Exclusive AVG has rejiggered the fake traffic it's spewing across the internet, causing new headaches for the world's webmasters.

In late February, AVG paired its updated anti-virus engine with a real-time malware scanner that vets search engine results before you click on them. If you search Google, for instance, this LinkScanner automatically visits each address that turns up on Google's results page.

According to the company, more than 20 million people have downloaded the new AVG 8, and this has caused a huge up-tick in traffic on sites across the web, including The Register. Because the scanner attempts to disguise itself as a real live human click, webmasters who rely on log files for their traffic numbers may be unaware their stats are skewed. And others complain that LinkScanner has added extra dollars to their bandwidth bill.

Daniel Brandt, who runs Wikipedia Watch, estimates that LinkScanner traffic to the site has outstripped legitimate clicks by nearly ten times. In this graph, the pink line represents suspected LinkScanner scans, the blue line legitimate clicks:

LinkScanner meets Wikipedia Watch

When we first told the tale of AVG's fake traffic earlier this month, we pointed out that if webmasters were wise to the problem, they could filter LinkScanner visits from their log files. Each scan left a unique user agent: "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;1813)."

But over the weekend, the company changed this user agent on the for-pay version of AVG 8. It appears that scans now use these agents as well:

Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;1813)

Judging from the log files of two separate web sites, including Wikipedia Watch, the first agent is by far the most common. Which is bad news for webmasters. That's also the Internet Explorer 6 user agent. Unlike the other two - and the original "1813" agent - it's a perfectly valid agent that may turn up with real clicks.

AVG's chief of research Roger Thompson says the for-pay LinkScanner is only using the IE6 user agent. Presumably, the company believes this is more likely to fool malware exploits. "There are still ways for concerned web masters to filter LinkScanner requests out of their statistics," he told us over email. But he did not divulge these methods and did not say whether they might clip legitimate traffic as well.

Many webmasters may have no choice but to abandon log file analysis, adopting alternative tools from companies like Google, Yahoo!, comScore, or Nielsen NetRatings. And these tools have their drawbacks. comScore's service tends to underestimate traffic from daytime work machines. And if you go with Google Analytics, you have to tag your pages with JavaScript - and share your traffic numbers with Google.

Plus, these tools won't solve the bandwidth issue.