Bypassing firewall using SSH

Friday, September 7, 2007

Voila

Protect yourself from someone looking over your shoulder

Here's a great application that fits in perfectly with the theme of this page. It's called Ghostzilla; http://www.ghostzilla.com/ . The idea is that you want to surf the web, but have it look like you are doing normal work to people walking by your computer. Ghostzilla is a browser that hides itself in your normal work applications, like Excel, or Word, or Visual Studio... anything. With a swish of the mouse, Ghostzilla pops up and you can surf the web. If you see someone coming, simply move the mouse away, and it disappears, leaving no trace. Plus, you can easily configure it to use the shunnel as described here, for total privacy!

A Simpler Solution

Buzzsurf has teamed up with HTTP-Tunnel Corp to encourage users to try the HTTP-Tunnel Client as a simplier alterntive to the procedure described here. Using HTTP-Tunnel , you don't need a computer at home to leave turned on all day. And you don't need to know how to install SSH or Putty. All the network communication is encrypted and sent over standard webserver ports, just like I describe, so it offers just as much protection without the hassle. Try it for free at HTTP-Tunnel.com

Configure Internet Explorer

Now we have to configure Internet Explorer at work to use a SOCKS proxy server.

First, at school/work, go to http://www.whatismyip.com . Write down the number. This is your IP address WITHOUT your shunnel enabled.

In Internet Explorer;

Open the Tools menu, then click Internet Options.
Click the Connections tab, then click LAN Settings.
Check "Use a proxy server ...", then click the Advanced button.
If "Use the same proxy for all protocols" is checked, uncheck it.
Delete anything from the "Proxy address to use" and "Port" boxes.
On the Socks line, enter "127.0.0.1" for the address, and "8080" for the Port.
Click OK a couple times, then close Internet Explorer and restart it.

First go to http://www.whatismyip.com again. If everything worked correctly, the page should have changed to show your HOME IP address, NOT your work IP address. If it shows your home IP Address, congratulation, your surfing the web securely and privately from work.

If your intent is to access MySpace, and MySpace was blocked before, try it now.

Configuring other applications to use the private connection

Most applications that access the Internet can be configure to use the shunnel. For it to work, they have to support a SOCKS 4 or SOCKS 5 proxy connection. Instant messaging programs like AIM, ICQ, Yahoo IM, and mIRC all support this.

Setup is different for all application, but the settings will be the same. You want to configure the application to use a SOCKS 4 or SOCKS 5 proxy server, Host should be 127.0.0.1, and Port should be 8080.

Getting your hands dirty

Start/Stoping the SSH Server

On your home computer, open a command prompt. To start your SSH server, type the following:

net start opensshd

To stop your SSH server, type the following:

net stop opensshd

To make it easy, you can create a .bat file that will this command. If you make a shortcut to the .bat file in your Windows Startup program group, then when you turn on your home computer in the morning, the servers will startup automatically, and be ready for you when you get to work.

If you have a wired or wireless router at home (Linksys, D-Link, Netgear, etc)

Some routers call it port forwarding and others call it virtual servers, but the setup is very similar no matter what brand you use. You will need to configure your router to route port 443 to the computer where you're running the SSH server. I not going to go into details, but there is usually a browser based interface directly to the router, which will have a page to setup virtual servers. Configure it to forward port 443 to your SSH server computer, port 443.

Setup Putty at Work/School

Copy putty.exe to somewhere on your hard drive at work. c:\ will do fine, or anywhere else you want. Your desktop is convenient but kind of obvious. If you don't have permissions to write files to your hard drive, just copy putty.exe and shunnel.bat to a floppy disk or burn them onto a CD. Take the disk to work and run Putty from the appropriate drive.

Open Notepad and copy the following into it, change the bold part where necessary;

putty -D 8080 -P 443 -ssh homeIP

homeIP should be the IP address of your home machine that you wrote down in the Addresses section above.

Save the file as shunnel.bat in the same directory that you saved putty.exe.

Note for advanced users: If your computer at work is already configured to use a proxy server, you need to configure Putty a little differently, but this may still work.

Open Putty in graphical mode, input your connection setting, and also copy the proxy settings from Internet Explorer to Putty's proxy configuration screen. Putty should now create a secure tunnel through the proxy at work to your computer at home... pretty neat trick.

Create your tunnel

At work, simply double click shunnel.bat to initiate the shunnel. A Putty window will popup asking for a login name and password. Type the user name and password you created above on the Windows account. If it works, you will be presented with a DOS prompt waiting for a command. This is actually a command prompt to your HOME machine. You can use it if you want, but as long as this command prompt is open, your tunnel is alive. To close the tunnel, type exit or close the window.

For Advanced Users
If you are very familiar with SSH and know what you are doing, you can set this up so you don't have to enter a password each time you create the shunnel. You have to install OpenSSH as your SSH client and then setup key based authentication by creating a public and private key on your work computer. Install the public key on the SSH server on your home computer. Thanks to Robert W. for this suggestion. I may go into more detail on how do set this up in the future.

Installation and configuration

The OpenSSH installer comes in a zip file. Unzip the file, then run setupssh.exe. Choose to install both the Client and the Server. It will ask you to install into C:\Program Files\OpenSSH. If you choose to install into a different location, that fine, but be aware I will use the above path in this document.

Configure

OpenSSH for Windows uses Windows' user database for login authentication. That mean you must have a User name and Password setup to login to your home computer. If you don't, you have 2 choices. 1, set a password on your Windows account, or 2, create a new local account that you will use to login from SSH. I know a lot of people out there don't use logins or passwords on their home computer, but if you're using NT, 2000, or XP, the functionality is there, even if you don't use it.

There are many different flavors of Windows, with different methods of creating a local user. There's no way I can cover all of them, but here are a few examples;

To create a new account on your home machine (Windows XP):

Start Menu, open Control Panel, then User Accounts.
Click Advanced tab, then the Advanced button.
Highlight Users, then click Actions, then New User.
Enter a User name, and a Password twice. I recommend you use a User name and Password that is different than anything you have ever used at work. Obviously, your employer probably knows your password, so there's no security if you use the same password at home.
Deselect User must change password at next logon.
Check Password never expires.
Click Create.
Close the Windows, close Control Panel.

You should now have a new local Windows user on your home machine. Remember the Login name and password for later.

Configure the SSH Server
We want to configure your SSH server to allow access using User name and Passwords, and to listen on port 443 instead of port 22.

Why port 443 instead of port 22? In most cases your employer will block almost all outgoing network ports except for port 80 and port 443, which are the 2 ports that webservers run on. I used to tell people to run SSH on port 80 because that's the standard webserver port, but now I recommend you run it on 443. Port 443 is used for encrypted websites, which is what your shunnel traffic will look like as it passes through the firewall. If you have trouble on port 443, try it on port 80 instead. If neither work, you're probably out of luck.

Open Windows Explorer, navigate to C:\Program Files\OpenSSH\etc. Open the file sshd_config using Wordpad. (That's sshd_config not ssh_config!)

Change the line

#Port 22

to

Port 443

Save the file.

Now open a command prompt. Change to C:\Program Files\OpenSSH\bin. We are going to create a user and group database from your Windows user database. Type the following;

mkgroup -l > ..\etc\group

Then

mkpasswd -l > ..\etc\passwd

These 2 commands will create group and password files at C:\Program File\OpenSSH\etc

Software

Before we start installing and configuring software, you need to find out the following things;

Your home IP Address
Your work/school external IP Address

The easiest way to get your IP Addresses is to go to www.whatismyip.com at home and at work. Write down the numbers.

Software
We're going to be using 2 fairly simple pieces of software; an SSH Server and an SSH Client.

There are a few flavors of SSH Server's out there, but we're going to be using OpenSSH because it's free. The website for OpenSSH is http://www.openssh.com . But wait! OpenSSH doesn't run on Windows unfortunately... But there is a site that converted OpenSSH to run on Windows, which is what we want! http://sshwindows.sourceforge.net/ .

Download OpenSSH for Windows from http://sshwindows.sourceforge.net . The version I wrote this document using was 3.7.1p1-1. The latest version should work for you, plus it will have less security holes.

For the SSH Client I recommend using Putty. Putty is a small single executable SSH client with the ability to setup a tunnel. The newer version also support Dynamic Forwarding, which is essential. It's possible to use OpenSSH as your client as well as your server, but Putty is much easier to setup and use. Download putty.exe from http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html .