SSH Tunneling Explained

13Sep 2020 by Amine G No Comments

This video is mainly about different techniques about how to use SSH tunneling serving and maintaining access when not possible .

i am actually adding this video particularly because of the graphic approach followed explaining how communication in the internet work , more precisely , IP PORTS .

Types of Port Forwarding

SSH’s port forwarding feature can smuggle various types of Internet traffic into or out of a network. This can be used to avoid network monitoring or sniffers, or bypass badly configured routers on the Internet. Note: You might also need to change the settings in other programs (like your web browser) in order to circumvent these filters.

There are three types of port forwarding with SSH:

  • Local port forwarding: connections from the SSH client are forwarded via the SSH server, then to a destination server
  • Remote port forwarding: connections from the SSH server are forwarded via the SSH client, then to a destination server
  • Dynamic port forwarding: connections from various programs are forwarded via the SSH client, then via the SSH server, and finally to several destination servers

Local port forwarding is the most common type. For example, local port forwarding lets you bypass a company firewall that blocks Wikipedia.

Remote port forwarding is less common. For example, remote port forwarding lets you connect from your SSH server to a computer on your company’s intranet.

Dynamic port forwarding is rarely used. For example, dynamic port forwarding lets you bypass a company firewall that blocks web access altogether. Although this is very powerful, it takes a lot of work to set up, and it’s usually easier to use local port forwarding for the specific sites you want to access.

Port-forwarding is a widely supported technique and a feature found in all major SSH clients and servers, although not all clients do it the same way. For help on using a specific client, consult the client’s documentation. For example, the PuTTY manual has a section on port forwarding in PuTTY.

To use port forwarding, you need to make sure port forwarding is enabled in your server. You also need to tell your client the source and destination port numbers to use. If you’re using local or remote forwarding, you need to tell your client the destination server. If you’re using dynamic port forwarding, you need to configure your programs to use a SOCKS proxy server. Again, exactly how to do this depends on which SSH client you use, so you may need to consult your documentation.

Local Port Forwarding

Local port forwarding lets you connect from your local computer to another server. To use local port forwarding, you need to know your destination server, and two port numbers. You should already know your destination server, and for basic uses of port forwarding, you can usually use the port numbers in Wikipedia’s list of TCP and UDP port numbers.

For example, say you wanted to connect from your laptop to http://www.ubuntuforums.org using an SSH tunnel. You would use source port number 8080 (the alternate http port), destination port 80 (the http port), and destination server www.ubuntuforums.org. :

ssh -L 8080:www.ubuntuforums.org:80 <host>

Where <host> should be replaced by the name of your laptop. The -L option specifies local port forwarding. For the duration of the SSH session, pointing your browser at http://localhost:8080/ would send you to http://www.ubuntuforums.org/.

In the above example, we used port 8080 for the source port. Ports numbers less than 1024 or greater than 49151 are reserved for the system, and some programs will only work with specific source ports, but otherwise you can use any source port number. For example, you could do:

ssh -L 8080:www.ubuntuforums.org:80 -L 12345:ubuntu.com:80 <host>

This would forward two connections, one to www.ubuntuforums.org, the other to www.ubuntu.com. Pointing your browser at http://localhost:8080/ would download pages from www.ubuntuforums.org, and pointing your browser to http://localhost:12345/ would download pages from www.ubuntu.com.

The destination server can even be the same as the SSH server. For example, you could do:

ssh -L 5900:localhost:5900 <host>

This would forward connections to the shared desktop on your SSH server (if one had been set up). Connecting an SSH client to localhost port 5900 would show the desktop for that computer. The word “localhost” is the computer equivalent of the word “yourself”, so the SSH server on your laptop will understand what you mean, whatever the computer’s actual name.

Remote Port Forwarding

Remote port forwarding lets you connect from the remote SSH server to another server. To use remote port forwarding, you need to know your destination server, and two port numbers. You should already know your destination server, and for basic uses of port forwarding, you can usually use the port numbers in Wikipedia’s list of TCP and UDP port numbers.

For example, say you wanted to let a friend access your remote desktop, using the command-line SSH client. You would use port number 5900 (the first VNC port), and destination server localhost:

ssh -R 5900:localhost:5900 guest@joes-pc

The -R option specifies remote port forwarding. For the duration of the SSH session, Joe would be able to access your desktop by connecting a VNC client to port 5900 on his computer (if you had set up a shared desktop).

Dynamic Port Forwarding

Dynamic port forwarding turns your SSH client into a SOCKS proxy server. SOCKS is a little-known but widely-implemented protocol for programs to request any Internet connection through a proxy server. Each program that uses the proxy server needs to be configured specifically, and reconfigured when you stop using the proxy server.

For example, say you wanted Firefox to connect to every web page through your SSH server. First you would use dynamic port forwarding with the default SOCKS port:

ssh -C -D 1080 laptop

The -D option specifies dynamic port forwarding. 1080 is the standard SOCKS port. Although you can use any port number, some programs will only work if you use 1080. -C enables compression, which speeds the tunnel up when proxying mainly text-based information (like web browsing), but can slow it down when proxying binary information (like downloading files).

Next you would tell Firefox to use your proxy:

  • go to Edit -> Preferences -> Advanced -> Network -> Connection -> Settings…
  • check “Manual proxy configuration”
  • make sure “Use this proxy server for all protocols” is cleared
  • clear “HTTP Proxy”, “SSL Proxy”, “FTP Proxy”, and “Gopher Proxy” fields
  • enter “127.0.0.1” for “SOCKS Host”
  • enter “1080” (or whatever port you chose) for Port.

You can also set Firefox to use the DNS through that proxy, so even your DNS lookups are secure:

  • Type in about:config in the Firefox address bar
  • Find the key called “network.proxy.socks_remote_dns” and set it to true

The SOCKS proxy will stop working when you close your SSH session. You will need to change these settings back to normal in order for Firefox to work again.

To make other programs use your SSH proxy server, you will need to configure each program in a similar way.

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