Add kernel side support for Secure Neighbor Discovery (SeND), RFC 3971.
The implementation consists of a kernel module that gets packets from
the nd6 code, sends them to user space on a dedicated socket and reinjects
them back for further processing.
Hooks are used from nd6 code paths to divert relevant packets to the
send implementation for processing in user space. The hooks are only
triggered if the send module is loaded. In case no user space
application is connected to the send socket, processing continues
normaly as if the module would not be loaded. Unloading the module
is not possible at this time due to missing nd6 locking.
The native SeND socket is similar to a raw IPv6 socket but with its own,
internal pseudo-protocol.
Approved by: bz (mentor)
1db8d1f843