How to forward users through ssh

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http://stackoverflow.com – I need to distribute the users over many ssh servers but I would like to have only one head server and do that in a transparent way for the users. For example, I have a server Z that can be accessed from internet with two network cards. One card face internet and the other card face my internal network. In my internal network there is 2 ssh servers. I assigned the users a, b and c to use the internal server X and the users d, e, and f to use the internal server Y. if any user (a, b or c) makes a ssh to server Z the ssh connection should be forwarded (using NAT or other method) to the server (HowTos)