I´m very interested in blocking URLs, too. Not long ago an employee in our firm mailed "know-how-documents" via GMX to a competitor, where he was employed a few weeks later. So he cheated our mailsystem. I tried to block GMX, WEB.DE, ... via spam-filter but had no access, because those freemailers have to many URLs with to many domains.
We need URL filtering so bad for schools I would be happy if someone just had a step-by-step installation of how to put squidgaurd on Astaro. Web interface or not it's badly needed. How about smartfilter?
You can use the acl functions unside the squid.conf-default file. I configured mine to hold domains, domain patterns, & url patterns in files in the /var/chroot-squid/etc/ directory. It works fine on a P2-266 machine. Search the web for squid access control. You'll find plenty of examples. I was able to do it and I'm a linux retard compared to some folks here.