Anatomy of user-agent strings

By: Seb Harvey, seb@omegasoft.co.uk

User-agent strings offer web administrators an insight into the setup of a visitors computers. Information such as web browser type, version, operating system are among key pieces of information made available.

A user-agent string is sent from your web browser via a specialised header within the HTTP response. This string adopts the following format (these strings may vary slightly):


Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727)

Breaking this down into its component pieces;

Mozilla application name

4.0 application version

compatible compatibility flag

MSIE 6.0 version token, identifies the browser name and version

Windows NT 5.1 platform token, identifies the users operating system

In addition to these tokens, other tokens may appear. SV1 identifies Windows XP users who have the Server Pack 2 installed. .NET CLR identifies that the Microsoft .NET framework is installed and the version of it.

You can view your own user-agent string by typing (or copy & pasting) the following case-sensitive JavaScript prompt into your web browsers address bar:


javascript:alert(navigator.userAgent)

Microsoft uses the following tokens for its Internet Explorer web browser:

MSIE 7.0 Internet Explorer 7

MSIE 7.0b Internet Explorer 7 (Beta 1 pre-release only)

MSIE 6.0 Internet Explorer 6

MSIE 6.0b Internet Explorer 6 (pre-release)

MSIE 5.5 Internet Explorer 5.5

MSIE 5.01 Internet Explorer 5.01

MSIE 5.0 Internet Explorer 5

MSIE 5.0b1 Internet Explorer 5 (pre-release)

MSIE 4.01 Internet Explorer 4.01

And operating systems:

Windows NT 6.0 Windows Vista

Windows NT 5.2 Windows Server 2003; Windows XP x64 Edition

Windows NT 5.1 Windows XP

Windows NT 5.01 Windows 2000, Service Pack 1 (SP1)

Windows NT 5.0 Windows 2000

Windows NT 4.0 Microsoft Windows NT 4.0

Windows 98; Win 9x 4.90 Windows Millennium Edition (Windows Me)

Windows 98 Windows 98

Windows 95 Windows 95

Windows CE Windows CE

Note: Some internet users may alter their user-agent string.

Web bots, such as OmegaSoft’s PathSearch Fifobot and Google’s Googlebot contain their own user-agent strings to help identify their applications.


PathSearch Fifobot 2.1 BETA (http://pathsearch.omegasoft.co.uk/fifobot)

These may contain the bot name, version number, web address and/or email address.

Developer Comments

Paul Atkinson
There, are of course a whole host of other user agents for our Mac based browers. Most of them are quite self explanitory. A Mac user will no doubt be using Safari or Firefox. Internet Explorer for Mac depreciated at v5.