The residential Swisscom Internet Box 2 does not allow to autoconfigure an Unify Dream Machine with IPv6.

After several hours of research I found a workaround to get IPv6 support.

Initially I tried several prefix lengths and I used tcpdump to see what happens on the network.

I could see the router advertisments of the Internet Box but the Unify Dream Machine was looking for DHCPv6.

When I configured the Dream Machine to allow these RA's (sysctl -w net.ipv6.conf.eth8.accept_ra=2) the UDM got an global IPv6 address, but the ubios-udapi-server removed the unknown dynamic address and unknown dynamic route within seconds.

Luckily I found a Unify community post about getting a Dream Machine work with DS-Liste IPv6:

https://community.ui.com/questions/Dream-Machine-DS-Lite-IPv6-how-to-make-it-work/63d025ff-3026-4a39-9285-00a48fb75533#answer/02109822-27f7-4314-ad46-b292f4a852c2

The described solution worked for me:

  1. I configured a static WAN IP address (Based on the diagnostic informations of the Swisscom Internet Box 2 where the LAN subnet and the LAN IP of the Internet Box is shown)
  2. I generated an local network using https://cd34.com/rfc4193/
  3. I used the generated subnet to configure the LAN IPv6 network with neabled RA and DHCPv6
  4. I created /mnt/data/on_boot.d/01-ipv6-nat.sh as executable shell script with the following content: 
    #!/bin/bash
ip6tables -t nat -A POSTROUTING -o eth8 -j MASQUERADE

Note: the on_boot.d script is executed by on-boot-script from boostchicken/udm-utilities which has to be installed first.