A special host name "odst.-" in rURL (or in the SERVER
parameter) represents the original destination host of the TCP connection
from the client via NAT (provided by iptables on Linux).
It can be used to configure a transparent proxy (or gateway)
for arbitrary protocols.
The original destination port number can be referred with "-" as "odst.-:-".
The number can be mapped with an offset value as "-8000" or "+8000" for example.
The name "odst.-" can be used in the "rserv" MountOption