Add Web Porn Filtering and Other Content Filtering to Linux Desktops
Ubuntu comes with Firefox as the preferred client browser, so the instructions here are specific to Firefox. Other client browsers will likely have similar capabilities and documentation to show how to mimic these instructions.
This last installation step points the browser at port 8080, so it sends data only through DansGuardian and Tinyproxy. With Firefox, go to Edit→Preferences→General tab→Connection Settings to see the screen shown in Figure 1. As shown, select manual proxy configuration, enter localhost and port 8080. This assumes you are going to install and use DansGuardian and Tinyproxy on every workstation. If you set up DansGuardian and Tinyproxy on a separate server, then you need to enter the name or IP address of the server machine that runs DansGuardian and Tinyproxy instead of the word localhost in the HTTP Proxy: line.
Restart your browser and test how well the filter works.
When testing the new filter, you should see an access denied screen similar to the one shown in Figure 2. Before going any further, it's a good idea to look for problems you may find with the default filter settings. For example, I often download .tar and other executable files. The default configuration file stops these files from download. To fix this problem, you need to edit the bannedextensionlist.txt file, and place a # to comment out the file extensions you want to let through the filter.
To be thorough, you should look through all default configuration .txt files with DansGuardian to tailor how you want the filters to react. You won't know all the situations you'll run into at first, but this is a good opportunity to gain an understanding of this application's powerful features.
No system is perfect, and there are several obvious ways to defeat DansGuardian and Tinyproxy. The most noteworthy is how easily users can bypass the proxy and filters. Without further protection, a user can restore Firefox's preferences back to Direct Connection, which bypasses DansGuardian and Tinyproxy. Once reversed, users have unrestricted access to the Internet.
However, there are more ways to secure the DansGuardian filters further by forcing all communication with the Internet through port 8080. A link on the DansGuardian documentation Web page explains a well-thought-out method of using FireHol to force this condition on all Internet thoroughfares (see Resources).
For the novice user, an easier approach is to set up a filtering plan that includes restricted user privileges, locked browser preferences and making sure the proxy filters start each time the computer reboots.
For test purposes, I created a new user account on Ubuntu Dapper Drake (Figure 3). Using the checklist features, I severely limited the capability of the user test. Although these privileges could be just right for anyone who has no computer experience or who is plainly not trustworthy. Utilities like update-rc.d and fcconf define certain programs to start at the system boot. I used a bootup manager called BUM to make DansGuardian and Tinyproxy start at each boot.
Finally, I decided to lock down the preferences of Firefox. Restricting Firefox's preferences is not as difficult as it may sound. An older copyrighted article titled “HOWTO Lock Down Mozilla Preferences for LTSP” by Warren Togami (see Resources) describes how to carry this out in great detail. Although, I didn't want to mess with byte shift coding to achieve similar results.
After rummaging through Mozilla.org's Web site, I chose to add lockPref statements to my Firefox configuration file to keep users from changing connection settings. I edited the file /usr/lib/firefox/firefox.cfg to appear as the one shown in Figure 5. The last three lines force a manual proxy selection on localhost, port 8080. After saving this file and restarting Firefox, you can't reset the connection settings. Further, other users without administrative privileges could not quickly change the settings and bypass the filters.
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