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Re: [Xen-users] Xen bridge allows to sniff traffic destined to other domUs in same dom0

On 01/14/2013 12:21 PM, Sherin George wrote:
Hi Guys,

I am working as syadmin for a hosting company.

Recently one of our customers came to me saying that he can view
traffic not destined to his VPS(domU) which are not broadcast.

I created a test VPS(domU) in the hardware node(dom0) and found that
what customer claimed may be correct.

I did tcpdump in my tes VPS testvps.example.com and I could see
traffic as customer explained. So I think my customer was true about
what he said.

I tried to access the website customer-website.net hosted in the
customer VPS server1.customer-server.net( Then I logged
into testvps.example.com&  checked tcpdump. I found that traffic from
my office IP to server1.customer-website.net server is
showing in testvps.example.com.

336630167 2230533262>
07:10:38.479684 IP> . ack 8368 win 454
07:10:38.482157 IP> P
1960:2456(496) ack 8368 win 454
07:10:38.520554 IP> . ack 8093 win 408
07:10:38.522452 IP> P
1493:1990(497) ack 8169 win 408
07:10:38.637627 IP> . ack 9827 win 454
07:10:38.643413 IP> . ack 11167 win 499
07:10:38.704186 IP> . ack 7627 win 363
07:10:38.744250 IP> . ack 7954 win 408

I was under the impression that domU(VPS) will get only broadcast
traffic other than packets actually destined to them. Bridge is
supposed to send packets to MAC address than broadcasting. So, this
behavior is interesting, something that need to be investigated
further and may be fixed if possible.

Could anyone please provide any insight into what might be happening ?

Note: I replaced actual IP addresses for privacy

Thanks in advance.

Hi Sherin,
all that is just expected and it just shows that your bridge is working correctly.
Once you are interested in reading about Linux bridging read some of these:
 - http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge
- https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/s2-networkscripts-interfaces_network-bridge.html
 - http://wiki.debian.org/BridgeNetworkConnections
You didn't mentioned what OS do you use for dom0, but I anticipate it is Linux. In that case the ebtables should help you to secure your network environment and restrict the packet flow only to the interfaces they are related to.

Best regards,
Peter Viskup

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