The ndproxy(4) kernel module implements IPv6 Neighbor Discovery
proxying with many options to handle several use-cases.

It replies to a neighbor solicitation with a specific neighbor
advertisement, in order to let the PE uplink router send further
packets to a CPE downlink router, that may or may not be the same
node that the one which runs ndproxy.

The main difference with the ndp(8) command-line tool is that, with
ndproxy(4), the host running ndp can be used only to redirect
packets to another IPv6 internal router, for instance a dedicated
one with hardware support of IPv6 routing processes.

WWW: http://www.fenyo.net/newweb/ndproxy.html