pfSense

Will do, not familiar with gre tunnels, assuming someone else has a test case for this I don't see why not.

I'll pick this up this week together with the link local gateways.

Only thing I can find in the verification is the subnet mask length enlarged from 32 to 128.
Otherwise it does accept ipv6 addresses.

gre0: flags=9051<UP,POINTOPOINT,RUNNING,LINK0,MULTICAST> metric 0 mtu 1476
tunnel inet 192.168.11.248 --> 192.168.11.253
inet6 fe80::20c:29ff:fedc:7b9f%gre0 prefixlen 64 scopeid 0x8
nd6 options=3<PERFORMNUD,ACCEPT_RTADV>

Maybe this then.
Jan 15 14:48:29 php: /interfaces_gre_edit.php: The command '/sbin/ifconfig gre0 2001:db8:1::2 2001:db8:1::1 netmask 255.255.255.255' returned exit code '1', the output was 'ifconfig: 2001:db8:1::2: bad value'

Ok, backend code fixed to create the GRE tunnel. Although it accepts the prefixlen /64 command it does not set anything other then /128.

gre0: flags=9051<UP,POINTOPOINT,RUNNING,LINK0,MULTICAST> metric 0 mtu 1476
tunnel inet 192.168.11.248 --> 192.168.11.253
inet6 fe80::20c:29ff:fedc:7b9f%gre0 prefixlen 64 scopeid 0x8
inet6 2001:db8:1::2 --> 2001:db8:1::1 prefixlen 128
nd6 options=3<PERFORMNUD,ACCEPT_RTADV>

Confirmed fix, works in march 13th snapshot