My systems are usually configured to allow ssh connections from a small set of trusted hosts or a bastion host. This is decent security practice but can be a pain when you want to scp a file or grab the stdout of a command from a host outside the trusted area. It also can be problematic if you have hosts on a private subnet and only one host (bastion host or jump box) to get in through. This method will enable transparent access to a host while behind the scenes tunneling through another host. No modification of the server is required. It just involves a few adjustments to the .ssh/config (ssh client config file) in your home directory.

Here’s how it works

A connection is established to the bastion host

+-------+            +--------------+
|  you  | ---ssh---> | bastion host |
+-------+            +--------------+

Bastion host runs netcat to establish a connction to the target server

+--------------+                +--------+
| bastion host | ----netcat---> | server |
+--------------+                +--------+

Your client then connects through the netcat tunnel and reaches the target server

+-----+                  +--------------+                +--------+
| you |                  | bastion host |                | server |
|     | ===ssh=over=netcat=tunnel======================> |        |
+-----+                  +--------------+                +--------+

So there are 3 things we need to have happen behind the scenes:

1. Ssh to bastion host.
2. Run netcat command on bastion host.
3. Connect to netcat tunnel.

Here’s how to use the ssh proxycommand

#~/.ssh/config
 
Host superchunk.example.org
	ProxyCommand  ssh [email protected] nc %h %p

In the above we are telling ssh that when it establishes a connection to superchunk.example.org to do so using the stdin/stdout of the ssh ProxyCommand as a transport. The ssh ProxyCommand then tells the system to first ssh to our bastion host and open a netcat connection to host %h (hostname supplied to ssh) on port %p (port supplied to ssh).

The result is a connection as if you were connecting from a trusted host:

$ ssh superchunk.example.org
Password: 
[email protected]'s password: 
Last login: Wed Jun 25 12:05:47 2008 from 10.0.0.221
[user@superchunk ~]$

Now you may be wondering why it prompted me for two passwords. This is because we are effectively sshing into two systems one right after the other. This can be resolved through the use of pre-shared ssh keys or with more advanced methods such as kerberos ticket forwarding.

More info about ssh proxycommand

For more detail you can read the full ssh_config man page here: http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/ssh_config.5.