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Strange Problem – Unable to Connect to the ASP.Net WebServer on Localhost

Introduction

This problem appeared out of the blue in Visual Studio 2005 on my work laptop. Google didn’t help with a solution, so I thought a brief blog post was in order.

The Issue

Trying to launch any ASP.Net application on my local machine, either in debug mode or starting without debugging, started the Cassini ASP.Net Webserver just fine, but the browser just threw up an “unable to connect” error. Google seemed to suggest it might be a corrupt webserver exe, but it seemed to be running fine so I ignored that option. I tried manually specifying a port in the project settings, and even using telnet to try access the port on localhost from the command prompt, but even though the webserver was running, nobody was listening!

The Fix

After clutching at random straws for a while I did a “ping localhost” from the command prompt and noticed it was returning replies from ::1 (the IPv6 “loopback” address). I popped open my hosts file (c:\windows\system32\drivers\etc\hosts) and noticed that localhost was mapped to ::1, but not to 127.0.0.1. I have no need for IPv6 on my machine so I commented out that line and added:

127.0.0.1       localhost

Fired up Visual Studio, pressed F5 and voila – all fixed! I’m not sure why this was suddenly a problem, it just appears that Cassini just doesn’t like being accessed via IPv6, but this seems to be the solution if this issues bites you :-)

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Posted in ASP.Net, Visual Studio.

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