Problem: Unexpected 400 Bad Request Error from IIS Express
Updated on Nov 6, 2025
When requesting http://ipv4.fiddler:2468 from an IIS Express instance, the server returns the following unexpected error message:
HTTP
HTTP/1.1 400 Bad Request
Content-Type: text/html;charset=us-ascii
Server: Microsoft-HTTPAPI/2.0
Date: Wed, 22 Aug 2012 19:59:52 GMT
Connection: close
Content-Length: 334
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN""https://www.w3.org/TR/html4/strict.dtd">
<HTML><HEAD><TITLE>Bad Request</TITLE>
<META HTTP-EQUIV="Content-Type" Content="text/html; charset=us-ascii"></HEAD>
<BODY><h2>Bad Request - Invalid Hostname</h2>
<hr><p>HTTP Error 400. The request hostname is invalid.</p></BODY></HTML>Solution:
Replace http://ipv.fiddler:2468 with http://localhost.fiddler:2468.