IE 10 send different user-agent header to same page on different domain, one domain is resolved from hosts file

Question

I have a browser identification web page. On that page, the visitors will be asked to modify their hosts file, and add a fake domain and address entry in it.

When IE 10 user using the normal domain (resolved from DNS server) to visit the page, the server got user-agent like the following

Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)

When using the fake domain to visit the page, the server got user-agent like the following

Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.2; WOW64; Trident/6.0)

Because the identification logic need these two user-agent values to be identical, so IE 10 can't pass the identification.

I thought it maybe related the page content which may cause IE 10 use different mode to render the page, but then I realize it shouldn't, because when browser send HTTP request, it does not get response yet.

So, what's the possible reason?

Answer 1

It looks like the "Download updated compatibility list from Microsoft" option & the fake domain caused this behavior together

Turn the option off, then IE 10 does not identify itself as IE 9 for the fake domain anymore.