What are User Agents?

The term 'user agent' is an umbrella word that is used to describe any application through which a user accesses any kind of network. The term is most commonly used when describing the client application through which a user accesses the world wide web.

When the term is used technically in specific reference to a certain application purpose, such as a user using the internet to access their e-mail through a certain site, the user agent means the application used to access the specific network protocol used for the purpose the user is using it for. In other words in the case of the e-mail example, the user agents would be the internet service provider, the browser, and the secure log-in page that only works for a specific protocol (accessing the user's e-mail).

As the term is most often used in regards to internet access and web surfing, some of the most common user agents are web browsers, search engine crawlers (spiders), and e-mail clients. User agents can also be physical devices that access certain networks such as a cell phone accessing a certain network only, or a computer that accesses a secure network (such as in a closed or password protected network).

The interesting thing about a user agent is that it has a unique text string that is sent to the server it is trying to access which basically identifies the user agent and client information to the network server. This information can include IP address, operating system, specific program requesting access, and more. It is valuable because it allows not only for secure access points to remain secure, but also allows host sites and networks to run parallel sites. This means that specific user agents will be able to access different material then other user agents. The text string identifies the agent and this identification information allows a network server to show different content/information to one agent or another based on what is in the string.

Of course the downside to this is that malicious code and viruses can interfere with this communication by imitating a user agent, sending false identification information allowing access to content not meant to be accessed by the fake agent, or even reversing it and posing as the specific network protocol for the transmitted string to collect user data (such as log in information) illegally. Encryption is used by browsers to help reduce miscommunication between user agents and networks but isn't perfect quite yet.

No matter who you are or what you do, if you use cell phones, computers, or any device that connects to any kind of network to access content or to communicate to other devices then you are a user of client applications that can be considered user agents.