Use-agent
See also: Agent
A to use agent is a customer application used with a protocol particular network; the expression is more generally employed as reference for those which reach the World Wide Web. User Agents of the Web goes from the range of the navigator S to the robots of research, while passing by the readers of screen or the navigators Braille for the people having a Handicap.
When a Net surfer visits a Web page, a chain of the text type is generally sent to the waiter to identify the agent of the user. It is included in the request HTTP, with like Préfixe " Use-agent: " or " To use-agent" and it gives information such as for example: the name of the application, the version, the operating system, the language.
The robots of research often include a URL and/or an email address if the webmaster wants to contact the operator of the robot.
The chain " To use-Agent" one of the criteria is used to exclude a certain number of pages or part of a Web site by using the " Robots Standard Exclusion " ( robots.txt ).
This allows the webmasters which estimate that certain parts of their Web site should not be included in the data collected by a robot in particular, or that a robot in particular exhausts too the band-width, to invite it not to visit these pages.
Mystification of Use-Agent
At various times of its history, the use of the Web was dominated by a navigator insofar as many sites were conceived for to function with this one in particular, rather than to follow the standards such as those of the W3C and IETF.At the level of the code of these sites, a part is which is dedicated to the detection of the navigator. According to what this part returns, the site turns over different contents, often poorer, less complex with the less popular navigators whereas they could treat this one correctly. In extreme cases, there exist also sites completely inaccessible.
To counter these practices, the various navigators masked or diverted the chain, in order to be identified like another thing on the level of the code of detection; often, the true identity of the navigator is included more far in the chain.
The oldest example is that of Internet Explorer which uses one
chain Use-Agent starting with " Mozilla/
Opera and Safari, of the rivals of Internet Explorer then put
in place of the systems where the user can choose a false chain
Use-agent to send it. Some, for example like Safari, reproduce
entirely the chain Use-Agent; others, as Opera reproduce
the chain Use-Agent but add the true name of the navigator to sound
end. This last approach, naturally, leads to a chain
containing three names and versions: In first, the chain Use-Agent claims to be
" Mozilla" (Netscape Navigator); then MSIE (Internet Explorer) and finally
the true navigator " Opera".
With share navigators, other programs using the protocol HTTP,
like the tools of remote loading or the navigators in differed mode, have
capacity to change the chain Use-Agent sent to the waiters. This for
to be accepted by waiters which refused to serve these programs very of
continuation, or at all because they are considered most of the time
like person in charge of the overload.
This vicious circle is not close stopping in the field of the navigators
Web. Some developers using the standards began a campaign
entitled " With Any Browser" to encourage the creators of Web page to respect the official standards, and not the characteristics of the navigators.
In 2005, much of Web sites became in conformity with the standards than before.
A result of the mystification of the chain Use-Agent is that the share of
gone of the navigators like Opera and Safari is often minimized, whereas
the market share of the more popular navigators like Mozilla Firefox and Internet
To explore is often exaggerated.
Versions of Before Browser: ELinks 0.4pre5 under Linux - ELinks (0.4pre5; Linux 2.4.27 i686; 80x25) Versions of Firefox: Versions of Internet Explorer: Versions of Konqueror: Versions of Links: Lynx 2.8.4rel.1 under Linux - Lynx/2.8.4rel.1 libwww-FM/2.14 Minimo : Versions of Mozilla SeaMonkey: Versions of Netscape Navigator (owner): Versions of Netscape (based on code not-owner divided on the level of the base by Mozilla): Off By One 3.5a under Windows XP - Mozilla/4.7 (compatible; OffByOne; Windows 2000) Versions of OmniWeb: Versions of Opera: Versions of Pocket Internet Explorer: Versions of Safari:
Netscape Navigator used these two letters to specify the level of coding, because the US government did not authorize one
coding higher than 40 bits outside the USA. " U" for the " USA" (version with coding with 128 bits) and " I" for the International one
(the navigator has a coding of 40 bits and can be used everywhere in the world). At the origin, the version " U" was authorized and downloaded
only from the USA, but since the government stopped this policy and raised coding now authorized (see
Export off cryptography for more information), it is not obligatory any more to have two versions.
Netscape and Mozilla distributes their navigators only in the version " U" ,
since an international version is not required any more.
This text is included in the to use agent Internet Explorer to specify which framework Microsoft .NET was installed on
the computer. One can often find in the chains if it is about Windows XP or of Windows Server 2003.
Site referring all UserAgentStrings
Example of chain use-agent
Navigators
Robots
Letter " U" /" I"
.NET CLR
See too
Related article
External bonds
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