How to display localhost traffic in Fiddler while debugging an ASP.NET application?

Question

How do I display localhost traffic in Fiddler while debugging an ASP.NET application?

Answer 1

Using Fiddler v4:

1. Check your IE proxy settings

IE->Tools->Internet Options->Connections->Lan Settings

2. Check your settings in Fiddler:

Fiddler -> Options-> Connections & Https

Check the Fiddler port, default is 8888

3. In Fiddler-Menu:

File -> Capture Traffic is checked

The following solution worked for me, when using a

• HttpClient or
• WebClient from inside an ASP.NET application.
  

Web.config

Code:

var resourceServerUri = new Uri("http://localhost.fiddler:YourAppServicePort");
var body = c.GetStringAsync(new Uri(resourceServerUri)).Result;

---

Check if your request actually reaches fiddler by customizing the Fiddler Rules script

Fiddler->Rules->Customize Rules

and hook into the OnBeforeRequest event:

static function OnBeforeRequest(oSession: Session) {

if (oSession.hostname.Contains("localhost:YourPortNumber")
{
 System.Windows.Forms.MessageBox.Show(oSession.hostname);  
} 

Or explicitly by setting a web proxy

WebClient wc = new WebClient();

WebProxy proxy = new WebProxy();
// try one of these URIs
proxy.Address = new Uri("http://127.0.0.1:8888");
proxy.Address = new Uri("http://hostname:8888");
proxy.Address = new Uri("http://localhost.fiddler");
proxy.Address = new Uri("http://ipv4.fiddler");
// https://en.wikipedia.org/wiki/IPv6
proxy.Address = new Uri("http://ipv6.fiddler");

proxy.BypassProxyOnLocal = false; wc.Proxy = proxy;
var b = wc.DownloadString(new Uri(YourResourceServerBaseAddress));