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I can understand that people buy their software, or service or whatever, to make sure their content isn't being stolen (at least that's what their website says), but what does that have to do with testing the "404 response" of my website?
Any idea? Someone afraid I stole their 404 page?
Could they be testing whether you have taken the time to adapt your error page? -perhaps hoping to find a link there to your regular pages? (which will be moast likely called index.html or inde.htm)
Who, in turn can today afford to give a 404 not found when someone simply calls [awebsite.com(...]
Could they in turn try to figure out what webserver you are running, to get a point of attack?
I'm sure some people do that, but digital-integrity is a legit site, offering a legit service (hunting down online copyright infringement), which desn't seem to have anything to do with testing 404 pages, so I was just wondering what the heck this specific spider was up to....