FiddlerCore behavior when network is disconnected

Question

I'm handling local requests by using FiddlerCore like this:

private static void FiddlerApplication_BeforeRequest(Session session)
{
    if (session.hostname.ToLower() == "localhost")
        ProcessRequest(session);
}

Everything works well but when the Internet network is down, I'm getting the following message:

"[Fiddler] DNS Lookup for "www.google.com" failed. The system reports that no network connection is available. No such host is known"

My question is:
How should I configure FiddlerCore so when the network is down, I will receive the regular 404 page?

Answer 1

You're noting: When a proxy is present, the browser doesn't show its default error pages. That is a correct statement, and it's not unique to Fiddler.

You're confusing folks because you're talking about "regular 404 response"s. That's confusing because the page you're talking about has nothing to do with a HTTP/404-- it's the browser's DNS Lookup Failure or Server Unreachable error page, which it shows when a DNS lookup fails or a TCP/IP connection attempt fails. Neither one of those is a 404, which is an error code that can be returned by a server only after the DNS lookup succeeds and the TCP/IP connection is successful.

To your question of: How can I make a request through a proxy result in the same error page that would be shown if the proxy weren't present, the answer is that, in general, you can't. You could do something goofy like copying the HTML out of the browser's error page and having the proxy return that, but because each browser (and version) may use different error pages, your ruse would be easily detectable.

One thing you could try is to make a given site bypass the proxy (such that the proxy is only used for the hosts you care about). To do that, you'd create a Proxy Autoconfiguration script file (a PAC file) and have its FindProxyForURL function return DIRECT for everything except the site(s) you want to have go through the proxy. see the Configuration Script section of this post.

But, stepping back a bit, do you even need a proxy at all? Can't you just run your web server on localhost? When you startup Fiddler/FiddlerCore, it listens on localhost:[port]. Just direct your HTTP request there without setting Fiddler as the system proxy.