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User agent


A user agent is a string of text that is sent by a web browser to a web server to identify itself and provide information about the browser's capabilities. The user agent string can include information such as the browser type and version, the operating system, and the device type.[1]


Web servers use the user agent string to determine how to serve content to the browser. For example, a web server may use the information provided in the user agent string to serve a different version of a website to a mobile device than it would to a desktop computer.

The user agent is transmitted to a web server through the HTTP header of a request made by a web browser.

When a web browser makes a request to a web server (for example, to load a web page), it sends an HTTP request message to the server. This message includes an HTTP header, which contains a variety of information about the request. One of the pieces of information in the HTTP header is the user agent string.

Here is an example of an HTTP request message with the user agent string in the HTTP header:

GET /index.html HTTP/1.1

Host: www.example.com

User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.79 Safari/537.36

In this example, the user agent string is Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.79 Safari/537.36.

The web server receives the HTTP request message and reads the HTTP header, including the user agent string. It can then use this information to determine how to serve content to the browser.

Types of user agents

There are many different types of user agents, including:

  • Web browsers: These are the most common type of user agents. Examples include Google Chrome, Mozilla Firefox, and Microsoft Edge.
  • Mobile apps: Many mobile apps act as user agents, making requests to servers and displaying the responses to the user.
  • Desktop applications: Some desktop applications, such as email clients and chat clients, act as user agents when they communicate with servers.
  • Bots and crawlers: These are automated user agents that are often used for tasks such as web indexing[2], site monitoring, and data mining.
  • Internet of Things (IoT) devices: These are user agents that represent devices connected to the Internet, such as smart thermostats, security cameras, and home appliances.
  • Custom user agents: Some user agents are designed specifically for a particular purpose and are not generally available to the public. For example, a company might develop a user agent to perform automated testing of its website.
    user agent settings in Rank Tracker

    The image above shows how you can set up custom user agents to control rank tracking in the Rank Tracker tool[3].


User agents play a crucial role in the way the World Wide Web functions, as they enable users to access and interact with online content and services. Some of the key benefits and importance of user agents include:

  • Interoperability: User agents enable different devices, platforms, and software to communicate with each other and access the same online content and services.
  • Accessibility: User agents enable users with disabilities to access the web by providing features such as text-to-speech, screen magnification, and keyboard navigation.
  • Personalization: User agents can store user preferences and settings, such as language and display options, which allow for a more personalized web experience.
  • Security: User agents can implement security measures, such as SSL/TLS encryption and certificate verification, to protect user data and prevent malicious attacks.
  • Innovation: User agents enable the development of new and innovative technologies, such as virtual and augmented reality, which can enhance the user experience.

Overall, user agents are an essential component of the web ecosystem, enabling users to access and interact with online content and services in a convenient and secure manner.

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