Background

Imagine a scene: there is a server running in friend’s house, U want to ssh to that machine. But the LAN break your dream.

Is there anyway to achieve this? The answer is YES, as long as U have another server that running on cloud.

Let’s do it

the solution is very simple, just a one-line command needs to be executed on friends server, as below:

ssh  -N -R *:11111:localhost:22 root@CLOUD_SERVER_IP

-R [bind_address:]port:host:hostport
         Specifies that the given port on the remote
         (server) host is to be forwarded to the given host
         and port on the local side.  This works by allocat‐
         ing a socket to listen to port on the remote side,
         and whenever a connection is made to this port, the
         connection is forwarded over the secure channel,
         and a connection is made to host port hostport from
         the local machine.
         Specifying a remote bind_address will only
         succeed if the server's GatewayPorts option is
         enabled (see sshd_config(5)).

 -N      Do not execute a remote command.  This is useful
         for just forwarding ports (protocol version 2
         only).

After the command executed, the data will be forwarded in this way:
my_pc –> CLOUD_SERVER –> friend’s server

Now run below in my_pc (USERNAME is login-able user that on friend’s server)

ssh  USERNAME@CLOUD_SERVER_IP  -p 11111 

Bingo~

Troubleshooting

• Like manual doc said, GatewayPorts option must be enabled on CLOUD_SERVER
• if U feel the password prompt appears slow, try set UserDNS no and GSSAPIAuthentication no on CLOUD_SERVER
• add -vvv parameter to ssh command will show the detail info that may helpful
• execute ‘netstat -anplut’ on CLOUD_SERVER will show the connection info, to ensure that the port has been start listening
• Example use port 11111 on CLOUD_SERVER, please make sure this port is not used by other process.

The end

ssh is powerful, isn’t it?