Hello,

I am dealing with a problem on a virtual environment where a VM is moved from one datacenter to another, let's call them DC1 and DC2. This VM is providing services to clients and I want to keep the packets flowing from DC1 to DC2. There is a proxy Ubuntu VM at each datacenter and they can communicate to each other, but other VMs are not configured to do so.

The proxy on DC1 is acting as a proxy ARP so when the VM is moved to DC2 it will answer any ARP request to its IP in order to forward the packets to DC2 and keep the clients happy even when the VM has changed its physical location. The VM has been configured with two IPs compliant with DC1 and DC2 respectively.

Here is a simplified network diagram:

DC1

192.168.5.0/24
+-------------+
|
| eth0: 192.168.5.45
+-------------+
| Proxy 1 |
+-------------+
| eth1: 192.168.7.10
|
|
DC2 |
| eth1: 192.168.7.11
+-------------+
| Proxy 2 |
+-------------+
| eth0: 192.168.6.45
|
+-------------+
192.168.6.0/24

When the VM is moved to DC2 I cannot ping it. The proxy ARP is configured correctly on Proxy 1 and the packets are forwarded to Proxy 2, which gets the packets on eth1 but for some reason is not transferring the packet to its eth0 interface.

The VM has two IPs: 192.168.5.130 and 192.168.6.130.

Any clue why is Proxy 2 not forwarding the packets from eth1 to eth0?, I have added an iptable rule to specifically forward that IP but nothing happens. IP forward is enabled on Proxy 2 as well.


Thanks,



Juan.

Sorry that I didn't try to understand the whole set up but only two questions/remarks.

Have you tested to get replies from an ARP ping. I am not sure but I had the impression that some Linux desktop distributions haven't set up ARP at all. They simply won't do anything with ARP until you enable this explicitely.

Then, isn't all that forwarding etc reserved to TCP/IP and ARP as simple low-protocol is part of TCP/IP too in this regard?

Hi Dilbert,

The proxy arp works fine, I added the entry to the arp table and I can see from a traffic capture that the ARP request is answered by proxy 1. My problem is at layer 3 I believe, since proxy 1 forwards the packets to proxy 2 but they are not forwarded on proxy 2 to the network 192.168.6.0/24.

Is forwarding enabled in the kernel on the server? Is IPTables configured properly? Does a simple ping work properly?

IP forwarding is enabled on both proxy servers. When a machine on 192.168.5.0/24 wants to talk to another on 192.168.6.0/24 the traffic goes through the proxies and the communication works perfectly, the problems show up when I am trying to forward packets that are destined to the VM 192.168.5.130 that was on DC1 but currently is on DC2.