I ran into a weird problem. I set up my tunnel and radvd on a linux router (2.4.21). The autoconfig worked nicely, I got an ipv6 address on my client machine (linux 2.6.0-test9-mm1). But then my client didn't receive icmp replies when I ping6'd noc.sixxs.net. On the router, I could see the outgoing packets on the tunnel interface and the replies on the tunnel, but then the machine somehow decided not to forward packets to the client. Weird. Really weird. Then I tried, as part of the debugging, to ping6 the local interface from the client, and all of sudden traffic came trough on the client! I didn't change anything at all, just pinged the router's internal interface once... Things work now, but it might be a hint for other folks.