SSH tunneling

You need to log in to a server on an internal network, but you can only access the internal network through a gateway machine. If both these machines are running SSH, it is convenient to set up an SSH tunnel.

Commands
On the client machine (where you are):

where
 * GATEWAY = hostname/IP address of the gateway machine
 * GWUSERNAME = username on the gateway (optional if this username is the same as on the client)
 * SERVER = hostname/IP address of the server you wish to log into
 * SUSERNAME = username on the server (optional if this username is the same as on the client)
 * SPORT = port number on which the server SSH daemon is listening, usually 22
 * CPORT = port number of your choosing on which the tunnel will be receiving connections on the client machine (should be greater than 1024 unless you are invoking as root)

In the first invocation, -f instructs the ssh instance to go into the background, and -N instructs it to not launch a shell.

You can scp files from the server as you would normally by specifying the tunnel port:

Similarly for sending files to the server:

Tips for reducing work
In order to make this tunneling process less onerous in the future:
 * Set these commands as shell aliases (in Bash, usually in ~/.bashrc).
 * To avoid typing passwords, copy the client key to the gateway, and the client and gateway keys to the server. This process is described in the official Gentoo documentation OpenSSH key management.

X11 forwarding
To enable X11 forwarding in your client connection, add the -Y switch to the second invocation above:

The following is required for the forwarding of X11 connections from the server to client to work:
 * The SSH daemon on the gateway machine must have TCP forwarding must be enabled, otherwise X11 connections won't be forwarded:


 * The xauth tool must be present on the server. On Gentoo, install  with the X USE flag set to pull it in.
 * X11 forwarding must be enabled in the server SSH daemon configuration: