FreeBSD IPv6 Tunnel Config

In /etc/rc.conf:

ipv6_enable="YES"
ipv6_gateway_enable="YES"
ipv6_defaultrouter="-interface gif0"
ipv6_default_interface="gif0"
gif_interfaces="gif0"
gifconfig_gif0="A.B.C.D 81.187.81.6"
# substitute the IPv4 address of your machine for A.B.C.D
ipv6_ifconfig_gif0="2001:08b0:XXXX:1::1 prefixlen 64"
# 2001:08b0:XXXX should be replaced by your /48 allocation
# in this case I have used IP 1 on subnet 1 as the IPv6
# address - you can (and probably should) adjust as desired.

Don't forget to configure an IPv6 firewall (this is left as an exercise for the reader).