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.
Http, SIP, SMTP, …
In HTTP, SIP, and SMTP/NNTP protocols, the identifier:
- is known as the agent string
- is transmitted in a header field User-Agent
For an email signing agent, the identifier is the AUID
The user agent string is another name for the user agent identifier value.