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Re: [Xen-users] what the H is this virbr0?

To: Sadique Puthen <sputhenp@xxxxxxxxxx>
Subject: Re: [Xen-users] what the H is this virbr0?
From: Steven <xen@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 06 Dec 2007 14:16:49 -0500
Cc: Steven <xen@xxxxxxxxxxxxxxxxxxx>, xen-users <xen-users@xxxxxxxxxxxxxxxxxxx>
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Sadique Puthen wrote:
Steven wrote:
Sadique Puthen wrote:
Steven wrote:

I just tested in the lab the new CentOS 5.1 Xen and find out it breaks network connectivity, at least in my setup. It seems it is somehow making use of the 3.1 Xen version (altought I am not sure about that) but whatever changes it introduce is not documented at the location one would expect, the obvious new thing (at least for me) is this thing I will call out of guessing a virtual bridge, when I do an ifconfig on the dom0:

virbr0    Link encap:Ethernet  HWaddr FE:FF:FF:FF:FF:FF
         inet addr:  Bcast:
This seems to change everything with regard to networking. I googgled a bit and read the following links but I could not find any explanation about what it is, what it does and why it breaks my setup:

Centos 5.1 makes networking for guests a bit easier by adding virb0 and implementing a natted network connection for guests. This shouldn't break the networking that you had previously in Centos-5.0. At least it has not broken anything on my RHEL5 system when I upgraded that to RHEL-5.1

RHEL-5.1 makes it possible for you to put guests in a private network behind dom0 and nat packets to your LAN through dom0. This is the preferred set up for laptops. If you create guests using virt-manager, it defaults the network setup to use natting, unless you explicitly select to bridge the network to xenbrx. The problem is that RHEL-5.1 does not enable ip forwarding by default in the kernel. So if you create guests and bridge them to virbr0, the guest wouldn't be able to communicate to the external networks. This can simply be solved by enabling ip forwarding in Dom0.
Ok, thank you. Do you know how or where to disable it?



Thanks, that is what I was asking, to disable it in Xen, not forwarding with iptables.
Too bad this is not documented officially anywhere .


Steven Dugway

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