SSH: Secure Browsing Via SOCKS Proxy
April 10, 2009
It seems that not a week goes by any more that I don’t find some new, fun trick to do with SSH. A few weeks ago, I found one that to me has been especially useful.
I was sitting in the Tulsa International Airport, once again wishing that airports would just suck it up and provide free wireless access throughout their terminals. It’s a real pet peeve of mine, as layovers become incredibly more painful when I can’t waste away my time stumbling about the internet. I might even have to do something *shudder* productive…
Anyway, there I was, sipping some coffee and working on a project, when I noticed that there was an open wireless network available that was not one of those god forsaken Boingo hotspots. Being the curious person that I am, I decided to see if I could connect. Sure enough, it let me right on. Being the cautious person I am, I went to an HTTPS secured site to see what would happen. And sure enough, the normally valid certificate was invalid, pretty much guaranteeing someone was trying to listen in. I was still happy though, at least I still I had internet access and could keep myself mildly entertained with that.
However, I was feeling especially curious that day, so I decided to try to tunnel my traffic over SSH to a box back in my apartment, keeping my oh-so precious personal data away from prying eyes. Besides, beats working. After a little digging through man pages, this task, to my surprise, turned out to be much simpler than I had expected. All you need is one SSH command and an SSH server that you have access to and has forwarding enabled (the default OpenSSH installation on Ubuntu does).
If you don’t have an SSH server set up and you’re using Ubuntu at home, simply execute this on your home machine:
sudo apt-get install openssh-server
This will install and start the service. Make sure that a.) your user password is of decent strength (SSH is a common target for password bruteforcing) and b.) that you have port 22 forwarded on your router if you are behind a NAT so that you can access it from outside of your local network. The SSH client should already be installed on a default Ubuntu install (you can also do this using PuTTY on windows).
Once you have these two things ready, just open up terminal on your laptop/netbook/mobile device and type the following:
ssh -Nf -D randPortNum firstname.lastname@example.org
Replace randPortNum with a port number of your choosing (something above 1024 if you are not root, which is probable), remote-username with your username on the remote system, and ssh.server.com with the hostname or IP address of your SSH server. If you are using your home server, I’d suggest using DynDNS to get a simple domain name to access it with. If you do not feel very comfortable with the command line, or you are lazy like me (I hate having to close the window after I’m done…), you can execute this command using Alt+F2, and the SSH client will prompt you for your password.
Now let me explain what exactly this command is doing. The N and f flags both specify that the command is to be forked into the background, so that you can do whatever you want after you execute it. Close the terminal, keep using it for something else, anything you please (just not killall ssh!). The D flag is the one doing the really interesting stuff: the OpenSSH developers decided it would be cool to put SOCKS proxy functionality straight into the client, and the D flag is how you access it. Basically, you are just telling SSH to start “local dynamic application-level port forwarding” (SOCKS proxy) from the specified port on your local machine to the remote host. Now, any program on your computer that supports SOCKS proxies will be able to connect to that port on your machine and have its traffic automagically forwarded (and encrypted!) across the internet to your remote machine, where it will then go out to its destination.
To add to it, tons of programs do support SOCKS proxies, more than you might think. Firefox, Opera, Pidgin, Deluge, Transmission (Tracker only), the list goes on. On top of that, using some programs (like tsocks) you can actually use any TCP based program over it. Very cool stuff.
To go ahead and encrypt your web traffic, open up Firefox (if you need Opera instructions, they’re probably very similar). Go to Edit->Preferences->Advanced->Network->Settings (Configure How Firefox Connects To The Internet) . Select “Manual proxy configuration”, enter “localhost” for your SOCKS host and the port number you chose earlier as your port. Either SOCKS 4 or 5 should work (I use 5). Now, it should look similar to the picture below:
Now just click OK, close out the Settings dialog, and you’re done! Go here and check it out, your IP is now the same as the remote host’s. If you’re really paranoid, you can also make Firefox tunnel your DNS queries over the proxy. This prevents the nameserver of the local network feeding you bad DNS information or keeping tabs on what you are viewing (you are still relying on the remote nameserver being trustworthy though ) . To do this, open up a tab, enter the address “about:config”, search for “network.proxy.socks_remote_dns” and set it to true. And that’s it!
This trick can be immensely useful in many situations, from securing your traffic across untrusted local networks, to getting around packet shaping/filtering, to remaining anonymous online. I now use it all the time on my laptop, and very rarely trust the local network. A word of warning before I sign off though, I was lucky on that hotspot because the attacker was not trying to launch a MITM attack against my SSH traffic. If they had, the keys would not have matched my previous connection attempts to my SSH server, and I would have been warned in big bold letters that I was being listened in on, and the SSH client would have quit. In this situation, securing your traffic may be more difficult, but not impossible. I may post later on how one might go about this.
Anyway, hope someone else finds this as useful and interesting as I do. As always, feel free to ask if you have any questions.
UPDATE 04/15/2010: I have done a follow-up post to this article describing how you can use proxychains to allow any program that uses TCP sockets to tunnel traffic over SOCKS proxies, not just ones that have built-in proxy support. I also show how to chain multiple proxies together.