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that's the simple scenario: from pc A i open a tunnel SSH to a pc B. Can someone, on pc B, use this tunnel to enter or forwarding some protocol to pc A? Can i limit traffic to be generated only from pc A? Thanks.

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You haven't specified what tunnel you've created. Please give an example. –  MikeyB Sep 9 '11 at 14:51
    
ssh -L localPort:remoteHost:remotePort user@SSHserver (ssh server and remote host are the same) –  John Sep 9 '11 at 15:03

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I'm assuming you're talking about an SSH port-forward "tunnel" -- If you run (for example) ssh -L 8080:B:80 user@B on host A and haven't done anything funky to your SSH configuration the answer is that there is no risk of "backwash" from host B.

The command above opens a listener on host A's local loopback address (127.0.0.1 / ::1) on port 8080.
When you send traffic to to that listener it is sent over the SSH connection to the other side to Host B port 80.
Replies from host B will travel back along the tunnel, but host B cannot initiate new connections back to host A -- it is a single port forward, not a wide-open tunnel.

Regarding the second part of your question (limiting traffic), by default SSH binds forwarded ports to localhost, so if you use the command above host A is the only one that can use the connection. The full syntax for the -L option is -L LocalIP:LocalPort:RemoteHost:RemotePort, however the LocalIP part is typically omitted and defaults to localhost.

For more information consult the OpenSSH man pages.

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Yes, it's a port-forward tunnel and i use the -L option. Thanks for answer. –  John Sep 9 '11 at 15:01

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