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A user agent is an application which is used to browse the World Wide Web. Web user agents range from web browsers
to search engine spiders, but also include screen readers and braille browsers which can be used for
people with disabilities.
When internet users visit a web site, users leave, as a visit sign, an information string about their browsers. This
information string is the user agent. Different web browsers and different operating systems leave different user agent strings, and some of the most common are listed below.
Examples
- Internet Explorer 5.5 on Windows 2000: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)
- Internet Explorer 6.0 in MSN on Windows
98: Mozilla /4.0 (compatible; MSIE 6.0; MSN 2.5; Windows 98)
- Safari v125 on Mac OS X: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/124 (KHTML, like Gecko)
Safari/125
- Opera 6.03 on Windows 2000, cloaked as MSIE: Mozilla/4.0 (compatible; MSIE 5.0; Windows 2000) Opera 6.03
[en]
- Opera 7.23 on Windows 98: Opera/7.23 (Windows 98; U) [en]
- Mozilla Firefox 0.8 on Windows XP: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.6) Gecko/20040206 Firefox/0.8
- Netscape 7 on Sun Solaris 8: Mozilla/5.0 (X11; U; SunOS sun4u; en-US; rv:1.0.1) Gecko/20020920
Netscape/7.0
- wget : Wget/1.9
Standards Compliance
Some websites fail to comply to network (IETF) or Web (W3C) standards. Sometimes it is possible to still gain (limited) access to such sites by spoofing the user agent as
being Internet Explorer, for historical reasons. Another reason
web browsers allow users to cloak the real user agent is that many websites use badly written Javascript which locks out all browsers except Internet Explorer or Netscape. To combat this selective lock-out
browsers like Opera and Safari by default identify themselves as another browser, but they will still add the real browser
version in the user agent string (for example at the end). It should be noted that Internet Explorer itself claims to be a
"compatible" version of Mozilla, i.e. Netscape.
As of 2004 most websites are more standards-compliant than they were in
1997 when the 'browser sniffing' as outlined above began, but especially on smaller
non-corporate websites outdated Javascript which will still lock browsers other than MSIE or Netscape is still in use. This is
due to such programmers doing voodoo programming by cutting and pasting older code, without actually thinking
what this does to their website.
External links
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