Web - User Agent (UA)


A user agent is the internet name for a process.

It may be:

  • a software
  • an application
  • a service

They are called user agent because they are acting on behalf of a user.

The most well-known user agent is a web browser but it can also be:


A GUI user agent such as a browser may:

  • display a document,
  • read it aloud,
  • cause it to be printed,
  • convert it to another format, etc.


When a software agent operates, it identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer.

Http, SIP, SMTP, …

In HTTP, SIP, and SMTP/NNTP protocols, the identifier:


For an email signing agent, the identifier is the AUID


The user agent string is another name for the user agent identifier value.

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