Dynamic DNS for both IPv4 and IPv6 (using freedns.afraid.org or others)

Answering my own question as I have now found a solution.

I am now using the service nsupdate.info, which supports what I want: Both IPv4 and IPv6 are reached using the same name, for which both A and AAAA records are provided.

They had specific instructions for the Fritz!Box, which were shown when I was setting up my account. I repeat here what I have in my Fritz!Box settings now:

Dynamic DNS provider: User-defined

Update URL: https://ipv4.nsupdate.info/nic/update https://ipv6.nsupdate.info/nic/update

Domain name: mydomainname.nsupdate.info

User name: mydomainname.nsupdate.info

Password: [as provided by nsupdate.info]

I have been using this service for just over half a year now, and it works flawlessly so far (but I am not logging into my Fritz!Box daily, so your mileage may vary). Apologies for not replying earlier, but I completely forgot about this question, and just received a reminder e-mail as someone else wrote a comment.

Hope this helps others!

The answer is very simple: you just have to enter both update URLs with one blank in between:

https://freedns.afraid.org/dynamic/update.php?[MY-SECRET-TOKEN4]&address=<ipaddr> https://freedns.afraid.org/dynamic/update.php?[MY-SECRET-TOKEN6]&address=<ip6addr>

Works with my FRITZ!Box 7390.

First of all, I think it is a really stupid idea to give people dynamic IPv6 addresses. Those IPv6 addresses are used in people's internal networks, and changing them all the time makes it really hard to do manage them.

The difference between IPv4 and IPv6 is that you probably only have one IPv4 address. This address belongs to the Fritz!Box, which uses NAT to allow systems on the LAN to access the internet. With IPv6 every system has its own IPv6 address(es). In that situation it might be easier to have every system register their own IPv6 address in the dynamic DNS. It then of course depends on the system if they can actually do that...