But what is a User Agent? A User Agent is the client application used with a particular network protocol; the phrase is most commonly used in reference to those which access the Web. Web user agents range from web browsers and e-mail clients to search engine crawlers (spiders), as well as mobile phones, screen readers and braille browsers used by people with disabilities. When Internet users visit a web site, a text string is generally sent to identify the user agent to the server. This forms part of the HTTP request, prefixed with user-agent: and typically includes information such as the application name, version, host operating system, and language. Bots, such as web crawlers, often also include a URL and/or e-mail address so that the webmaster can contact the operator of the bot.

By simply typing about:version into Chrome’s address bar you will be presented with the following information:

Google Chrome
0.2.149.29 (1798)
Official Build
Google Inc.
Copyright © 2006-2008 Google Inc. All Rights Reserved.
Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/525.13 (KHTML, like Gecko) Chrome/0.2.149.29 Safari/525.13

As you can see Chrome’s version information provides limited detail about the browser. The last line is the important one. It is the HTTP User-Agent header:

Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/525.13 (KHTML, like Gecko) Chrome/0.2.149.29 Safari/525.13.

If you know the RFC 2616 specification on the HyperText Transfer Protocol — which incidentally, I gladly don’t — you would know that the User Agent, or more formally, product token, should be short and to the point:

Product tokens SHOULD be short and to the point. They MUST NOT be used for advertising or other non-essential information. Although any token character MAY appear in a product-version, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value).

Clearly this isn’t the case! One of Google’s reason’s behind creating the Chrome browser was to start afresh. It would have therefore been truely amazing if they had made the string simply Chrome/0.2.149.27.

Unfortunately, browser sniffing makes an ever-growing UA string the path of least resistance for browser vendors.

So, what does Chrome’s User Agent string actually mean:

• Mozilla/ – This means that browser has the kind of capabilities that Netscape 1.1 had compared to Mosaic and Lynx.
• 5.0 – This means that the browser engine is from the post-Browser War Web Standards era as opposed to being from the Browser War era.
• (Windows; – This means that general windowing system flavor the browser runs on is Windows (as opposed to, for example, Apple and X11).
• U; – This means that the browser has at least the level of cryptographic capability / encryption strength that U.S. versions of browsers had in the late 1990s.
• Windows NT 6.0; – This indicates the operating system the browser is running on. In this instance, the browser is running on Vista.
• en-US) – This indicates the user interface language of the browser (U.S. English in this case). This may be used to choose between different content languages even though HTTP has a different header for that purpose.
• AppleWebKit/ – This indicates that the engine of the browser is WebKit as opposed to being Gecko. Developers should not do user agent sniffing as a rule, but if they still do, this is what they should be sniffing.
• 525.13 – This is the WebKit version from which Chrome branched its copy. Site admins could use this to detect old versions with known bugs.
• (KHTML, like Gecko) – This introduces the substring Gecko into the UA string while pointing out to human readers that Webkit was forked from KHTML. Without this substring, Chrome might be put in the same category as IE and Netscape 4.
• Chrome/ – This string identifies the browser as actually Google Chrome.
• 0.2.149.27 – This is the Chrome version. This could be used to detect old versions with known bugs.
• Safari/ – This means that the browser is like Safari as opposed to being like Firefox.
• 525.13 – This just repeats the WebKit version in order to have some version but not the irrelevant Safari.app version.