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Before I start, I want to say that I know very little about the OS implementation of networking features, or about TCP/IP and basically most anything relevant to this question.

I thought that Fiddler worked at the operating system level, just capturing any and every outgoing piece of information sent by any process running on your system. Yet by default, Firefox traffic does not appear - you need to use the FiddlerHook plugin, and even then there's an option to not send traffic to Fiddler. How can this be? How does Firefox's traffic dodge past Fiddler as it exits my computer?

If Fiddler captures all traffic that leaves the computer, then Firefox's traffic should appear. If an application has to specifically send its traffic to Fiddler, then how is it that other applications do this without the need for a Fiddler plugin or an explicit "Send to Fiddler" option?

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Fiddler is a debugging Proxy - that means, traffic must be routed through it to be visible. I would assume that Internet Explorer is set to use the default system proxy, whereas Firefox doesn't use a proxy (by default) hence the plugin requirement.

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Oh, of course. I hadn't thought about the possibility of some sort of default system proxy that Fiddler could simply set itself as. –  Jack M Feb 28 '13 at 15:05

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