IPv6 only works after pinging the default gateway.

Solution 1:

I gave the whole problem another try today, a couple of weeks later. And what can I say, I fixed it. Can someone please explain me why adding a ipv6 loopback fixed my problem? Here is what I've added to my /etc/network/interfaces file:

iface lo inet6 loopback

I have no ideas why I've forgot to add it in the first place!^^ Thank you all for your responses!

Solution 2:

I am looking at the bit of your question where you say:

Now, my problem is that my IPv6 isn't working properly. If I try to ping an IPv6 address e.g. ping6 ipv6.google.com. I get: "Destination unreachable: Address unreachable"

Which looks like this in tcpdump -i eth0 ip6:

00:29:05.386500 IP6 2a01:4f8:a0:aaaa::2 > ff02::1:ff00:1: ICMP6, neighbor solicitation, who has fe80::1, length 32 00:29:05.390869 IP6
2a01:4f8:a0:bbbb::1 > 2a01:4f8:a0:aaaa::2: ICMP6, neighbor advertisement, tgt is fe80::1, length 32

This is strange. Your system sends a correct (as far as I can see from your tcpdump) neighbor solicitation and the router sends back a correct (again AFAICT) neighbor advertisement. That would indicate a problem on your local machine.

Do you do any firewalling on that machine? ICMPv6 filtering is very different than ICMP filtering. Much more parts of IPv6 communication need properly working ICMPv6. If you filter too much or in the wrong way you get problems exactly like you are seeing now.

If you want to know more details please take a look at RFC 4890.