From TheBestLinks.com
A user agent is an application that 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 that can be used for people with disabilities.
When Internet users visit a web site, users send, as a visit sign, an information string about their browsers. This information string identifies 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
- 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
- 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]
- Netscape 7 on Sun Solaris 8: Mozilla/5.0 (X11; U; SunOS sun4u; en-US; rv:1.0.1) Gecko/20020920 Netscape/7.0
- Safari v125 on Mac OS X: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/124 (KHTML, like Gecko) Safari/125
- 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)
- wget : Wget/1.9
Standards compliance
Some web sites 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, for historical reasons, Internet Explorer.
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 Navigator. To combat this selective lock-out, browsers such as Opera and Safari by default identify themselves as another browser but still add the real browser version in the user agent string (usually 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. However, outdated Javascript, which will still lock out browsers other than MSIE or Netscape, is still in use, especially on smaller, non-corporate websites. 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
ja:ユーザーエージェント
nl:User Agent
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