javascript

Chapter 9: Client Detection
With the release of Opera 8, the language part of the user - agent string was moved inside of the
parentheses to better match other browsers, as follows:
Opera/ Version ( OS-or-CPU ; Encryption ; Language )
Opera 8 on Windows XP yields the following user - agent string:
Opera/8.0 (Windows NT 5.1; U; en)
By default, Opera returns a user - agent string in this simple format. Currently it is the only one of the
four major browsers to use the product name and version to fully and completely identify itself. As with
other browsers, however, Opera found problems with using its own user - agent string. Even though it ’ s
technically correct, there is a lot of browser - sniffing code on the Internet that is geared toward user - agent
strings reporting the Mozilla product name. There is also a fair amount of code looking specifically for IE
or Gecko. Instead of confusing sniffers by changing its own user - agent string, Opera identifies itself as a
different browser completely by changing its own user - agent string.
As of Opera 9, there are two ways to change the user - agent string. One way is to identify it as another
browser, either Firefox or IE. When using this option, the user - agent string changes to look just like the
corresponding one for Firefox or IE, with the addition of the string ” Opera ” and Opera ’ s version
number at the end. Here ’ s an example:
Mozilla/5.0 (Windows NT 5.1; U; en; rv:1.8.1) Gecko/20061208 Firefox/2.0.0 Opera 9.50
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; en) Opera 9.50
The first string identifies Opera 9.5 as Firefox 2 while maintaining the Opera version information. The
second string identifies Opera 9.5 as Internet Explorer 6 and includes the Opera version information.
Although these user - agent strings pass most tests for Firefox and IE, the possibility of identifying Opera
is open.
Another option for identifying the browser is to mask it as either Firefox or IE. When masking
the browser ’ s identity, the user - agent strings are exactly the same as would be returned from the
other browsers — the string ” Opera ” does not appear, nor does any Opera version information. There is
literally no way to distinguish Opera from the other browsers when identity masking is used. Further
complicating the issue is Opera ’ s tendency to set site - specific user - agent strings without notifying the
user. For instance, navigating to the My Yahoo! site ( http://my.yahoo.com ) automatically causes
Opera to mask itself as Firefox. This makes identifying Opera by user - agent string very difficult.
Before version 7, Opera could interpret the meaning of Windows operating - system
strings. For example, Windows NT 5.1 actually means Windows XP, so in Opera 6,
the user - agent string included Windows XP instead of Windows NT 5.1. In an effort
to be more compatible with other browsers, version 7 started including the officially
reported operating - system version instead of an interpreted one.
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