- If `@' is specified as an interface for prefix delegation, it is assumed
as a tunnel interface.
- Add experimental support for an IPv6 over an IPv4 tunnel using Netgraph.
The relevant options are -b and -U.
- Make an IPv6 over an IPv4 tunnel NAT friendly. If -n option is specified,
it is enabled. It requires some modification to ng_ksocket of server side.
- Add $up_command and $down_command to run at up/down.
- Add -B option to specify NAT port.