How to SSH gate forwarding git user to gitserver

Public Domain (git, e.g. clone in ssh way) -> Public Server (Linux Based) -> Private Git Server

How to config 'Public Server' so user in Public Domain can access 'Private Git Server'?

Use iptable forwarding for 'Public Server'. See:
https://serverfault.com/questions/437952/how-to-ssh-gate-forwarding-git-user-to-gitserver

Setup iptable (Chinese)
http://s2.naes.tn.edu.tw/~kv/iptables.htm