|Anti-spam technique: URL filtering|
|Date of first use:||early 2000|
|Difficulty of implementation:||Medium|
The idea behind URL filtering is that most spams contains URLs to redirect people to some web site. So, it's enough to extract all URLs present in the body of the message and check them against a blacklist. Primitive filters can static flat file blacklists, and the efficiency and drawbacks are the same of a static list of keywords. Most modern filters use URL blacklists stored in DNS zones, as this is an easier way to distribute these lists.
As long as URLs found in spams change very frequently, the maintenance of this kind of blacklist is a hard work and, most of the time, use a lots of spamtraps to collect spams.
Efficiency of URL filtering is usually something between 50 % and 70 %. False positive rate can be as low as 0.1 %, but some lists are more agressive, and can present a higher false positive rate.