This is how to open a SSH connexion to a serveur hidden behind a NAT gateway.
We use a reverse SSH tunnel:
nated-host$ ssh -R 2222:localhost:22 anyuser@public-host anyuser@public-host$
This command opens 2222 port on public-host, forwarding it to local 22 port on nated-host.
Finally, from public-host we connect to 2222 local port with SSH, to end on nated-host:
public-host$ ssh -p2222 localhost nated-host$
References: