An HTTP header lets the client and the server to send extra information with either request or response. The HTTP headers can be classified according to their context, such as General Header, Request Header, Response Header and Entity Header.  In case of a request header, a case insensitive name is utilized which is followed by a colon and then the value is given. Headers are name/value pairs that appear in both request and response messages of HTTP. You can use the below line on your haproxy configuration file for rewriting http header