Why can't I find this agent string: Mozilla/5.0 (compatible; MSIE 6.0; Windows NT 5.0)

I was looking through my sites logs and by far the most popular useragent was: Mozilla/5.0 (compatible; MSIE 6.0; Windows NT 5.0) Being new at this I googled it but, other than other peoples stat pages, it's not explained anywhere in that particular combo Mozilla/5.0 a开发者_JAVA百科nd NT 5.0. Plenty of other combos but not this one. Why is this useragent so unique?

This string belongs to Thunder Downloader (aka Xunlei), a Chinese download manager.

One of the numerous nasty things it does is that it provides direct download link to resources.
For example, Anna visits your page and downloads justin-bieber-baby.mp4 with Xunlei. Xunlei saves the file url and hash value on its server.
Next day, Betty downloads B~A~B~Y.mp4 from another site using Xunlei, Xunlei also saves the file url and hash value. Now Xunlei knows these two files are identical.
On the third day, when Cathy downloads B~A~B~Y.mp4 from that site, Xunlei also downloads from your site as an accelerator.
That means, Cathy doesn't visit your website, yet you'll have to provide bandwidth for her.
Imagine a popular file (~100MB) which millions of people^*1 want to download. It could cost you ~100GB bandwidth but brought to you only one visitor.

So imho it's better to block this user agent.

_{*1: Xunlei has ~400 million users by 2012.}