Paranoid Penguin - Building a Secure Squid Web Proxy, Part IV
The last thing you need to do is reconfigure Squid to use squidGuard as a redirector and tell it how many redirector processes to keep running. The location of your squidGuard binary is highly distribution-specific; to be sure, you can find it like this:
bash-$ which squidGuard /usr/bin/squidGuard
As for the number of redirector processes, you want a good balance of system resource usage and squidGuard performance. Starting a lot of redirectors consumes resources but maximizes squidGuard performance, whereas starting only a couple conserves resources by sacrificing squidGuard performance. Ubuntu's default of 5 is a reasonable middle ground.
The squid.conf parameters for both of these settings (redirector location and number of processes) are different depending on with which version of Squid you're using squidGuard. For Squid versions 2.5 and earlier, they're redirect_program and redirect_children. For Squid versions 2.6 and later, they're url_rewrite_program and url_rewrite_program.
For example, on my Ubuntu 9.04 system, which runs Squid version 2.7, I used a text editor (run via sudo) to add the following two lines to /etc/squid/squid.conf:
url_rewrite_program /usr/bin/squidGuard url_rewrite_children 5
As with any other time you edit /etc/squid/squid.conf, it's probably a good idea to add custom configuration lines before or after their corresponding comment blocks. squid.conf, you may recall, is essentially self-documented—it contains many lines of example settings and descriptions of them, all in the form of comments (lines beginning with #). Keeping your customizations near their corresponding examples/defaults/comments both minimizes the chance you'll define the same parameter in two different places, and, of course, it gives you easy access to information about the things you're changing.
By the way, I'm assuming Squid itself already is installed, configured and working the way you want it to (beyond blacklisting). If you haven't gotten that far before installing squidGuard, please refer to my previous three columns (see Resources).
Before those changes take effect, you need to restart Squid. On most Linux systems, you can use this command (omitting the sudo if you're already in a root shell):
bash-$ /etc/init.d/squid reload
If you get no error messages, and if when you do a ps -axuw |grep squid you see not only a couple Squid processes, but also five squidGuard processes, then congratulations! You've now got a working installation of squidGuard.
But is it actually doing what you want it to do? Given the filters we just put in place, the quickest way to tell is, on some client configured to use your Squid proxy, to point a browser to http://www.gotomypc.com (a site in the remotecontrol blacklist). If everything's working correctly, your browser will not pull up gotomypc, but rather Google. squidGuard is passive-aggressively encouraging you to surf to a safer site!
squidGuard isn't the only Squid add-on of interest to the security-conscious. squidtaild and squidview, for example, are two different programs for monitoring and creating reports from Squid logs (both of them are available in Ubuntu's universe repository). I leave it to you though to take your Squid server to the next level.
This concludes my introductory series on building a secure Web proxy with Squid. I hope you're off to a good, safe start!
squidGuard home page, featuring squidGuard's latest source code and definitive documentation: squidguard.org.
OpenSUSE's squidGuard page: en.opensuse.org/SquidGuard.
squidGuard 1.2 RPMs for Fedora, CentOS and RHEL from Dag Wieers: dag.wieers.com/rpm/packages/squidguard.
squidGuard 1.4 RPM for CentOS 5, from Excalibur Partners LLC: www.excaliburtech.net/archives/46.
The Debian Wiki's “Rudimentary squidGuard Filtering” page: wiki.debian.org/DebianEdu/HowTo/SquidGuard.
Wessels, Duane: Squid: The Definitive Guide. Sebastopol, CA: O'Reilly Media, 2004 (includes some tips on creating and using a Squid chroot jail).
The Squid home page, where you can obtain the latest source code and binaries for Squid: www.squid-cache.org.
The Ubuntu Server Guide's Squid chapter: https://help.ubuntu.com/8.10/serverguide/C/squid.html.
The Squid User's Guide: www.deckle.co.za/squid-users-guide/Main_Page.
“Building a Secure Squid Web Proxy, Part I” by Mick Bauer, LJ, April 2009: www.linuxjournal.com/article/10407.
“Building a Secure Squid Web Proxy, Part II” by Mick Bauer, LJ, May 2009: www.linuxjournal.com/article/10433.
“Building a Secure Squid Web Proxy, Part III” by Mick Bauer, LJ, July 2009: www.linuxjournal.com/article/10488.
Mick Bauer (email@example.com) is Network Security Architect for one of the US's largest banks. He is the author of the O'Reilly book Linux Server Security, 2nd edition (formerly called Building Secure Servers With Linux), an occasional presenter at information security conferences and composer of the “Network Engineering Polka”.