CODEWITHSUNDEEP
network-programmingproxyfiddler
CodyBugstein
CodyBugstein

Reputation: 23322

Why can't Fiddler detect traffic through proxy servers?

I had Fiddler open recently with no filters on, and I was running a program from my command prompt that communicates with a server (it's a simple in-house proprietary program).

Interestingly, Fiddler was not picking up this communication, which is mostly made up of REST API calls. I did some research and found this quote, from here

With regard to why you're not seeing this traffic in Fiddler-- assuming you don't have any filters set, this suggests that whatever mechanism you're using to send the HTTP request isn't adopting the system's proxy settings. This means, for instance, that your code would fail if run on a corporate computer that requires a proxy server to reach the Internet.

However, I'm wondering why this would be the case. As far as I understand, my computer still needs to send data from my network card to the proxy server's network card. The traffic isn't bypassing my network card - it simply carries a different address. Why isn't Fiddler able to see this?

Upvotes: 2

Views: 659

Answers (1)

Marc
Marc

Reputation: 14277

The reason is that Fiddler works not by sniffing your network card but by installing itself as the system proxy. If you click start and type "Internet Options" and choose the tab "Connections" and the button "LAN Settings" you'll see that localhost:8888 is your system proxy. Now, most well-behaved clients (e.g. IE and Chromer) respect and use the system proxy but some don't. In particular Java programs have their own ideas about which proxy they will use and you would have to set that separately. For other programs which are just hard-coded to make their own direct HTTP request you cannot AFAIK monitor them with Fiddler.

Upvotes: 1

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