Sorry for digging this up, but here's my working rc.conf for /64:
ipv6_activate_all_interfaces="YES"
gif_interfaces="gif0"
gifconfig_gif0="X.X.X.X Y.Y.Y.Y"
ifconfig_gif0_ipv6="inet6 2001:XXX:XXXX:63::2 2001:XXX:XXXX:63::1 prefixlen 128"
ipv6_defaultrouter="2001:XXX:XXXX:63::2"
Where:
X.X.X.X =>
Client IPv4 Address (
note: if you have NAT this should be your NAT IP i.e. 192.168.0.100)
Y.Y.Y.Y =>
Server IPv4 Address2001:XXX:XXXX:63::2 =>
Client IPv6 Address2001:XXX:XXXX:63::1 =>
Server IPv6 AddressWorks like a charm even after rebooting