ipv4.fiddler, how does it work?

Question

I'm curious as to how fiddler is able to capture traffic when you use the URL ipv4.fiddler. Is ipv4 a special domain that resolves local and fiddler just registers with http.sys to proxy the call?

Any insight would be great.

Answer 1

Easy: Fiddler is a proxy, it gets to see almost(*) all of the requests. When it sees a request bound for "ipv4.fiddler", it simply changes it to 127.0.0.1.

(*)The only reason Fiddler needs this at all is that some HTTP stacks are hardcoded to bypass the proxy for "localhost" addresses. By using the magic string "ipv4.fiddler", you can circumvent that hardcoded restriction.