I'm testing a Windows desktop application, that requests data from a server during certain operations. I want to make sure the application can handle all possible responses, including bad ones.

What is the easiest way to accomplish this?

Will I have to create a test server, and use the hosts file to connect to that server instead of the real server? Is there a better way?

Are there tools available that can already do this?

3 Answers 3


Fiddler is a great tool for this kind of testing. It is an http proxy which allows you to send a request and then "fiddle" with both the request and the response between your app and going to the server. You can make small changes to a response, or create a response from scratch to send to your app.

  • I agree - fiddler is a good tool to use here. May 29, 2012 at 19:35
  • And here is a good video on using Fiddler: mehdi-khalili.com/advanced-web-debugging-with-fiddler May 29, 2012 at 22:56
  • Does fiddler work with desktop applications, or is it only useful with web applications? I'm not able to get it to see the request sent by my desktop application.
    – Tester101
    May 31, 2012 at 15:16
  • It works with any http traffic from any source. If the traffic is not over http then you would need a different tool. Did you set the proxy for your app? fiddler2.com/fiddler/help/hookup.asp#Q-ConfigureApp
    – Sam Woods
    May 31, 2012 at 19:01
  • Another thing I should mention, Fiddler has an API as well which makes it useful for automating these tests if you are looking at doing that.
    – Sam Woods
    May 31, 2012 at 19:07

Fiddler is a great (and free) tool for this, like Sam mentioned. It accomplishes a lot for a free app and even has a few features that others don't, like changing your user agent.

I do have Fiddler installed, but when I want these kinds of tools I tend to open Charles: http://www.charlesproxy.com/

It isn't free ($50/seat), but the additional tools available are really awesome. Map Local, Map Remote, DNS Spoofing, Port Forwarding, Reverse Proxies, etc.

If you're in more of a security mindset, there is also ZAP (Zed Attack Proxy). It will do many of your normal proxy tasks as well as quite a lot of other goodies. Just be careful where you point some of the tools :)

ZAP: https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project


If you want an in-browser solution, you can use the Firefox addon Tamper Data. Just like fiddler, you'll be able to modify each request being made in the browser.

From the addon description:

Use tamperdata to view and modify HTTP/HTTPS headers and post parameters.

Trace and time http response/requests.

Security test web applications by modifying POST parameters.

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