Google maintains the Google Safe Browsing databases, and an API that allows us to check URLs against Google’s lists of malware and phishing sites.

The gsb filter checks the URL that is being monitored, plus it creates a list of links from the page and checks each of them against the Google list.

By the time your URL appears on either of these lists, you’re already sunk because Firefox and Chrome browsers will be throwing up alert messages for visitors to your site. So our intent is to catch any links to malware sites before your site is flagged.

Note: The gsbdaily filter does a bit more thorough analysis once a day.