The User-Agent (UA) field contains information about the application type, operating system, software vendor, and version of the requesting system. User-Agent helps web servers know what content to send in response to a general request; for example, whether they should be the desktop or mobile versions, what scripting is supported, and which image types are available.
The UA string is normally in the following format:
Mozilla/<version> (<system-information>) <platform> (<platform-details>) <extensions>
Mozilla is common to all common browsers now, since browser engines are generally an evolution of Mosaic Netscape, or designed to emulate it (in the case of Internet Explorer-derived browser engines).
The system information generally lists the operating system, and in later browsers there are also device details. The platform is the engine that's being used, as well as any qualifiers (see the following examples). The extensions specify any enhanced features that are available (such as add-ons in Chrome):
| User-Agent string | Description |
| Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0 |
Mozilla Firefox 47 on a computer running 64-bit Windows 7 (Windows NT 6.1). The layout engine is Gecko, which is designed and built by Mozilla. 20100101 indicates that this is running on a desktop (non-mobile device). |
| Mozilla/5.0 (iPhone; CPU iPhone OS 10_3_1 like Mac OS X) AppleWebKit/603.1.30 (KHTML, like Gecko) Version/10.0 Mobile/14E304 Safari/602.1 |
Safari 10 on an Apple iPhone running iOS 10.3.1. Available features are all those supported by AppleWebKit, which operates like Gecko (the Mozilla Firefox engine). |
| Mozilla/5.0 (Linux; U; Android 4.4.2; zh-cn; GT-I9500 Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko)Version/4.0 MQQBrowser/5.0 QQ-URL-Manager Mobile Safari/537.36 |
QQ Browser on a Samsung Galaxy S4 (GT-I9500) running Android 4.4.2 (KitKat). The Layout Engine is AppleWebKit. |
| Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_4) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17 |
Safari 9.1 running on macOS X. The layout engine is AppleWebKit. |
| Mozilla/5.0 (compatible; MSIE 9.0; Windows Phone OS 7.5; Trident/5.0; IEMobile/9.0) |
Internet Explorer Mobile 9 on a Windows Phone 7.5. The layout engine is Trident, which is a Microsoft proprietary engine that has been discontinued. |
| Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 |
Thunderbird 45.8 on a computer running 64-bit Linux in the GUI (X11). The layout engine is Gecko, with 20100101 indicating that it is running in desktop mode. |
It is important to look for unexpected strings in the traffic (for example, an IE UA string on a system that only has Chrome installed). This could show that another application is masquerading as Internet Explorer (in order to access more complex content features).