Get ipv6 address of server for ddns update

Installing and Using OpenWrt Network and Wireless Configuration
16

Set up dynamic prefix forwarding:
Port forwarding to a dynamic IPv6 address - #2 by vgaetera

Then extract the prefix and append the suffix:

NET_IF="wan6"
NET_SFX="23"
. /lib/functions/network.sh
network_flush_cache
network_get_subnets6 NET_SUB "${NET_IF}"
for NET_SUB in ${NET_SUB}
do if [ "${NET_SUB::2}" != "fd" ]
then
echo "${NET_SUB%::*}::${NET_SFX}"
break
fi
done