I tried to use a website's API, but I always become a ProtocolError (UnsafeHeaderParsing did not help me) so I decide to capture the traffic of a program which is using that API, so that I know which attributes I have to set in the Header/POST...The problem is that Fiddler is currently only capturing the traffic of my browsers, how can I change that so, that it captures traffic of programs which are runnung on my localhost.
Message Analyzer can do a great deal of capture and analysis, correlating events and so on. You can download it here.
NetMon is the old alternative, now replaced by Message Analyzer.
An of course, Wireshark if you want a non-OS alternative.