Linux IPv6 Router Advertisement Daemon (radvd)
From DD-WRT Wiki
From the manpage:
radvd is the router advertisement daemon for IPv6. It listens to router solicitations and sends router advertisements as described in "Neighbor Discovery for IP Version 6 (IPv6)" (RFC 2461). With these advertisements hosts can automatically configure their addresses and some other parameters. They also can choose a default router based on these advertisements.
See the radvd homepage for more information.