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1. YOU!!! CCBot/2.0 (http://commoncrawl.org/faq/)
2. Mozilla/5.0 (compatible; AhrefsBot/5.0; +http://ahrefs.com/robot/)
3. Mozilla/5.0 (X11; Linux x86_64; rv:21.0) Gecko/20100101 Firefox/21.0
4. Mozilla/5.0 (compatible; Yahoo! Slurp; http://help.yahoo.com/help/us/ysearch/slurp)
5. Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36
6. Mozilla/5.0 (compatible; AhrefsBot/5.0; +http://ahrefs.com/robot/)
7. Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
8. Mozilla/5.0 (compatible; AhrefsBot/5.0; +http://ahrefs.com/robot/)
9. Mozilla/5.0 (compatible; AhrefsBot/5.0; +http://ahrefs.com/robot/)
10. Mozilla/5.0 (compatible; AhrefsBot/5.0; +http://ahrefs.com/robot/)
A user agent (or user-agent/useragent) is a particular name assigned to your browser (application you use to browse on Internet). A user agent contains vital information that identifies itself, the operating system is running on, name of software vedor, or software revision. This information is sent when a browser operates in a network protocol.
A website needs to know who you are, looking for a browser user agent. This information is sent with each and every request from your browser. This information can be used to provide different information back to the browser. For example, if you visit a website on a mobile device, it can show you smaller images and remove some areas that would be too wide for a mobile device to show. A user agent tells us which browser you are using. This way we know what are your's browser capabilities and what is supported and what not.
A user agent can be easily spoofed. Why? Because it can be modified from client side, from any major browser. Most browsers offer add-ons that can spoof it. Find out more information at Change My User Agent. Why do you need to spoof your user agent? Most websites show relevant information based on your user agent. If you want to see that hidden information you have to spoof your user agent.
If you want to show relevant information to your visitor, you have to sniff the user agent. For example, you can show different content for Internet Explorer or Firefox nerds. Also, user agent sniffing is a very good technique for mobile supported websites. If you have a new website and you want to implement the latest technology, but at the same time support older browsers, user agent sniffing is a very good technique.