The user-agent header

As mentioned in our overview of HTTP requeset headers, the user-agent header specifies information about the browser or robot that made the request. Here are some examples of user-agent strings:

1. Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 1.0.3705)
2. Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en) AppleWebKit/418.9 (KHTML, like Gecko) Safari/419.3
3. Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9) Gecko/2008052906 Firefox/3.0
4. Opera/9.27 (Windows NT 5.1; U; en)
5. Opera/9.50 (J2ME/MIDP; Opera Mini/4.1.11320/534; U; en)
6. BlackBerry8100/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/125
7. T-Mobile Dash Mozilla/4.0 (compatible; MSIE 4.01; Windows CE; Smartphone; 320x240)
8. msnbot/1.1 (+
9. libwww-perl/5.800

As you can see, there is some variety in the format of user-agent headers. However, the following generally apply:

Common client strings in the user-agent header

For reference, the following commonly occur in the non-bracketed section of the user-agent string. I've indicated the approximate frequency of given agents in requests to my server (taken over a sample of requests from approximately 12,000 unique clients). Obviously, these figures could differ according to the target audience of your site.

Agent name% occurrenceTypeExplanation
Mozilla99.5 %Compatibility

Essentially meaningless nowadays. Originally, this agent name denoted either a version of Netscape or the version of Netscape with which another agent wanted to claim compatibility. Nowadays, even the most trivial home-made bot tends to include Mozilla/4.0 or Mozilla/5.0 at the start of the user-agent string.

Gecko24.8 %Compatibility

Gecko is the rendering engine used by Firefox and a handful of other Mozilla spin-offs such as SeaMonkey. In principle, you could take notice of this field to alter page contents to work around some rendering bug, or offer enhanced content if you were aware of some feature that Gecko rendered better than other engines. In practice, I suspect practically all servers ignore this field.

The Safari web browser amusingly includes the string (KHTML, like Gecko) in its user-agent string.

Firefox24.4 %Browser

Identifies the Firefox web browser.

Safari7.0 %Browser

Apple's Mac OS browser. More information on the user-agent string sent by Safari is given in Apple's Safari FAQ.

AppleWebKit7.0 %Compatibility

What Apple describe as their "web technology framework", used by the Safari browser.

Version6.1 %Compatibility

Used by version 3.0 or greater of the Apple's Safari browser to indicate the "Safari family version number" (see entry for Safari above).

msnbot< 1 %Robot

Microsoft's search bot for their MSN Live Search engine.

Profile< 1 %Compatibility

Sent by some Java-copmpatible mobile devices to indicate which Java Mobile Information Device Profile (MIDP) they support. A typical specification would be Profile/MIDP-2.0. BlackBerries and Nokia and Sony Ericcson mobile phones (possibly among others) send this tag.

Configuration< 1 %Compatibility

Sent by some Java-copmpatible mobile devices to indicate which version of the Java Connected Limited Device Configuration they support.

BlackBerryXXXX< 1 %OS / Browser

BlackBerry devices appear to send as their browser/OS identifier a string that includes the model number (e.g. a BlackBerry 8310 with OS version 4.3 would send BlackBerry8310/4.3.0).

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Editorial page content written by Neil Coffey. Copyright © Javamex UK 2021. All rights reserved.