I have been using ipfilter in the past. Here is what I used there:
map-block tun0 192.168.1.0/24 -> 18.104.22.168/24
Some remote applications require that multiple connections all come from the same IP address. So, I want to tell iptables to have a static mapping between IP address and use the same IP address for a host (i.e. use some magic to choose a port). How do I do this?