Syntax:
ssh -L localport:host:hostport user@ssh_server -N
where:
- -L - port forwarding parameters (see below)
- localport - local port (chose a port that is not in use by other service)
- host - server that has the port (hostport) that you want to forward
- hostport - remote port
- -N - do not execute a remote command, (you will not have the shell, see below)
- user - user that have ssh access to the ssh server (computer)
- ssh_server - the ssh server that will be used for forwarding/tunneling
Without the -N option you will have not only the forwardig port but also the remote shell. Try with and without it to see the difference.
Note:
- Privileged ports (localport lower then 1024) can only be forwarded by root.
- In the ssh line you can use multiple -L like in the example...
- Of course, you must have ssh user access on secure_computer and moreover the secure computer must have access to host:hostport
- Some ssh servers do not allow port forwarding (tunneling). See the sshd man pages for more about port forwarding (the AllowTcpForwarding keyword is set to NO in sshd_config file, by default is set to YES)...
Example:
ssh -L 8888:www.linuxhorizon.ro:80 user@computer -N
ssh -L 8888:www.linuxhorizon.ro:80 -L 110:mail.linuxhorizon.ro:110 \
25:mail.linuxhorizon.ro:25 user@computer -N
The second example (see above) show you how to setup your ssh tunnel for web, pop3 and smtp. It is useful to recive/send your e-mails when you don't have direct access to the mail server.
For the ASCII art and lynx browser fans here is illustrated the first example:
+----------+<--port 22-->+----------+<--port 80-->o-----------+
|SSH Client|-------------|ssh_server|-------------| host |
+----------+ +----------+ o-----------+
localhost:8888 computer www.linuxhorizon.ro:80
...And finally: Open your browser and go to http://localhost:8888 to see if your tunnel is working. That's all folks!
The SSH man pages say:
-L port:host:hostport Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side. This works by allocating a socket to listen to port on the local side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the remote machine. Port forwardings can also be specified in the configuration file. Only root can for- ward privileged ports. IPv6 addresses can be specified with an alternative syntax: port/host/hostport
-N Do not execute a remote command. This is useful for just for- warding ports (protocol version 2 only).
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