Configuring clients

Can I monitor RAS, VPN, or dialup connections? 

Fiddler 2.2.0.3 and later introduces a new option which enables automatic monitoring of dialup or VPN connections:

Otherwise, you'll need to set the Proxy Configuration manually. 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.

Note: IE will always use the proxy settings from any active VPN connection, without regard to whether or not that VPN connects to the Internet. 

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

How can I configure a PHP/CURL application to use Fiddler?

curl_setopt($ch, CURLOPT_PROXY, '127.0.0.1:8888');

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?

Internet Explorer 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 simplest 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, 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 update 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.

Note: You shouldn't ever encounter the "Localhost traffic not captured" problem with Firefox. The FiddlerHook add-on for Firefox removes "localhost" from the "bypass proxy" list when Fiddler is in "Capturing" mode.

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:

On XP or below:

proxycfg -p http=127.0.0.1:8888;https=127.0.0.1:8888

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

proxycfg -u

On Vista or above:

netsh winhttp set proxy 127.0.0.1:8888

 

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?

Yes, all current versions of Fiddler support chaining to upstream proxies (either autodetected or manually specified).

So, you end up with an architecture like so: 

Note that Fiddler does not support upstream proxy configuration scripts that are accessed using the FILE:// protocol, only those accessed using the HTTP or HTTPS protocols (so far, no one seems to have hit this limitation in the last 6 years).