Xen users - Jan 2013 - Re: XenServer Q & A

I''m  working on a problem related to networking between two DomUs. 
Currently we''re using Xen 4.2.1 built on CentOS 6.

We''re trying to create a bridged network between two VM DomUs.  All
devices seem to set themselves up correctly with regard to the DomU and the
Dom0.  Each DomU configuration file defines a vif device and points it at the
bridge.  Additionally, upon viewing the bridge information, I can correctly see
the running DomU''s vif device attached to the bridge.

# brctl show
bridge name      bridge id                              STP enabled      
interfaces
xenbr0                  8000.feffffffffff                no                     
vif3.0
                                                                                
vif4.0

However, I cannot seem to ping between the two DomU machines (each was assigned
a 10.0.0.* address).

I have followed a number of guides that suggest the following:

-          Disabling netfilter on bridges (/etc/sysctl.conf).

-          Enable forwarding (/etc/sysctl.conf).  Rebooted to make sure this
took effect.

-          Disabled iptables.





Does anyone have any advice on how to remedy this situation?



Currently I''m just trying to keep network communications between the
DomU''s...not seeking external communication yet.



Thanks for your help.


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Hi Todd,

On Fri, Jan 4, 2013 at 9:23 PM, Wardzinski, Todd
<todd.wardzinski@progeny.net> wrote:
> We’re trying to create a bridged network between two VM DomUs.  All devices
> seem to set themselves up correctly with regard to the DomU and the Dom0.
> Each DomU configuration file defines a vif device and points it at the
> bridge.  Additionally, upon viewing the bridge information, I can correctly
> see the running DomU’s vif device attached to the bridge.
From the OS perspective, when you create a bridged network you specify
the interfaces on the Dom0.  With this in place, the interfaces on the
DomU will be able to appear on the same network as the physical NICs
on the Dom0.  You tell the Xen that the virtual nic should use the
bridge.

# virsh dumpxml g15
<domain type=''xen''>
  <name>g15</name>
  <uuid>d92286af-b53a-b5a0-6f62-8d83e4b6c8f7</uuid>
  <memory unit=''KiB''>524288</memory>
  <currentMemory unit=''KiB''>524288</currentMemory>
  <vcpu>1</vcpu>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type>linux</type>
  </os>
  <clock offset=''utc''
adjustment=''reset''/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <disk type=''block'' device=''disk''>
      <driver name=''phy''/>
      <source dev=''/dev/xendomu/g15''/>
      <target dev=''xvda'' bus=''xen''/>
    </disk>
    <interface type=''bridge''>
      <mac address=''00:16:3e:16:94:15''/>
      <source bridge=''br0''/>
      <script path=''/etc/xen/scripts/vif-bridge''/>
    </interface>
    <console type=''pty''>
      <target type=''xen'' port=''0''/>
    </console>
  </devices>
</domain>

So, your bridge should  show NIC[s] which correspond to your physical
host - so eth[0123...] (or em[1234...] or p<slot>p<ethernet port> if
you''re using hardware which supports consistent naming or you or your
hosting provider enforced it with biosdevname).

# brctl show br0
bridge name	bridge id		STP enabled	interfaces
br0		8000.50e5491753bd	no		p4p1

# # ip link show dev p4p1
2: p4p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master br0 state UP mode DEFAULT qlen 1000
    link/ether 50:e5:49:17:53:bd brd ff:ff:ff:ff:ff:ff

Note that the interface p4p1 and br0 are bridged.

I recommend you create your bridge using the standard Red Hat network
scripts at /etc/sysconfig/network-scripts/ifcfg-*

More reading on this is found at:
/usr/share/doc/initscripts<version>/sysconfig.txt

Take care to specify NM_CONTROLLED="no" as NetworkManager does not
support bridged networking under RHEL 6.  You might also want
explicitly to disable and stop NetworkManager and enable and start the
legacy network service.

S.