Unfortunately, this question is quite complex.
By default, Fiddler launches and adopts the system's proxy settings as an upstream proxy. If a host:ip is configured in those settings but that host:ip are not reachable when Fiddler starts, Fiddler ignores the proxy settings and defaults to sending traffic directly to the origin.
If a host:ip proxy is initially responsive when Fiddler boots, but later becomes unresponsive, requests from Fiddler will fail until that proxy becomes responsive again.
If Windows notifies Fiddler that the network has changed (e.g. you connected or disconnected for WiFi) Fiddler may redetect the upstream proxy settings.
If you manually specify a proxy using the X-OverrideGateway
flag on a Session, Fiddler 126.96.36.199 and earlier will fail the request if the proxy is unresponsive and
the proxy uses SOCKS; if the proxy is unresponsive and is not SOCKS, the request will be sent direct. In Fiddler 188.8.131.52 and later (to be released shortly), Fiddler will fail the request if the X-OverrideGateway proxy is unresponsive whether it uses SOCKS or not.
If you have a scenario where it makes sense for Fiddler to NOT ignore these proxy settings, please help explain it and I will consider adding a preference for that behavior in the next version.
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