The Mysterious IPv6 DNS AAAA-Resolver Bug

In my quest to do as many things over IPv6 now that it’s supported not only by my VDSL line at home but also by my mobile network operator, the last success story I reported here was about how to update my IPv6 DNS entry whenever the dynamic IPv6 address of one of my servers at home changed. The procedure works like a charm but unfortunately I found a big bug in the DNS resolver in my Fritzbox VDSL router that pretty much forestalls my IPv6 home server ambitions for the moment.

When I first activated IPv6 at home, everything looked bright and shiny. For external websites that were IPv6 capable the Fritzbox 7490 I use as my VDSL router dutifully returned both IPv4 and IPv6 addresses whenever Firefox made the corresponding DNS requests. So far, so good.

Mysteriously, however, the DNS resolver on the router returns nothing when Firefox queries the IPv6 address for my local server. It doesn’t even return an error message it just doesn’t answer the request at all. The result is that Firefox waits for 10 seconds and then just gives up and takes the IPv4 address to connect to the server. To make sure this is not a Firefox or Linux issue I also tested the scenario on a Windows machine and also with Internet Explorer. Same thing there as well.

This is very strange as all other DNS servers I tried return the IPv6 address of my home server just fine. On the fixed line side I tried Google’s DNS servers both over IPv4 and IPv6 and my fixed line provider’s DNS server. On the mobile side I tried the DNS server of my mobile network operator that is configured by default for the connection and all of them work just fine as well.

So why doesn’t that Fritzbox answer? I’ve opened a ticket for the issue two weeks ago and had some correspondence with AVM, the manufacturer of the device but so far no solution has been offered.

For me personally, the fix is simple, I just configure my devices to go directly to other DNS servers, case closed. But that’s not an option for others who, if they had the same router and same configuration, would always have to wait 10 seconds before my webpage showed up. Totally unacceptable.

So let’s see I hope AVM will find what’s going wrong in their router and release a fix for this at some point.