(Solved) GT-BE98 Pro refusing to enable Stateless IPv6 configuration for LAN

[MrKiasu]

Hi all.

I recently picked up at GT-BE98 Pro to replace an RT-AC88U (running Merlin). I managed to get everything I had previously working except that it's refusing to enable stateless IPv6. Whether I have the stateless or stateful option set, it generates the same dnsmasq.conf with an IP range for dhcp and none of the stateless config options set (ra-stateless,ra-names). confusingly, when I manually edited the config and restarted dnsmasq, it still didn't work. Has anyone had any experience with this? Is it some known behaviour of the stock ASUS rom that's been true for ages but I just never had to confront it before? The practical implications are not massive but it means my android devices will not get an IPv6 address, as they only support stateless configuration.

Thanks!

 

[MrKiasu]

I managed to find the cause of the behaviour. I got the clue I needed from this thread.

Although it's a bit of a confusing discussion, that thread indicated that it's not possible to handle RA messages when you've got a single vlan and subnet being shared between the main network and one or more guest networks. In that discussion it talked about "guest network 1" being on a separate vlan and subsequent ones being on the main one. I had set up a guest network for IoT devices and I used the default asus "IoT network" option to do this. It turns that that creates one of these non-vlan guest networks and so even though it was my first and only guest network, it threw me into this broken-RA state.

I deleted and re-created my IoT network with a separate subnet which then used a separate vlan, and then immediately my main dnsmasq configuration was updated to have the proper stateless configuration and slaac started working.

Interestingly, the fact that the firmware refused to write out a stateless dnsmasq config means that asus are aware of the incompatibility and are intentionally forcing stateful mode to provide ipv6 support. But the UI doesn't provide any indication that this happening or why. So that's mighty poor on their part.

With respect to the hardware, I assume it's not specific to my router and would happen with any device running basically any firmware where these kinds of guest network configurations are possible.

Hopefully this will help other people in the future.