Port mapping: 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