Don't use this, it's bad.

#!/bin/bash

sysctl -w net.ipv6.conf.all.forwarding=1
sysctl -w net.ipv6.conf.wan0.accept_ra=2

dhclient -1 -6 -v -P -N wan0

unset IPv6PREFIX
unset IPv6ADDR
unset IPv6PREFIXLEN

IPv6PREFIX=`grep "iaprefix" /var/lib/dhcp/dhclient6.leases | tail -n 1 | cut -d" " -f 6`
IPv6ADDR=`echo ${IPv6PREFIX} | cut -d"/" -f 1`1
IPv6PREFIXLEN=`echo ${IPv6PREFIX} | cut -d"/" -f 2`

#echo $IPv6PREFIX
#echo $IPv6ADDR

cat << EOF > /etc/radvd.conf
interface lan0
{
        ## (Send advertisement messages to other hosts)
        AdvSendAdvert on;
        ## (Fragmentation is bad(tm))
        #AdvLinkMTU 1280;
        MaxRtrAdvInterval 300;
        ## (IPv6 subnet prefix we've been assigned by our PoP)
        prefix ${IPv6ADDR}/${IPv6PREFIXLEN}
        {
                AdvOnLink on;
                AdvAutonomous on;
                DeprecatePrefix on;
                AdvRouterAddr on;
        };
};
EOF

ifconfig lan0 inet6 add ${IPv6ADDR}/${IPv6PREFIXLEN}

/etc/init.d/radvd start