PROXY_UNABLE_TO_CONNECT_TO_DESTINATION

An attempt to proxy a request failed because the proxy was not able to successfully connect to the destination. This likely indicates an issue with the request itself (for instance, the hostname failed to resolve to an IP address or the destination server refused the connection). This error code is used to indicate that the error is outside the control of the proxy server and thus the proxy chain should not be marked as bad. This is in contrast to TUNNEL_CONNECTION_FAILED which is used for general purpose errors connecting to the proxy and by the proxy request response handling when a proxy delegate does not indicate via a different error code whether proxy fallback should occur. Note that for IP Protection proxies this error code causes the proxy to be marked as bad since the preference is to fail open for general purpose errors, but for other proxies this error does not cause the proxy to be marked as bad.