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A Prisoner of Windows

By Lew Pitcher

Imprisoned in a Windows jail? Make your escape to Linux through an SSH tunnel and remote X Windows.

Those of us who use Linux desktops sometimes forget that most of our peers do not have our luxuries. At times, we find ourselves forced to devolve, and use a Windows desktop to do our work. When we do, we pine for the vast variety of tools we had under Linux and curse the clumsiness of the software prison that we are locked into.

But there is hope for Prisoners of Windows. If our Microsoft Windows environment is network-enabled, and if we can connect to a suitably configured network-enabled Linux box, we can still work on our platform of choice. So, how do we free ourselves? By giving ourselves secure remote access to Linux, both at the command line and at the GUI level with a combination of standard Linux tools and a couple of Open Source applications for Windows.

My goal is to show you how to let a Windows desktop user log into and run both command-line and GUI applications on a remote Linux system, with Windows handling the presentation (and only the presentation). I will show you how to set up OpenSSH on a Linux system in order to provide secure access and facilitate GUI interaction, and how to configure and use an Open Source SSH client (PuTTY) and an Open Source X server (Xming) on Windows. With these tools, Windows captives can securely and easily escape to a Linux system and work on it remotely as if it were the only system they were on.

The escape plan: Setting up the OpenSSH Server

Before you can escape, you have to prepare a "safe haven" on a Linux system. You will need to set up an OpenSSH server on a Linux system and make it reachable from the outside world.

As most Linux distributions come with (or make available) a native version of OpenSSH, I will not cover the distribution-specific details of installation from native packages, or the tedious technical details of installation from source. Instead, I will assume that you already have located and installed the appropriate package, and are ready to configure it.

Configuring the OpenSSH Server

You can find the default configuration file for the OpenSSH server in /etc/ssh/sshd_config. This text file contains three types of text: blank lines, comments (which start with a #) and lines of keyword/argument pairs which control the behaviour of the SSH server. You only need concern yourself with the keyword/argument lines, since 'sshd' ignores comments and blank lines.

Even though the default values for the 'sshd' parameters (the values assumed when no options are specified) are usually adequate for general SSH use, I'd like you to explicitly set the values of four of the parameters to ensure proper access.

Specifically, set:

controls whether or not the SSH daemon will permit generic TCP forwarding. This feature permits SSH clients to direct selected network traffic through the SSH tunnel so that it moves securely from client to server (or vice versa). While this parameter defaults to "yes", you may find that the default sshd_config explicitly sets it to "no", and since we wish to ensure that TCP forwarding is permitted, you need to change it.
controls whether or not the SSH daemon will explicitly handle SSH forwarding of X11 data streams. This parameter defaults to "no" and you should explicitly set it to "yes" in sshd_config in order to permit your Windows detainee to easily interact with Linux GUI applications. Note that, with AllowTcpForwarding enabled, X11Forwarding is a convenience, and is not absolutely necessary for X11 tunnelling.
specifies the starting DISPLAY number for SSH-transported X11 datastreams. This parameter should be set to a value higher than the highest 'real' (locally attached) X11 display on the Linux system in order to avoid interfering with the local GUI users. The default value for this parameter is 10, but you may wish to assign a larger value if your Linux system supports a large number of X users.
controls whether SSH should "bind" X11 forwarding to the local loopback address or to the less secure wildcard address. If bound to the local loopback address, some older X11 clients may not function properly. The default for this parameter is "yes" (which is the value I'd like you to explicitly set it to), but your initial configuration file may have a different value.

Of the above four parameters, two are typically unnecessary (since their default values are sane anyway), and one of the other two may be redundant. Specifically, X11DisplayOffset and X11UseLocalhost default to sane values, and you don't need to alter them in sshd_config. Also, AllowTcpForwarding and X11Forwarding offer clients similar abilities; if AllowTcpForwarding is already specified, then it is simple (but not obvious) for a client to perform X11 forwarding without the X11Forwarding option. If X11Forwarding alone has been set, our incoming clients can forward X11 sessions, but not other types of data streams. For your prisoner of Windows, you want to allow access to all network data streams (hence the explicit AllowTcpForwarding setting), and painlessly permit him to forward X11 specifically (hence the explicit X11Forwarding setting).

Once you've made the appropriate changes to your /etc/ssh/sshd_conf file (see Figure 1), save your editing efforts and continue to the next step: generating your SSH server keys.

Generate your Server Keys

Before you can use the SSH server, you must generate encryption keys for it. Some Linux distributions arrange for the first execution of the SSH server to build these keys, but some do not. In the next step, you will explicitly build three sets of keys (one set for each method of conversation that SSH understands), and place these key sets in the standard files.

As root, first execute the ssh-keygen command to build your rsa1 key into the /etc/ssh/ssh_host_key file:

/usr/bin/ssh-keygen -t rsa1 -f /etc/ssh/ssh_host_key -N ''

Next, again execute the ssh-keygen command, but this time to build your dsa key into file /etc/ssh/ssh_host_dsa_key:

/usr/bin/ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key -N ''

Finally, execute the ssh-keygen command for a third time, this time to build your rsa key into file /etc/ssh/ssh_host_rsa_key:

/usr/bin/ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key -N ''

With these commands, you have given your SSH daemon both ears and a voice through which your Windows captive can communicate with the Linux environment.

Start the OpenSSH server

Finally, it is time to start your OpenSSH server, which will use the configuration and key files you've prepared to conduct conversations with the outside world. If your system starts the SSH server automatically, you can just "signal" it to have it reread its configuration. To do this, as root, you

/bin/kill SIGHUP `cat /var/run/sshd.pid`
to send the SIGHUP signal to the currently running ssh daemon. On the other hand, if your system hasn't started the SSH daemon, you (again as root) can run the sshd server with a simple command:

[ In most distributions, launching standard system daemons such as SSH is best done via the scripts in '/etc/init.d'. The above two tasks, for example, would be handled by '/etc/init.d/ssh restart' and '/etc/init.d/ssh start' executed as root. -- Ben ]

Now you have completed the extra Linux configuration, created a secure door to your Linux "safe haven" (did you remember to adjust your firewall to permit SSH connections?), and have taken the first step to freeing that Prisoner of Windows from his oppressive confines. Of course, he will still require an account on the Linux system, which you will need to create with the standard tools and methods of your distribution. Initially, he will log into the Linux system using his assigned username and password; after that, he will have the option of logging in through the secure facilities of SSH.

A foot out the door - Linux Commandline with PuTTY

Fully locked-down Windows systems, where you can't even run unapproved programs from floppy disk or USB "thumb drive" memory stick, are the worst prisons of all. You can't do anything about these dungeons without the co-operation of the Windows system administrator. Let's hope that your Windows captive is not locked in one of these "solitary confinement" cells, and instead has a little latitude in his actions.

The first step to freedom, from the Windows side, is to install the "PuTTY" SSH client application. "PuTTY" is a Windows GUI application written and maintained by Simon Tatham, and released (both in executable form, and in source code) under a BSD-like Open Source license. When installed, it takes about 2800K of disk space, and creates a couple of (not absolutely required) Windows Registry entries. The 6 programs in the PuTTY suite provide the client tools necessary for our captive to escape to our Linux "safe haven".

PUTTY.EXE This is the SSH client program used for "command-line" access to outside systems, and is a secure replacement for the telnet client command.
PUTTYGEN.EXE This program builds client "public key" keys, for use in identifying the client to the ssh server.
PAGEANT.EXE This tool is a "key agent" that allows the Windows user to "make available" selected keys on an ongoing basis to PuTTY applications. This is a convenience (to reduce the number of times a user must enter key information), and not a requirement.
PSCP.EXE An SSH replacement for the rcp client command, and allows secure copying.
PSFTP.EXE An SSH replacement for the ftp client command.
PLINK.EXE This tool is an SSH client program (like PUTTY.EXE) suitable for use in automated processes (like .BAT and .CMD files). Other GUI programs (like Xming) also use PLINK.EXE to establish their own SSH connections to the target systems.

Your first task will be to configure PuTTY to connect to the Linux system. This will be a simple connection that depends on Linux userid and password confirmation; it will not (yet) use the power of SSH public key authorization to give you immediate access to the Linux system. We'll save that little flourish for later.

So, from the Windows Start -> All Programs -> Putty menu, you now run PuTTY. The PUTTY.EXE program will start, and present you with a PuTTY Configuration window. Because this is your first time running PuTTY, and you have not yet set any configuration information, the window will be mostly empty. On the left hand side of the window, you will see a tree of PuTTY options, and on the right, the settings for the "Session" option. It is here that you provide the basic information necessary to get out of Windows and into your Linux commandline.

In the Host Name field, you enter the DNS name or IP address of your Linux system. If you intend to return to this system (and you likely do), you can save the session information by entering a meaningful name in the Saved Sessions entry box and selecting the Save button (see Figures 2a-2c). From this point on, you can launch PuTTY, select and Load the Linux system session information from the list of Saved Sessions, and then select the Open button to immediately connect to the Linux system.

Because PuTTY has never conversed with your Linux system before, it will show you the SSH key that the "Safe Haven" OpenSSH server provided (see Figure 3a), and ask you if you trust it. Answer "Yes" to complete the SSH handshake with the Linux system; if you answer "No" instead, then PuTTY will continue with the handshake, but will not remember that you trust the Linux system, and if you answer "Cancel", Putty will abandon the connection altogether. Once you've completed the handshake, it is now just a simple matter of completing the text-based login with your Linux userid and password (Figures 3b-3d), and you are done; the world of your Linux system awaits you, from a character-based interface (like in Figure 3e).

Free as a Bird - Linux GUI with Xming

But a character-based interface is just "a shadow on the wall" to our GUI-acquainted eyes. While it is possible to use Linux this way, you likely prefer a less limited interaction, and that means X. Your Windows captive will better enjoy his freedom if you install the "Xming" X server, and that is your next step.

Xming is a port of the X.org X11 server to the Windows environment, currently maintained by Colin Harrison who took over from Alexander Gottwald. The latest release of Xming (at the writing of this article) is based on the new X.org X11R6.9 X server, and (like PuTTY) is released under an Open Source licence.

Unlike PuTTY, however, the Xming install comes in multiple parts. There are two different X servers (one that uses OpenGL for rendering, and one that uses the much slower Mesa renderer) and one font package. You will need to install one of the two X servers, and some of the fonts from the font package. The installations are very straightforward, consisting of a familiar graphical installation wizard that takes care of all the mechanics of the install. Configuration (at this point) is not an issue; the basic Xming services that the install builds will suit our needs perfectly. With the OpenGL server and all the fonts (both obligatory and optional) installed, Xming takes about 45Mb of disk space (and a few optional registry entries) to install.

Once you have the X server and fonts installed, you will start a "rootless" Xming server by traversing the Windows Start -> All Programs -> Xming menu, and selecting the Xming option. This will start the X server, and leave you with an Xming icon (see Figure 4) in your toolbar. Xming does not come with the xauth or xhost commands (although they are available through an add-on package), and the X server does not, by default, permit unauthenticated remote applications to connect to and use the display. You could add in some of the add-on toolset in order to get the X authority tools, but instead, I'll show you how to use the X11 forwarding capabilities of your OpenSSH server and PuTTY client to cause your remote X11 applications to appear as local applications to Xming. So, back to PuTTY we go.

When you launch PUTTY.EXE, you are always first presented with a configuration window. When you first connected to your Linux "safe haven", you (hopefully) saved the PuTTY configuration for that connection. You now want to Load that Saved Session, and alter it a bit. If you scroll down the "Category" list on the left hand side of the configuration window, you will come to a leaf titled "X11". Select this leaf and alter its settings (see Figure 5a) to Enable X11 Forwarding. Then, scroll back to the Sessions leaf, and Save the altered definition before you Open the session to your Linux system.

When you do open the session, you will find that, on the Linux side of the connection, the $DISPLAY environment variable is now set (see Figure 5b) to a display on the Linux system. This display doesn't actually physically exist; it is one of the X11 tunnels that you permitted with your OpenSSH X11Forwarding and X11DisplayOffset settings. Your Linux X client applications will interact with OpenSSH instead of a real X server, and OpenSSH (on Linux) will relay the X protocol messages to PuTTY (on Windows), which will then pass them on to your Xming X server running on Windows. The X server will respond in reverse, passing data to your PuTTY client, which passes it back to OpenSSH to be finally handed over to the X client application. It is a round-about route to be certain, but it does provide several advantages:

  1. it provides a layer of security around your GUI traffic,
  2. it gives your X client applications an assured route to your X server, guaranteeing that they will not be stopped by firewall rules or routing problems that may inadvertently block the X protocol traffic, and
  3. it eliminates the need to use the X authorization tools (like xauth or xhost) because, to your X server, all the X protocol traffic is local traffic.

As Figure 5c shows, from this PuTTY and Xming setup, you can run even the most sophisticated X application. Truly, almost any X application that is available to you in Linux can be run this way. Only those applications that require services not implemented in the Xming server (such as OpenGL when you use the Mesa version of Xming) cannot be run this way.

A Person In Danger

With this, I've met my goal and given you the tools to free your Prisoner of Windows. Now, all the facilities of Linux are open to him, command-line and GUI tools alike. He no longer needs to curse the confines of his local working environment, and can luxuriate in the freedom of the remote Linux system.

The I Ching says "A person in danger should not try to escape at one stroke. He should first calmly hold his own, then be satisfied with small gains, which will come by creative adaptations." We've given our Prisoner of Windows a small gain achieved by a creative adaptation. The next steps you take will lead to his complete escape.


OpenSSH SSH server (Linux)

PuTTY SSH client (Windows)

Xming X server (Windows)

X Foundation (X server reference site)

Remote X Apps mini-HOWTO (Linux Documentation Project)

Usenet Newsgroup for SSH matters (the authors of PuTTY sometimes hang out here)

Usenet Newsgroup for general X Window system matters

Usenet Newsgroup for Linux X Window system matters

Talkback: Discuss this article with The Answer Gang

Bio picture Canadian by birth, and living in Brampton, Ontario, I am a career techie working at a major Canadian bank. For over 25 years, I've programmed on all sorts of systems, from Z80 CP/M up to OS/390. Primarily, I develop OS/390 MVS applications for banking services, and have incorporated Linux into my development environment.

Copyright © 2006, Lew Pitcher. Released under the Open Publication license unless otherwise noted in the body of the article. Linux Gazette is not produced, sponsored, or endorsed by its prior host, SSC, Inc.

Published in Issue 129 of Linux Gazette, August 2006

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