Custom DNS on IPv6 while tracking interface

[symon]

Hello Everyone,
in my Network I have a custom DNS server that is configured to reply differently based on the host that uses it. My current setup is:

Code: [Select]

____________________________________________________________________
|                                                                   |
|                             |---------------- Internal Clients    |
|                             |                                     |
|                             |                                     |
------- ISP Router-------- OPNsense Box-------- External Clients    |                                                                 
|                             |                                     |
|                             |                                     |
|                             |----------------- DNS & Server       |
|                                                                   |
|____________________________________________________________________

The clients, external clients and servers do not share interfaces; they are on VLANs attached to different physical NICs.
My setup is pretty easy: The DNS has a fixed IPv4, it is announced to all IPv4 clients via DHCP.

On IPv6 I have to track the WAN-Interface. Now since I do this, the default behavior seems to be that OPNSense will announce itself as the DNS server on DHCPv6. This is not what I want, since I want all clients to directly use the custom DNS server. But it seems like I cannot announce it as it's IPv6 Address will change based on the tracked interface and I cannot find a way to set it dynamically.

I thought about assigning ULA to all clients manually but I stumbled upon this discussion: https://forum.opnsense.org/index.php?topic=33902.0 wich states that ULA has problems ansd should not be used.

So I'm curious: What's the correct way to solve this? I thought about using the IPv4 DNS as the upstream DNS server of OPNsense but it will not solve my problem as the DNS Server itself is meant to be filtering based on the requesting host. This won't work if it only sees the gateway doing all the requests.

Thanks in Advance
symon