-L [bind_address:]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, optionally bound to the specified bind_address. 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. IPv6 addresses can be specified with an alternative syntax: [bind_address/]port/host/hostport or by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of “localhost” indicates that the listen- ing port be bound for local use only, while an empty address or ‘*’ indicates that the port should be available from all interfaces.
-N Do not execute a remote command. This is useful for just forwarding ports (protocol version 2 only).
-R [bind_address:]port:host:hostport
Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote 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 local machine.
Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote 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 local machine.
Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in square braces or using an alternative syntax: [bind_address/]host/port/hostport.
By default, the listening socket on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address. An empty bind_address, or the address ‘*’, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server’s GatewayPorts option is enabled (see sshd_config(5)).
If the port argument is ‘0’, the listen port will be dynamically allocated on the server and reported to the client at run time.
By default, the listening socket on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address. An empty bind_address, or the address ‘*’, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server’s GatewayPorts option is enabled (see sshd_config(5)).
If the port argument is ‘0’, the listen port will be dynamically allocated on the server and reported to the client at run time.
-f Requests ssh to go to background just before command execution.
This is use-ful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm.
If the ExitOnForwardFailure configuration option is set to “yes”, then a client started with -f will wait for all remote port forwards to be success- fully established before placing itself in the background.
Difference between Reverse tunneling and Normal tunneling.
Normal tunneling
ssh -L 8888:www.linux.ro:80 user@computer -N
ssh -L 8888:www.linux.ro:80 -L 110:mail.linux.ro:110 \
25:mail.linux.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.linuxon.ro:80
For example, if the remote server in question was myserver.example.com you could run the following command on your local system to create a tunnel as described above:
ssh -T -N -L 3308:localhost:3306 myserver.example.com
The meat of the command is the -L option, which tells ssh to listen on port 3308 locally and then on the remote side to forward all traffic on that port to localhost:3306. Note that the localhost here is not referring to the local system but rather where to forward things to on the remote side, in this case to localhost on the remote side.
ssh -T -N -L 3308:private.local:3306 myserver.example.com
Here, ssh listens on port 3308 on the local system and it forwards that data to port 3306 on private.host, but it does that via the server myserver.example.com. In other words the local traffic on port 3308 gets transferred first to the remote system which then transfers it to port 3306 on private.host. Of course, if private.local's mysql server is only listening on its local interface this won't work, you'll need something more involved.
ssh -R 9001:intra-site.com:80 home (Executed from 'work')
Once executed the SSH client at ‘work’ will connect to SSH server running at home creating a SSH channel. Then the server will bind port 9001 on ‘home’ machine to listen for incoming requests which would subsequently be routed through the created SSH channel between ‘home’ and ‘work’. Now it’s possible to browse the internal site
by visiting http://localhost:9001 in ‘home’ web browser. The ‘work’ will then create a connection to intra-site and relay back the response to ‘home’ via the created SSH channel.
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