OpenVZ: Bridged IPv6 subnets

OpenVZ: Bridged IPv6 subnets

I’ve been working on a gre tunneling interface for a while, but had wishes to make my OpenVZ host take care of the services that should host the tunnel – like for example, instead of assigning each single IPv6-address manually via vzctl, addressing should be handled from the container. And as long as vzctl and the venet-interfaces is used, it has to be done this way. With OpenVZ this is not entirely obvious, since documentation is not always collected in the same place.

As a matter of fact, after searching half of the day, I think I’ve got it covered. First, make sure you’re not using vzctl –ipadd, when you’re adding a larger subnet. Let’s use an example:

vzctl set <ctid> ---ipadd 2a01:299:a0:7000::/64

The example above will only assign one ip – 2a01:299:a0:7000:: – to your container, not the entire subnet. To have more addresses in this case, you have to make vzctl set them up in the same way: 2a01:299:a0:7000::1/64, 2a01:299:a0:7000::2/64, etc. The real magic occurs when you’re starting to use veth and brctl correct. To make it quick:

vzctl set <ctid> --netif_add eth0 --save

# Find the right veth-interface and ...
brctl addif br0 veth-interface

In the OpenVZ release I use, the bridging is set up by linking br0 with the created vethinterface – how to identify this interface when having more containers than one is currently undicovered ground as, again, documents are not very clear on this. I’ve found names like veth101.0 have been used, but in my case – with Virtuozzo 7.x – I get interfaces like veth123a4bcd, and they are a bit hard do identify and connect to the right containter. This should be automatically fixed with scripts that is running during the openvz startup sequences, but that part is still undiscovered.

Instead, I’ve created a cron-job that makes sure that all the interfaces are really linked up after the server boot (which is probably a high security issue if you host many containers for many users as bridging opens up networks a bit more than you probably wish):

#!/bin/bash PATH=/bin:/usr/bin:/sbin:/usr/sbin:/usr/local/bin:/usr/local/sbin 

veth=$(ifconfig|grep veth|sed 's/:/ /'|awk '{printf $1 " "}')
echo "Bridging VE interfaces..."

for interface in $veth
hasInterface=$(brctl show | grep ${interface})
if [ "" = "$hasInterface" ] ; then
echo brctl addif br0 ${interface}
brctl addif br0 ${interface

With this little ugly one, we’re making sure that unbridged interfaces are really bridged, and only once, so brctl does not need to rerun creation all the time. I think there are much better ways of doing this, however. The last thing to do after this setup is to actually assign the subnets properly. On the host server:

ip route add <A:B:C:D::/64> dev br0

At the container:


# Local bridge
ip -6 addr add A:B:C:D::/64 dev eth0

# Gateway
ip -6 route add <gateway> dev eth0

# Do not route ipv6 via venet0
ip -6 route del default dev venet0

# Route default via this router
ip -6 route add default via <gateway> dev eth0

# Route ipv6 via eth0
ip -6 route add default via <gateway> dev eth0