HTTP Header User-Agent Field Format Required?

Question

I could not find anything in the RFC docs about whether the User-Agent field needed to be formatted the way it commonly is. For example, could I just do:

User-Agent: Mozilla windows NT

and still pass a valid request?

Answer 1

The formal definition of the User-Agent header is defined in RFC 2616 Section 14.43 as follows:

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

product is defined in Section 3.8 as follows:

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

token and comment are defined in Section 2.2 as follows:

token          = 1*<any CHAR except CTLs or separators>
comment        = "(" *( ctext | quoted-pair | comment ) ")"

Whitespace is a separator, so to answer your question, Mozilla windows NT is NOT a valid product by the above token definition. If you want to follow the spec, you could instead format it more like this: Mozilla (Windows NT).

Answer 2

Sure, you can 'fake' a user agent. Check for yourself with this tool for instance: https://chrome.google.com/webstore/detail/user-agent-switcher-for-c/djflhoibgkdhkhhcedjiklpkjnoahfmg