Recently at home, my testing Firefox started hanging when I tried to visit a particular website that I browse every so often (experimentation eventually showed that it would display the site, but very very slowly). I could browse it from work, my syndication feed reader at home was still talking to it, and in fact my regular Firefox instance could still see it. It was rather odd, or even outright mysterious.

My home machine has a native IPv6 address and connectivity (not just a 6to4 setup); my work machine only has 6to4 connectivity. The particular website I was having problems with has both IPv4 and IPv6 addresses, and, I believe, has had them for some time. And, you guessed it, there is some sort of connectivity problem between me and their IPv6 address (or addresses, since they have various sub-sites for static media and so on).

This IPv6 connectivity issue affects only my testing Firefox because only that Firefox runs in an environment that allows it to make IPv6 connections; my regular Firefox and my syndication feed reader both run through setups that force IPv4 only traffic. I don't have a general IPv6 connectivity problem, since I can visit places like ipv6.google.com and test sites such as this one say that my setup is fine.

Now, I'm an experienced system administrator and I know that I have IPv6 enabled (because I configured it specifically). It still took me a fair while to make the leap from 'weird inability to browse this website in Firefox' to 'IPv6 related issue', and I still don't know exactly what's wrong between hither and yon (although testing here suggests that it is a routing reachability issue).

This is the world we have to look forward to as IPv6 becomes more widely deployed and used. In a sense this is a good development, because it will smoke out a lot of configuration issues and so on. But in another sense it is a terrible world, because what users see is things breaking when you (or they) turn on IPv6.

One consequence of this is that I don't feel very optimistic about World IPv6 Day; I expect it to be notable mostly for mysterious problems and lots of complaints. I am frankly amazed that Google and other major websites could be talked into it.

(My bad workaround for my problem was to tell my system that the IPv6 addresses associated with this website were unreachable. This causes an immediate connection failure and the browser immediately falls back to IPv4.)