Using Fiddler for Mobile HTTP Capture, Response Modification, and Network Speed Simulation

Fiddler is an HTTP debugging proxy tool that can inspect the communication between a computer and the Internet, set breakpoints, and view incoming and outgoing HTTP data.

01. Introduction

Fiddler can be used to capture traffic from mobile devices.

02. Capturing on Mobile

Open Tools → Fiddler Options → Connections , check “Allow remote computers to connect”, set the listening port (default 8888) and click OK.

Ensure the phone and PC are on the same network, then configure a manual proxy on the phone: host = PC’s IP address, port = the port set above.

Launch the phone; HTTP traffic will appear in Fiddler.

03. Modifying Returned Data

Use AutoResponder to change response results:

Step 1: Write the desired response content into a text file.

Step 2: Open AutoResponder in Fiddler and add a rule that maps the target URL to the text file.

Step 3: Set the matching conditions and enable the rule.

Example: modify the response of www.baidu.com to return a custom JSON string.

04. Simulating Network Speed

Testing often requires simulating different network conditions; Fiddler provides throttling capabilities.

Enable throttling via Rules → Performances → Simulate Modem Speeds , which immediately slows down the traffic.

For custom speeds, edit the script under Rules → Customize Rules (see screenshot below). The script allows you to introduce delays before sending or receiving data, effectively limiting upload/download rates.

The calculation for throttling is simple: for a target upload speed of 100 kbps, 1 KB / 100 KBps = 0.01 s = 100 ms of delay.