HTTP requests may include a ‘referer’ field, which is used to tell from which URL the client got to this particular page. Many programs/scripts check the referer field of requests to check the source of the request. You can simulate the referer field by the following command.

curl -e http://some_referring_site.com  http://www.example.com/

All HTTP requests may set the User-Agent field. It names what user agent or client that is being used. Many web applications use this information to decide how to display web pages or use it to track browser usage. You can impersonate a particular browser by the following method:

curl -A "Mozilla/5.0 (compatible; MSIE 7.01; Windows NT 5.0)" http://www.example.com