HTTP request header: UserAgent variable

Question

When sending a HTTP request, IE sends the User-Agent variable to the server. A possible value (as seen by the network debugger):

User-Agent: Mozilla /

Answer 1

The Wikipedia article on User Agent states that:

The User-Agent string format is currently specified by Section 14.43 of RFC 2616 (HTTP/1.1) The format of the User-Agent string in HTTP is a list of product tokens (keywords) with optional comments.

So I recommend you read the RFC 2616 section about it:

14.43 User-Agent

The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user
agent limitations. User agents SHOULD include this field with
requests. The field can contain multiple product tokens (section 3.8) and comments identifying the agent and any subproducts which form a
significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the
application.

   User-Agent     = "User-Agent" ":" 1*( product | comment )

Example:

   User-Agent: CERN-LineMode/2.15 libwww/2.17b3

The section 4.8 about products token states the following:

3.8 Product Tokens

Product tokens are used to allow communicating applications to
identify themselves by software name and version. Most fields using
product tokens also allow sub-products which form a significant part
of the application to be listed, separated by white space. By
convention, the products are listed in order of their significance
for identifying the application.

   product         = token ["/" product-version]
   product-version = token

Examples:

   User-Agent: CERN-LineMode/2.15 libwww/2.17b3
   Server: Apache/0.8.4

Product tokens SHOULD be short and to the point. They MUST NOT be
used for advertising or other non-essential information. Although any token character MAY appear in a product-version, this token SHOULD
only be used for a version identifier (i.e., successive versions of
the same product SHOULD only differ in the product-version portion of the product value).

So according to the RFC 2616, the contents of the User Agents is up to the browser. For instance, here is how Mozilla Firefox builds its user agent: https://developer.mozilla.org/en-US/docs/Gecko_user_agent_string_reference