SSH to Restricted Server via SSH Tunnel
If you have a server, Restricted_Server, which is only accessible via SSH from a gateway machine say, Gateway_Server, then to ssh to the Restricted_Server, the command you can use is
ssh -L 10000:Restricted_Server:22 Gateway_Server -N
The above command says: create a ssh tunnel to the Gateway_Server and forward all localhost traffic on port 10000 to the Restricted_Server on its port 22.
ssh user@localhost -p 10000
Connect to the localhost as "user" on port 10000. Since the localhost port 10000 traffic gets forwarded to the Restricted_Server port 22, this allows us to connect to the Restricted_Server's ssh service