This how-to describes the possibility to redirect a (server) remote port, e.g. 80 to a client local port, e.g. 8888 via SSH. This is sometimes useful, e.g. if you need to access a web server (e.g. an internal website) but only have SSH access to the network, which is otherwise sealed off from the outside (internet).
The instructions (screenshots) described here refer to a macOS environment but can also be used under Windows or Linux.
It is assumed that access to the remote server via SSH is available.
Here is an example of a simple diversion with the corresponding explanation:
ssh -L 8888:localhost:80 user@example-remote-host-873383.ex -p 49111The command means: Log on to the remote server example-remote-host-873383.eu. Then forward the standard web server port 80 to the port 8888 of my client.
The optional parameter -p defines the port on which the SSH service on the remote server listens (49111). In a standard SSH configuration, this is port 22.
Important: For SSH accesses via the Internet, the default port 22 should never be (open) used.
The syntax for port forwarding has the following meaning:
my-localport:localhost:server-port
And this is what the result of a redirected port of the web server looks like in the local browser window.
The same example with a slightly different notation:
ssh -L 8888:127.0.0.1:80 user@example-remote-host-873383.ex -p 49111This example corresponds exactly to the example above. 127.0.0.1 represents the loopback IPv4 address of the localhost entry.
You can also redirect other service ports, e.g. those of a MySQL service running on the remote server.
ssh -Ng -L 33306:127.0.0.1:3306 myUser@example-remote-host-873383.ex -p 49111The command means: Log on to the remote system and execute each command remotely (-Ng) and perform port forwarding (-L). In this case, it means that all commands executed locally and routed via port 33306 are tunneled to port 3306 of the remote server.
I know that the examples are only a tiny sample of what is possible. However, the topic is very fascinating and allows you to do cool things.
For more info, check out the manual pages on SSH
man sshThis manual or parts of it are provided "as is" without warranty of any kind.
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