IPv6 support

[pgj]

Make the necessary changes to have IPv6 fully working. Tasks:

• [x] Check how to configure the Wifibox guest from the host to pass around IPv6 traffic
• [x] Verify if the Linux configuration has all the required bits enabled
• [x] Verify if udhcpd supports dual-stack configuration or it shall be replaced for dhcpcd as suggested by the Alpine Linux Network Configuration page
• [x] Verify if ip6tables works
• [x] Provide forwarding rules for ip6tables
• [x] Implement the changes according to the findings
• [x] Write documentation

[pgj]

I have got a proof-of-concept solution working. Random notes about it:

• It needs IPv6 NAT to translate the unique local address on the (FreeBSD) host to that of the had the (Wifibox) guest assigned. Currently the Linux kernel is not configured to provide IPv6 NAT, Wifibox/Alpine will need a version of that.

• ip6tables rules are not loaded automatically, this has to be fixed in the guest.

• The host can use the fd00::1/64 unique local address. In rc.conf(5):

ifconfig_wifibox0_ipv6="inet6 fd00::1/64 auto_linklocal"

• ip6tables rules:

  
  *nat
  :PREROUTING ACCEPT [0:0]
  :INPUT ACCEPT [0:0]
  :OUTPUT ACCEPT [0:0]
  :POSTROUTING ACCEPT [0:0]
  [0:0] -A POSTROUTING -o wlan0 -s fd00::/64 -j MASQUERADE
  COMMIT

*filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] [0:0] -A FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT [0:0] -A FORWARD -i eth0 -o wlan0 -j ACCEPT

COMMIT

- `dhcpd` needs to have installed for the Wifibox guest.  Not sure about the contents of the `dhcpd.conf`, defaults could just work fine.

- `radvd` is not needed.

[pgj]

A static route is needed in the guest too:

ip -6 route add fd00::/64 dev eth0

[pgj]

Automated propagation of information on DNS servers for the host should be solved somehow.