This quick guide will show you how you can add and modify headers in Fiddler and them to all requests.
There will be times where you will need to test performance or functionality of web apps which may require the modification of request headers – for example, if you were to move to a shared hosting environment and want to test the functionality and performance of the new host without changing DNS records, you will need to modify the HOST header otherwise the host will not know which site you want to visit.