth0


    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 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 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


    _______________________________________________
    CentOS-virt mailing list
    CentOS-virt at centos.org
    http://lists.centos.org/mailman/listinfo/centos-virt
    Apologies for replying to my own post, but after running arp a few more
    times on the VM it now shows the MAC and IP of the subnet's gateway and the
    KVM host. Still not entirely sure what all this information means, but it's
    a start.

    Thanks

    - Trey
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postedJul 15, '11 at 5:07p
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