The source are from xl2tpd-1.3.1(master branch), and I applied IPv6 patch from http://blog.lifetoy.org

The ipv6 branch with version 1.2.8 seems not work well in some situations, even cannot compile.
This one is tested with Openwrt SDK Toolchain. If you are compile it with Openwrt Toolchain, please read http://wiki.openwrt.org/doc/devel/crosscompile first. Environment variables is needed.