Problem

I am attempting to configure an SSH tunnel to a NexentaStor appliance from either a Windows or Linux computer so that I can connect a Crashplan Desktop GUI to a headless Crashplan server running on the Nexenta box, according to these instructions on the Crashplan support site: Connect to a Headless CrashPlan Desktop.

So far, I've failed to get a working SSH tunnel from from either either a Windows client (using Putty) or a Linux client (using command line SSH). I'm fairly sure the problem is at the receiving end with NexentaStor. A blog article - CrashPlan for Backup on Nexenta - indicates that it could be made to work only after "after enabling TCP forwarding in Nexenta in /etc/ssh/sshd_config" - although I'm not sure how to go about that or specifically what I need to do.

Things I have tried

Ensuring the Crashplan server on the Nexenta box is listening on port 4243

$ netstat -na | grep LISTEN | grep 42
127.0.0.1.4243             *.*                0      0 131072      0 LISTEN
      *.4242               *.*                0      0 65928      0 LISTEN

Establishing a tunnel from a Linux host:

$ ssh -L 4200:localhost:4243 admin:10.0.0.56

and then, from another terminal on the Linux host, using telnet to verify the tunnel:

$ telnet localhost 4200
Trying ::1...
Connected to localhost.
Escape character is #^]'.

with nothing more, although the Crashplan server should respond with something.

From Windows, using PuTTY have followed the instructions on the Crashplan support site to establish an equivalent tunnel, but then telnet on Windows gives me no response at all and the Crashplan GUI can't connect either. The PuTTY log for the tunnelled connection shows reasonable output:

...
2011-11-18 21:09:57 Opened channel for session
2011-11-18 21:09:57 Local port 4200 forwarding to localhost:4243
2011-11-18 21:09:57 Allocated pty (ospeed 38400bps, ispeed 38400bps)
2011-11-18 21:09:57 Started a shell/command
2011-11-18 21:10:09 Opening forwarded connection to localhost:4243

but the telnet localhost 4200 command from Windows does nothing at all - it just waits with a blank terminal.

On the NexentaStor server I've examined the /etc/ssh/sshd_config file and everything seems 'normal' - and I've commented out the ListenAddress entries to ensure that I'm listening on all interfaces.

How can I establish a tunnel, and how can I verify that it is working?