Hetzner don’t provide an IPv6 default route. Instead, you use the Neighbor Discovery Protocol by specifying the link-local address for defaut IPv6 gateway:
And thats it, all done. Test it actually works:
And then make these changes persistent by adding the following to /etc/rc.conf:
Also don’t forget to update your firewall rules (you do run a firewall right?) to allow IPv6 packets to actually pass through.
Update 2014-05-05 - the earlier version of this post mentioned the need to accept Router Advertisement messages by setting the net.inet6.ip6.accept_rtadv sysctl to 1. That step is actually unnecessary, as the configuration works via Neighbor Discovery. Another blogpost explaining that in more detail is coming up.