Following the documentation on online.net won't get IPv6 to work fully as they are missing some ipv6 forwards on sysctl.conf to make IPv6 works on Debian based systems, or Proxmox.

• Change Module Options to Enable IPv6:

In the following file:

/etc/modprobe.d/local.conf

enable IPv6 by changing the 1 to 0:

options ipv6 disable=0

• Load IPv6 Kernel Module at the boot: Edit the following file:

/etc/modules

Add ipv6:

ipv6

• Reboot the server to load IPv6 Modules

Now the system has IPv6 modules loaded and enabled, we need to configure and allow IPv6 forwarding.

Note: You can skip this step if you have simple Debian installation without Virtualization.

• Enable Forwarding by adding them into the system variables:

Edit the system variables file:

/etc/sysctl.conf

Disable autoconf for ipv6 on interface vmbr0, enable forwards on default interface and all other interfaces, enable NDP proxy on default and all interfaces, as the following:

net.ipv6.conf.vmbr0.autoconf=0
net.ipv6.conf.vmbr0.accept_ra=2

net.ipv6.conf.default.forwarding = 1
net.ipv6.conf.all.forwarding = 1

net.ipv6.conf.default.proxy_ndp = 1
net.ipv6.conf.all.proxy_ndp = 1

Read the new system variables, by executing the command:

sysctl -p

At this point we enabled IPv6 on the system by loading the required modules, and enabling forwarding and ndp proxy on the host, so that our guest machines can obtain IPv6 addresses from the host.

We will use the default DHCP client to issue and get the IP Block from the router using DUID given in online.net console.

Edit or create the following file if not exists:

/etc/dhcp/dhclient6.conf

Add the following lines:

interface "vmbr0" {
   send dhcp6.client-id DUID;
}

Where vmbr0 represents the main interface, and DUID obtained from Online.net IPv6 Panel.

Edit the interfaces file:

/etc/network/interfaces

And add the following lines:

iface vmbr0 inet6 static
    address YOUR_IPv6
    netmask YOUR_SUBNET
    accept_ra 1
    pre-up dhclient -cf /etc/dhcp/dhclient6.conf -pf /run/dhclient6.vmbr0.pid -v -nw -6 -P vmbr0
    pre-down dhclient -x -pf /run/dhclient6.vmbr0.pid

Note: YOUR-IPv6 is the created subnet from online.net IPv6 panel, for example:

iface vmbr0 inet6 static
    address 2001:bc8:50ca:101::1
    netmask 64