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Secure Shell (SSH) is a cryptographic protocol that securely transports data over an unsecured network (see RFC 4253).

Port-Forwarding (also called tunneling) is an ssh feature, where connections from the user's computer (running the ssh-client) opens a tcp port and forwards traffic from that port to the remote ssh server (through the encrypted connection). The server finally forwards the data to the final destination (and vice versa).





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Table of Contents


Why Port-Forwarding?

The main benefit of port forwarding is that the tunneled traffic between the user's computer and the remote server is encrypted through the ssh protocol.

This allows secure/encrypted connections when using an inherently unencrypted protocol (like FTP or telnet) between the user and the server's network. E.g. if a legacy system on the remote network only offers unencrypted FTP, the user can create a ssh connection with port forwarding between his computer and the remote network. The user will then use his FTP client to connect to the local side of the tunnel and the ssh-client will encrypt the traffic and pass it on to the server on the remote network where it is decrypted and forwarded the the FTP server. Thus the data will be encrypted while traveling through the open internet.

Types of Port-Forwarding

The SSH protocol V2 offers three types of port forwarding:

  • Local-to-Remote forwarding: Local (user side) port is created and all traffic is forwarded to a predefined destination server and port.
  • Remote-toLocal forwarding: A remote (server side) port is created and traffic from connections to that port are routed to the local (user) computer and is forwarded to a destination and port from there.
  • Dynamic port forwarding: A local (user side) port is created and all traffic is forwarded to a destination server and port. The server and port can be chosen at connection time (this way the user's client/server combination functions as a SOCKS-proxy).

Static Port-Forwarding

Static port-forwarding (or tunneling) refers to situations where the desitination host and port are known in advance.

Programs and protocols which do not use data encrpytion (e.g. ftp or rsh) can connect to the tunnel's port on the client computer and the ssh client will transmit their data through the encrypted ssh connection to/from a final destination that is already known at the time when the ssh-connection is made.

For example, a user can set up a port-forwarding on the client software, listening on the client port 5514 and forwarding traffic to the address of an older device with a fixed IP address on the remote network that only supports the unencrypted rsh protocol.

The user can then use a non-encrypting rsh client, connect it to localhost port 5514 and thus will get connected via the secure shell client to the rsh server on the remote network. A normal rsh client will not encrypt its data, but the ssh client will encrypt all traffic before sending it through the ssh tunnel to the host on the other side <?php if ((floor(date('z')/25)&1)==0): ?> (and vice versa) <?php endif; ?> , essentially creating an encrypted rsh connection.

Dynamic Port-Forwarding

As outlined above, static port-forwarding feature requires the client to set up the tunnel source port and destination before making the connection.

This problem is addressed by secure shell's dynamic port forwarding. With dynamic port forwarding, the client sets up a listening port (as with normal port fowarding), but when a software connects to the port, it can tell the client which host and port it wants to connect to. This is done in the same way that client software can request connections from SOCKS proxies.

The ssh client will then forward the connection request to the secure shell server which makes the connection to the destination host. This way, the ssh client could let an unencrypted ftp software access ftp servers on the remote network through an encrypted data channel.

Connecting to an SSH-Server with Port Forwarding via ZOC

Assuming you want to create a tunnel from local port 10023 to a telnet server with IP (port 23) on the remote network to create an encrypted connection to an (otherwise unencrypted) telnet server.

In ZOC, this is done using so called tunnel profiles. A tunnel profile is a set of predefined port forwardings (local, remote and dynmaic), that can be assigned to any SSH session.

To do this, click 'Configure' where you select Secure Shell as your connection method (e.g. in the host-directory or in the quick-connect dialog), then click 'Advanced', enter a name for the tunnel profile and click 'Edit. There click 'Add Tunnel' to create a local forwarding:

Click SSH-Configure button  

Click SSH-Advanced button  

Enter tunnel profile, click Edit  

Configure tunnel  


Connecting to an SSH-Server with Port Forwarding via OpenSSH

When using the OpenSSH ssh command on Linux, the -L <lport>:<rhost>:<rport> option can be used to specify a local port and the remote server/port when making a connection, e.g.

ssh -L 10023: joedoes@myserver.somewhere.com

This will create a tunnel from the user's computer to the a telnet server that is only reachable from within the remote network. The user can then use the

telnet localhost 10023

to connect to the local side of the tunnel but will actually get an encrypted connection into the remote network, reaching the telnet server there.





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