Support Duplicate Address Detection in Gatekeeper's ND

[AltraMayor]

The implementation of the Neighbor Discovery protocol of IPv6 (ND) in Gatekeeper doesn't support Duplicate Address Detection (DAD). While this missing feature is not causing an issue for anyone, it's nice to have.

See this comment in the code for a reference.

[AltraMayor]

Pull request #511 may have indirectly solved this problem by shipping the ICMPv6 packets to the kernel through the KNI interfaces. Tests must be done to confirm that this issue has been addressed before this issue can be closed.