+------+

+--->Server| | +------+ | | | +---v----+ | |Firewall| | +--------+ | | | +---v---------+ | |Public Server| | +-^-----------+ | | | +---+ +---+You| +---+

You have a server behind NAT and with to ssh into your server without having to mess with the NAT. The configuration is as follow

[Public Server] vim /etc/ssh/sshd_config

add this line

GatewayPorts yes

[Server] ssh -R 2222:localhost:22 public-server-user@public-server-ip

[you] ssh -p 2222 server-user@public-server-ip