1) Download and Install ParosGrab the download from the Paros site. Your install process will differ depending on your O/S, but they've provided some install instructions here. Everybody will need the Java Runtime Environment 1.4 or above.
2) Configure ParosOnce installed, launch Paros and find the configuration options (on OS X they are under Tools -> Options). Paros is configured by default to listen on localhost only, but we are going to route our iPhone's traffic through Paros, so we need to set it to listen on the IP address of the interface connected to the same LAN as the iPhone.
My LAN's network is 126.96.36.199/16, so I'll configure the Local Proxy address accordingly:
3) Configure iPhoneOn the iPhone, open the "Settings" app and navigate to the Wi-Fi page. Once there, edit the settings for the wireless network you are currently connected to (this needs to be the same network where your proxy is running). To do this, click the little blue arrow on the right side of the screen.
4) Using ParosThe main section of Paros is the "Request/Response/Trap." As the iPhone talks through Paros to Internet sites, it will display the iPhone's request and the server's response. The "trap" functionality allows you to stop either the request or the response and view/modify it before sending it along to the recipient. Trapping is very cool, and why Paros is used for security auditing, but for our purposes we just want to see what is going on, so I won't explain it any further.
For now, let's see what happens when we fire up my iPhone's "App Store" app:
In the bottom section of the screen is the history viewer. There we can see that my iPhone made 4 requests to different servers ( 3 GETs and 1 POST):
Sniffing traffic like this can help you understand how different iPhone apps work behind the scenes or it can help debug interaction for an app that you're writing. Hope this helps you get started!