Like so many other bloggers who allow comments on their websites and blog articles, I was facing increasing comment spam as my blog got noticed by more spammers. The size of this problem is illustrated by this Google query for "comment spam" that returned 1.5 million hits. For the uninitiated comment spam is like email spam for blogs; the spammer inserts fake comments in a blog where either the comment text contains from one to dozens of links to the spammer's websites. When web search sites "spider" the blog the links to the spammer's site are treated as "endorsements" of the spammer's sites and the spammer's sites are raised to the top of the search site's result lists.

There is another growing type of blog spam called referer spam (yes it is officially misspelled). When a web surfer clicks on a link in a web page that sends them to another web page most web browsers fill in the URL of the referring page into the request called the HTTP_REFERER. Some websites and blogs capture that page link information when they are on the receiving end of a web request. These sites might have a section on each page indicating the sites that link to that page. These links are referer links.

Referer spam uses the same mechanism as comment spam to raise the search sites ranking of the spammer's websites. But referer spammer's don't post comments; they post fake referrals to a website. The are hoping that the website or blog displays links of the sites that refer to them. So when the website is spidered the search ranking is raised.

Blosxom Plugins Addressing Spam

Like so many bloggers once I started getting comment spam I was able to manually delete them as they occurred. But that got old fast. After some Googling I discovered Doug Alcorn's Blosxom writeback blacklist plugin. I had been using the original writeback plugin. Doug's improvements provided enough protection (so far) with less than a dozen regular expressions removing all my comment spam.

Referer spam started hitting me three weeks ago. What was most infuriating was that I don't display any links of referring sites on my site at all. So all these spammers were succeeding in doing was skewing my site statistics and using my bandwidth with their fake referer attacks several times a day.

Of course all the referer spam site's addresses contained one or more of the same dozen blacklisted words I had already configured for comment spam. About the same time I saw Jason Clark post his deferer plugin to return a 301 permanent redirect for the IP address of one particular referer spammer who was attacking his site. I thought that by combining Doug's blacklisting plugin with Jason's immediate redirect plugin I could reduce the referer spam from my log files.

This plugin hasn't removed the entries entirely from my logs, since the initial request is still logged with a 301 status. But it has stopped the subsequent downloading of images for the pages whose content is now not served. Now that the log contains 301 status messages for these requests they are ignored by my host's statistics program (Advanced Web Statistics - AWStats).

On a re-reading of Jason's blog entry for deferer he also mentions the idea of white and black lists for referer filtering. So I might have subliminally remembered his idea and implemented my refererblock plugin based on his idea. In any event, I fully credit Jason and Doug for giving me the ideas and code with which to put together my plugin.

Referer Block Plugin

The refererblock plugin's tar file can be downloaded here. It contains the refererblock plugin already named 000refererblock so that it runs before all other plugins (you want to discard the blacklisted requests before all legitimate requests). A sample blacklist.txt file is provided and contains some example regexs. It uses the same blacklist file format and file name as Doug's writeback modification (I took the code from his plugin with only cosmetic changes). See Doug's website for links to the Movable Type and other blacklists.

The only configuration variable you can set is $log_blacklisted. If set to a full path file name the script logs the UTC date/time, referer string, and the page to which they were referring. You could use the frequency of words in the rejected referer strings to fine tune the content and ordering of your blacklist.txt file to match the spammers hitting your site. Be aware that this file isn't trimmed so you might want to keep an eye on its size.

Lastly, the zip file contains a simple Perl script you can use to test the plugin. Execute it as:
where the first URL is your website and the second URL is the referer

to be sent in the request. This script uses HTTP::Request to send the request. The script returns the status "200 OK" for the requested page if it isn't blacklisted and "301 Moved Permanently" if the referer is blacklisted.

Please let me know if you use this plugin or if you have comments or suggestions for improving it.