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

[Peter Steele]

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.

Peter