I try to do source natting with Zeroshell 3.1 as well.
As far as I can see, this is not possible to configure in the ui.
I know to configure iptables on a normal Linux system, but I don’t understand the answer of 777maxism:
What does mean
Zahodish (Startup / Krohn)there choose section (NAT and Virtual Servers) and Enable section, insert the rules, change the value of their own, at the end restart Zeroshell.
The first bold part I do not understand.