Site news: No referrer, no comment

by Volker Weber

If your personal firewall* blocks referrers, you can no longer post comments to the site. This is another effort to block malicious bots:

RewriteCond %{REQUEST_FILENAME} mt-cmmnt.cgi [NC]
RewriteCond %{HTTP_REFERER} !^http://vowe\.net.*$ [NC]
RewriteRule (.*) - [F,L]

Comment spam bots call cgi without going through the site. For now they are coming in with a blank referrer. Using mod_rewrite it is rather simple to divert them with a 403 return code before they hit cgi, thus avoiding any unnecessary load on the server.

Maybe I will add another rule for those smartass people that send a long string of ++++ instead of a referrer.

*) Technically you are not using a firewall but rather a local proxy server that takes requests from your browser, processes them and then relays the request to the site. Symantec calls the stripping of referrers Browser-Datenschutz in german. No idea how they call this in english.


Hi Volker,
there have been bits and pieces of defense information published on your blog. Do you have a summary? Kind of problem/solution table?
:-) stw

Stephan H. Wissel, 2004-12-20

I guess, that would probably be read by spam bot authors pretty soon and would be a comprehensive compendium on which countermeasures to by-pass to even access a well-protected blog. In order to be one step ahead, this might not be such a good idea.

Ragnar Schierholz, 2004-12-20

I published a summary of anti-spam measures here (in German):

Heiko Hebig, 2004-12-20

I'm using this rule since two weeks, no more spam so far - but hundreds of HTTP 403s in my log file.

Since you have to allow the comment script's URL as a referrer (because of comment previewing), it will be easy for spambots to fake a correct referrer - they just have to insert the script address. Guess it won't take too long until "smart" spambots will exploit that weakness.

Wolfgang Sommergut, 2004-12-20

I guess in English it´s call privacy protection. :-)

Adalbert Duda, 2004-12-20

I can live without previews. Personally, I find them annoying. I proof read in the edit form and I an error slips through, well, that's life.

We could enforce the referer rule by using a regex like


to ensure only requests made from entry archives are accepted.

And to check for either an entry archive or the comment cgi, we could use this (untested, but should work after some testing ;) )

RewriteCond %{REQUEST_FILENAME} mt-cmmnt.cgi [NC]
RewriteCond %{HTTP_REFERER} !^http://vowe\.net/([0-9]+)\.html$ [OR]
RewriteCond %{HTTP_REFERER} !^http://vowe\.net/cgi-bin/mt-cmmnt\.cgi$ [NC]
RewriteRule (.*) - [F,L]

If not this way, mod_rewrite also provides chaining, which also could solve the problem.

Sascha Carlin, 2004-12-20

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