I have a Hurricane Electric tunnel set up on my Debian Squeeze Xen VPS. It seems to work half the time, and drop incoming traffic to the tunnel half the time. I did what [this question's](https://serverfault.com/questions/234840/how-to-allow-6to4-through-iptables-firewall) answer said, but to no avail: it still seems to intermittently stop accepting traffic. When I ping it when it's not accepting the traffic it gives me a "Destination host unreachable" error, so it could simply be a routing issue that's on HE's end, but I don't know.