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

Try with http://127.0.0.1. <-- note the . at the end

So you can still connect to Casini and debug easily (I'm currently debugging page on http://127.0.0.1.:1718/login/Default.aspx ).

Answer 2

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

<system.net>
    <defaultProxy
                enabled = "true"
                useDefaultCredentials = "true">
      <proxy autoDetect="False" bypassonlocal="False" proxyaddress="http://127.0.0.1:8888" usesystemdefault="False" />
    </defaultProxy>

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));

Answer 3

To make Fiddler work on localhost with IIS Express you should use this form of URL

http://localhost.fiddler:50262/

This puts correct Host header value (localhost) which satisfies IIS Express.

Answer 4

Probably the easiest way to monitor traffic to localhost is to replace "localhost" with "localhost." in the browser's URL bar. E.g.

http://localhost./MyApp/default.aspx

Answer 5

Ensure that in your Fiddler Connections that localhost isn't in the "IE should bypass Fiddler for URLs that start with:" box.

Answer 6

Check out this link...the 'workaround' is hacky, but it does work:

Tip for using Fiddler on localhost