ndppd, or NDP Proxy Daemon, is a daemon that proxies NDP (Neighbor Discovery Protocol) messages between interfaces.
ndppd currently supports only Neighbor Solicitation Messages and Neighbor Advertisement Messages.
Before an IPv6 packet can be sent to a host, that host's link-layer address must first be discovered. This is done by sending a Neighbor Solicitation message containing the requested target IPv6 address to a specific multicast address. If a host has configured a matching IP, that host will then respond with a Neighbor Advertisement message, and provide its link-layer address.
For Proxy NDP to work on a Security Gateway that processes IPv6 traffic, it is necessary to create a special file