Add IPv6 router settings to ufw-framework

"man ufw-framework" already gives simple instructions on how to set up a NAT firewall using ufw's before.rules -- a similar section on setting up routing for IPv6 should be added to ufw-framework. It should be a lot simpler than explaining NAT, since NAT is gone. :-)

The applicable rules that would be added to the man page are (something like) :

To allow connections to servers on the inside at 2001:0DB8::a and 2001:0DB8::b add the following lines before COMMIT in before6.rules:

-A ufw6-before-forward -o eth0 -d 2001:0DB8::a -m state --state NEW -j ACCEPT
-A ufw6-before-forward -o eth0 -d 2001:0DB8::b -m state --state NEW -j ACCEPT</pre>

To allow routing to the Internet for hosts on the internal network 2001:0DB8::/32 on interface eth0 add the following lines before COMMIT in before6.rules:

# ok icmp codes
-A ufw6-before-forward -p icmpv6 --icmpv6-type destination-unreachable -j ACCEPT
-A ufw6-before-forward -p icmpv6 --icmpv6-type packet-too-big -j ACCEPT
-A ufw6-before-forward -p icmpv6 --icmpv6-type time-exceeded -j ACCEPT
-A ufw6-before-forward -p icmpv6 --icmpv6-type parameter-problem -j ACCEPT
-A ufw6-before-forward -p icmpv6 --icmpv6-type echo-request -j ACCEPT
-A ufw6-before-forward -p icmpv6 --icmpv6-type echo-reply -j ACCEPT

-A ufw6-before-forward -i eth0 -s 2001:0DB8::/32 -o eth1 -m state --state NEW -j ACCEPT
-A ufw6-before-forward -m state --state RELATED,ESTABLISHED -j ACCEPT

To allow DHCPv6 for the local network, run the following command

ufw allow in on eth0 proto udp from fe80::/10 port 546 to any port 547