Activate ipv6 but not RA

Hi list,

setup is: 2 VMs, one running UTM 9.705-3 for Internet access and FW, the other one doing the network stuff like DHPC, VPN, NAT64, ... ipv4 and ipv6 enabled on the network.

First of all, I have troubles with Android and Chromebooks who have difficulties to access Internet ith ipv6, that's another story at this time.

I would like that the server running DHCP (dnsmasq, Debian Buster, v2.85 self compiled) take care of all DHCP stuff, ipv4 and ipv6. It's working like expected when devices have both dual-stack. Why dnsmasq for ipv6? it can create dynamic DNS entry for ipv6 devices (option ra-names).

I setted a third VM -Ubuntu 18.04- with only ipv6 activated: the ipv6 stuff is done by UTM concerning RA so everything goes there. I could manage NAT64 and DNS64 to work, problem is that this machine has no DNS entry on the network, can't be easely controlled.

Other info: the DHCPv6 setup on the UTM doesn't allow the same interface as client and server. And we can't tell to forward request to another device. Perhaps should I use FW rules ...

Does anyone have any hint to solve this problem ?

Cheers, Daniel.