Friday, July 4, 2008

Secure HTTP over SSH proxy with Linux

In an previous post I made I detailed how to create a secure your browser's HTTP communications by tunneling the HTTP session over an SSH proxy using Putty.

Putty is what you would use if you use a Windows desktop. If you're on a Linux Desktop you do not need Putty since you should have OpenSSH with the distribution you use.

Doing a man ssh on your Linux Desktop should give you the manual on how to use your SSH client:

SSH(1)                                                         BSD General Commands Manual                                                         SSH(1)

NAME
     ssh - OpenSSH SSH client (remote login program)

SYNOPSIS
     ssh [-1246AaCfgKkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D  [bind_address:]port] [-e escape_char] [-F configfile] [-i identity_file] [-L
         [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-R  [bind_address:]port:host:hostport]
         [-S ctl_path] [-w local_tun[:remote_tun]] [user@]hostname [command]

... etc ...
The synopsis gives you the format of the command and the options that can be used with the ssh command. Of interest is the -D option. This allows you to bind the SSH session to a local address and port. Below is the part of the manual explaining the D option:
     -D [bind_address:]port
             Specifies a local “dynamic” application-level port forwarding.  This works by allocating a socket to listen to port on the local side,
             optionally bound to the specified bind_address.  Whenever a connection is made to this port, the connection is forwarded over the secure
             channel, and the application protocol is then used to determine where to connect to from the remote machine.  Currently the SOCKS4 and
             SOCKS5 protocols are supported, and ssh will act as a SOCKS server.  Only root can forward privileged ports.  Dynamic port forwardings can
             also be specified in the configuration file.

             IPv6 addresses can be specified with an alternative syntax: [bind_address/]port or by enclosing the address in square brackets.  Only the
             superuser can forward privileged ports.  By default, the local port is bound in accordance with the GatewayPorts setting.  However, an
             explicit bind_address may be used to bind the connection to a specific address.  The bind_address of “localhost” indicates that the listen‐
             ing port be bound for local use only, while an empty address or ‘*’ indicates that the port should be available from all interfaces.
Basically it means that you can start an SSH session using the OpenSSH client with a command such as:
ssh -D localhost:8000 user@example.com
and it will create a SOCKS proxy on port 8000 that will tunnel your HTTP connection over SSH to the server at example.com under the username user.

Now you can configure your applications that access the internet to use the secure HTTP tunnel you've created to your remote SSH server. The applications are not limited to web browsers, you can configure your Instant Messenger, Skype, Games etc. to use the socks proxy, as long at the communication protocol is supported.

Configuring Firefox to use the Socks Proxy

  • Tools -> Options -> Advanced -> Network
  • Under Connection click on the Settings button
  • Choose Manual Proxy configuration, and SOCKS v5
  • Fill in localhost for the host, and 8000 (or the port number you used) for the port
  • Click OK and reload the page

Now what you can do is have the the ssh session start up when you start your desktop. Thats if you want to use your secure tunnel every time you use Firefox or whatever program you have configured to use it. On Ubuntu (Debian) you'd add a shell script to your home directory.
Example:

#!/bin/sh
ssh -D localhost:8000 user@example.com
That should start up the ssh connection and create the socks proxy when you log in. The other alternative is to create a launcher and use ssh -D localhost:8000 user@example.com as the command, allowing you to launch the proxy whenever you need.

You can also set up an ssh key for authentication instead of having to log in. This is detailed in other posts: http://pkeck.myweb.uga.edu/ssh/ and http://sial.org/howto/openssh/publickey-auth/. This allows you to use the proxy transparently in the background without having to start it and log in.

For Firefox you can switch between proxy and direct connection using the switchproxy extension.

Disclaimer: Please note that it is your responsibility to use the information in this article within the legal laws of your country. Some countries do not allow encryption of internet traffic, therefore you SHOULD NOT use this resource if you live in such a country. I provide this information without warranty and free of charge and will not be held accountable for any damages lost due to its use.. etc etc.

3 comments:

Kais said...

Great guide, gave me exactly what I needed.

Chris said...

I think I am trying to do what you explain here, but I am not having any success. I have a workstation at work (call it work:) that is on a network from which certain sites can be accessed with full privileges by browser. At home I have a laptop (home:) on a dsl line from which those privileged sites cannot be accessed. One solution I tried is to ssh from home: to work: and run a browser. This works but the Xwindow display is tedious at best. I tried the
ssh -D home:8000 work
and the connection is made but when I set the home browser proxy to http://home:8000 I get for eg "Connection to host www.google.com is broken". Am I fully understanding what is going on? Should there actually be a web server or proxy running on work?

bucabay said...

Hi chris,

"I set the home browser proxy to http://home:8000"

Which browser is this? It wouldn't be a http proxy but SOCKS 5 proxy (TCP).