How to allow 6to4 through iptables firewall?
When using 6to4, you do not transmit IPv6 packets. Instead, special IPv4 packets are wrapped around IPv6 payloads. These packets have IPv4 protocol 41, as defined by RFC 2473.
iptables -t filter -I INPUT -p 41 -j ACCEPT iptables -t filter -I OUTPUT -p 41 -j ACCEPT