If a portNum is prefixed with "-" or "+",
it means mapping the port number of the entrance port
adding the specified offset,
or using the port number as is by "-" with empty portNum.
Example: forwarding multiple ports to an another host
A special hostname "odst.-" can be used
to refer the original destination host when the incoming data is
forwarded by NAT.
Example: forwarding NAT to the original destination via a SOCKS proxy