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Vasanthdev2004
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Asking the Proxy function a question of our own invention answered for a request the transport never made. With NO_PROXY excluding a host, the real request to that host goes direct while a synthetic probe still comes back proxied, so a direct dial whose address happened to equal the configured proxy endpoint skipped the guard and reached whatever the name resolved to at connect time. A probe on the previous head dials rebind.example:80 with zero safety lookups; the guard now runs and refuses its private answer. Host-dependent proxy callbacks make the mismatch general, and no spelling of the probe host closes it, because the two calls are about different requests. The decision is now recorded where it is made. proxydial.Transport wraps the transport, asks its Proxy function about the actual request, and puts the answer in that request's context; the transport derives the dial context from the request context, so the dialer reads back the route this very connection is being opened for. Per-request, so concurrent requests cannot overwrite each other, and the context key is unexported so nothing outside the package can plant one. Address matching is gone with it. A transport opens one connection dial, and when a proxy is selected that dial goes to the proxy, so the recorded route already identifies the dial. Comparing the address on top could only disagree with the transport, which is what a Unicode proxy hostname did: Go dials xn--bro-hoa.example while the URL still spells it as configured, and the proxy was refused as an ordinary private address. Both spellings are now one endpoint with no comparison to get wrong. Documents where destination enforcement ends: with a proxy configured, the target hostname is sent to the proxy and the proxy resolves it, so the local pre-request checks no longer bind the address that is reached. A forward proxy already sees and can rewrite every request through it, so that trust comes with configuring one; the alternative is refusing to use proxies at all, which is the outage in #569.