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Re: [libvirt-users] Networking issues with lxc containers in AWS EC2

On 04/12/2016 01:37 PM, Peter Steele wrote:
On 04/11/2016 11:33 AM, Laine Stump wrote: I wouldn't be too quick to judgement. First take a look at tcpdump on the bridge interface that the containers are attached to, and on the ethernet device that connects the bridge to the rest of Amazon's infrastructure. If you see packets from the container's IP going out but not coming back in, check the iptables rules (again - firewalld uses iptables to setup its filtering) for a REJECT or DISCARD rule that has an incrementing count. I use something like this to narrow down the list I need to check:

while true; do iptables -v -S -Z | grep -v '^Zeroing' | grep -v "c 0 0" | grep -e '-c'; echo '**************'; sleep 1;

If you don't see any REJECT or DISCARD rules being triggered, then maybe the problem is that AWS is providing an IP address to your container's MAC, but isn't actually allowing traffic from that MAC out onto the network.

I'll get this test setup. Unfortunately I'm not particularly knowledgeable with iptables; we don't use it in our product so I've never had to deal with it. I think you are right though about what's happening--AWS doesn't recognize the MAC addresses for containers running under another instance.

I did this test and there were no REJECT or DISCARD rules being triggered. I did discover something interesting though. I had two AWS instances running with some libvirt containers on each. I did a ping from one AWS instance to an IP assigned to a container on another AWS instance. The ping failed, and when I checked the source host's arp table the mac address that was recorded for the container being pinged was that of the container's host instance's br0 interface, not the mac address of the container's eth0 interface.

Doing the same test on premise using KVM based instances, when a ping was run from one VM to a container hosted on another VM, the arp table of the source VM contained the mac address of the eth0 interface bound to the container, not the mac address of its host VM.

This indicates to me that AWS thinks all of the IP addresses that have been allocated to an instance will be bound to that instance and it doesn't try to go any further than that. I'm not exactly sure how to get AWS to route these addresses properly, but it doesn't seem to be an issue with libvirt per se.


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