Problem: Traffic sent to http://localhost or http://127.0.0.1 is not captured

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.

  • This behavior was changed for Internet Explorer 9 in the Release Candidate build. IE9 RC allows Fiddler to proxy traffic sent to localhost or 127.0.0.1 without additional steps on your part.

  • You should never 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.

Solution 1: Use Machine Name or Hostname

The simplest workaround is to use your machine name as the hostname instead of Localhost or 127.0.0.1. For example, rather than hitting:

http://localhost:8081/mytestpage.aspx

Instead visit:

http://machinename:8081/mytestpage.aspx. 

Solution 2: Use http://ipv4.fiddler

Use http://ipv4.fiddler to hit localhost on the IPv4 adapter. This works especially well with the Visual Studio test webserver (codename: Cassini) because the test server only listens on the IPv4 loopback adapter. Use http://ipv6.fiddler to hit localhost on the IPv6 adapter, or use http://localhost.fiddler to hit localhost using "localhost" in the Host header. This last option should work best with IIS Express.

Solution 3: Updated Rules File

  1. Update your Rules file as follows:

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

  2. Make requests for http://myapp, which will act as an alias for 127.0.0.1:8081.

See Also