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If you are Experiencing issues with a Pi-hole install that has non-standard elements (e.g you are using nginx instead of lighttpd, or there is some other aspect of your install that is customised) - please use the Community Help category.
Expected Behaviour:
-operating system
-hardware]_ Raspberry pi 4
Actual Behaviour:
no address range available for DHCPv6 request via eth0
This warning can be printed when you have enabled the IPv6 variant of DHCP but don't have upstream IPv6 connectivity. In this case, Pi-hole can not derive the necessary prefix assigned by your ISP (because there is none) and, hence, cannot generate a valid DHCP configuration, ultimately leading to this warning.
Solution: Disable the IPv6 support of the Pi-hole DHCP server if you don't have upstream IPv6 connectivity.
Furthermore, your debug log shows that you have two DHCP server active at the same time. The one embedded into your Pi-hole and your router. This can lead to all sorts of obscure networking issues as there is no guarantee collisions won't happen. Furthermore, your router distributes itself as DNS server rendering Pi-hole blocking ineffective for those devices picking up the DHCPOFFER of your router.
Solution: Use either Pi-hole's internal DHCP server or the one in your router. If you want to use the one of your router, ensure you set the DNS server to your Pi-hole's IP address (this isn't the case right now).