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How is IPv6 DNS server determined by a client?

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I have a problem with IPv6 DNS: I am using Comcast via an ASUS RT-AC86U router with IPv6 enabled. On my LAN I have a Pi-hole on a Raspberry Pi 3B+ and want all DNS requests to go to it either by its IPv4 or IPv6 address. The router WAN IPv4 DNS settings are manually configured for server 1 and 2 to OpenDNS. The router LAN DNS IPv4 setting is manually configured to server 1 as the Pi - the router appears to add itself as a second server - as seen by running DHCPtest (https://github.com/CyberShadow/dhcptest). The router IPv6 DNS settings are manually configured to server 1 as the Pi and server 2 as OpenDNS. I haven't found a way to simulate a IPv6 DHCP request and see the response, if there is such a thing.

All of my LAN devices go to the Pi for IPv4 requests but they appear to go to the router for IPv6 requests. So while (I think) IPv6 requests end up at the Pi, they don't go there directly but rather first to the router and then get bumped up to the Pi. The issue is that the Pi log shows where a given request came from and I can't tell which device initiated the IPv6 request as all appear to originate in the router. I would like to know which devices asked about certain domain names.

If I look at a Windows 10 client via ipconfig, I don't even see the Pi's IPv6 address as a DNS server - only the router's IPv6 address. Same when I look at a FreeNAS machine. I'm not sure where to look on other devices (like an iPhone). I'm hoping this has a simple fix.
 
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