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 bot
• the process that signs email

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.

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.

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