Blocking old user agents

I’ve been looking through awstats and the logs of my blog today after talking about that with Petteri the other day. And I noticed quite a few interesting thing in the list of browser versions.

Beside hits reporting “Firefox/8.10” as version (because of most likely a broken packaging in Ubuntu that reports as user agent “Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/2008101315 Firefox/8.10 (intrepid) Firefox/3.0.3”, I got a fair number of pre-2 versions of Firefox, as well as pre-5 versions of Internet Explorer and Netscape. and Firebird/Phoenix branded browsers.

A rapid check shows that stuff like “Firefox/0.10.0” is just spammers. so this is giving me an idea: what if I modify the blog so that comments result disabled if the user agent is too old? Or a known spammer one, or an RSS reader (which cannot leave comments)? Optionally it could reject requests without an User-Agent field too.

Now, I know this is not going to be free of false positive since there are people out there who think that the whole User-Agent header is ruining their privacy and thus intentionally remove or make it invalid. I sincerely don’t give a crap. I don’t see how User-Agent is a privacy invasion when it’s needed for proper technical reasons.

So anyway, does anybody know if there is anything like this already or if I should be starting from scratch?