Using SSH Tunnels
Port forwarding via SSH (SSH tunneling) creates a secure connection between a local computer and a remote machine through which services can be relayed. Because the connection is encrypted, SSH tunneling is useful for transmitting information that uses an unencrypted protocol, such as IMAP, VNC, or IRC.
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.
Warning: Filtering and monitoring is usually implemented for a reason. Even if you don't agree with that reason, your IT department might not take kindly to you flouting their rules.
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.example.com 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.example.com. :
ssh -L 8080:www.example.com:80 <host>
<host>should be replaced by the name of your laptop.
-Loption specifies local port forwarding.
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.example.com:80 -L 12345:example.com:80 <host>This would forward two connections, one to www.example.com, the other to example.com.
- Pointing your browser at http://localhost:8080/ would download pages from www.example
- Pointing your browser to http://localhost:12345/ would download pages from example.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
ssh -R 5900:localhost:5900 guest@joes-pc
-Roption 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
-Doption 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.
-Cenables 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:configin 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.
Forwarding GUI Programs¶
SSH can also forward graphical applications over a network, although it can take some work and extra software to forward programs to Windows or Mac OS.
If you are logging in from a Unix-like operating system, you can forward single applications over SSH very easily, because all Unix-like systems share a common graphics layer called X11. This even works under Mac OS X, although you will need to install and start the X11 server before using SSH.
To forward single applications, connect to your system using the command-line, but add the
-X option to forward X11 connections:
ssh -X laptop
Once the connection is made, type the name of your GUI program on the SSH command-line:
Your program will start as normal, although you might find it's a little slower than it would be if it were running locally. The trailing
& means that the program should run in "background mode", so you can start typing new commands in straight away, rather than waiting for your program to finish.
If you only want to run a single command, you can log in like this:
ssh -f -T -X laptop firefox
That will run Firefox, then exit when it finishes. See the SSH manual page for information about
If you start an application and it complains that it cannot find the display, try installing the
xauth package from the Main repository. Xauth is installed by default with desktop installations but not server installations.
If you suspect that programs are running slowly because of a lack of bandwith, you can turn SSH compression on with the
ssh -fTXC joe@laptop firefox
-fTXC here is identical to
-f -T -X -C.
If you get a message like this when you try to forward a port:
bind: Address already in use
channel_setup_fwd_listener: cannot listen to port: <port number>
Could not request local forwarding.
Then someone is already listening on that port number. You won't be able to listen on that port until the other person has finished with it.
If forwarding doesn't seem to work, even though you didn't get a warning message, then your SSH server might have disabled forwarding. To check, do the following:
grep Forwarding /etc/ssh/sshd_config
If you see something like this:
Then forwarding is disabled on your server. See the SSH configuration page for more information.