Setup Raspberry Pi IPv6 Gateway with Router Advertisement Daemon

A Raspberry Pi is supposed to make IPv6 available on a local network as a gateway. To do this, Raspberry Pi must be set up as an IPv6 tunnel endpoint. The starting point is that you have a 6in4 tunnel from SixXS and want to make the over Raspberry Pi available to all other participants.  Raspberry Pi is set up as an IPv6 gateway, which passes its assigned prefix to the other IPv6-enabled participants.
Note: Please note that the operation of an IPv6 gateway must include an IPv6 firewall, otherwise the IPv6 clients will be unprotected connected to the Internet.
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