It depends what server it is.
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Private servers are Illegal, i would not advice playing and/or making one.
I don't think it is ILLEGAL to play Private Servers. However, to own a Private Server is illegal.
Running a Runescape private server is illegal. Just enjoy the game for what it is.
how do i make an private server flyff
You are talking about an illegal action. Flooding a server constitutes a Denial of Service attack and if traced back to you can get you into a whole world of hurt. Even if it is just a private server.