If you run a website or a blog, you’ve probably trawled through your analytics data, whether that was using the excellent GetClicky, Google Analytics, or the logging system provided by your web host (Awstats, Analog, or Webalizer, for instance). You may even have poured over raw log files at some point or imported them into a desktop program like WLexpert to make some pretty charts of your traffic.

One feature that is useful when you’re designing your blog is to check which browsers your visitors are using. You’ll probably see a list topped by MSIE, Microsoft Internet Explorer, followed by Firefox (the Mozilla browser), then Netscape, Safari, Opera, various newsreaders that access your RSS feed, dozens of other less familiar browsers, mobile devices, and possibly even Google Chrome (now just 0.1% of visitors to Sciencetext).

Some analytics programs will allow you to break down the list of browsers by version number. You’d expect to see the latest versions of Firefox, MSIE, etc featured prominently, perhaps with the previous legacy version still hanging in there too. What you might not expect to see are browser versions that don’t exist. Firefox is currently at version 3.x and MSIE is in beta at 8, but commonly being used at version 7.

However, 2 visitors this month have hit the Sciencetext site claiming to use MSIE version 999.1. Now, I know for a fact this doesn’t exist, it’s not even some future Windows 7 beta version or similar, because people have discussed version 999.1 on forums for years. It’s a spoofed browser identification. Bizarrely, Firefox gets spoofed in this way even more often, with 31 users claiming to be using version 8.1.0. There is no version 8.1.0.

Even that old stalwart of the earliest graphical web, Netscape, gets spoofed with dozens of visitors claiming some odd versions of the program.

So, what’s going on? Well, it’s possible that a few proxies, VPNs and other systems are causing errors in the headers from legitimate browsers visiting Sciencetext, but equally possible is that witty visitors are deliberating changing the user-agent setting in their browser to make it appear is if they are using some odd version of the software. PCTools explains how you can change the user-agent in MSIE, while Johnbokma has an explanation of how it’s done for Firefox. There is even a plugin available on the Mozilla site that you can install to change the Firefox user-agent setting on the fly.

Now, aside from spooking webmasters with odd versions of browsers in their analytics, why might you want to spoof your browser user-agent? Well, there are various reasons:

- First, might be to access a site that only opens properly in one version of a particular browser, without you having to switch browser.

- Secondly, you might want to test how your site responds to lots of different browser headers, again without having to install all those browsers.

- Thirdly, you might want to disguise your browser as a mobile device either to test your own site for mobile compatibility, for instance, or to access resources available only to users of mobile devices (please do keep within the law though).

- Fourth, you might want to pretend to be a search engine spider or bot, to test how a site behaves when confronted by an access from Google and its cohorts.

- Finally, you might be a spammer intent on filling someone’s stats with links to your website. If those stats are inadvertently spidered by the search engines then their links will be spidered from your site and leech some of your pagerank. Nasty buggers, leeches.

User agent switcher is a neat addon for Firefox that allows you to pretend to be Firefox 8, MSIE 999.1 or even Safari 666.