How to set up IPv6 for Pihole correctly?

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Just because this is not known to anyone - and because the world of IPv6 is really different than IPv4 - let me stress again what @Bucking_Horn said above, just to avoid any possible further frustration: While DHCPv6 may work, it is certainly not the right way of assigning IPv6 addresses. I say this because IPv6 addresses should not be assigned at all. IPv6 is a protocol made for auto-configuration. While DHCPv6 may work, there are many devices that do not support getting configured by DHCPv6. They may only partially pick up the assignment or chose to ignore it altogether.

This is very much in contrast to the IPv4 world with DHCP. (I'm simplifying below to make it easier to understand):

In the IPv4 world a new client actively asks the network:

Help, I'm new, I don't know anything. Please tell me what to do.

A DHCP server (or multiple if you have multiple, but that's more involved, assume only one here) replies to the client and tells it:

Don't worry, here is what you need. I also already have an address for you. Use this one plus that DNS server, etc.

With IPv6 it is somewhat different. Instead of coming and actively asking for help, not being able to do anything until you get the response from the DHCP server, the process is a multi-step one, without the need for one DHCP server.

  1. The client comes up with a (hopefully unique) link-local (LL) address. This is typically build from it's MAC address (which should be unique).
  2. It sends out a neighbor solicitation (NS) packet to the broadcast address to check if there is already another device with this address. If a neighbor advertisement (NA) is received, it tries again with a new LL address. If not, the LL address is not used and can be used for this client.
  3. Since the client can now communicate within our local network (we have a valid LL address), it wants to also get a global address for communication beyond its local network (typically the Internet). Hence, it sends out a router solicitation (RS) packet (from the LL address to ff02::2 which is the "All Routers Multicast Address").
  4. All routers on the link will reply with router advertisement (RA). Containing various bits of information, but always the address prefix to be used and, most often, the DNS information for the network.
  5. The client uses the prefix to generate its own global address (global unicast address, GUA).
  6. The client repeats the duplicate address check already described in step 2.
  7. The auto-configuration is done.

Nowhere in the steps above, we needed a DHCP server. This is what the standard wanted to have and what we call SLAAC (Stateless Address Autoconfiguration).

I'm also writing this text here to be used in an introduction/tutorial later to - hopefully - improve understanding of IPv6. It is unfortunate now that you cannot change the DNS settings broadcasted by the router without rebranding. This is a clear defect of the router. If you can disable/change the DHCP server for IPv4, you should also have the ability to do the same/similar for IPv6's RAs.

Coming back to where I started (DHCPv6): This SLAAC is (must!) be supported by all the devices according to the standards. DHCPv6 has been standardized for clients that need more than can be provided by RAs, e.g., network boot options. It somewhat organically grew over time to be able to do even more, now including assigning addresses and handing out DNS server addresses. However, a lot of possibilities showed that the developers did not always understood the entirety of the new SLAAC way of handling addresses and a lot was rolled back later on. As a consequence, we have a vast amount of diverging implementations out there. Linux does it differently than Windows, Android does special things and Macs just ignore a lot of other options.

There is no one way to do it right. SLAAC is the best one but if you cannot change the router's configuration in what it broadcasts as DNS server, you'll really have a hard time.

TL;DR:

if you're experiencing issues. Yes, you will loose some performance bits here and there, however, the vast vast majority of the Internet will be reachable over IPv4-only for a very long time. Once the trend goes towards IPv6-only, router vendors will give you more options because their customers will ask for them.

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