Which one is the right place to ask an iptables usage related question?
Some topics are accepted on multiple sites, as your question does here. Sometimes it is hard to determine what exactly is the best site to ask. It often depends on the specifics of your question.
Since you link the Linux documentation on IP tables, I would guess you have the best result on Unix & Linux, although Server Fault could do too. Unless it is about programming, I would not ask on Stack Overflow.
Completely agree with other answers and comments here: this depends on the nature of the question, and some topics are accepted on multiple sites.
Stack Overflow only deals with programming. Unless you are a developer writing code for use with
iptables, you need to ask somewhere else.
Regarding Server Fault, make sure your question deals with managing information technology systems in a business environment. Even if you have a question that is about networking and computer hardware, if you can do it at home on your PC, the question will probably be considered off-topic.
Regarding Super User, this handles questions from a broad range of power users. Even without knowing the nature of your question, general purpose questions about the use of
iptables ought to be on-topic.
Naturally, if your question is less general purpose, and specifically for Unix & Linux, it would be better to ask there.