Unlike the tools we've discussed so far, DansGuardian relies on a separate proxy server (usually Squid) to function. Many proxies offer some manner of filtering capabilities themselves, so choose carefully.
Apart from blocking the list of specified URLs, DansGuardian also supports matching words and phrases in the page content and filtering based on file and MIME types. Also, like WebCleaner, it too can work with ClamAV to protect users from viruses, and remains one of the most trusted content filtering tools on offer after years in business.
With the extensive official and unofficial documentation, wikis, mailing lists et al, configuring DansGuardian and tweaking it to best serve your environment shouldn't be a problem, if you're willing to invest some time.
DansGuardian is very peculiarly set up, and with the default configuration you're bound to run into more false positives then you'd expect. Browse into the /etc/dansguardian/lists/ directory and go over each of the files in each of the subdirectories. Thankfully, the files are well commented and you won't be lost.
There are lists that cover all sorts of offensive material including sex and nudity, violence and gambling. You can easily configure DansGuardian to prevent users from downloading certain types of files, either by using a whitelist or blacklist.
The lists directory is a massive collection of banned items, and there's a corresponding exceptions file for most. Tread carefully here, or you can easily render the system useless – whether it's because of an overzealous content filter or one that's too lax depends on how well you perform surgery on the configuration files.
Price: Free under GPL
Takes time to configure, but can handle all types of material and is ideal for all environments.
Although still in its infancy, Gnome Nanny has already attracted much attention. In addition to blocking content, you can even time-restrict the users' browser, chat and email tools, effectively controlling how many hours they spend on the internet each day.
It's the only tool on our list with a graphical interface independent of a browser and designed especially to serve as a parental control system. Once installed, you can launch Nanny by clicking System > Administration > Parental Control.
From the PC Use Time tab of the Nanny Admin Console window, you can restrict the computer access of logged in users to a specified number of hours or even split the hours throughout the day if you want. There's a similar option under each of the remaining three tabs that let you limit the number of hours users can spend browsing using the mail client or on the IM clients.
Clicking the Use Web Content Filtering checkbox under the Web Browser tabs enables filtering, and you can click on Configure to define some of the sites you wish to block.
You can also download the Nanny blacklist from here. Unfortunately, Nanny comes with very limited documentation, and although the tool is easy enough to use, the lack of any documentation mentioning how to create your own blacklist, or adapting a blacklist to the Nanny format, is disappointing.
The blacklist is made up of many categories. You can choose to enable only certain categories, but Gnome Nanny doesn't let you examine any of the rules or modify them in any fashion.
Gnome Nanny 2.29.3
No documentation, impossible to create new blacklists or modify others