Configuring clients

Can I monitor RAS, VPN, or dialup connections? 

IE will always use the proxy settings from any active VPN connection, without regard to whether or not that VPN connects to the Internet.  You'll need to set the Proxy Configuration manually (or make a configuration change to Fiddler; see below). 

Set the proxy for your VPN/dialup connection as follows: 

    

Alternatively, you can set the "Use automatic configuration script" option as described in the "Other browsers" tip below.

Note: If you always have an active VPN or dialup connection, set the HookConnectionNamed registry value to the name of the Connection (e.g. "Dial-up Connection" in the above screenshot).  When you do this, Fiddler will hook this connection instead of the LAN connection.

How can I configure a .NET v1.1 application to use Fiddler?

How can I configure a  .NET v2.0 application to use Fiddler?

How can I configure a Java application to use Fiddler?

Per http://www.davidreilly.com/java/java_network_programming/#2.4 you should be able to do something like:

jre -DproxySet=true -DproxyHost=127.0.0.1 -DproxyPort=8888 MyApp

Why don't I see traffic sent to http://localhost or http://127.0.0.1?

IE7 and the .NET Framework are hardcoded not to send requests for Localhost through any proxies, and as a proxy, Fiddler will not receive such traffic.

The workaround is to use your machine name as the hostname instead of Localhost or 127.0.0.1. So, for instance, rather than hitting http://localhost:8081/mytestpage.aspx, instead visit http://machinename:8081/mytestpage.aspx. 

...Or, if you're using Fiddler v2.1.8 or later, just use http://ipv4.fiddler to hit localhost on the IPv4 adapter, or use http://ipv6.fiddler to hit localhost on the IPv6 adapter.  This works especially well with the Visual Studio test webserver (codename: Cassini) because the test server only listens on the IPv4 loopback adapter.

Lastly, you could Customize your Rules file like so:

static function OnBeforeRequest(oSession:Fiddler.Session){
  if (oSession.HostnameIs("MYAPP")) { oSession.host = "127.0.0.1:8081"; }
}

...and then just hit http://myapp, which will act as an alias for 127.0.0.1:8081.

How can I configure a WinHTTP application to use Fiddler?

You can either directly configure the WinHTTP application to point to Fiddler, in code, or you can use the following command at the command prompt to tell WinHTTP to use Fiddler:

proxycfg -p http=localhost:8888;https=localhost:8888

...or this one to force WinHTTP to use WinINET's proxy settings:

proxycfg -u

Can I configure IEMobile on PocketPC to use Fiddler?

Yes.  This works over desktop-passthrough (ActiveSync connection).

On the device, set the HTTP proxy to be “ppp_peer:8888”
On your desktop, open port 8888 in Windows Firewall (Control Panel)
ActiveSync->Connection Settings… This Computer is connected to: “Work Network”
(Automatic will clobber the setting you did from #1)
Start Fiddler as you normally would
Tools->Fiddler Options… check “Allow remote clients to connect” (Restart Fiddler)
Connect your device to ActiveSync, fire up IEMobile and browse away…

Can I configure Fiddler to debug traffic from another machine (even a Mac or Unix box)?

Yes. 

1. Start Fiddler on a Windows machine named (for example) WINBOX1.
2. In Fiddler, click to make sure Tools / Fiddler Options / Allow remote clients to connect is checked.
(Restart Fiddler if this box wasn't already checked.)
3. On the other machine (Mac/Unix/Windows) set the proxy settings to WINBOX1:8888.

The second machine will now send its traffic through Fiddler running on WINBOX1.

How can I configure other browsers to use Fiddler?

Can Fiddler "chain" to an upstream proxy that is "Automatically detected"?

Yes, all current versions of Fiddler support this configuration.